Mechanism for the First-Order Magnetic Transition in the FeRh System

Measurements of the field dependence of the critical temperature (Tcrit) for the first-order antiferromagnetic-ferromagnetic transition in FeRh established that the total entropy change (ΔST) at Tcrit is much larger than the change in lattice entropy (ΔSL). Doping experiments (Pd, Pt, or Ir) showed that ΔST−ΔSL=ΔS varies almost linearly with Tcrit. Apparently the transition involves a change in the electronic density of states and thus in the electronic entropy, S=γTcrit, where γ is the electronic heat coefficient. To test this mechanism, low-temperature specific-heat measurements were made on several compositions in the Fe(Rh, Pd) system. The compositions and γ values are Fe53Rh47 (59 μJ/g·°K2), Fe51Rh49 (60 μJ/g·°K2), Fe48Rh46Pd6 (64 μJ/g·°K2), Fe49Rh51 (16 μJ/g·°K2), and Fe48Rh49Pd3 (31 μJ/g·°K2). The first three are ferromagnetic at T=0; the latter two are antiferromagnetic with Tcrit of 310° and 210°K, respectively. The insensitivity of γ to Fe concentration in the ferromagnetic samples suggests a broad peak in the d-band at the Fermi surface. The resulting entropy changes, ΔS=(γF−γA)Tcrit, agree with Kouvel's measurements and confirm the electron gas entropy as the main driving force for the transition.