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

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

“Calorimeter on a Chip” Specific heat of thin films –30nm-200nm –2K - 500K –0T - 8T 2006 APS Keithley Instrumentation Award 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 Target Ion Source Substrate Ion 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, (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 SiN x membrane Matches MgO C P 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, (2011)

Growth of FeRh on IBAD MgO FeRh undergoes AF>FM transition just above RT CsCl structure Lattice constant = a MgO /√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 O Rh 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 (1973) Studied two Fe-Rh alloys with different magnetic properties Low T data explores contributions from electrons and lattice Difference at ~120K?

Entropic Contributions: ΔS latt = -5.3+/-1.5 J/mol/K ΔS el = 1.3+/-0.2 J/mol/K ΔS mag = 6.6+/-3.6 J/mol/K C latt is approximated with Debye models combining low T data and sound velocity measurements C el is obtained from γT, as measured in low T C P 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(ε F ) 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