Investigation of the Itinerant Electron Ferromagnetism of Ni(2+x)MnGa(1−x) and Co(2)VGa Heusler Alloys

Experimental investigations into the field dependence of magnetization and temperature dependences of magnetic susceptibility in Ni(2+x)MnGa(1−x) (x = 0.00, 0.02, 0.04) and Co(2)VGa Heusler alloy ferromagnets were performed following the spin fluctuation theory of itinerant ferromagnetism, called as “Takahashi theory”. We investigated the magnetic field dependence of magnetization at the Curie temperature T(C), which is the critical temperature of the ferromagnetic–paramagnetic transition, and also at T = 5 K, which concerns the ground state of the ferromagnetic state. The field dependence of the magnetization was analyzed by means of the H vs. M(5) dependence, and the field dependence of the ground state at 5 K was investigated by means of an Arrott plot (H/M vs. M(2)) according to the Takahashi theory. As for Ni(2+x)MnGa(1−x), the spin fluctuation parameter in k-space (momentum space, T(A)) and that in energy space (T(0)) obtained at T(C) and 5 K were almost the same. On the contrary, as for Co(2)VGa, the H vs. M(5) dependence was not shown at T(C). We obtained T(A) and T(0) by means of an Arrott plot at 5 K. We created a generalized Rhodes–Wohlfarth plot of p(eff)/p(S) versus T(C)/T(0) for the other ferromagnets. The plot indicated that the relationship between p(eff)/p(S) and T(0)/T(C) followed Takahashi’s theory. We also discussed the spontaneous magnetic moment at the ground state, p(S), which was obtained by an Arrott plot at 5 K and the high temperature magnetic moment, p(C), at the paramagnetic phase. As for the localized ferromagnet, the p(C)/p(S) was 1. As for weak ferromagnets, the p(C)/p(S) was larger than 1. In contrast, the p(C)/p(S) was smaller than 1 by many Heusler alloys. This is a unique property of Heusler ferromagnets. Half-metallic ferromagnets of Co(2)VGa and Co(2)MnGa were in accordance with the generalized Rhodes–Wohlfarth plot with a k(m) around 1.4. The magnetic properties of the itinerant electron of these two alloys appeared in the majority bands and was confirmed by Takahashi’s theory.