Magnetic anisotropy in reentrant spin-glass NiMn multilayer film

The magnetization curve in the multilayer sample containing the reentrant ferromagnet NiMn layer was characterized, based on the hysteresis half-width Hr and the center of hysteresis loop H-d in the thickness range of 30-13,000Angstrom, in order to discuss (1) the difference in the magnetization process between the thin film and the bulk sample, (2) the magnetization process around the critical thickness of D-c for disappearance of ferromagnetic order, and (3) the nature of magnetic anisotropy in the thin thickness region. The value of H-r in the film sample is large compared with that in the bulk sample. This implies that the mobility of the domain wall motion is lowered in the film sample. This dependence is indicative of the significant role of domain wall motion in the magnetization process in the thin film of reentrant ferromagnet NiMn, which has not been taken into account in the established domain-anisotropy model for the bulk sample. The value of Hr increases as the thickness decreases, and rapidly decreases below similar to65Angstrom. In contrast, the value of Hd shows a monotonic increase that is weakly dependent on the thickness down to 30Angstrom. The behavior of H-r and H-d is consistent with findings in a recent report on the size-dependent magnetic phase transition in the reentrant ferromagnet NiMn, i.e., the ferromagnetic phase and the ferromagnetic order in reentrant spin-glass phase simultaneously disappear at D-c, although the spin-glass order is stable above 30Angstrom. The thickness dependence of H-r is expressed by the sum of two terms: a component inversely proportional to the film thickness and a constant. This implies the existence of surface anisotropy having uniaxial component, the origins of which can be attributed to the enhanced spin-orbital interaction at the film surface. (C) 2002 Elsevier Science B.V. All rights reserved.