Department of Physics, University of California at Berkeley, Berkeley, California 94720-7300, USA.
Phys Rev Lett. 2012 Dec 21;109(25):255901. doi: 10.1103/PhysRevLett.109.255901. Epub 2012 Dec 18.
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 (ΔS = 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 (ΔS(latt) = -33 ± 9 J/kg/K) and electronic contribution (ΔS(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 (ΔS(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.
FeRh 在室温以上经历了一种不寻常的反铁磁到铁磁（AFM-FM）转变，通过成分的微小变化可以对其进行调谐甚至完全抑制。这种转变的驱动力是竞争的 AFM 和 FM 状态之间随温度变化的依赖于熵差，通过两种近等原子外延 Fe-Rh 薄膜的比热作为温度的函数来测量，其中一个具有铁磁基态（富 Fe），另一个具有反铁磁基态（富 Rh）。FM 状态在近 100 K 处表现出过剩的热容，与 AFM 状态不存在的磁激发有关。两种合金的积分熵和焓差一直到 T(AFM>FM)与之前测量的转变熵（ΔS = 17 ± 3 J/kg/K）一致，并产生 3.4 J/g 的 0 K 能量差，与文献计算和实验数据一致；这种一致性支持使用富 Fe 的 FM 样品作为 AFM 富 Rh 样品的（不稳定）FM 状态的代理。从低温比热，以及声速和光发射测量中，晶格对差的贡献（ΔS(latt) = -33 ± 9 J/kg/K）和电子对差的贡献（ΔS(el) = 8 ± 1 J/kg/K）被计算出来，从中可以看出，FM 相中的过剩热容和由此产生的熵差主要由磁涨落决定（ΔS(mag) = 43 ± 9 J/kg/K）。过剩的磁热容主要由 FM 相的磁热容决定，它可以拟合为一个类似于 Schottky 的异常，具有 16 ± 1 meV 的能隙分裂和每个单位细胞的 1 倍多重性。
