Magnetic North II : Competing Interactions in Magnetic Materials

June 10-12, 2011

Murray Premises

St. John’s, Newfoundland

Organizing Committee: Martin Plumer, John Whitehead, Stephanie Curnoe Memorial University of Newfoundland This second of a series of annual workshops is devoted to research related to the materials properties of magnetic systems with a focus on behavior resulting from competing interactions. Four themes are covered, Competing Interactions, Neutron Scattering, Dynamics, and Exotic Spin States, from areas of both classical and quantum magnetism involving aspects of experiment, simulation, and theory. A particular goal of the workshop is to bring together researchers who are active in fundamental and applied areas of magnetism in order to explore and benefit from mutual interests. Presentations will be from a dozen invited speakers, seventeen contributed talks and nine posters. It is hoped that the modest-sized venue and the organization of presentations and activities will foster informal interaction and future collaborations. Magnetic North is an organization of magnetism researchers in Canada and their international collaborators: http://www.magneticnorth.mun.ca/. It is a forum for information exchange on individual and group research activities. Magnetic North also serves as the basis for the organization of regular magnetism sessions within the annual congress of the Canadian Association of Physicists as well as stand-alone Magnetic North workshops. There are a broad range of magnetic researchers in Canada, spanning various academic departments and government laboratories using a variety of experimental, theoretical and computational techniques. Research interests encompass geometrically constrained systems (thin films and wires), molecular magnets, dipolar systems, frustration, quantum effects, phase transitions, magneto-electric materials, etc. A goal of Magnetic North is to facilitate the exchange of ideas between researchers and to reveal overlapping areas of interest that can foster useful collaborations, serving also to strengthen the magnetism research community in Canada as a whole. It is anticipated that the next workshop in this series (Magnetic North III) will take place in Banff, Alberta in June 2012. A list of the sponsors of the Magnetic North II workshop is presented at the back of this booklet.

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PROGRAM Invited talks, listed in the Program as I.1 to I.12, are 40 minutes long which includes 5 minutes for questions. Contributed talks (listed as C.1 to C17) will be 20 minutes in length, including 3 minutes for questions. Abstracts for the posters Posters P.1 to P.9 are included in this Program. The session chairs will ensure that these times are adhered to. All talks (along with the breakfasts, lunches, refreshment breaks, reception, and banquet that are included in the registration fee) will take place at the Murray Premises. There are two social events in the program a Welcome Reception will be held Friday June 10 starting at 5:15 pm, which will coincide with the Poster Session, in the Murray Premises (conference room). As well, there will be a Workshop Banquet on the evening of Saturday June 11, for dinner at 7:00 pm in the Gypsy Tea Room, Murray Premises. The Workshop will end after lunch on Sunday June 12 at 1:30 pm. Thursday, June 9 6:00 pm to 8:30 pm Registration in the Murray Premises lobby. 6:00 pm to ? Informal gathering: Yellow Belly brew pub, see map below. Friday, June 10 Murray Premises conference room 8:00 am to 9:00 am BREAKFAST 8:00 am to 9:15 am REGISTRATION SESSION 1 Session chair: Byron Southern 9:25 am to 9:30 am Welcome and opening remarks 9:30 am to 10:10 am Paper I.1 Speaker: Mike Coey 10:10 am to 10:50 am Paper I.2 Speaker: Johan van Lierop 10:50 am to 11:10 am BREAK SESSION 2 Session chair: Kimberley Hall 11:10 am to 11:50 am Paper I.3 Speaker: Hong Guo 11:50 am to 12:10 am Paper C.1 Speaker: Mike Cottam 12:10 pm to 12:30 pm Paper C.2 Speaker: Tim Fal 12:30 pm to 2:00 pm LUNCH SESSION 3 Session chair: Johan van Lierop 2:00 pm to 2:40 pm Paper I.4 Speaker: Stephen Nagler 2:40 pm to 3:00 pm Paper C.3 Speaker: Carl Adams 3:00 pm to 3:20 pm Paper C.4 Speaker: David Menard 3:20 pm to 3:40 pm BREAK

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SESSION 4 Session chair: Uli Nowak 3:40 pm to 4:20 pm Paper I.5 Speaker: Dominic Ryan 4:20 pm to 4:40 pm Paper C.5 Speaker: Martin Leblanc 4:40 pm to 5:00 pm Paper C.6 Speaker: David Venus 5:15 pm to 7:00 pm POSTER SESSION and RECEPTION. Conference room. Session chair: Martin Plumer P1. Oktay Aktas P2. Niloufar Faghihi P3. Tianheng Han P4. Vahid Hemmati P5. Travis Redpath P6. Renan Villarreal P7. Wiqar Hussain Shah P8. Amir Roohi Noozadi P9. Michael Coates

Saturday, June 11 Murray Premises conference room 8:00 am – 9:00 am BREAKFAST SESSION 5 Session chair: Stephanie Curnoe 9:00 am to 9:40 am Paper I.6 Speaker: Leon Balents 9:40 am to 10:20 am Paper I.7 Speaker: Oleg Tchernyshyov 10:20 am to 10:40 am BREAK SESSION 6 Session chair: Stephen Nagler 10:40 am to 11:20 am Paper I.8 Speaker: Tsuyoshi Kimura 11:20 am to 11:40 am Paper C.7 Speaker: Rogerio de Sousa 11:40 am to 12:00 pm Paper C.8 Speaker: Ion Garate 12:00 pm to 12:20 pm Paper C.9 Speaker: Shaoyan Chu 12:20 pm to 1:40 pm LUNCH SESSION 7 Session chair: John Whitehead 1:40 pm to 2:20 pm Paper I.9 Speaker: Kimberley Hall 2:20 pm to 2:40 pm Paper C.10 Speaker: Larysa Tryputen 2:40 pm to 3:00 pm Paper C.11 Speaker: Kris Poduska 3:00 pm to 3:20 pm BREAK SESSION 8 Session chair: Can-Ming Hu 3:20 pm to 4:00 pm Paper I.10 Speaker: Burkard Hillebrands 4:00 pm to 4:20 pm Paper C.12 Speaker: Byron Southern 4:20 pm to 4:40 pm Paper C.13 Speaker: Arthur Yelon

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Saturday, June 11 6:45 pm - Banquet at The Gypsy Tea Room, Murray Premises.

Sunday, June 12 Murray Premises conference room 8:00 am to 9:00 am BREAKFAST SESSION 9 Session chair: Mark Freeman 9:00 am to 9:40 am Paper I.11 Speaker: Jacob Burgess 9:40 am to 10:00 am Paper C.14 Speaker: Can-Ming Hu 10:00 am to 10:20 am Paper C.15 Speaker: Bartek Kardasz 10:20 am to 10:40 am BREAK SESSION 10 Session chair: David Venus 10:40 am to 11:20 am Paper I.12 Speaker: Uli Nowak 11:20 am to 11:40 am Paper C.16 Speaker: Jason Mercer 11:40 am to 12:00 pm Paper C.17 Speaker: John Whitehead 12:00 pm to 1:30 pm LUNCH

Yellow Belly brew pub. 

 

288 Water Street, corner George St. 

 

Across from Murray Premises. 

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Dilute ferromagnetic oxides; are they possible ?

J. M. D. Coey,1,

1School of Physics and CRANN,

Trinity College, Dublin 2, Ireland.

Ten years have passed since the publication of the Science paper by Matsumoto et al [1]

claiming that TiO2 (anatase) thin films doped with 7 % Co were ferromagnetic at room

temperature. The paper has since been cited 1150 times, and there is a voluminous literature

literature, probably now just past its peak, on many insulating, semiconducting or metallic oxides

which exhibit weak ferromagnetic-like magnetization curves with little hysteresis or temperature

dependence. between helium temperatures and room temperature. The samples are thin films or

nanoparticles. either doped with a few percent of 3d transition metal ions, or even undoped.

The results, which have been reviewed recently by the author [2], are completely at odds with

traditional understanding of the magnetic behaviour of oxides. We review the data, which

demonstrate that only a few percent at most of the volume of the samples can be magnetically

ordered. A typical sample moment is very small, 10-7

– 10-8

A m2

so we first discuss if there is

convincing evidence that it is intrinsic to the samples, rather than an experimental artefact or

contamination.

Several models that have been proposed are then critically examined. The dilute ferromagnetic

semiconductor picture is incompatible with the data. Two models remain. One is the charge-

transfer ferromagnetism model, where electrons are present in a spin-split band at the surfaces or

grain boundaries. The Stoner ferromagnetism of such a magnetic ‘grain-boundary foam’ may be

very high. The other picture is of giant orbital paramagnetism associated with surfaces or grain

boundaries. In this case there is no collective magnetic order. Future steps are suggested to

resolve the issue.

