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 Hellman Physics Department, University of California, Berkeley Stephanie Moyerman Eric E. Fullerton Physics Department University of California, San Diego Peter Kruger Nanosciences Department University of Bourgogne, France Alex X. Gray Chuck S. Fadley Physics Department University of California, Davis Jean Juraszek Materials Physics Group University of Rouen, France

Why Fe-Rh? Superparamagnetic limit – K U V ~ k B T Large K? Alternative: FePt / FeRh bi-layer Thiele, J.-U., Maat, S., and Fullerton, E.E. APL 82, 2859 (2003) FeRh undergoes an AFM>FM transition at T crit ~ 50ºC RT < T < T crit : AFM FeRh; large K fixes FePt moment T crit < T < T C : FM FeRh reduces H C to flip FePt via coupling → Large H or T ~ T C MgO (001) FePt (111) FeRh (001) University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 23, 2011

FeRh Magnetic Phases AFM II T < T crit FM T > T crit T crit 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 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 (1973)

DFT Calculations of N( ε ) Theoretically predicted large difference in N(ε F ) Slater splitting: In AFM doubling of the lattice causes large drop at ε F Have itinerant AFM – does this difference persist at the transition? Turn to photoemission to examine electronic structure experimentally 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, must scale bands by scattering cross section and broaden by instrumentation resolution and core-hole lifetime broadening Photoemission taken above and below T crit Confirms large difference in electronic DOS persists up to T crit 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 Keithley Instrumentation Award 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

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 (1973) γ FM = 8.3±0.5 mJ/mol/K 2 γ AFM = 3.5±0.3 mJ/mol/K 2 ΔS el = 1.3±0.2 J/mol/K ΔS meas = 2.9 J/mol/K!

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 (1973) What about the lattice? Different slopes yield different Θ D … Θ T,FM = 354±20K Θ L,FM = 615±12K Θ T,AFM = 304±13K Θ L,AFM = 591±10K Softer AFM phase → Lattice resists transition!

Thermal Fluctuation Model 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, (2003) T crit T curie 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 (1973) γ FM = 8.3±0.5 mJ/mol/K 2 γ AFM = 3.5±0.3 mJ/mol/K 2 What about the lattice? Different slopes yield different Θ D … Θ T,FM = 354±20K Θ L,FM = 615±12K Θ T,AFM = 304±13K Θ L,AFM = 591±10K

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 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