Forced Magnetostrictions and Magnetizations of Ni(2+x)MnGa(1−x) at Its Curie Temperature

Experimental investigations into the field dependence of magnetization and the relationship between magnetization and magnetostriction in Ni(2+x)MnGa(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+x)MnGa(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 Ni(2)MnGa. 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 (T(0)) were obtained. The relationship between p(eff)/p(S) and T(C)/T(0) 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)/T(0) for other ferromagnets. The plot indicates that the relationship between p(eff)/p(S) and T(0)/T(C) follows Takahashi’s theory. We also measured the magnetostriction for Ni(2+x)MnGa(1−x) (x = 0.02, 0.04). As a result, at T(C), the plot of the magnetostriction (ΔL/L) versus M(4) 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 Ni(2)MnGa system alloys, the maximum value of magnetostriction is almost proportional to the e/a.