University of California at Berkeley – Physics Department

1. 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 University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010

2. FePt (111) FeRh (100) MgO (100) Why Fe-Rh? 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, Portland, OR – March 17, 2010

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

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

5. Electrons in AF/FM States FM Alloys allow for tuning of Tcrit, eventually pushing Tcrit= 0 yielding a FM ground state AFM 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

6. Electron-driven model Pros of electron-driven model: • Difference in N(EF) seen in DFT for AF/FM states because of splitting of d-bands leaving gap at EF • Assuming fixed N(EF) of T, matches experimental ΔS at Tcrit for a number of alloys Cons of electron-driven model: • Fe49.5Rh45.5Ir5 has higher Tcrit but γAFM ~2γFM! Outstanding questions: • No model of field/alloy dependence of Tcrit • No DFT work explaining lack of Tcrit in certain alloys University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010

7. Tcrit Tcurie 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

8. “Calorimeter on a Chip” • 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, Portland, OR – March 17, 2010

9. IBAD MgO Calorimeter • IBAD MgO(100) On Devices • Current calorimeters limited to amorphous/polycrystalline films • Use IBAD MgO on SiNX as template to grow biaxially-oriented films! Target Ion Source 45º • MgO grows (100) out of plane • 45º to substrate yields (110) in-plane due to channeling • Provides biaxially oriented substrate • Well-studied for high-Tc materials • Can use as template for STO, etc. 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, 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 (100) out-of-plane Azimuthal scan • 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

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

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

14. Future Work • Conclusions: • We have grown IBAD MgO on our unique a-SiNx-based microcalorimeters • We have confirmed growth of high quality Fe.49Rh.51 films on IBAD MgO • Specific heat data obtained on Fe.49Rh.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 CP 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 University of California at Berkeley – Physics Department March APS Meeting, Portland, OR – March 17, 2010