Magnetic properties of the ordered VPt3 alloy. I. — Ferromagnetic behaviour

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Abstract
We have shown that the VPt3 (V0.25Pt0.75) alloy becomes ferromagnetic in each of both atomically ordered states with Cu3Au and TiAl3 crystalline structure, respectively. The atomically disordered VPt3 alloy (with a Cu like fcc structure), remains paramagnetic over the whole temperature range 4.2 K to 1 100 K. By means of remanent magnetization measurements, we found the ferromagnetic Curie points Tc ≃ 240 K for VPt3 ordered in the Cu3Au like structure (VPt3 (1) sample) and Tc ≃ 210 K for VPt 3 ordered in the TiAl3 structure (VPt3 (2) sample). Magnetization measurements performed at low temperatures (4.2 K to 160 K) and in moderate fields (0 ≤ H ≤ 20 kOe) showed that both VPt3 (1) and (2) samples behave as classical ferromagnets. The high field (0 < H ≤ 150 kOe) magnetization measurements performed on both VPt3 (1) and (2) samples at 4.2 K, allowed an accurate determination of the saturation magnetization σ s (4.2 K) and of the high field χH (4.2 K) susceptibility. The order of magnitude of the χH susceptibility (at 4.2 K, χH ∼ 1 x 10-6 emu . g-1. Oe -1) is consistent with band paramagnetism of transition metals. Both VPt3 (1) and (2) may be considered as weak ferromagnets, in view of their small ferromagnetic moment (μ0 ∼ 0.1 μB per V atom-gram). The paramagnetic behaviour of VPt 3 (1) and (2) above their respective Curie points, is well described by a Curie-Weiss law yielding small paramagnetic moment values (μ ' ∼ 0.7 μB per V atom-gram). In spite of all experimental results obtained in this work, we could not settle with certainty whether the ferromagnetic behaviour of both ordered VPt3 alloys has to be interpreted in a localized model of ferromagnetism rather than in an itinerant one. Nevertheless, we have interpreted the ferromagnetic behaviour of VPt 3 (1) and (2) within a model of localized ferromagnetism : the ferromagnetism in VPt3 (1) and (2) is tentatively ascribed to a mechanism of indirect exchange interaction between the V moments through the conduction electrons of the Pt atoms. In this mechanism, the Pt electrons acquire a magnetic polarization antiparallel to the V moments

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