[1] Y. Matsumoto, M. Murukami, T. Shono et al, Science 291 854 (2001)

[2] J. M. D. Coey, Dilute Magnetic Oxides, Ch 20 in Handbook of Spin Transport and

Magnetism, E Tsymbal and I Zutic, editors, Taylor and Francis, London, 2011

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Increased surface spin stability in γ-Fe2O3 nanoparticles with a Cu

shell

R. D. Desautels,1 E. Skoropata,1 Y.-Y. Chen,2 H. Ouyang,2 J. W. Freeland,3 J. van Lierop1

1 Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB, R3T 2N2,

Canada2 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu,

Taiwan 300, R.O.C.3 Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439

Progress in nanotechnology depends upon the continued improvement of control over material

properties required for applications. At the nanoscale, control is provided via tuning the finite

size and surface effects. The physics of these two effects govern the nanomagnetism of a material,

and point to new ways of investigating processes fundamental to magnetism and undiscovered

applications. By coating γ-Fe2O3 nanoparticles with a nonmagnetic Cu shell, we demonstrate an

innovative way to alter the surface nanomagnetism. We have discovered that CuO has formed

at the interface between the γ-Fe2O3 nanoparticle coated fully with at least ∼0.5 nm Cu, and

transforms the intrinsic magnetism of the nanoparticle system so as to alter the typical disor-

dered surface Fe spin configuration of γ-Fe2O3 nanoparticles. Element specific x-ray absorption

spectroscopy (XAS) and magnetic circular dichroism (XMCD) at the L-edges for Cu and Fe

revealed that the magnetic moment of the Cu in the shell interacted with the Fe surface mag-

netic moments, and this seemed to have canted the moments of the CuO (an antiferromagnetic

typically) so that a Cu magnetization occurred that in turn modified the γ-Fe2O3 surface mag-

netism. The Cu coating altered the surface anisotropy of γ-Fe2O3 nanoparticles and apparently

modified the Fe(octahedral)–O–Fe(tetrahedral) superexchange (the dominant exchange pathway

in γ-Fe2O3). This thin layer of CuO had a Cu moment aligned with the octahedral Fe3+. Since

the ferrimagnetic order of γ-Fe2O3 arises from superexchange through the oxygen atoms via their

d-orbitals, the exchange path is likely Cu–O–Fe superexchange.

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Helical states of topological insulator Bi2Se3

Yonghong Zhao1,2, Yibin Hu1, Lei Liu3, Yu Zhu3 and Hong Guo1

1 Centre for the Physics of Materials and Department of Physics, McGill University, Montreal,

QC H3A 2T8 Canada2 College of Physics and Electronic Engineering, Sichuan Normal University, Chengdu 610068,

China3 Nanoacademic Technologies Inc. 7005 Blvd. Taschereau, Brossard, PQ, J4Z 1A7 Canada

We report density functional theory analysis of the electronic and quantum transport properties

of Bi2Se3 topological insulator, focusing on the helical surface spin states at the Fermi level

EF . The calculated Dirac point and the tilt angle of the electron spin in the helical states are

compared quantitatively with the experimental data. The calculated conductance near EF shows

a V-shaped spectrum, consistent with STM measurements. The spins in the helical states at EF

not only tilts out of the two dimensional plane, they also oscillate with a 3-fold symmetry going

around the two dimensional Brillouin zone. The helical states penetrate into the material bulk,

where the first quintuple layer contributes 70% of the helical wave functions. By projecting the

calculated helical states onto atomic orbitals, rather peculiar orbital symmetry was found.

7

A Microscopic Theory for Collective Spin Waves in Magnonic Nanostructures

Hoa T. Nguyen and Michael G. Cottam

Department of Physics & Astronomy, University of Western Ontario, London, Ontario, Canada

By analogy with photonic crystals, the spectrum of the collective spin waves in dense periodic arrays of interacting magnetic nanoelements gives rise to magnonic crystals in which there are allowed magnonic bands alternating with forbidden band gaps. Reviews of the emerging field of magnonics were featured in a special edition of Journal of Physics D (volume 13, number 26) in 2010. In general, the calculations of magnonic band structures have so far proceeded using macroscopic (continuum) and/or simulation techniques, whereas in the work described here we employ a microscopic (or Hamiltonian-based) approach for the competing short-range exchange and long-range dipole-dipole interactions. We have generalized our previous studies (see, e.g., [1, 2]) for individual magnetic nanoelements and small finite arrays of elements to cases where there are effectively-infinite periodic arrays of elements in one or two dimensions. This is achieved by introducing one (or two) Bloch wave numbers that are associated with the periodicity properties and by reformulating the microscopic dipole-dipole and exchange sums for the periodic structure to include the inter-stripe and intra-stripe contributions. In particular, the theory is compared with published Brillouin light scattering data for different magnonic crystals consisting of Permalloy stripes alternating withnonmagnetic spacers, as well as with the macroscopic theories. Good agreement with the experimental data is found, especially for smaller stripe sizes where the microscopic dipole-exchange approach seems to have some advantages over the macroscopic methods, which are more practical for large stripes sizes. Applications to magnonic arrays of periodically-spacedmagnetic “dots” or “antidots” in one and two dimensions are also discussed.

[1] H. T. Nguyen, T. M. Nguyen and M. G. Cottam, Phys. Rev. B 76, 134413 (2007). [2] S. Tacchi, M. Madami, G. Gubbiotti, G. Carlott, S. Goolaup, A. O. Adeyeye, H. T. Nguyen and M. G. Cottam, J. Appl. Phys. 105 (2009).

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Domain wall and microwave assisted switching in an exchange spring bilayer

T. J. Fal1, K. L. Livesey1,2,* and R. E. Camley1

1Center for Magnetism and Magnetic Nanostructures, University of Colorado at Colorado

Springs, 1420 Austin Bluffs Parkway, Colorado Springs, CO 80918

2 University of Western Australia, 35 Stirling Hwy, Crawley WA 6009, Australia

* Currently at Commonwealth Scientific and Industrial Research Organisation, 26 Dick Perry

Ave, Kensington WA 6151, Australia

Abstract

We explore the response of a magnetic bilayer to a driving microwave field using micromagnetic

simulations. The bilayer consists of 8 nm of a material with a high uniaxial anisotropy and 56 nm

of a material with a lower uniaxial anisotropy. The width and length of the structure is 100X100

square microns. A small applied field, opposite to the magnetization, switches most of the lower

anisotropy material but not the higher anisotropy material, forming a domain wall between the

two materials. We evaluate the frequencies of the magnetic eigenmodes for the entire system

using Fourier analysis and then drive the structure with an oscillating magnetic field at each of

the eigenfrequencies. When the oscillating microwave field is added, the static switching field

required to align both layers is decreased compared to the undriven case. With a driving field

strength of 120 Oe the switching field is reduced by about 40%, from 1.12 kOe for the undriven

case to 0.55 Oe for the driven case.

9

Competing Interactions and a Tale of Two Spinels

Stephen E. Nagler1,*

1 Neutron Scattering Science Division, Oak Ridge National Laboratory

The cubic spinels AB2O4 are frequently cited as examples of geometrically frustrated spin systems when the pyrochlore lattice forming octahedrally coordinated “B” site is occupied by a magnetic ion. The tetrahedrally co-ordinated “A” sites form a diamond lattice. The diamond lattice nearest neighbour antiferromagnet is not geometrically frustrated, but a small amount of antiferromagnetic next nearest neighbour interaction has a dramatic effect, and the system is a three dimensional prototype for spin frustration induced by competing interactions. Diamond lattices with competing interactions have been predicted to exhibit novel physics, including the possible formation of a ‘spiral spin liquid’ with phase transitions driven by the classical order-by-disorder mechanism. This talk describes recent elastic and inelastic neutron scattering studies of single crystals of two A-site spinels: MnAl2O4 and CoAl2O4. Both exhibit varying degrees of first (J1) and second (J2) neighbour antiferromagnetic interactions. MnAl2O4 is less frustrated (i.e. with a smaller ratio J2/J1) and exhibits conventional ordering with a continuous transition to a long-range ordered Néel state. On the other hand CoAl2O4 shows more subtle behavior arising from a higher degree of frustration. At a temperature that has been associated with the onset of glassy behavior there is in fact a transition to a kinetically frozen ordered state.The evidence for and significance of this scenario is discussed.

(*) All work discussed is in collaboration with G. J. MacDougall. Other collaborators on some or all related work (not all of which is discussed here) include A. Aczel, G. Ehlers, D. Gout, T. Hong, D. Mandrus, M. A. McGuire, J. Niedziela, A. Podlesnyak, A. Schneidewind, W. Schweika, Y. Su, and J. L. Zarestky. Work at ORNL supported by the US DOE BES, divisions of Scientific User Facilities and Materials Research.

