Unusual Nature of the Abrupt Magnetic Transition in FeRh and Its Pseudobinary Variants

TLDR

FeRh’s antiferromagnetic–ferromagnetic transition exhibits a total entropy change far exceeding the lattice contribution. ΔS was extracted from the field dependence of Tcrit, and the anomalous magnetic entropy is attributed to thermal excitation of Rh moments induced by Fe exchange fields in the ferromagnetic state, which vanish in the antiferromagnetic state. Pseudobinary FeRh variants with Pd, Pt, or Ir substitutions shift Tcrit markedly, and the ΔS–ΔS_lat difference peaks near 500 K, indicating that the excess entropy originates from magnetic contributions.

Abstract

Previous measurements of the critical temperature (Tcrit) for the first-order antiferromagnetic-ferromagnetic transition in FeRh as a function of magnetic field have established that the total entropy change at the transition (ΔS) is much larger than the estimated change in lattice entropy (ΔS)lat. This study has now been extended to pseudobinary variants of FeRh where the Rh is partially replaced by Pd, Pt, or Ir, resulting in a large decrease or increase of Tcrit from its value (about 330°K) for FeRh. In each case, a value for ΔS was deduced from the measured field dependence of Tcrit. The difference between each ΔS and an estimated (ΔS)lat value, when plotted vs Tcrit, defines a smooth curve with a maximum at about 500°K, which is just below the Curie points of these alloys. It is therefore concluded that ΔS−(ΔS)lat represents an entropy change of magnetic origin. This anomalous change in magnetic entropy is attributed to thermal excitation of the Rh moments which in the ferromagnetic state are induced by the net exchange field from neighboring Fe moments; in the antiferromagnetic state this net exchange field vanishes, and the induced Rh moments and their contribution to the entropy go essentially to zero.