Ferro-spin-glass domain model for disordered Ni-Mn

A ferro-spin-glass (FSG) domain model, recently developed for a Ni-Mn reentrant spin-glass (SG) alloy, is applied here to several Ni-Mn compositions on either side of the multicritical point (MCP). From displaced hysteresis loops measured after cooling in fields strong enough to saturate the thermoremanence, the values deduced for both the average domain magnetization ($M_S$) and the unidirectional domain anisotropy field ($H_K$) decrease steadily with increasing Mn concentration, showing no anomaly at the composition of the MCP. However, as deduced from symmetrical loops measured after zero-field cooling (assuming the same $M_S$ and $H_K$), the average exchange field for the net antiferromagnetic coupling between adjacent domains ($H_E$) remains small (<600 Oe) in the reentrant SG regime but rises very rapidly as the Mn concentration increases past the MCP composition. Nevertheless, in this critical composition region, $H_E$ is consistently much weaker than the average exchange field within the domains, implying that the FSG domain boundaries coincide with surfaces over which the spin frustration is especially severe. Essentially the same characteristic variation of $H_E$ with alloy composition is deduced separately from the observed changes in the displaced hysteresis loops for different fields applied during cooling. Recent electron-spin-resonance data for Ni-Mn cooled in zero and large fields are seen to be consistent with a FSG-domain-model interpretation of these different thermomagnetic conditions.