10

Singlet-triplet excitations in TiOBr: an unconventional spin-Peierls

compound

Carl Adams,1 J.P. Clancy,2 B.D. Gaulin,2,3,4 G.E. Granroth,5 A.I. Kolesnikov,5 T.E. Sherline,5

A.A. Aczel,5 F.C. Chou6

1 Department of Physics, St. Francis Xavier University, Antigonish, NS2 Department of Physics and Astronomy, McMaster University, Hamilton, ON3 Brockhouse Institute for Materials Research, McMaster University4 Canadian Institute for Advanced Research, Toronto, ON5 Neutron Scattering Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN6 Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan

TiOBr is a spin-Peierls compound where spin-1/2 Ti3+ ions along chains dimerize into spin singlet

states below 48 K. Spin-Peierls behaviour is quite rare, especially in inorganic compounds, and

involves significant magnetoelastic coupling. In TiOBr the lattice distortions arising from the

dimerization make a discontinuous incommensurate to commensurate transition at 27 K. A key

parameter in these materials is the singlet-triplet energy gap. We have used the time-of-flight

neutron spectrometer SEQUOIA to make the first direct measurements of both the one and

two triplet excitation modes. We find the gap to be 21.2 ± 1.0 meV. We will also present our

preliminary inelastic scattering results from Sc-doped materials, where the spin-Peierls phase is

strongly suppressed.

11

Critical behavior of heterogeneous GaP:MnP magnetic systems

C. Lacroix, N. Nateghi, N. Schmidt, R. A. Masut, D. Ménard,

Département de génie physique and Regroupement Québécois sur des Matériaux de Pointe, École Polytechnique Montréal, Canada

Experimental studies of magnetic phase transitions in nanoparticles are often difficult to model and to interpret due to various finite-size effects such as thermally activated reversal (superparamagnetism), surface magnetism and the interplay between correlation length and particle sizes. Manganese phosphide (MnP), with its rich magnetic phase diagram, [1] is an interesting candidate for such studies, as it can be grown into nanoclusters of various sizes embedded into GaP epilayers (GaP:MnP) [2]. Here we show that MnP nanoparticles exhibit a considerably modified critical behavior as compared to its well-known bulk ferromagnetic-to-paramagnetic phase transition at TC = 291 K [1].

A series of GaP:MnP epilayers have been grown at three different temperature using metal-organic-vapor-phase-epitaxy on GaP (001) substrates. Chemical and structural characterization indicates that the films consist of an epitaxial GaP matrix with semi-coherently embedded MnP nanoclusters. The slightly ellipsoidal clusters are uniformly distributed and exhibit average diameters ranging from 19 to 28 nm, depending upon the growth temperature. Angle-dependent ferromagnetic resonance spectroscopy, combined with angle-dependent remanent magnetometry, indicates that the clusters are highly-textured and weakly-interacting MnP monodomains. The distributions of crystallographic orientations of the MnP clusters are thus inferred, [3] enabling us to model both the magnetometric and ferromagnetic responses as a function of field (angle and intensity) and temperature, with excellent accuracy. However, we have to account for a fraction of the clusters becoming superparamagnetic as we approach the Curie temperature.

In the critical region, we observe a size-dependent asymptotic decrease of the remanent magnetization as a function of the temperature. Depending upon the clusters’ size, the temperature for which the remanent magnetization vanishes, varies between 300 and 315 K, much higher than the bulk MnP TC. We emphasize that the magnetization decrease is at zero-applied-field, not to be confused with the smoothed phase transition observed in bulk samples due to small applied fields. We speculate that the observed behavior may be due to magnetoelastic effects, combined with thermal reversal of the smallest clusters.

[1] E. Huber and D. Ridgley, Phys. Rev. 135, A1033 (1964). [2] S. Lambert-Milo, et al., J. Appl. Phys. 104, 083501 (2008). [3] C. Lacroix, et al., J. Appl. Phys. 105, 07C119 (2009).

12

Neutron diffraction and magnetic order:

Doing it right, keeping them honest

D.H. Ryan,1 J.M. Cadogan2

1 Physics Department and Centre for the Physics of Materials, McGill University, Montreal,

H3A 2T8, Canada2 Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, R3T

2N2, Canada

Automation of basic bulk measurements of susceptibility, magnetisation and heat capacity, etc.,

has made it possible to obtain extensive, high-density data sets. However, such techniques provide

a somewhat limited and often indirect view of the magnetic behaviour of a system. The results

can be distorted by even low levels of impurities, and they are flexible enough that preconceived

notions can occasionally be force-fitted to provide support. This problem is particularly acute

when competing interactions or frustration are either present or expected.

In this cautionary tale, I will draw on several recent examples where we have used neutron diffrac-

tion and Mossbauer spectroscopy to investigate magnetic order in systems where transitions have

been mis-identified, or even missed entirely; where the nature of the magnetic order has been

suspected, but not confirmed; and where the observed ordering may be the result of the mea-

surement. I will concentrate on systems containing samarium, europium and gadolinium, which

received wisdom inaccurately maintains cannot be studied using neutron diffraction, and where

assertions are perhaps made more freely when contradiction is less likely. The examples will

illustrate where our attempts to “put the record straight” have led to some surprises – keeping

us honest.

13

Simulations of inter- and intra-grain spin structure on a Heisenberg

model for magnetic recording media

Martin LeBlanc, Martin Plumer, John Whitehead, Jason Mercer

Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St.

John’s, NL, Canada

In order to keep supplying computer hard disk drives with increasing storage capacity, it is

essential to have smaller bits. Smaller bits, however, are susceptible to superparamagnetism,

the spontaneous flipping of the magnetic moments of bits caused by thermal fluctuations which

interferes with data stability. Bits consist of grains of about 10 nanometers which interact

between each other and are made up of thousands of atomic spins which also experience intra-

grain exchange. The interplay between these two interactions needs to be better understood for

the applications of a proposed technology called Heat Assisted Magnetic Recording (HAMR),

where grains are heated to assist with head field reversal. Building on previous work on the Ising

model [1], computer simulations using Monte Carlo and Landau-Lifshitz-Gilbert (LLG) methods

are performed on a quasi-2D Heisenberg model with strong anisotropy and a strong intra-grain

exchange interaction J as well as a weak inter-grain exchange J′. The results on the critical

temperature, Tc, show a strong deviation from traditional expected behavior when J

′ is large

enough. M-H hysteresis loops are also determined which measure the coercivity, Hc, where a

high value represents a strong resilience to the superparamagnetic effect. It is seen that taking

into account the internal degrees of freedom has a significant effect on Hc(T ).

[1] M. D. Leblanc, M. L. Plumer, J. P. Whitehead and J. I. Mercer, Monte Carlo simulations of

inter- and intra-grain spin structure of Ising and Heisenberg models for magnetic recording

media, Phys. Rev. B. 82, 174435 (2010).

14

Competing anisotropies in a 2D ferromagnetic phase transition

K. Fritsch, R.D’Ortenzio and D. Venus

McMaster University, Hamilton, ON

An ultrathin film with a thickness that is not a complete number of monolayers contains by definition many inhomogeneities that can affect the local crystalline anisotropy. These include changes in the effective surface anisotropy in regions where the thickness differs from the nominal value, edges around the perimeter of such regions, as well as structural defects and dislocations or domain walls created during the growth of the film. Since the surface makes up a large proportion of the material in the systems, these films formally have competing, or mixed anisotropies. The competing anisotropy due to inhomogeneities may affect the second-order phase transition, depending upon conditions set out in the Harris criterion[1]. Recent experiments[2] have investigated the magnetic susceptibility of Fe/W(110) films in the paramagnetic phase close to the critical temperature, TC, in the thickness range between 1.6 and 2.4 monolayers (ML) Fe. Since this system is described by the 2D anisotropic Heisenberg model, it is particularly sensitive to perturbations in the crystalline anisotropy. This is because TC is not finite in the absence of anisotropy. Uniaxial anisotropy creates a finite TC with a cross-over to the 2D Ising model in the critical region.[3]

In addition to the expected easy-axis susceptibility, the measurements reveal a hard-axis susceptibility that is qualitatively inconsistent with a homogeneous 2D anisotropic Heisenberg system. The hard-axis susceptibility is sharply peaked, and only one or two orders of magnitude smaller than the easy-axis susceptibility. The peak amplitude and temperature depend systematically on the degree of layer completion, with the peak temperature displaced systematically up to 10 K into the paramagnetic region. These results can be described naturally by the perturbed anisotropy due to inhomogeneities of a characteristic size d in the presence of a ferromagnetic correlation length, �(T)��that changes quickly with temperature near TC. A likely candidate for the inhomogeneities are the closed lines of atomic step edges that form the perimeters of regions of 1 or 3 ML in the nominally 2 ML Fe film. The Harris criterion is used to relate these results to previously reported findings[4] on the variation of the experimentally determined critical exponent �eff of the susceptibility in this same film system.

[1] A.B. Harris, J. Phys. C 7, 1672 (1974). [2] K. Fritsch, R. D’Ortenzio and D. Venus, Phys. Rev. B (in press). [3] M. Bander and D.L. Mills, Phys. Rev. B 38, 12015 (1988); P.A. Serena, N. Garciá, and A. Leyanyuk, Phys. Rev. B 47, 5027 (1993). [4] M.J. Dunlavy and D. Venus, Phys. Rev. B 69, 094411 (2004).

15

Quantum Spin Liquids In Quantum Spin Ices

Lucile Savary,1 Kate A. Ross,2 Bruce D. Gaulin,2,3,4 Leon Balents,5

1 Ecole Normale Superieure de Lyon, 46, allee d’Italie, 69364 Lyon Cedex 07, France2 Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1,

Canada3 Canadian Institute for Advanced Research, 180 Dundas St. W., Toronto, Ontario, M5G 1Z8,

Canada4 Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario, L8S

4M1, Canada5 Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA, 93106-

4030, U.S.A.

