University of California at Berkeley – Physics Department

1. 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 Peter Kruger Nanosciences Department University of Bourgogne, France Jean Juraszek Materials Physics Group University of Rouen, France Stephanie Moyerman Eric E. Fullerton Physics Department University of California, San Diego Alex X. Gray Chuck S. Fadley Physics Department University of California, Davis University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 23, 2011

2. Why Fe-Rh? FePt (111) FeRh (001) MgO (001) Superparamagnetic limit – KUV ~ kBT 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 Tcrit ~ 50ºC RT < T < Tcrit: AFM FeRh; large K fixes FePt moment Tcrit < T < TC: FM FeRh reduces HC to flip FePt via coupling → Large H or T ~ TC University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 23, 2011

3. FeRh Magnetic Phases Tcrit Fe Fe Rh Rh AFM II T < Tcrit FM T > Tcrit University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 23, 2011

4. Origin of the transition? Possible contributions: • Electronic • Entropy relates to N(E) • Lattice • Debye approximation • Magnetic • Magnons? • Thermal excitation model, related to Rh moment 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 23, 2011

5. DFT Calculations of N(ε) Slater splitting: In AFM doubling of the lattice causes large drop at εF Theoretically predicted large difference in N(εF) 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

6. Photoemission Photoemission taken above and below Tcrit In order to compare PE to theory, must scale bands by scattering cross section and broaden by instrumentation resolution and core-hole lifetime broadening Compare features in FM-AFM difference and see good agreement 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

7. “Calorimeter on a Chip” Need to measure epitaxial thin film Grow IBAD MgO template on device For more info, see talk W.2100015 Specific heat of thin films • 30nm-200nm • 2K - 500K • 0T - 8T 2006 APS KeithleyInstrumentation Award University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 23, 2011

8. Specific Heat Measurements (AFM)(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! * = 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 23, 2011

9. Specific Heat Measurements (AFM)(1973*) What about the lattice? Different slopes yield different ΘD… ΘT,FM = 354±20K ΘL,FM = 615±12K ΘT,AFM = 304±13K ΘL,AFM = 591±10K Softer AFM phase → Lattice resists transition! * = 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 23, 2011

10. Thermal Fluctuation Model Tcrit Tcurie • 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) University of California at Berkeley – Physics Department March APS Meeting, Dallas, TX – March 23, 2011

11. Specific Heat Measurements (AFM)(1973*) γFM = 8.3±0.5 mJ/mol/K2 γAFM= 3.5±0.3 mJ/mol/K2 What about the lattice? Different slopes yield different ΘD… ΘT,FM = 354±20K ΘL,FM = 615±12K ΘT,AFM = 304±13K ΘL,AFM = 591±10K * = 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 23, 2011

12. 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 23, 2011

13. 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