Anisotropy in the Zero-Field-Cooled States of ${\mathrm{Ni}}_{79}$${\mathrm{Mn}}_{21}$ and ${\mathrm{Au}}_{81}$${\mathrm{Fe}}_{19}$: Irreversibility Effects

The relationship $m$ vs $H$ for the alloys ${\mathrm{Ni}}_{79}$ ${\mathrm{Mn}}_{21}$ and ${\mathrm{Au}}_{81}$ ${\mathrm{Fe}}_{19}$ has been investigated at 1.5 and 4.2 K. Below a certain field $H_a$, it is linear, reversible, and governed by anisotropy fields even in the zero-field-cooled state. These fields are correlated with the spatial distribution of the spontaneous magnetization during cooling. A procedure allowing their suppression is presented. It leads to a radical change of the hysteresis loops without affecting significantly the degree of irreversibility.