Thermodynamic Measurements of Fe-Rh Alloys

FeRh undergoes an unusual antiferromagnetic-to-ferromagnetic (AFM-FM) transition just above room temperature (T-AFM>FM) that can be tuned or even completely suppressed with small changes in composition. The underlying temperature-dependent entropy difference between the competing AFM and FM states that drives this transition is measured by specific heat as a function of temperature from 2 to 380 K on two nearly equiatomic epitaxial Fe-Rh films, one with a ferromagnetic ground state (Fe-rich) and the other with an antiferromagnetic ground state (Rh-rich). The FM state shows an excess heat capacity near 100 K associated with magnetic excitations that are not present in the AFM state. The integrated entropy and enthalpy differences between the two alloys up to T-AFM>FM agree with the previously measured entropy of the transition (Delta S 17 +/- 3 J/kg/K) and yield a T = 0 energy difference of 3.4 J/g, consistent with literature calculations and experimental data; this agreement supports the use of the Fe-rich FM sample as a proxy for the (unstable) FM state of the AFM Rh-rich sample. From the low-temperature specific heat, along with sound velocity and photoemission measurements, the lattice contribution to the difference (Delta S-latt = -33 +/- 9 J/kg/K) and electronic contribution (Delta S-el = 8 +/- 1 J/kg/K) to the difference in entropy are calculated, from which the excess heat capacity in the FM phase and the resulting entropy difference are shown to be dominated by magnetic fluctuations (Delta S-mag = 43 +/- 9 J/kg/K). The excess magnetic heat capacity is dominated by the magnetic heat capacity of the FM phase, which can be fit to a Schottky-like anomaly with an energy splitting of 16 +/- 1 meV and a multiplicity of 1 per unit cell.