A flurry of recent theory and experiments has highlighted exotic physics in the spin ice materials,

Ho2Ti2O7 and Dy2Ti2O7 , which comprise classical Ising spins on a pyrochlore lattice. There

are a few related materials in which quantum fluctuations of spins are significant on the same

lattice. I will discuss a general microscopic model for these materials, and specifically the case of

Yb2Ti2O7, where experiments have revealed a puzzling low temperature state in low field, and

present a case that this indeed is an example of quantum spin ice. The ground state of this

material may well be a quantum spin liquid, with even more exotic physics than in the classical

spin ices. I will describe this quantum spin liquid state, its properties, and how this proposal

may be further pursued.

16

Dynamics of magnetic charges in artificial spin ice

Paula Mellado,1 Olga Petrova,2 Yichen Shen,2 Oleg Tchernyshyov2

1 School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA2 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA

Artificial spin ice has been recently implemented in two-dimensional arrays of mesoscopic mag-

netic wires [1-4]. We propose a theoretical model of magnetization dynamics in artificial spin ice

under the action of an applied magnetic field [5]. Magnetization reversal is mediated by domain

walls carrying two units of magnetic charge. They are emitted by lattice junctions when the

the local field exceeds a critical value Hc required to pull apart magnetic charges of opposite

sign. Positive feedback from Coulomb interactions between magnetic charges induces avalanches

in magnetization reversal. We take into account quenched disorder, induced by imperfections

in experimental samples, and assume overdamped dynamics of magnetic charges. The resulting

model predicts two distinct regimes of magnetization reversal distinguished by the presence of

avalanches for certain orientations of the applied magnetic field in agreement with experiments

[6]. However, the model with overdamped dynamics fails to predict the observed branching

of avalanches. We speculate that this discrepancy is caused by inertia of domain walls [7-8],

stemming from precessional dynamics of magnetization.

[1] R. F. Wang et al., Nature (London) 439, 303 (2006).

[2] M. Tanaka et al., Phys. Rev. B 73, 052411 (2006).

[3] Y. Qi, T. Brintlinger, and J. Cumings, Phys. Rev. B 77, 094418 (2008).

[4] S. Ladak et al., Nature Phys. 6, 359 (2010).

[5] P. Mellado et al., Phys. Rev. Lett. 105, 187206 (2010).

[6] S. Daunheimer and J. Cumings, March Meeting of the American Physical Society, Abstract

V18.3 (2011).

[7] E. Saitoh, H. Miyajima, T. Yamaoka, and G. Tatara, Nature (London) 432, 203 (2004).

[8] D. J. Clarke et al., Phys. Rev. B 78, 134412 (2008).

17

Hexagonal ferrites as room-temperature magnetoelectrics

Tsuyoshi Kimura1

1 Division of Materials Physics, Graduate School of Engineering Science, Osaka University, Toy-

onaka, Osaka 560-8531, Japan

Magnetoelectric multiferroics are old but emerging class of materials that combine coupled elec-

tric and magnetic dipole order. In these materials, ferroelectric and magnetic ordered states

coexist or compete with each other. The interaction leads to a so-called magnetoelectric ef-

fect, which is the induction of magnetization by an electric �eld or electric polarization by a

magnetic �eld. The magnetoelectric e�ect has attracted much interest for a long time, as the

coupling between the magnetism and ferroelectricity can provide an additional degree of freedom

in magnetoelectric device design. However, there have been no applications using ME couplings

developed to date, due mainly to materials limitations and the small magnitude of the magne-

toelectric interaction. In recent years, a new class of multiferroics such as TbMnO3 has been

discovered. These systems exhibit gigantic magnetoelectric e�ects accompanied by a magnetic

phase transition into a spiral magnetic ordered phase. However, their magnetoelectric e�ects

usually occur at temperatures that are too low to be practically useful. The quest for robust

room-temperature magnetically-induced ferroelectrics is a major challenge in magnetoelectric re-

search. Last year, it was found that a Z-type hexagonal ferrite Sr3Co2Fe24O41 exhibits a low-�eld

magnetoelectric e�ect at room temperature [1,2]. In this presentation, I show recent development

of magnetoelectric hexagonal ferrites working at room temperature.

This work has been done in collaboration with Y. Kitagawa, Y. Hiraoka, T. Ishikura,K. Okumura,

T. Honda, M. Soda, H. Nakamura, Y. Wakabayashi, T. Asaka, and Y. Tanaka.

[1] Y. Kitagawa et al., Nature Mater. 9, 797 (2010).

[2] M. Soda et al., Phys Rev. Lett. 106, 087201 (2011).

18

Electric-field control of spin waves in multiferroic BiFeO3

Rogerio de Sousa1

1 Department of Physics and Astronomy, University of Victoria, B.C., Canada

A recent experiment [1] demonstrated gigantic (30%) electric-field tuning of magnon frequencies

in multiferroic BiFeO3. We present a theory that shows that this effect occurs due to at most two

linear magnetoelectric interactions that couple the component of electric field perpendicular to

the ferroelectric vector to a quadratic form of the Neel vector. We calculate the magnon spectra

due to each of these interactions and show that only one of them is consistent with experimen-

tal data. At high electric fields, this interaction induces a phase transition to a homogeneous

magnetic state. As a result, the multi-magnon spectra of the inhomogeneous (cycloidal) mag-

net is fused into just two magnon modes representing the excitations of the homogeneous state.

We discuss the possible microscopic mechanisms responsible for this novel interaction and the

prospect for applications in magnonics.

[1] P. Rovillain, R. de Sousa, Y. Gallais, A. Sacuto, M. A. Measson, D. Colson, A. Forget, M.

Bibes, A. Barthelemy, and M. Cazayous, Nature Materials 9, 975 (2010).

19

Excitons and Photoluminescence on the Surface of a Strong Topological Insulator with a Magnetic Energy Gap

Ion Garate ,1,2 M. Franz2

1 Canadian Institute for Advanced Research, Toronto, ON M5G 1Z8, Canada2 Department of Physics and Astronomy, University of British Columbia, Vancouver, V6T 1Z1, Canada

We present a theoretical study of interacting electron-hole pairs located on a magnetized surface of a strong topological insulator. The excitonic energy levels and the optical absorption on such surface display unique and potentially measurable attributes such as (i) an enhanced binding energy for excitons whose total angular momentum is aligned with the magnetic exchange field, (ii) a stark dependence of the optical absorption on the direction of the magnetic exchange field as well as on the chirality of the impinging light, and (iii) a tunable center-of-mass motion of spinful excitons induced by particle-hole asymmetry in the exchange field.Our predictions are relevant to magnetically doped (coated) topological surfaces, in addition to topological insulator nanowires placed under longitudinal magnetic fields.

20

Magnetic properties of kagome spin system of Cu4–xMx(OH)6Cl2 (M = Cu, Zn and Mg)

Shaoyan Chu1, Peter Müller,2 Daniel G. Nocera2, and Young S. Lee3

1 Center for Materials Science and Engineering, Massachusetts Institute of Technology,

Cambridge, MA, USA 2 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA, USA 3 Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA

A hydrothermal method for growing millimeter-sized crystals of the quantum magnets of Cu4–

xMx(OH)6Cl2 (M = Cu, Zn and Mg) is presented in this work [1]. The crystal structure of these

compounds is determined by single crystal XRD technology. It has been found that

Cu3M(OH)6Cl2 (M = Cu) is monoclinic (space group P21/n). In this phase, three-fourths of the

Cu ions form deformed kagome layers and the others form deformed triangle interlayers between

the kagome layers. When more than one-third of the Cu2+ on the interlayer sites of the P21/n

phase are substituted by Zn or Mg, an excellent 2D kagome spin system (R-3m phase) results.

When all of the ions (Cu/Zn or Cu/Mg = 3/1) are on the kagome layers, a polymorphic phase (P-

3m1) forms [2, 3]. Magnetization measurements show that, due to geometrical frustration, both

of P21/n phase and the R-3m phases are antiferromagnets and likely present spin liquid behavior

at temperature below the Néel temperature. However, the P-3m1 phase of Cu3Mg(OH)6Cl2,

displays weak ferromagnetic characters [4]. Spins in this phase start to freeze out at temperature

of ~ 4.7 K. Moreover, in and out-of kagome plane susceptibilities of this 2D system present spin

ice and spin glass characters respectively owing to competing magnetic interactions.

[1] Shaoyan Chu, et al., Appl. Phys. Lett. 98, 092508 (2011). [2] W. Krause, et al., Miner. Mag.

70, 329 (2006). [3] Malcherek T, et al., Acta Cryst. B 63, 157 (2007). [4] Shaoyan Chu, J. Phys:

Conf. Ser. 273, 012123 (2011).

