Ferromagnetism Inhomogenous magnetization Magnetic vortices Dynamics Spin transport
Magnetism on the Move
US-Spain Workshop on Nanomaterials
Ferromagnetism is rare……
B = 25 nm (<3 nm), W=150 nm, t = 14 nmdata rate ~ GHz
Direction of Disk Motion
Inductive Write Element
GMR Read Sensor
t
W
B
“Compass” that responds to local magnetic field and varies the resistance
…. but useful
Write Coil
Write Pole2
Read Head
<D> = 8.5 nm+/- 2.5 nm100 nm
Disk Head
Courtesy of Eric Fullerton
Recording Media
<D> = 8.5 nm+/- 2.5 nm100 nm
1000 nm
NSNR # grains/bitCourtesy of Eric Fullerton
Why is ferromagnetism neither common nor “perfect”?
Microscopic
Macroscopic
R. Schaefer, Dresden
Magnetostatics (Not as bad as it looks)
Magnetostatics: equilibrium condition
Variational method to find the equilibrium condition
, where
Torque
= 0
W. F. Brown, Jr., Micromagnetics (Interscience Publishers, New York, 1963)
Micromagnetics Simulation
Excitations
[Equilibrium State] [Excited State] [Dynamic motion]
Landau-Lifshitz-Gilbert Equation
Bg
= 17.6 GHz/kOe
Spin waves
q
Uniform precession (q = 0 spin wave)
Spin Waves
ij
nn
jijiexch JSSJE cos2
2Aq
The simple case (no magnetocrystalline anisotropy)
Magnetic vortex
Of course there are intermediate cases - such as the S-state
K.L. Metlov et. al., J. Magn. Magn. Mater. 242-245 (2002) 1015
LE = exchange length = 22/ sMA
Four different configurations of the vortex state
Schematic illustration of four different vortex states
P= Polarity (the magnetization direction of the vortex core)
C= Chirality (the winding direction of in-plane magnetization)
The magnetostatic energies are obviously identical….
Magnetic vortices
1) Lorentz Microscopy on 200 nm Co disk
2) MFM on 1 m Permalloy disk
3) SP-STM on 200 nm wideand 500 nm long Fe island
Observation of magnetic vortices
1)J. Raabe et al. J. Appl. Phys. 88, 4437 (2000)
2)T.Shinjo et al. Science 289, 930 (2000)
3)A. Wachowiak et al. Science 298, 577 (2002)
What about the dynamics?
Vortex-core dynamics (gyrotropic motion)
A.A. Thiele, Phys. Rev. Lett. 30, 230 (1973).See also B. Argyle et al., Phys. Rev. Lett. 53, 190 (1984).
Gyromagnetic force acting on a shifted vortex
where = static force for an applied field H = gyrovector (antiparallel to the direction of vortex polarity P) = magnetic energy dissipation dyadic
W
G
D�
01
dt
MdM
MHM
dt
Md
seff
Landau-Lifshitz-Gilbert Equation:
Equivalent force equation
0)()(
)(
))((
dt
tXdD
dt
tXdG
tX
tXW
�
When P changes sign, changes sign!G
Gyrotropic Mode
16
The lowest frequency excitation: Gyrotropic mode
1 s 1 ns
[Will be replaced with a movie:Gyrotropic motion in simulation]
10-14 sec(chemical reaction
dynamics)
10-12 sec(semiconductors)
10-9 sec(magnetism)
10-7 sec
Time Scales
How do you make a movieon picosecond time scales?
Time-resolved Kerr microscopy (stroboscopic)
[Freeman et al. J. Appl. Phys. 79, 5898 (1996)]
What we measure:
Polar Kerr Rotation Mz
as a function of time delay, probe-beam position, and applied field
Also Back, Hicken, and others.....This is a stroboscopic technique.
Experimental Setup
20
Pinning potential
Different equilibrium positions
Not at a pinning site
At a pinning site
Excitation off
Large Amplitude: Core SwitchingLarge Amplitude: Core Switching
21
Counterclockwise orbit
Clockwise orbit
B. Van Waeyenberge et al., Nature 444, 461 (2006)1 s 0.5 ns
22
Core reversal
Phase Diagram of Vortex DynamicsPhase Diagram of Vortex Dynamics
23
Pinned & Depinned
Magnetic Heterostructures
New Technologies:
• Magnetic Random Access Memory• Magnetic tunnel junction sensors• Patterned media• Semiconductor spintronics • Highly polarizable materials
Diskdrives
MagneticRandom AccessMemory
Field sensing(medical devices,security)
• The electrical response of the device depends on the magnetic state of two or moreelectrodes (field sensors, read heads)
• The magnetic state of the device can be changedby an electrical current (memory, oscillators)
F F
Integration of ferromagnets with insulators, semiconductors, and normal metals
Example: the spin valve
Magnetic Heterostructures
Read Head Technology
Pole2
Pole1
Gap
Read
WriteShield2
Shield1
Scale: 50 nm
MRLeads Leads
Compound PtMn
Free Layer
Pinned Layer
Cu
Magnetic Tunnel Junctions
FM 1
FM 2
Insulator
Spin transfer torque oscillators
• MgO-based tunnel junction devices for maximizing signal and reducing threshold current
• Built-in hard-axis polarizer enhancesoutput power and allows for zero-fieldoperation
• Influence of CoFeB on damping (withData Storage Institute, Singapore)
• Modification of CoFeB/MgO interfaceanisotropy (with DSI)
• Spin transfer torque FMR (with DSI)
J. Appl. Phys. 109, 07D307 (2011)J. Appl. Phys. 109, 07C714 (2011)Appl. Phys. Lett. (accepted, 2012)Wang, Crowell
Materials science of magnetic heterostructures
Co2MnGe
GaAs
TEM
Electronic StructureCalculations
Growth and characterization
Interfacial characterization
Transport
Simulations
Spin dynamics
Epitaxial Fe/InxGa1-xAs heterostructures
• Epitaxial structures: low temperaturegrowth to minimize interfacial reactions
• Transport and modeling techniques developed by the IRG
• Increase spin-orbit coupling by shiftingto InxGa1-xAs
Palmstrøm, Crowell
Summary
• Magnetism is ubiquitous, although ferromagnetism is relatively rare
• Ferromagnetism is useful if not always easy to understand
• Imperfect magnets are more interesting than perfect ones
• Dynamics are accessible by new tools
• Integration of ferromagnets with other materials yields new physicsand new devices