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. M5 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 /pS versus T _C /T ₀ for the other ferromagnets. The plot indicated that the relationship between p _eff /pS and T ₀ /T _C followed Takahashi's theory. We also discussed the spontaneous magnetic moment at the ground state, pS, which was obtained by an Arrott plot at 5 K and the high temperature magnetic moment, pC, at the paramagnetic phase. As for the localized ferromagnet, the pC/pS was 1. As for weak ferromagnets, the pC/pS was larger than 1. In contrast, the pC/pS 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.