21

Coherent and Incoherent Carrier Response in GaMnAs K. C. Hall,1 M. Yildirim,1 T. De Boer,1 S. March,1 A. Gamouras,1 R. Mathew,1 X. Liu,2 and J. K. Furdyna2 1 Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada 2 Department of Physics, University of Notre Dame, Notre Dame, Indiana, USA The III-V diluted magnetic semiconductors (DMS) exhibit a unique combination of semiconducting and ferromagnetic properties, offering the ability to control magnetic characteristics through modification of the carrier density. This feature makes DMS materials of interest for developing semiconductor-based magneto-sensitive electronic and photonic devices [1]. Early demonstrations of manipulation of fundamental magnetic properties using electrical gates [2] and continuous wave optical excitation [3] have recently been extended to control of ferromagnetism on femtosecond time scales [4]. As the dynamics of the optically-injected carrier population dictate the resulting magnetization kinetics, it is of crucial importance that these dynamics are well understood. In this presentation, a comprehensive set of experiments characterizing the coherent and incoherent ultrafast dynamics of carriers in GaMnAs will be described. We observe a pronounced red shift and broadening of the coherent carrier response with increasing Mn concentration, reflecting a dramatic modification of the nature of the optical nonlinearity in the near-IR. Differential reflectivity measurements over a wide range of carrier energies (1.4-2.0 eV) indicate the dominant influence of mid-gap trapping states on the carrier kinetics in the incoherent regime. Our results have implications for understanding the electronic band structure and ultrafast response of GaMnAs. [1] Hall et al. Appl. Phys. Lett. 83, 2937 (2003); Hall et al., Appl. Phys. Lett. 88, 162503 (2006); Rudolph et al. Appl. Phys. Lett. 82, 4516 (2003);Onadera et al. Electron. Lett. 30, 1954 (1994). [2] Ohno et al., Nature (London) 408, 944 (2000); Chiba et al. Science 301, 943 (2003). [3] Koshihara et al. Phys. Rev. Lett. 78, 4617 (1997); Oiwa et al. Appl. Phys. Lett. 78, 518 (2001). [4] Hall et al. Appl. Phys. Lett. 93, 032504 (2008); Zahn et al. J. Appl. Phys. 107, 033908 (2010).

22

Point Contact Surface Spin-Valve with an Exchange Bias I.K. Yanson1, Yu.G. Naidyuk1, O.P. Balkashin1, V.V. Fisun1, L.Yu. Tryputen1, S. Andersson2, V. Korenivski2, Yu.I. Yanson3, H. Zabel3

1 Verkin Institute for Low temperature Physics and Engineering of National academy of Sciences of Ukraine, Kharkiv, Ukraine 2 Nanostructure Physics, Royal Institute of Technology, Stockholm, Sweden 3Lehrstuhl für Experimentalphysik/Festkörperphysik, Ruhr-Universität Bochum, Bochum, Germany Magnetoresistance R(H) at V=0 and differential resistance R(V) (R=dV/dI) at H=0 of point contacts between nonmagnetic Cu tips and single ferromagnetic films (FM - Co) exchange-pinned by antiferromagnetic films (AFM – Fe50Mn50) have been investigated. Analysis of measured R(V) and R(H) characteristics confirms recently proposed model of the point contact surface spin-valve (SSV) [1], where spin orientation at the interface can be different from spin orientation in the F-film bulk. Magnetoresistance R(H) of SSV in the point contacts to ferromagnetic films exchange-pinned by antiferromagnets shows an exchange offset that depends on a mutual orientation of the applied field Н in respect to a pinned magnetization of the AFM/FM layer М. We have found that switching of this ferromagnet bulk occurs at lower fields than switching of surface spin layer. Origin of such higher switching field can be caused by a higher coercivity due to morphological imperfections and defects in the contact core. In addition, it has been shown that point contact SSVs based on an amorphous alloy Co40Fe40B20 (20,9,6,3 nm) also have the same properties as spin-valves with a geometrically controlled structure. The experiments showed that an increase of an exchange bias under decreasing of CoFeB films thickness is observed both at the surface and in the SSV bulk. A negative magnetoresistance of such point-contact SSVs based on CoFeB was also observed. [1] I.K. Yanson, Yu.G. Naidyuk, V.V. Fisun, A. Konovalenko, O.P. Balkashin, .Yu. Tryputen, and V. Korenivski, Nano Letters, 7, 927 (2007).

23

Colloidal patterning of magnetic materials

Matthew Seymour, Ian Wilding, Anand Yethiraj, Kristin M. Poduska, Martin Plumer

Department of Physics and Physical Oceanography, Memorial University of Newfoundland

St. John’s, NL, Canada

Patterned magnetic materials are being evaluated for the next generation of data storage in both

computer hard disks and random access memory (RAM) devices. A variety of techniques are

being explored within the industry to optimize their performance and effectiveness. The oppor-

tunity to develop new methods to fabricate these materials has catalyzed an active and growing

collaboration at Memorial University that draws from expertise in three distinct areas: electro-

chemical production of materials, self-assembly of patterned materials, and computer simulations

of magnetic material properties. We have demonstrated a promising new avenue for preparing

patterned magnetic materials by pairing exceptionally fast and economical materials synthesis

techniques [1]. The resulting patterned materials exhibit complex shapes, and we propose that

they may offer advantages for retaining high integrity data storage even as device dimensions

shrink. We show preliminary experimental and magnetic simulation results toward this goal.

[1] C. Arcos, K. Kumar, W. Gonzalez-Vinas, R. Sirera, K.M. Poduska, A. Yethiraj (2008) Phys.

Rev. E Rapid Communication 77, 050402R/1-4.

24

The concept of magnon spintronics

Burkard Hillebrands

Fachbereich Physik and Forschungszentrum OPTIMAS, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany

Spintronics is concerned with the development of devices which exceed the applicability, performance, and energy efficiency of conventional charge-based electronics by exploiting the electron's spin degree of freedom. Spin angular momentum, which is the information carrier in spintronics, can be transferred not only by the flow of electrons, but also by magnons: the quanta of spin waves (collective excitations of the spin lattice of a magnetic material). This opens a new research direction: magnon spintronics, a sub-field of spintronics in which information is transferred and processed using magnons. In my talk I will concentrate on the main constructing blocks of magnon spintronics: (1) converters between information coded into the spin or charge of electrons and into magnons, (2) magnon conduits, (3) physical phenomena allowing information processing by magnons.

The most promising converters for magnon spintronics are based on the spin pumping effect (which transforms spin waves into pure spin currents) and the inverse spin Hall effect (iSHE) (which converts spin currents into charge currents). Our study concentrates on magnetic insulator yttrium iron garnet (YIG) –nonmagnetic platinum (Pt) structures. We have shown that different dipolar spin-wave modes have different spin pumping efficiencies [1]. Additionally, we have investigated spin pumping for purely exchange, sub-micron wavelength magnons. We found that spin pumping is possible for magnons even without an associated dipolar field. The studies of the temporal behavior of the iSHE-voltage in YIG/Pt structures show that the slowest processes in these converters are associated with the processes inside the magnon system.

Magnon conduits will be presented by examples of meso-sized YIG-based and micro-sized Permalloy-based magnonic crystals (MCs). A MC, which is an artificial media with spatially periodic variation in itsmagnetic properties, can serve as a magnon conduit combined with information processing elements allowing, e.g., filtering or phase shifting.

Magnon physics is very rich providing unique opportunities for magnon spintronics. As an example, I will show the experimental results where all-linear spectral transformations, including frequency inversion and time reversal, have been realized by a dynamic magnonic crystal [2].

[1] C. W. Sandweg, Y. Kajiwara, K. Ando, E. Saitoh and B. Hillebrands, Appl. Phys. Lett. 97, 252504 (2010).

[2] A. V. Chumak, V. S. Tiberkevich, A. D. Karenowska, A. A. Serga, J. F. Gregg, A. N. Slavin, and B. Hillebrands, Nat. Commun. 1:141 doi: 10.1038/ncomms1142 (2010).

25

Nonlinear Magnetic Excitations in Microstrips

M.P. Wismayer1 and B.W. Southern1

1 Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB, Canada

Recent developments in the technique of electrical detection of spin wave modes in ferromagnetic

microstructures allows a direct mapping of spin wave evolution both in the linear and nonlinear

regimes. We have performed micromagnetic simulations to model nonlinear spin-wave modes

within a normally magnetized permalloy strip (NiFe). The spin-wave excitations are excited by

applying a time dependent sinusoidal field of fixed frequency along the strip plane and perpen-

dicular to the applied static field. Fourier analysis is used to map out the spin-wave spectrum

as a function of the applied static field and the amplitude of the transverse field. The spin-wave

spectrum in the linear/nonlinear regime shows good qualitative agreement with measurements

obtained from the photo-voltage technique.

