university of california at berkeley – physics department

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 24, 2011

Calorimetry of epitaxial thin films

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

Stephanie MoyermanEric E. Fullerton

Physics DepartmentUniversity of California, San Diego

Bruce M. ClemensDepartment of Applied Physics

Stanford University

J. Randy GrovesSuperconductivity Technology

CenterLos Alamos National Laboratory

Rev. Sci. Instr. 82, 023908 (2011)

Why calorimetry?

• What can we study with heat capacity?– Lattice contribution

– Electronic contribution

– Phase transitions– Magnons, Two-state system, etc.

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University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 24, 2011

“Calorimeter on a Chip”

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

2006 APS KeithleyInstrumentation Award

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C

T

P

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 24, 2011

Limitation?

Many thin films are of interest because can use epitaxy/strain to alter bulk-like properties, but need membrane to reduce heat link while having small addenda!

What about a crystalline sample area?

Ion-Beam-Assisted Deposition

IBAD MgO(001)

J.M.E. Harper, et al., J. Appl. Phys. 82, 4319 (1997)

45º

MgO Target

Substrate

TargetIon Source

SubstrateIon Source

Figure adapted from L.S. Yu, et. Al., J. Vac. Sci. A 4, 443 (1986)

• MgO grows (001)-textured out-of-plane

• 45º to substrate yields (110) in-plane due to channeling

• Provides biaxially-oriented substrate

• Can be grown on any substrate

• Used by the superconductivity community for years

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 24, 2011

Synchrotron XRD of IBAD MgO

• Peaks displaced from MgO (hkl) values

• Distortion along beam axis ([202]):• a = .4228nm [020]• b = .4202nm [200]• c = .4212nm [002] (same as bulk)

hk reciprocal space map. RSI 82, 023908 (2011)

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 24, 2011

Heat Capacity of IBAD MgO

• ~25nm IBAD MgO grown on 200nm SiNx membrane

• Matches MgO CP well at high T

• Softening of lattice observed

Total addition to background < 6.5%

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 24, 2011

Specific heat of IBAD MgO layer. RSI 82, 023908 (2011)

Growth of FeRh on IBAD MgO

• FeRh undergoes AF>FM transition just above RT• CsCl structure

• Lattice constant = aMgO/√2 means

well-matched to MgO (rotated by 45º)

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 24, 2011

Mg

ORh

Fe

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 24, 2011

Specific Heat of FeRh

(AFM)(1973*)

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

Studied two Fe-Rh alloys with different magnetic properties

Low T data explores contributions from electrons and lattice

Difference at ~120K?

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 24, 2011

Specific Heat Measurements

Specific heat of Fe/Cr MMLs• Measured Fe/Cr MMLs on IBAD MgO to study interfacial enhancement of N(εF)

• XRD: Well-preserved superlattice order and four-fold symmetry indicating epitaxy

• Film on IBAD MgO shows lowest enhancement, indicating role of disorder

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 24, 2011

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

University of California at Berkeley – Physics Department

March APS Meeting, Dallas, TX – March 24, 2011