Irreversibilities in low-field magnetization of site-disorderedNi75Al25

Abstract

The results of extensive ``zero-field-cooled'' ${(M}_{\mathrm{ZFC}})$ and ``field-cooled'' ${(M}_{\mathrm{FC}})$ magnetization and hysteresis measurements performed in the magnetic field (H) and temperature (T) ranges $2.5\mathrm{Oe}<~H<~3\mathrm{kOe}$ (10 kOe) and $14K<~T<~{1.4T}_{C}$ (Curie temperature) on ${\mathrm{Ni}}_{75}{\mathrm{Al}}_{25}$ samples with varying degree of site disorder and on samples with composition in the range ${\mathrm{Ni}}_{74.31}{\mathrm{Al}}_{24.69}$ to ${\mathrm{Ni}}_{75.98}{\mathrm{Al}}_{24.02}$ having the same degree of site disorder, are presented and discussed in the light of the existing theoretical models. The difference, ${M}_{\mathrm{irr}}{(T)=M}_{\mathrm{FC}}(T)\ensuremath{-}{M}_{\mathrm{ZFC}}(T),$ is taken to be the direct measure of irreversibility in magnetization. As the temperature is lowered from $T\ensuremath{\gg}{T}_{C},$ ${M}_{\mathrm{irr}}$ as a function of temperature at a fixed H, (i) deviates from zero at a temperature ${T}_{\mathrm{WI}}$ (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 ${T}_{\mathrm{SI}}$ (the temperature at which a crossover to strong irreversibility occurs). While the occurrence of a peak in ${M}_{\mathrm{irr}}(T)$ has not been theoretically addressed yet, the observed variations of ${T}_{\mathrm{WI}}$ and ${T}_{\mathrm{SI}}$ with H as well as the observation that ${T}_{\mathrm{WI}}\ensuremath{\gg}{T}_{C}$ and ${T}_{\mathrm{SI}}\ensuremath{\simeq}{T}_{C}$ 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 ${T}_{\mathrm{WI}},$ ${T}_{P},$ and ${T}_{\mathrm{SI}}$ 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.