26

Linear and non-linear magnetoimpedance in microwires D. Seddaoui, D. Ménard, and A. Yelon

Département de génie physique and Regroupement Québécois sur des Matériaux de Pointe, École Polytechnique Montréal The phenomenon of magnetoimpedance (MI) in metallic ferromagnetic wires or ribbons has been intensively investigated, due to its application to magnetic sensors. Experimental studies have been carried out in many parts of the world, since measurements can be performed, using equipment which is relatively easy to obtain and to use, from very low frequencies up to about 10 GHz. Modeling MI requires the approach which was developed in the 1950s for modeling FMR in metals: simultaneous solution of the Landau Lifshitz (LL) equation and Maxwell’s (M) equations, accounting for the “combined” spatial dispersion due to both skin effect (M) and exchange (LL). The solutions are then used to satisfy the boundary conditions on the electromagnetic fields and on the magnetization. When the LL equation is linearized, by neglecting the product of the dynamic magnetization and field, this requires some effort (which some workers in the field are unwilling to make), but analytic solutions are known. However, many experiments are performed under circumstances for which this approximation it is not justified. In order to model these cases, it is necessary to perform numerical calculations. Our group has developed and exploited the methodology for such calculations, under the assumption that a wire is a perfect cylinder whose properties and behavior vary only with the radius (thickness for a perfect ribbon). We present the approach used for the calculations, and some results. We show how magnetization reversal may propagate through a sample in a solitary-wave-like manner. The failure of the linear approximation at very low effective field is discussed. At low static field and large ac current and field, bifurcation, and eventually, chaotic behavior, is predicted. We discuss future plans. The computer program, which is being made as user-friendly as possible, is available for other users.

27

Intermediate timescale methods to probe the energetic landscape of nanoscale magnetic elements Jacob A. J. Burgess,1,2 Alastair E. Fraser,1 Joseph E. Losby1,2, Stephen K. Portillo1, Fatemeh Fani1, David C. Fortin1, John P. Davis1, Mark R. Freeman1,2 1 Department of Physics, University of Alberta, Edmonton, Canada 2 National Institute for Nanotechnology, NRC, Edmonton, Canada New detection techniques have increased the detail in which magnetic systems can be examined. As a consequence, an exciting new regime of magnetic behaviour is undergoing intensive investigation. We report studies of simple magnetic textures interacting with minute inhomogeneities in the component films, as well as with subtle, large scale shape variations, using a combination of optical and micromechanical inspection methods. The goal is to develop a complete quantitative picture of the energy landscape of micro or nanomagnetic elements. Technological demands for magnetic devices also necessitate an improved understanding of the dynamics and pinning mechanisms of the magnetic textures. To accomplish this, we employ a pair of sensitive experimental techniques that provide very complementary information. The first technique employs a simple magneto-optical Kerr effect magnetometer modified with a small electromagnetic coil to function as a susceptometer. The resulting instrument enables high signal-to-noise investigation of arrays of magnetic elements and yields statistical information quickly and conveniently. This technique is particularly well-suited to the study of hysteretic shifts and, in its inaugural application, was applied to measure vortex annihilation barriers in permalloy disks [1]. Because arrays obscure the variations within individual objects, we also apply nanomechanical torque magnetometry to obtain complementary insight through single-element measurements. Permalloy disks are fabricated onto silicon nitride or silicon-on-insulator based torsional paddles, allowing for a direct measurement of the quasi-static magnetization. Initial work focused on statistical measurements of vortex creation and annihilation [2]. Subsequent work exploits the extraordinarily high sensitivity of these devices to perform a quantitative study of vortex pinning sites [3]. In conjunction with analytical work and simulation, the two techniques combined provide a cohesive experimental foundation for the statistical investigation of detailed models of pinning, depinning, annihilation and creation of vortices. Concurrently, we apply the same approach of study to domain wall motion in magnetic wires fabricated onto microcantilevers [4]. This geometry gives us a highly sensitive method of studying domain wall propagation and pinning potentials, opening up the possibility for novel magneto-mechanical devices. An overview of our studies of magnetic vortices will be presented with a focus on pinning behaviour. Analogous work on domain wall propagation and pinning will also be presented. [1] J. A. J. Burgess et al, Phys. Rev. B. 81, 144403 (2010). [2] J. P. Davis et al, New J. Phys. 12, 093033 (2010). [3] A. E. Fraser et al, in prep. [4] J. E. Losby et al, J. Appl. Phys. 108, 123910 (2010).

28

Achilles heel in the study of electrical detection of ferromagnetic resonance

M. Harder, Z. X. Cao, X.L. Fan, Y.S. Gui, and Can-Ming Hu

Department of Physics and Astronomy, University of Manitoba, Winnipeg, R3T 2N2, Canada

By using spin rectification effects of ferromagnetic materials and devices, electrical detection of ferromagnetic resonance (FMR) has become a powerful new tool for investigating spin dynamics, spin pumping, spin torque, spin diode, and spin Hall effects. However, the treatment of the line shape of FMR measured by spin rectification effects has remained the “Achilles heel” in some of the most recent studies. This talk aims to exam this critical issue and to outline a unified picture for understanding spin rectification effects of magnetic single layer, bilaryer, and tunnelling junctions.

29

Interface properties of GaAs/Fe/Au(001) and YIG/Au - spin pump-

ing, interface Gilbert damping and magnetic anisotropies

Bartek Kardasz1, Eric Montoya1, Capucine Burrowes1, Erol Girt1, and Bret Heinrich1

1 Department of Physics, Simon Fraser University, Burnaby, BC, Canada

GaAs/(d)Fe/20Au(001) structures were deposited using Molecular Beam Epitaxy (MBE), where

Fe thickness d=(5...90) atomic layers (AL). Interface anisotropies were investigated using the

in-plane angular dependence of Ferromagnetic Resonance (FMR). Intrinsic and extrinsic contri-

butions to magnetic damping were investigated using the FMR linewidth (ΔH) measurements at

9, 24, 36, and 72 GHz (in-plane configuration) and 9, 24, and 36 GHz (perpendicular configura-

tion). The in-plane cubic and uniaxial perpendicular anisotropies were well described by the bulk

and interface contributions. For d >(20)Fe the strength of the in-plane uniaxial anisotropy K‖u,eff

deviated from a simple interface contribution. For the Fe films with d ≥(90)Fe the rotation of the

K‖u,eff

axis from the [110] to [110] crystallographic direction showed that a gradual disappearance

of K‖u,eff

is a complex process involving the lattice strains caused by the interface lattice shear

and onset of misfit dislocations. The frequency dependence of ΔH(f) was analyzed using the

Gilbert damping, two magnon scattering, and long range magnetic inhomogeneity contributions.

These analysis has shown that the GaAs/Fe interface leads to an appreciable contribution to

Gilbert damping. The thickness dependence of the damping parameter α(d) was well described

by the bulk Gilbert damping parameter Gbulk

=6.7x107s−1 and the (1/d)n interface term with

the power law parameter n ∼ 1 indicating its interface character. The magnetic disorder at the

GaAs/Fe interface [1] can be expected to introduce disorder in the spin-orbit interaction and

change the interface electron band structure, which play an essential role in spin dynamics, and

most likely result in the observed interface Gilbert damping.

Spin injection across the ferromagnetic insulator (YIG)/normal metal (Au) interface was studied

using FMR. The Gilbert damping parameter α of YIG was measured from the slope of FMR

ΔH(f) at f= 10, 14, 24 and 36 GHz in the YIG and YIG/Au/Fe/Au samples. The additional Fe

layer acted as a spin sink. The YIG/Au/Fe/Au samples displayed an increased α compared to

that in the bare YIG films. This provides the direct evidence for spin pumping at the YIG/Au

interface. The transfer of spin momentum across the YIG interface is surprisingly efficient with

the spin mixing conductance g↑↓ � 1.5×1014/cm2.

[1] B. Kardasz, J. Zukrowski, O. Mosendz, M. Przybylski, B. Heinrich, and J. Kirschner, J. Appl.

Phys. 101, 09D110 (2007).

30

Complex spin dynamics at finite temperatures

S. Wienholdt1, F. Schlickeiser

1, D. Hinzke

1, U. Atxitia

2, O. Chubykalo-Fesenko

2, and U.

Nowak1

1Department of Physics, Universität Konstanz, D-78457 Konstanz, Germany

2Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain

Ultrafast magnetisation dynamics has been extensively studied recently as a possibility to

improve the storage density as well as the writing speed in magnetic data storage. The direct,

ultrafast manipulation of the magnetisation by femtosecond laser pulses promises to become a

real alternative to those techniques where magnetic field pulses are used. Recently it was

demonstrated that a sub 100 femtosecond, circularly polarised laser pulse is able to reverse

magnetisation on a time scale of some picoseconds, as if it acts as an equally short magnetic field

pulse pointing along the direction of light caused by the inverse Faraday effect [1,2]. In

femtosecond single-shot time-resolved imaging of magnetic structures [3] it has been shown that

the magnetisation reverses via a so-called linear pathway [4], a high-temperature switching mode

without any precession. However, so far, ultrafast all-optical magnetisation switching has been

demonstrated experimentally only in ferrimagnetic materials like GdFeCo. A reason for this

restriction seems to be the antiferromagnetic coupling of the two sublattices in these materials,

which may lead to completely different dynamics as compared to a ferromagnet. It was

speculated that the special properties of the ferrimagnet close to the compensation point could be

relevant [3].

