FIELD-INDUCED DIRECTIONAL SURFACE ANISOTROPY IN AN NIMN ALLOY

Magnetic properties of the reentrant Ni76Mn24 alloy have been studied as a function of temperature by using both ferromagnetic resonance (FMR) and DC magnetization techniques. The experiments were performed either by applying the DC field after cooling the sample to 4 K (ZFC case) or by cooling the sample in the presence of an external field (FC case). In the FC case, the field-induced unidirectional anisotropy field acting on the surface spins is much stronger than that acting on the inner spins, probably due to the lower symmetry of the structure at the surface. This effect causes surface-induced modes to appear in addition to usual FMR bulk mode, when the measuring field is applied opposite to the field in which the sample is cooled. Introducing a unidirectional surface anisotropy energy into the (Rado and Weertman) general exchange boundary conditions, we analysed the experimental data and obtained magnetic parameters such as the surface-induced exchange anisotropy, bulk anisotropy and exchange stiffness constant as a function of temperature. A strong correlation has been observed between the field-induced exchange anisotropy deduced from FMR and the hysteresis effect observed in DC magnetization data.