Structural change and straining in Fe–Pd polycrystals by magnetic field

The effect of a magnetic field on martensite structures in Fe–30 and –30.5at.% Pd has been studied using optical and electron (TEM) microscopy. No appreciable change to the martensite structure is observed with optical microscopy when a magnetic field of ∼8×105 A m−1 is applied to a sample in the fully martensite state. The fractions of elementary martensite units (Bain correspondence variants—BCVs), existing as alternately stacked twins in a martensite plate and observed by electron microscopy, remain almost the same whether a TEM foil is under practically no magnetic field or under a field of more than ∼8×105 A m−1. Fe–Pd is more difficult to magnetically saturate in the martensite state than in the austenite state, even though the saturation magnetization is only slightly smaller in the former state. Hysteresis in a magnetization–magnetic field relationship is small. From these observations, it is concluded that the magnetization in the martensite state is mostly achieved by rotation of the magnetization vector, with hardly any contribution from the movement of magnetic domain walls. This is reasoned using the argument that the interfaces of BCVs offer large resistance to the movement of magnetic domain walls.