To understand the dynamics in materials with complex spin structures, we perform atomistic spin

model simulations of antiferromagnets as well as ferrimagnets, driven either by external

magnetic field pulses or variation of the spin temperature and investigate possible switching

mechanisms as well as their typical time scales in detail. Furthermore, we calculate the effective

frequencies and damping parameters in order to compare them with experimental findings and

analytical results.

We acknowledge financial support by the EU through the collaborative research project

ULTRAMGNETRON and by the Center for Applied Photonics (CAP).

[1] A. V. Kimel, A. Kirilyuk, P. A. Usachev, R. V. Pisarev, A. M. Balbashow, and Th. Rasing,

Nature 435, 655 (2005)

[2] C. D. Stanciu, F. Hansteen, A. V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, and Th.

Rasing, Phys. Rev. Lett. 99, 047601 (2007)

[3] K. Vahaplar, A. M. Kalashnikowa, A. V. Kimel, D. Hinzke, U. Nowak, R. W. Chantrell, A.

Tsukamoto, A. Itoh, and Th. Rasing, Phys. Rev. Lett. 103, 117201 (2009)

[4] N. Kazantseva, D. Hinzke, R. W. Chantrell, and U. Nowak, Europhys. Lett. 86, 27006 (2009)

31

Scripted Micromagnetics Environment

Jason I. Mercer1, John P. Whitehead2

1 Department of Computer Science, Memorial University of Newfoundland2 Department of Physics and Physical Oceanography, Memorial University of Newfoundland

Simulating systems with competing interaction can present a considerable computational challenge, both in terms of dimensionality of phase space involved and widely divergent time

and length scales.

The talk will introduce MagLua, a scriptable micromagnetics environment. It allows both rapid

prototyping of problems and execution of production level HPC simulations. The modular design and flexibility enable both experimenting with algorithms and the creation of novel simulation

methods that can combine different methodologies to simulate complex systems.

In this talk we will demonstrate how MagLua can and has been used for multiscale simulations,

minimum energy pathway calculations as well as @Home and MPI parallelism.

32

Microdomain Formation in Ultra-Thin Magnetic Films

J. P. Whitehead,1 J. I. Mercer ,

2 A. B. MacIsaac

3

1Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St.

John's, Canada 2Department of Computer Science, Memorial University of Newfoundland, St. John's, Canada

3Department of Applied Mathematics, University of Western Ontario

Ultra thin magnetic films (UTMFs) consist of several layers of magnetic atoms deposited on a

non-magnetic substrate. Advances in the fabrication and characterization of these materials allow

researchers to fabricate films comprising multiple layers, each with a distinct and well-defined

atomic structure. By carefully selecting the substrate, the atomic composition and number of

layers it is possible to produce materials that exhibit a fascinating and diverse range of magnetic

properties.

The ability to finely tune the magnetic properties of these materials by varying the number and

composition of the layers makes it possible to create UTMFs in which the magnetic moments are

aligned perpendicular or parallel to the surface and which can exhibit a reorientation transition

between these two orientations. In addition, a number of systems with a net magnetisation

perpendicular to the plane have been shown experimentally to manifest a stripe phase consisting

of elongated domains of alternating magnetisation direction.

This talk will summarise some of the key properties of UTMFs and discuss how the stripe phase

can be understood as the result of the competition between the ferromagnetic exchange and the

antiferromagnetic dipolar interactions. I will present results from a recent series of simulation

studies that, together with some analytical calculations, provide some insight into the nature of

the stripe phase close to the reorientation transition.

33

Ultrasonic and Raman studies on delafossite, magnetoelectric CuFeO2

and CuCrO2: Possible ferroelastics

O. Aktas,1 K. D. Truong,2 T. Otani,3 S. Jandl,2 G. Balakrishnan4, O. Petrenko4, Maynard J.

Clouter,1 T. Kimura,3 G. Quirion1

1 Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St.

John’s, Canada2 Departement de Physique, Universite de Sherbrooke, Sherbrooke, Canada3 Division of Material Physics, Graduate School of Engineering Science, Osaka University, Toy-

onaka, Japan4 Department of Physics, University of Warwick, Coventry, UK

Delafossite ABO2 (A: Cu, Ag and B: Fe, Cr) type compounds bare rich magnetic properties

at low temperatures owed to geometrical frustration and strong spin lattice coupling. Among

these crystals, the multiferroic compounds CuFeO2 and CuCrO2 display an electric polarization

induced upon the stabilization of a proper-screw magnetic structure, making these compounds

magnetoelectric. Moreover, recent sound velocity measurements on CuFeO2 show clear evidence

of an R3m ⇀ C2/m pseudoproper ferroelastic transition at TN1= 14 K, in coincidence with one

of the antiferromagnetic transitions observed at zero field [1]. Thus, understanding the elastic

properties of CuFeO2 and CuCrO2 might help to elucidate the role played by the spin-lattice

coupling in the magnetic properties of this class of frustrated systems. The comparison between

these two isostructural compounds is particularly relevant as their magnetic ground states are

different. For this study, the elastic properties are obtained via sound velocity measurements as a

function of temperature. Results on CuCrO2 reveal softening on the longitudinal and transverse

bulk acoustic modes as the temperature is reduced down to TN1 = 24.3 K, while similar softening

is observed down to TN1 = 14 K for CuFeO2. Our sound velocity measurements are consistent

with an R3m ⇀ C2/m pseudoproper ferroelastic transition which coincides with the emergence

of a magnetic order at TN1. According to group theory, the driving mechanism of such a transition

must belong to the Eg irreducible representation of the high symmetry R3m space group. As

a result, we performed Raman measurements on CuFeO2 and CuCrO2 in order to determine

whether any Raman modes play an active role in the ferroelastic transition. Our results show

an increase in the frequencies of all modes down to 5 K, in accord with anharmonic phonon-

phonon interactions. Thus, we must conclude that the Raman active Eg modes are not the

order parameters, leaving the driving mechanism of the ferroelastic transitions observed at TN1

unresolved.

[1] G. Quirion, M. J. Tagore, M. L. Plumer, and O. A. Petrenko, Phys. Rev. B 77, 094111

(2008).

34

Modelling The Relation Between Magnetic And Elastic Properties Of

Magnetic Materials

Niloufar Faghihi1, Nikolas Provatas2, Ken Elder3, Mikko Haataja4, Mikko Karttunen1

1 Department of Applied Mathematics, The University of Western Ontario, London, Canada2 Department of Materials Science and Engineering , McMaster University, Hamilton, Canada3 Department of Physics, Oakland University, Rochester, USA4 Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, USA

We study the interrelation between magnetization and density order parameters in ferromagnetic

materials using a time dependent Ginzburg-Landau formalism i.e., we extend the free energy of

the Phase Field Crystal model [1] to study how magnetic domains affect the microstructures

in magnetic materials and vice versa. In our simulations, we used Finite Difference and Fast

Fourier Transform methods to solve dynamical equations of motion. We also calculated the

phase diagram of the model by minimizing the free energy in terms of the parameters of an

approximate solution of the system and then applying the common tangent construction to find

the equilibrium states. The results of the simulation are in agreement with the calculated phase

diagram and let us analyze the effect of the magnetization-density coupling.

[1] K.R. Elder, M. Grant, Phys. Rev. E 70 051605 (2004)

35

Quantum spin liquid state of S=1/2 kagomé lattice single crystals Tianheng Han1,Joel S.Helton1,Andrea Prodi1,Claudio Mazzoli2,Peter Müller3,Deepak K.Singh4, Jose A.Rodriguez4,Collin Broholm5, Daniel G.Nocera3,Shaoyan Chu6 ,Young S. Lee1

1Department of Physics, MIT, Cambridge, MA 02139 USA 2ESRF, 6 rue Jules Horowitz, 38043 Grenoble, France 3Department of Chemistry, MIT, Cambridge, MA 02139 USA 4NCNR NIST, Gaithersburg, Maryland 20899 USA 5Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218 USA 6Center for Material Science and Engineering, MIT, Cambridge, MA 02139 USA The Zn-paratacamite mineral family, ZnxCu4−x(OH)6Cl2, presents a promising system for studies

of frustrated magnetism on a S=1/2 kagomé lattice. Here we report a new synthesis method, by

which high quality single crystals of Zn-paratacamite can be produced [1]. The x = 1

herbertsmithite is a spin-liquid candidate. This compound displays a magnetic susceptibility that

is anisotropic at high temperatures. Based on the observed anisotropy, spin Hamiltonian terms in

additional to the isotropic Heisenberg exchange will be discussed. Synchrotron x-ray scattering

performed on a single crystal sample puts restrictions on the proposed valence bond solid state

and rules out the long debated Zn-Cu antisite disorder [2]. Inelastic neutron scattering has been

performed and we will discuss the observed structure factor in the context of various theoretical

expectations.

[1] T. H. Han et al. Phys. Rev. B 83, 100402(R) (2011) [2] D. E. Freedman et al.,JACS 132,16185(2010)

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Monte Carlo Simulations to Investigate the Magnetic Properties of Anti ferromagnetic IrMn3.

