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 (FM R) and DC magnetization techniques. The experiments were performed eit her 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 ca se). 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 th e surface. This effect causes surface-induced modes to appear in addit ion to usual FMR bulk mode, when the measuring field is applied opposi te to the field in which the sample is cooled. Introducing a unidirect ional surface anisotropy energy into the (Rado and Weertman) general e xchange boundary conditions, we analysed the experimental data and obt ained magnetic parameters such as the surface-induced exchange anisotr opy, 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.