André Strydom Physics Department, University...

YFe2Al10

…unravelling the origin of quantum criticality

André Strydom

Physics Department, University of Johannesburg

Acknowledgements

Frank Steglich (MPI‐CPfS, Dresden)

Michael Baenitz and co‐workers (MPI‐CPfS, Dresden)

Meigan Aronson (BNL & Stony Brook Univ., USA)

Devashibhai Adroja and Adrian Hillier (ISIS, UK)

Pararajasingham Peratheepan (Eastern Univ., Sri Lanka)

Overview of properties of the 1:2:10 series

Data on YFe2Al10 : a candidate for criticality of T=0 ferromagnetism?

Ferromagnetic quantum criticality: an enigmatic rarity

Examples of itinerant ferromagnets tuned into quantum criticality …

by doping (UNi1xCoxSi2, Ca1xSrxRuO3, CeRu1xFexPO,… )

or pressure (ZrZn2, MnSi, UGe2)– role of superconductivity?– is the end‐point continuous or not?

quantum critical fluctuations above TC = 3K in UCoGe…

…but magnetic field always seems malevolent to ferromagnetic quantum criticality!

Ferromagnetic quantum criticality: an enigmatic rarity

Case of NbFe2:

…proximity to ferrimagnetism

…proposed mechanism of vanishing carrier velocity

…competing q=0 and q ≠ 0 instabili es producing non‐Fermi liquid critical exponents

…and the key role of chemical doping

Haynes et al., PRB 85 (2012) 115137 Neal et al., PRB 84 (2011) 085133Tompsett et al., PRB 82 (2010) 155137Alam and Johnson, PRL 107 (2011) 206401

Ferromagnetic quantum criticality: a rarity

Tuning of a ferromagnet turns out to be more complex…

FM‐QCP reached through a tricritical point…

Belitz, Kirkpatrick, Rollbühler

PRL 94 (2005) 247205

YFe2Al10 : a candidate for accidental criticality of T=0 ferromagnetism?

RT2Al10: (R=Y, La, Ce,…Yb, Lu; T=Fe, Ru, Os) [1]

CeRu2Al10: a collection of very unusual properties [2]

Orthorhombic Cmcm,Al unique sites for Ce, Ru

Ce – Ce: 5.2 ÅCe

Ru

1. Thiede, Ebel, Jeitschko, J Mater Chem 8 (1998) 2. Strydom, Physica B 404 (2009) 2981

Two different anisotropic Kondo insulators…

CeRu2Al10

TN = 27 K… ; ord = 0.34 B ; k=(1,0,0) [3]8 meV spin gap [4] with strong c–f hybridization; depletion of

magnetic moment and low electrical conductivity along b–axiseasy magnetization along a–axisyet ordered moment and high conductivity along c–axisfavours description in terms in terms of ac–plane dimensionality

CeFe2Al10

Strong and highly anisotropic c–f hybridization, pseudogap from NMR 11 meV [5] or from neutrons 12‐15meV [6]

3. Khalyavin et al, Phys Rev B 82 (2010) 100405 4. Robert et al, Phys Rev B 82 (2010) 100404 5. Chen & Lue, Phys Rev B 81 (2010) 0751136. Adroja et al., unpublished

CeRu2Al10

A phase transition with a commonthread in resistivity, specific heat, and susceptibility…

Strydom, Physica B 404 (2009) 2981

CeRu2Al10

Characteristic Kondo Insulator (or Heavy‐Fermion semiconducting)susceptibility…

Nishioka et al., J. Phys. Soc. Japan 78 (2009) 123705

YFe2Al10

Exploratory susceptibility [1] and Mössbauer [7] reported absence of magnetic ordering above 2K

Our studies exposed T– and B– dependencies in various properties below 5 K…

Sample quality : elemental purity > 99.99 wt.‐%

homogeneity and composition using WDX:Y : Fe : Al = 1.00(1) : 2.00(1) : 9.55(3)a=8.97 Å, b=10.15 Å, c=9.02 Å

powder‐XRD profile refinement

Mössbauer to check for single species of Fe

7. Waerenborgh et al, J. Alloys Comp. 323‐324 (2001) 78

YFe2Al10

Sample quality (continued)

nearest‐neighbour FeFe : 4.48 Å

Fe in octahedral site symmetry

YFe2Al10

Sample quality (continued) : recent single‐crystal analysis [8]reported no significant deviation from ideal Fe and Al site compositions through refinement of site occupations

Y environment… Fe environment…

8. Kerkau et al., Z. Kristallogr. NCS 227 (2012) 289

YFe2Al10: physical properties: Thermal behaviour

Susceptibility

Curie‐Weiss at high T (> 120 K), eff ≈ 0.4 Bac shows no frequency dependence (1 – 1000 Hz)…

Strydom & Peratheepan, PSS‐RRL 4 (2010) 356


Susceptibility

Park et al., PRB 84 (2011) 094425 Strydom & Peratheepan, PSS‐RRL 4 (2010) 356


Susceptibility

Park et al., PRB 84 (2011) 094425

(T ) T 1.28(T ) T 1.28


Susceptibility

Key results of Park et al.: YFe2Al10 can be tuned into a Fermi liquidusing magnetic field,