Vahid Hemmati, Martin Plumer, John Whitehead Memorial University of Newfoundland

The antiferromagnet IrMn3 is a material that is widely used in stabilizing the magnetization of the soft ferromagnetic layer employed in rewritable memories. The theories that underly exchange bias phenomena that provides the magnetic field to stabilize the soft ferromagnetic layer confirm the importance of the details of the spin-configuration of the IrMn3 [1].

IrMn3 is composed of ABC stacked layers of the Kagome lattice [2]. Previous theoretical studies on the two-dimensional kagome lattice have shown that the ground state spin configuration strongly depends on not only the nearest neighbors but also second and third neighbors in magnetic interaction[3].In the current work, the Metropolis Monte Carlo simulation method is used to determine thermal equilibrium properties and spin configurations of IrMn3. The specific heat and spin configuration of the Heisenberg and XY models are examined and the results show that the magnetic interaction only between the nearest spins is enough to achieve a stable ground state for the three-dimensional crystal.

[1] Spray J and Nowak U J. Phys. D: Appl. Phys. 39 4536-4539 (2006)

[2] Tomeno I, Fuke H N and Tsunoda Y J. Appl. Phys. 86 3853-3855 (1999)

[3] Harries A B, Kallin C and Berlinsky A J J. Phys. Rev. B 45 2899-2919 (1992)

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Dimensional Crossover of a Frustrated Distorted Kagome Heisenberg

Model; Application to FeCrAs

Travis Redpath,1 John M. Hopkinson,2,1 Alton A. Leibel,1, and Hae-Young Kee3

1 Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB, Canada2 Department of Physics and Astronomy, Brandon University, Brandon, MB, Canada3 Department of Physics, University of Toronto, Toronto, ON, Canada

FeCrAs is an unusual magnetically frustrated iron pnictide that has recently garnered interest as

a possible novel non-Fermi liquid. In an experiment conducted by Wu et al [1] FeCrAs was shown

to have Fermi liquid behaviour with a specific heat coefficient which indicates that it may be

metallic but this contrasts with a resistivity which increases almost isotropically. FeCrAs consists

of two sublattices: a sublattice of Cr atoms that form distorted kagome planes and a sublattice of

Fe trimers. The dominant magnetic interaction is believed to be an antiferromagnetic coupling

between local moments in the Cr sublattice. In between the Cr planes lie Fe atom trimers which

have a small moment (if any) and lie equidistant directly below and above the centre of the

distorted hexagons of Cr in adjacent planes. We posit a nearest neighbour coupling between the

distorted kagome Cr planes and the Fe trimers that lie between. In order to show this Monte Carlo

simulations using a classical J1−J2 Heisenberg model were run with differing values of the Cr-Cr

coupling constant (J1) and Cr-Fe coupling constant (J2). The general phase diagram for material

with this structure was investigated. There are three distinct regions in the phase diagram that

correspond to different ranges of the ratio of the coupling constants (J2

J1

). Large and small

values of the ratio have a first order phase transition to a ferrimagnetic and 120 degree rotated

state respectively. For values of the ratio near unity both phase transitions occur and the lattice

contains a combination of both states over some temperature range. The heat capacity, magnetic

susceptibility and ordering wavevectors are calculated within this approach and compared to the

experimental data of Wu et al. The magnetic ordering transition temperatures extracted from

the heat capacity were checked against the Binder cumulant to check for finite size effects of the

Monte Carlo simulation.

[1] W. Wu, A. McCollam, P.M.C. Rourke, D.G. Rancourt, Ian Swainson, S.R. Julian, EPL 85

17009 (2009)

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Ultrasonic velocity measurements of the multiferroic CuO under mag-netic fields

R. Villarreal,1 T. Usui,2 T. Kimura2 and G. Quirion1

1 Department of Physics and Physical Oceanography, Memorial University, St. John’s, Canada2 Division of Materials Physics, Osaka University, Toyonaka, Osaka, Japan

Multiferroics have been of great interest due to the possible technological applications of their

magnetoelectric (ME) effect, wherein magnetism and ferroelectricity are coupled. Among these

new compounds, the multiferroic cupric oxide (CuO) is particularly interesting as the ME

coupling is observed close to room temperature. According to recent magnetic and dielectric

measurements1, CuO adopts an incommensurate spiral antiferromagnetic spin configuration be-

low TN2 = 230 K while it transforms into a collinear antiferromagnetic state below TN1 = 213 K.

The spin-lattice coupling, which is believed to play a significant role in some ME compounds, can

be probed via sound velocity measurements. For this study, longitudinal and transverse modes

propagating along the principal crystallographic directions have been used. In summary, our

measurements reveal two anomalies which coincide with the magnetic phase transitions observed

at TN1 and TN2. The anomaly at TN1 clearly reflects a first-order transition, whereas the tem-

perature dependence around TN2 is consistent with a second order phase transition. However,

our high resolution measurements reveal a third anomaly 0.5 K below TN2. This new feature is

clearly visible when a field of 7 T is applied along the b-axis of the monoclinic structure, while

no changes are observed for other directions.

[1] T. Kimura et al., Nature Materials 7, 291 (2008).

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Competing Interactions and Spin Orbit Coupling in

Doped Rare-earth Manganites Wiqar Hussain Shah

Department of Physics, College of Science, King Faisal University,

Hofuf, 31982, Saudi Arabia ABSTRACT

The effect of Fe doping in La0.65Ca0.35Mn1-xFexO3 (where 10.00 �� x ) compound on the Mn site in the ferromagnetic phase has been studied in detail. The XRD results showed that all compounds crystallized in a tetragonal phase and no appreciable change is observed in the lattice parameters with increasing Fe concentration. Resistivity measurements of the compound from ambient temperature down to 77 K exhibit a peak at temperature Tp, which decreases with increasing Fe content. Substantial rise in resistivity corresponding to the Tp and increase spin disorder are also observed with increasing doping. Two models, variable range hopping (VRH), and polaronic have been used to explain the DC transport mechanism in the insulating region above Tp. The VRH model shows better fit to the resistivity data as compared to the polaronic model. The localization length is found to decrease by increasing Fe concentration. The activation barrier, W, has been calculated and found to increase with the increase of Fe content. In the metallic region (T<Tp) a linear decrease of W with temperature has been observed. It is observed that near to the metal-insulator transition temperature, transport in these compounds may be described in terms of carrier hoping between states, which are localized as a result of magnetic and non-magnetic disorder. The variations in the critical temperature Tp, Tc, confinement length, magnetic moment and magnetoresistance show a rapid change at about 4-5% Fe. Colossal magnetoresistance has been shifted to lower temperature, and enhanced by Fe doping. The maximum magnetoresistance is seen to increase consistently with the addition of Fe and increases upto 400% for 8% Fe concentration. However, conduction and ferromagnetism have consistently suppressed by Fe doping. The effect of Fe is seen to be consistent with the disruption of the Mn-Mn exchange possibly due to the formation of magnetic clusters. The formation of ferromagnetic and antiferromagnetic clusters and the competition between them with the introduction of Fe3+ ions, which do not participate in the double exchange (DE) process, have been suggested to explain the low value of magnetization at higher Fe concentration.

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Anisotropic Exchange Interactions on the Pyrochlore Lattice

Amir A. Roohi,1 Stephanie Curnoe1

1 Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St.

John’s, Canada

We investigate nearest neighbour exchange interactions on the pyrochlore lattice using the most

general (i.e. anisotropic) form allowed by symmetry. The pyrochlore structure consists of vertex-

sharing tetrahedra; there are two different types (A and B) of tetrahedra that differ by their

orientations within the pyrochlore lattice. Each edge of each tetrahedron corresponds to a pair

of nearest neighbours in the exchange interaction. Our model assumes that the pyrochlore space

group symmetry is broken such that the exchange constants on the A tetrahedra are different

than those on the B tetrahedra. The Hamiltonian describing exchange interactions on the A

tetrahedra has an exact solution; we include the exchange interactions on the B tetrahedra using

perturbation theory. We apply our results to Er2Ti2O7.

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Accurate Bandgaps and DFT: Semilocal Exchange for Electronic Structure. Michael Coates and Hong Guo Centre for the Physics of Materials and Department of Physics, McGill University, Montreal, Québec H3A 2T8 Canada. Computing accurate bandgaps in semiconductors and insulators is one of the greatest challenges in density functional theory (DFT) calculations. Much of this difficulty arises from the formulation of the exchange potential. Standard techniques, including hybrid functionals and the GW method, boast strong agreement with experiment but are computationally expensive, prohibiting the study of realistic systems. A novel approach proposed by Tran and Blaha [1] also correlates strongly with experimental data but is shown to be extremely efficient. It was implemented using a complete electronic basis set. We demonstrate the first known adaptation of this functional under the pseudopotential approximation and a valence-only basis set, further enhancing its efficiency. Using such a technique, entire semiconductor devices may be studied with physically accurate bandgaps. [1] Tran, F. and Blaha, P. PRL 102, 226401 (2009).

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