Magnetization and electrical resistivity measurements on an iron-rhodium alloy of approximate composition FeRh, having CsCl-type structure, confirm recent x-ray and neutron diffraction evidence for a first-order antiferromagnetic-ferromagnetic transition at about 350°K. From 350°K down to 77°K, an essentially constant, field-independent susceptibility of about 1×10−4 emu/g is observed. Above 350°K, the alloy behaves like a normal ferromagnet with a Curie point of 675°K and a saturation magnetization (extrapolated to 0°K) of 130 emu/g. This σ0 value is equivalent to an average atomic moment of 1.85 μB, or to 3.85 μB per iron atom if the rhodium moment is assumed to be zero. Alternatively, if the iron moment is taken to be 2.2 μB, the rhodium moment has to be about 1.5 μB. Qualitative results are given for the effect of high magnetic fields and pressures on the first-order transition temperature of this alloy.
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.
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.
Well-annealed bulk samples of the intermetallic compound FeRh exhibit a first-order transition from antiferromagnetic to ferromagnetic order on heating near 330°K. Plastic deformation has been found to convert the normal CsCl-type ordered structure of FeRh to a disordered fcc structure, which is only weakly magnetic and does not exhibit the first-order transition. On annealing the disordered material at Ta=510°K, three stages of effects have been observed. In the first, the fcc phase converts to a highly ordered CsCl-type structure, the magnetization at Ta increasing with a time constant of ∼10 min; the material is strongly ferromagnetic down to low temperatures. In the second stage, there is a much slower (τ≈400 min) increase in magnetization at Ta with a gradual return of the first-order transition at lower temperatures. In the third stage, there is no further increase in magnetization at Ta, but the first-order transition continues to increase in completeness and sharpness. The slow return of the first-order transition is attributed to the gradual attainment of perfect long-range order or, alternatively, to the annealing out of defects produced by plastic deformation.
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