(T ) T 1.28

Susceptibility scaling exponents are not reconcilable with either mean‐field nor with disorder or impurity models

Park et al., PRB 84 (2011) 094425


Magnetization

Continuous curvature towards T=0…

yet / | → is still finite at 0.5 K

Strydom & Peratheepan, PSS‐RRL 4 (2010) 356


Magnetization

Strydom et al., J. Phys. Soc. Japan 80 (2011) SA043

T=0.78 K


Magnetization

Strydom et al., J. Phys. Soc. Japan 80 (2011) SA043

26Al‐NMR : suppression of spin fluctuations commences from B 1T…

T=0.78 K

YFe2Al10 physical properties: Magnetism

Inelastic neutron scattering

T = 4 K T = 100K

Ei = 25 meV

Ei = 6 meV

Adroja & Strydom (25 – 30 Oct 2012, ISIS, UK)

0 1 2 3 4 5 6 70

2

4

6

8

10

12

14

16

18

20

22

|Q| (Å-1)

Ene

rgy

(meV

)

mar17975.spe, symd(1,2,3), s=4, Ei=25 meV0<|Q|<7, 0<Energy<22

0

0.2

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1.6

1.8

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0 1 2 3 4 5 6 70

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|Q| (Å-1)

Ene

rgy

(meV

)


0

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1.2

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0 0.5 1 1.5 2 2.5 3 3.50

0.5

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1.5

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2.5

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3.5

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4.5

5

|Q| (Å-1)

Ene

rgy

(meV

)

mar17976.spe, symd(1,2,3), s=4, Ei=6 meV0<|Q|<3.5, 0<Energy<5

0

0.1

0.2

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0 0.5 1 1.5 2 2.5 3 3.50

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|Q| (Å-1)E

nerg

y (m

eV)


0

0.1

0.2

0.3

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0.5

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1


Inelastic neutron scattering

T = 4 K T = 100K

Ei = 25 meV

Ei = 6 meV

Adroja & Strydom (25 – 30 Oct 2012, ISIS, UK)

0 1 2 3 4 5 6 70

2

4

6

8

10

12

14

16

18

20

22

|Q| (Å-1)

Ene

rgy

(meV

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 1 2 3 4 5 6 70

2

4

6

8

10

12

14

16

18

20

22

|Q| (Å-1)

Ene

rgy

(meV

)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.5 1 1.5 2 2.5 3 3.50

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

|Q| (Å-1)

Ene

rgy

(meV

)


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.50

0.5

1

1.5

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2.5

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3.5

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4.5

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|Q| (Å-1)E

nerg

y (m

eV)


0

0.1

0.2

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0.9

1

Further experiments: lower T, E, Q…


26Al NMR:

Baenitz et al., 2012

0.1 10


Muon spin resonance

Adroja, Hillier, Strydom, (27 – 28 Oct 2012, ISIS, UK)

T = 0.05 K

BL = 2500 G2000G1500G1000 G500 G50 G0 G


Muon spin resonance

absence of cooperative magnetic effects confirmed down to 0.05 K

Complete absence of small‐field induced effects

Are spin fluctuations at 50 mK still too short‐lived for the ISIS SR time window…?

Adroja, Hillier, Strydom, (27 – 28 Oct 2012, ISIS, UK)

YFe2Al10: physical properties: ThermalSpecific heat : low T

Below 2K, Cp(T )/T = 0.0382(2) ∙ T 0.31(1)

YFe2Al10: physical properties: ThermalSpecific heat : the role of Fe stoichiometry


Far from stoichiometric Fe… Fermi liquid



At stoichiometric Fe… non‐Fermi liquid


CP(T )/T = + bT 2 + [dT 2 . ln(T/Tsf)] , Tsf = 15(2) K



Near stoichiometric Fe… spin fluctuating


non‐Fermi liquid behaviour does not scale with Fe concentration…



Near stoichiometric Fe… spin fluctuating

YFe2Al10: physical properties: ThermalElectrical resistivity

0 ≈ 100 .cm, RRR ≈ 3 (polycrystal) 0 ≈ 75 .cm, RRR ≈ 2 (single crystal) [Aronson et al.]

TK ≈ 20 K

Conclusions

YFe2Al10 is a good metal, paramagnetic at room temperature

No magnetic ordering found down to 0.05K

Correlated phenomena develop below 2K in electronic specific heat, or below 20K in electrical resistivity

Notions of itinerant FM quantum criticality:Specific heat : Cp(T )/T T 0.33 (2D clean FM1)Susceptibility : (T ) T 4/3 (3D clean FM2)

1. Moriya & Takimoto, JPSJ 64 (1995) 960, Lonzarich, The Electron (1997) Cambridge2. Hertz PRB 14 (1976) 1165, Millis PRB 48 (1991) 7183

Questions

How to reconcile the dimensionality issue?

Is there magnetic ordering below T = 50 mK?

…or is the observed nFL scaling merely a spectator segment en route to a very lowT Fermi liquid?

Current work

(T) and (T) under pressure

(T) at low temperatures

THANK YOU !!

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