Influence of palladium on the unidirectional anisotropy field in NiMn disordered systems

Abstract

We have carried out AC susceptibility, hysteresis cycle and torque measurements on (Ni_76−xPd_x)Mn₂₄, with 0 ⩽ x ⩽ 5, bulk alloys in the temperature and field ranges of 4.2–260 K and 0–20 kG, respectively. The AC susceptibility curves, χ′(T), present a maximum at the same freezing temperature, T_g, as the isoelectronic alloy Ni₇₆Mn₂₄. These alloys exhibit also the same Curie temperature, T_c, and the same saturated magnetization, M_s. However, both the unidirectional anisotropy field, H_a, and the coercivity field, H_c, increase considerably with the palladium content. For instance, H_a is 1.5 times larger for (Ni₇₄Pd₂)Mn₂₄ than for Ni₇₆Mn₂₄. The torque measurements give approximately the same H_a at 4.2 K as the hysteresis cycle. We conclude that Pd impurities enhance unidirectional anisotropy but have practically no effect on T_c, T_g and M_s. This is ascribed to the greater spin-orbit coupling of palladium.