Investigation of the Itinerant Electron Ferromagnetism of NiMnGa and Co₂VGa Heusler Alloys.

Experimental investigations into the field dependence of magnetization and temperature dependences of magnetic susceptibility in NiMnGa ( = 0.00, 0.02, 0.04) and Co₂VGa 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 , which is the critical temperature of the ferromagnetic⁻paramagnetic transition, and also at = 5 K, which concerns the ground state of the ferromagnetic state. The field dependence of the magnetization was analyzed by means of the vs. ⁵ dependence, and the field dependence of the ground state at 5 K was investigated by means of an Arrott plot ( vs. ²) according to the Takahashi theory. As for NiMnGa, the spin fluctuation parameter in -space (momentum space, ) and that in energy space (₀) obtained at and 5 K were almost the same. On the contrary, as for Co₂VGa, the vs. ⁵ dependence was not shown at . We obtained and ₀ by means of an Arrott plot at 5 K. We created a generalized Rhodes⁻Wohlfarth plot of / versus /₀ for the other ferromagnets. The plot indicated that the relationship between / and ₀/ followed Takahashi's theory. We also discussed the spontaneous magnetic moment at the ground state, , which was obtained by an Arrott plot at 5 K and the high temperature magnetic moment, , at the paramagnetic phase. As for the localized ferromagnet, the / was 1. As for weak ferromagnets, the / was larger than 1. In contrast, the / was smaller than 1 by many Heusler alloys. This is a unique property of Heusler ferromagnets. Half-metallic ferromagnets of Co₂VGa and Co₂MnGa were in accordance with the generalized Rhodes⁻Wohlfarth plot with a 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.