1. University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010 Thermodynamic measurements of iron-rhodium alloys David W. Cooke, Frances Hellman Physics Department, University of California, Berkeley Stephanie Moyerman, Eric E. Fullerton Physics Department, University of California, San Diego

2. 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 University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010 MgO (100) FePt (111) FeRh (100)

3. FeRh Magnetic Phases AFM II T < T crit FM T crit < T T crit University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010

4. Electrons in AF/FM States Tu, P. et al. J. Appl. Phys. 40, 1368 (1969) University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010 AFM FM Koenig, C. J. Phys. F 12, 1123 (1982) εFεF εFεF

5. Electrons in AF/FM States Tu, P. et al. J. Appl. Phys. 40, 1368 (1969) University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010 AFM FM Alloys allow for tuning of T crit, eventually pushing T crit = 0 yielding a FM ground state

6. Electron-driven model Pros of electron-driven model: Difference in N(E F ) seen in DFT for AF/FM states because of splitting of d-bands leaving gap at E F Assuming fixed N(E F ) of T, matches experimental Δ S at T crit for a number of alloys Cons of electron-driven model: Fe 49.5 Rh 45.5 Ir 5 has higher T crit but γ AFM ~2γ FM ! Outstanding questions: No model of field/alloy dependence of T crit No DFT work explaining lack of T crit in certain alloys University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010

7. 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, 064415 (2003) University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010 T crit T curie

8. “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, Portland, OR – March 17, 2010

9. IBAD MgO Calorimeter 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) IBAD MgO(100) On Devices Current calorimeters limited to amorphous/polycrystalline films Use IBAD MgO on SiN X as template to grow biaxially-oriented films! MgO grows (100) out of plane 45º to substrate yields (110) in-plane due to channeling Provides biaxially oriented substrate Well-studied for high-T c materials Can use as template for STO, etc. University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010

10. FeRh Magnetization Data University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010

11. FeRh XRD on IBAD MgO Similarly high crystal quality with some relaxation on IBAD Slightly more mosaicity than MgO sample Maintains four-fold symmetry University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010 (100) out-of-planeAzimuthal scan

12. Specific Heat of Fe.49 Rh.51 University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010

13. Specific Heat of Fe.49 Rh.51 University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010

14. Future Work University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010 Conclusions: We have grown IBAD MgO on our unique a-SiN x -based microcalorimeters We have confirmed growth of high quality Fe.49 Rh.51 films on IBAD MgO Specific heat data obtained on Fe.49 Rh.51 matches that in the literature Growth of an Fe-rich alloy on IBAD MgO was confirmed to be FM down to 2K Future work: Measure C P of this FM FeRh as a function of H, T to examine two-state Rh theory Ongoing collaboration to examine domain formation during AF>FM transition in using our devices as an in-situ heater stage in magnetic soft x-ray transmission microscope at the Advanced Light Source (LBNL) DFT calculations of non-stoichiometric FeRh alloys