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

1
Department of Mechanical and Systems Engineering, Faculty of Science and Technology, Ryukoku University, Otsu, Shiga 520–2194, Japan
2
Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311–1313, Japan
3
Institute for Materials Research, Tohoku University, Sendai, Miyagi 980–8577, Japan
4
Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai, Miyagi 980–8579, Japan
5
Kurume Institute of Technology, Kurume, Fukuoka 830–0052, Japan
6
Research Institute for Engineering and Technology, Tohoku Gakuin University, Tagajo, Miyagi 985–8537, Japan
*
Author to whom correspondence should be addressed.
Materials 2019, 12(4), 575; https://doi.org/10.3390/ma12040575
Received: 20 January 2019 / Revised: 12 February 2019 / Accepted: 12 February 2019 / Published: 14 February 2019
(This article belongs to the Special Issue Fundamental Science and Applications of Highly Functional Alloys)
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Abstract

Experimental investigations into the field dependence of magnetization and temperature dependences of magnetic susceptibility in Ni2+xMnGa1−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 TC, 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 Ni2+xMnGa1−x, the spin fluctuation parameter in k-space (momentum space, TA) and that in energy space (T0) obtained at TC and 5 K were almost the same. On the contrary, as for Co2VGa, the H vs. M5 dependence was not shown at TC. We obtained TA and T0 by means of an Arrott plot at 5 K. We created a generalized Rhodes–Wohlfarth plot of peff/pS versus TC/T0 for the other ferromagnets. The plot indicated that the relationship between peff/pS and T0/TC 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 Co2VGa and Co2MnGa were in accordance with the generalized Rhodes–Wohlfarth plot with a km 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. View Full-Text
Keywords: ferromagnetic Heusler alloy; magnetization; itinerant electron ferromagnetism; half-metal ferromagnetic Heusler alloy; magnetization; itinerant electron ferromagnetism; half-metal
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MDPI and ACS Style

Sakon, T.; Hayashi, Y.; Fukuya, A.; Li, D.; Honda, F.; Umetsu, R.Y.; Xu, X.; Oomi, G.; Kanomata, T.; Eto, T. Investigation of the Itinerant Electron Ferromagnetism of Ni2+xMnGa1−x and Co2VGa Heusler Alloys. Materials 2019, 12, 575.

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