Magnetic anisotropies of FeO and NiMn films

Abstract

Epitaxial Fe3O4 films on MgO(100) substrate and polycrystalline reentrant Ni77Mn23 films of various thickness (50, 100, 300, 600, 1000, 3700 Angstrom) on quartz substrate have been studied by Spin Wave Resonance (SWR) as a function of temperature. The SWR spectra have unusual uni-directional character upon cooling the sample in an external field H,. All films show spin-glass properties in the temperature range below freezing temperature T-f (about 120 K for NiMn and 70K for Fe3O4). The number of peaks increases with film thickness, indicating long spin wave excitation across the film thickness. The angular dependent SWR spectra were successfully analyzed by usual classical SWR theory. Using a field induced unidirectional exchange surface and bulk anisotropy together with usual magneto-crystalline anisotropy energy, we have simulated the experimental spectra and deduced magnetic parameters. The effective exchange stiffness parameter (D) remains practically the same for all films above T-f, while its value surprisingly increases below T-f, showing a strong correlation with spin-glass properties. The bulk anisotropy was found to strongly enhance with decreasing temperature and film thickness, while linearly temperature dependent surface anisotropy shows very weak thickness dependence. The easy-plane surface anisotropy on both substrate and free surfaces has been attributed to an intrinsic property for the films rather than accidental surface condition.