A SPIN-WAVE RESONANCE STUDY ON REENTRANT NIMN FILMS

The spin-wave resonance (SWR) spectra, at X-band, has been studied in the temperature range 4.2-300 K for the reentrant Ni76Mn24 amorphous thin films coated with Ni and Ag layers. At room temperature, the spectra taken as a function of the dc field orientation have been extensively analyzed. The SWR spectra at all temperatures consist of both the volume and the surface modes. The spectra also exhibit hysteresis effects below the canting temperature, T(f) = 120 K. In addition, the macroscopic unidirectional anisotropy is induced along the cooling field direction when the sample is cooled down to 4.2 K through T(f) in a sufficiently high field. However, this anisotropy vanishes after a few cycles of the applied measurement fields, whereas this anisotropy is quite stable in the polycrystalline specimens. It was concluded that introducing the uniaxial volume anisotropy energy density up to the forth order only, E(ani) = - K1 cos2theta - K2 cos4theta, into the Landau-Lifshitz dynamic equation of motion for magnetization, where theta is the angle between the magnetization and film normal, would account for the observed angular behaviour of the spectra. Using the Rado-Weertman general exchange boundary condition with the surface energy density, E(s) = k(s) cos2theta, we have derived an expression for spin wave absorption curves with Bloch type damping. The surface and bulk anisotropy constants, k(s), K1, K2, the exchange stiffness constant, A, and damping parameter, 1/T2, have all been deduced by comparing the simulated theoretical absorption curves to the experimental ones. It was also found that the bulk mainly has an easy axis perpendicular anisotropy and a relatively weak easy plane shaped anisotropy, while the surface anisotropy essentially has an easy plane nature.