Irreversibilities in low-field magnetization of site-disordered Ni₇₅Al₂₅

Abstract

The results of extensive "zero-field-cooled" (M_ZFC) and "field-cooled" (M_FC) magnetization and hysteresis measurements performed in the magnetic field (H) and temperature (T) ranges 2.5Oe≤H≤3kOe (10 kOe) and 14K≤T≤ 1.4T_C (Curie temperature) on Ni₇₅Al₂₅ samples with varying degree of site disorder and on samples with composition in the range Ni_74.31Al_24.69 to Ni_75.98Al_24.02 having the same degree of site disorder, are presented and discussed in the light of the existing theoretical models. The difference, M_irr(T)=M_FC(T)-M_ZFC(T), is taken to be the direct measure of irreversibility in magnetization. As the temperature is lowered from T»T_C, M_irr as a function of temperature at a fixed H, (i) deviates from zero at a temperature T_WI (which marks the onset of weak irreversibility), (ii) goes through a peak at T_P (a new feature, to our knowledge not reported in the literature so far, observed in all the samples except for the quenched one), and (iii) exhibits a steep increase below T_SI (the temperature at which a crossover to strong irreversibility occurs). While the occurrence of a peak in M_irr(T) has not been theoretically addressed yet, the observed variations of T_WI and T_SI with H as well as the observation that T_WI»T_C and T_SI≈T_C are in conflict with the predictions based on the mean-field vector-spin models. By establishing a clear link between the magnetic field variations of T_WI, T_P, and T_SI and the temperature dependences of H_C (coercive field), the present work asserts that the pinning of domain walls at the magnetic (exchange) inhomogeneities present in the samples under consideration is at the root of the observed irreversibilities in magnetization.