University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

Examining the AF>FM transition in Fe-Rh thin films through

photoemission and specific heat measurements

David W. Cooke, Catherine Bordel, Frances HellmanPhysics Department, University of California, Berkeley

Stephanie MoyermanEric E. Fullerton

Physics DepartmentUniversity of California, San Diego

Peter KrugerNanosciences Department

University of Bourgogne, France

Alex X. GrayChuck S. Fadley

Physics DepartmentUniversity of California, Davis

Jean JuraszekMaterials Physics Group

University of Rouen, France

Why Fe-Rh?

Superparamagnetic limit – KUV ~ kBT

Large K?

Alternative: FePt / FeRh bi-layerThiele, J.-U., Maat, S., and Fullerton, E.E. APL 82, 2859 (2003)

FeRh undergoes an AFM>FM transition at Tcrit ~ 50ºC

RT < T < Tcrit: AFM FeRh; large K fixes FePt momentTcrit < T < TC: FM FeRh reduces HC to flip FePt via coupling

→ Large H or T ~ TC

MgO (001)

FePt (111)

FeRh (001)

m

m

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

FeRh Magnetic Phases

AFM IIT < Tcrit

FMT > Tcrit

Tcrit

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

Fe

Rh

Fe

Rh

Origin of the transition?

Possible contributions:• Electronic

– Entropy relates to N(E)

• Lattice– Debye approximation

• Magnetic– Magnons?

– Thermal excitation model, related to Rh moment

maglattelAFMFM SSSSSSdTT

C

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

M.J. Richardson, D. Melville, and J.A. Ricodeau. Phys. Lett. A 46 153-154 (1973)

DFT Calculations of N(ε)

Theoretically predicted largedifference in N(εF)

Slater splitting:In AFM doubling of the latticecauses large drop at εF

Have itinerant AFM – doesthis difference persist at thetransition?

Turn to photoemission to examine electronic structureexperimentally

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

Photoemission

Compare features in FM-AFM difference and see good agreement

In order to compare PE to theory, mustscale bands by scattering cross sectionand broaden by instrumentation resolution and core-hole lifetime broadening

Photoemission taken above and below Tcrit

Confirms large difference in electronic DOS persists up to Tcrit

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

“Calorimeter on a Chip”

Specific heat of thin films• 30nm-200nm• 2K - 500K• 0T - 8T

2006 APS KeithleyInstrumentation Award

t

e

C

T

P

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

Need to measure epitaxial thin film

Grow IBAD MgO template on device

For more info, see talk W.2100015

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

Specific Heat Measurements(AFM)(1973*)

* = M.J. Richardson, D. Melville, and J.A. Ricodeau. Phys. Lett. A 46 153-154 (1973)

γFM = 8.3±0.5 mJ/mol/K2

γAFM= 3.5±0.3 mJ/mol/K2

ΔSel = 1.3±0.2 J/mol/K

ΔSmeas = 2.9 J/mol/K!

critAFMFM TγγdTT

C

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

Specific Heat Measurements(AFM)(1973*)

* = M.J. Richardson, D. Melville, and J.A. Ricodeau. Phys. Lett. A 46 153-154 (1973)

What about the lattice?

Different slopes yielddifferent ΘD…

ΘT,FM = 354±20KΘL,FM = 615±12K

ΘT,AFM = 304±13KΘL,AFM = 591±10K

Softer AFM phase →Lattice resists transition!

Thermal Fluctuation Model

nnn

iknnnnn

iknni nnnnn

kiikikii rVrVSSrJSDU,

2

• Note the shoulder at ~200K• Two-state system (Schottky)

• FM – competition between non/magnetization of Rh• AFM – no such competition because Fe AFM cancels

Gruner, M.E., et al. Phys. Rev. B 67, 064415 (2003)

Tcrit Tcurie

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

Specific Heat Measurements(AFM)(1973*)

* = M.J. Richardson, D. Melville, and J.A. Ricodeau. Phys. Lett. A 46 153-154 (1973)

γFM = 8.3±0.5 mJ/mol/K2

γAFM= 3.5±0.3 mJ/mol/K2

What about the lattice?

Different slopes yielddifferent ΘD…

ΘT,FM = 354±20KΘL,FM = 615±12K

ΘT,AFM = 304±13KΘL,AFM = 591±10K

Entropic Contributions:ΔSlatt = -5.3+/-1.5 J/mol/KΔSel = 1.3+/-0.2 J/mol/KΔSmag= 6.6+/-3.6 J/mol/K

• Clatt is approximated with Debye models combining low T data and sound velocity measurements

• Cel is obtained from γT, as measured in low T CP

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 23, 2011

Specific Heat Measurements

Conclusions

• Photoemission: Observed change in electronic density of states between AF/FM phases

• Specific Heat: Observed Schottky-like anomaly suggesting dominant contribution of magnetism to entropy of transition