Magnetic Field-induced Martensitic Transformations in Disordered and Ordered Fe–Pt Alloys

Abstract

Magnetic field-induced martensitic transformations in disordered and ordered Fe-24 at%Pt alloys were studied to examine the effect of degree of order (S) on the transformations by measuring the magnetization and electrical resistivity, applying a pulsed ultra-high magnetic field. As a result, it is found that a magnetic field higher than a critical strength is needed to induce the martensitic transformations at temperatures above M_s, without regard to the degree of order. The critical strength increases with increasing temperature difference from M_s, ΔT, and those plotted as a function of ΔT are found to lie on a straight line for the non-thermoelastic alloys with SS=0.7–0.8, as if the curve diverges near the respective T₀ temperature. The divergence phenomenon seems to suggest that the T₀ temperature is a maximum one above which martensites can not be induced even though any high magnetic field is applied. The amount of magnetic field-induced martensites increases linearly with increasing strength of the magnetic field for any ΔT and any degree of order, and the smaller the ΔT for a given degree of order and the lower the degree of order, the larger the amount of martensites becomes. A thermodynamic calculation for the increase of M_s, temperature suggests that the effect of a magnetic field on martensitic transformations may result in not only the Zeeman effect but also another unknown effect.