Investigation of the Itinerant Electron Ferromagnetism of Ni2+ xMnGa1- x and Co₂VGa Heusler Alloys

Takuo Sakon; Yuhi Hayashi; Akihito Fukuya; Dexin Li; Fuminori Honda; Rie Y Umetsu; Xiao Xu; Gendo Oomi; Takeshi Kanomata; Tetsujiro Eto

doi:10.3390/ma12040575

Investigation of the Itinerant Electron Ferromagnetism of Ni_{2+ x}MnGa_{1- x} and Co₂VGa Heusler Alloys

Materials (Basel). 2019 Feb 14;12(4):575. doi: 10.3390/ma12040575.

Authors

Takuo Sakon¹, Yuhi Hayashi², Akihito Fukuya³, Dexin Li⁴, Fuminori Honda⁵, Rie Y Umetsu⁶, Xiao Xu⁷, Gendo Oomi⁸, Takeshi Kanomata⁹, Tetsujiro Eto¹⁰

Affiliations

¹ Department of Mechanical and Systems Engineering, Faculty of Science and Technology, Ryukoku University, Otsu, Shiga 520⁻2194, Japan. sakon@rins.ryukoku.ac.jp.
² Department of Mechanical and Systems Engineering, Faculty of Science and Technology, Ryukoku University, Otsu, Shiga 520⁻2194, Japan. t150289@mail.ryukoku.ac.jp.
³ Department of Mechanical and Systems Engineering, Faculty of Science and Technology, Ryukoku University, Otsu, Shiga 520⁻2194, Japan. t150299@mail.ryukoku.ac.jp.
⁴ Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311⁻1313, Japan. dxli@imr.tohoku.ac.jp.
⁵ Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311⁻1313, Japan. honda@imr.tohoku.ac.jp.
⁶ Institute for Materials Research, Tohoku University, Sendai, Miyagi 980⁻8577, Japan. rieume@imr.tohoku.ac.jp.
⁷ Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai, Miyagi 980⁻8579, Japan. xu@material.tohoku.ac.jp.
⁸ Kurume Institute of Technology, Kurume, Fukuoka 830⁻0052, Japan. geomi@kurume-it.ac.jp.
⁹ Research Institute for Engineering and Technology, Tohoku Gakuin University, Tagajo, Miyagi 985⁻8537, Japan. kanomata@mail.tohoku-gakuin.ac.jp.
¹⁰ Kurume Institute of Technology, Kurume, Fukuoka 830⁻0052, Japan. teto@kurume-it.ac.jp.

Abstract

Experimental investigations into the field dependence of magnetization and temperature dependences of magnetic susceptibility in Ni_2+xMnGa_1-x (x = 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 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. M⁵ dependence, and the field dependence of the ground state at 5 K was investigated by means of an Arrott plot (H/M vs. M²) according to the Takahashi theory. As for Ni_2+xMnGa_1-x, the spin fluctuation parameter in k-space (momentum space, T_A) and that in energy space (T₀) obtained at T_C and 5 K were almost the same. On the contrary, as for Co₂VGa, the H vs. M⁵ dependence was not shown at T_C. We obtained T_A and T₀ by means of an Arrott plot at 5 K. We created a generalized Rhodes⁻Wohlfarth plot of p_eff/p_S versus T_C/T₀ for the other ferromagnets. The plot indicated that the relationship between p_eff/p_S and T₀/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 Co₂VGa and Co₂MnGa 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.

Keywords: ferromagnetic Heusler alloy; half-metal; itinerant electron ferromagnetism; magnetization.