Forced Magnetostrictions and Magnetizations of Ni2+xMnGa1-x at Its Curie Temperature

Takuo Sakon; Yuhi Hayashi; Dexin Li; Fuminori Honda; Gendo Oomi; Yasuo Narumi; Masayuki Hagiwara; Takeshi Kanomata; Tetsujiro Eto

doi:10.3390/ma11112115

Forced Magnetostrictions and Magnetizations of Ni_2+xMnGa_1-x at Its Curie Temperature

Materials (Basel). 2018 Oct 28;11(11):2115. doi: 10.3390/ma11112115.

Authors

Takuo Sakon¹, Yuhi Hayashi², Dexin Li³, Fuminori Honda⁴, Gendo Oomi⁵, Yasuo Narumi⁶, Masayuki Hagiwara⁷, Takeshi Kanomata⁸, Tetsujiro Eto⁹

Affiliations

¹ Department of Mechanical and System Engineering, Faculty of Science and Technology, Ryukoku University, Otsu, Shiga 520-2194, Japan. sakon@rins.ryukoku.ac.jp.
² Department of Mechanical and System Engineering, Faculty of Science and Technology, Ryukoku University, Otsu, Shiga 520-2194, Japan. t150289@mail.ryukoku.ac.jp.
³ Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan. dxli@imr.tohoku.ac.jp.
⁴ Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan. honda@imr.tohoku.ac.jp.
⁵ Kurume Institute of Technology, Kurume, Fukuoka 830-0052, Japan. geomi@kurume-it.ac.jp.
⁶ Center for Advanced High Magnetic Field Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan. narumi@ahmf.sci.osaka-u.ac.jp.
⁷ Center for Advanced High Magnetic Field Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan. hagiwara@ahmf.sci.osaka-u.ac.jp.
⁸ Research Institute for Engineering and Technology, Tohoku Gakuin University, Tagajo, Miyagi 985-8537, Japan. teto@kurume-it.ac.jp.
⁹ Kurume Institute of Technology, Kurume, Fukuoka 830-0052, Japan. kanomata@mail.tohoku-gakuin.ac.jp.

Abstract

Experimental investigations into the field dependence of magnetization and the relationship between magnetization and magnetostriction in Ni_2+xMnGa_1-x (x = 0.00, 0.02, 0.04) alloy ferromagnets were performed following the self-consistent renormalization (SCR) spin fluctuation theory of itinerant ferromagnetism. In this study, we investigated the magnetization of and magnetostriction on Ni_2+xMnGa_1-x (x = 0.02, 0.04) to check whether these relations held when the ratio of Ni to Ga and, the valence electron concentration per atom, e/a were varied. When the ratio of Ni to Ga was varied, e/a increased with increasing x. The magnetization results for x = 0.02 (e/a = 7.535) and 0.04 (e/a = 7.570) suggest that the critical index δ of H ∝ M^δ is around 5.0 at the Curie temperature T_C, which is the critical temperature of the ferromagnetic⁻paramagnetic transition. This result confirms Takahashi's spin fluctuation theory and the experimental results of Ni₂MnGa. The spontaneous magnetization p_S slightly decreased with increasing x. For x = 0.00, the spin fluctuation parameter in k-space (momentum space; T_A) and that in energy space (T₀) were obtained. The relationship between p_eff/p_S and T_C/T₀ can also be explained by Takahashi's theory, where p_eff indicates the effective magnetic moments. We created a generalized Rhodes-Wohlfarth plot of p_eff/p_S versus T_C/T₀ for other ferromagnets. The plot indicates that the relationship between p_eff/p_S and T₀/T_C follows Takahashi's theory. We also measured the magnetostriction for Ni_2+xMnGa_1-x (x = 0.02, 0.04). As a result, at T_C, the plot of the magnetostriction (ΔL/L) versus M⁴ shows proportionality and crosses the origin. These magnetization and magnetostriction results were analyzed in terms of Takahashi's SCR spin fluctuation theory. We investigated the magnetostriction at the premartensite phase, which is the precursor state to the martensitic transition. In Ni₂MnGa system alloys, the maximum value of magnetostriction is almost proportional to the e/a.

Keywords: ferromagnetic Heusler alloy; itinerant electron magnetism; magnetization; magnetostriction; premartensite phase.