University of California at Berkeley – Physics Department

1. Calorimetry of epitaxial thin films Rev. Sci. Instr.82, 023908 (2011) David W. Cooke, Frances Hellman Physics Department, University of California, Berkeley J. Randy Groves Superconductivity Technology Center Los Alamos National Laboratory Stephanie Moyerman Eric E. Fullerton Physics Department University of California, San Diego Bruce M. Clemens Department of Applied Physics Stanford University University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 24, 2011

2. Why calorimetry? • What can we study with heat capacity? • Lattice contribution • Electronic contribution • Phase transitions • Magnons, Two-state system, etc. University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 24, 2011

3. “Calorimeter on a Chip” 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! • Specific heat of thin films • 30nm-200nm • 2K - 500K • 0T - 8T What about a crystalline sample area? 2006 APS KeithleyInstrumentation Award University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 24, 2011

4. Ion-Beam-Assisted Deposition IBAD MgO(001) • 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 J.M.E. Harper, et al., J. Appl. Phys.82, 4319 (1997) Target Ion Source 45º Substrate Ion Source Substrate MgO Target Figure adapted from L.S. Yu, et. Al., J. Vac. Sci. A4, 443 (1986) University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 24, 2011

5. 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

6. 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% Specific heat of IBAD MgO layer. RSI 82, 023908 (2011) University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 24, 2011

7. Growth of FeRh on IBAD MgO Mg • FeRh undergoes AF>FM transition just above RT • CsCl structure • Lattice constant = aMgO/√2 means well-matched to MgO (rotated by 45º) Rh Fe O University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 24, 2011

8. Specific Heat of FeRh (AFM)(1973*) Studied two Fe-Rh alloys with different magnetic properties Low T data explores contributions from electrons and lattice Difference at ~120K? * = M.J. Richardson, D. Melville, and J.A. Ricodeau. Phys. Lett. A46 153-154 (1973) University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 24, 2011

9. Specific Heat Measurements • 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 Entropic Contributions: ΔSlatt = -5.3+/-1.5 J/mol/K ΔSel = 1.3+/-0.2 J/mol/K ΔSmag= 6.6+/-3.6 J/mol/K University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 24, 2011

10. 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

11. 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