MAGNETIC ANISOTROPIES OF SPUTTERED FE FILMS ON MGO SUBSTRATES

Ferromagnetic resonance (FMR) and superconducting quantum interference device (SQUID) measurements have been used to study the magnetic prop erties of rf sputtered Fe films on MgO(001) substrates. The dependence s of the FMR spectra parameters on the direction of the de magnetic fi eld turning in the plane of the films were measured in a wide temperat ure range (20-400 K) for films with thickness L in the range 25-500 An gstrom. The analysis of the angular dependence of the resonance field H-0 allowed us to determine the fourfold cubic anisotropy constant K-1 and the effective magnetization value M(eff). It was found that both values decrease with decreasing L and approach a constant value below a certain thickness. A theory of FMR is outlined demonstrating that fo r the case of the de magnetic field lying in a film plane, the anisotr opy constant can be interpreted as a combination of a volume anisotrop y contribution and a 1/L-dependent contribution from the surface aniso tropy up to the thickness L greater than or equal to 10(3) Angstrom. T his means that for the experimentally studied thickness range the film s may be considered as ''dynamically thin films'' with respect to surf ace perturbations. Then the peculiar thickness dependence of the K-1 v alue can be explained assuming that the relaxation of the strain due t o the mismatch between him and substrate extends to distances as far a s 45 Angstrom from the film-substrate interface. Since our SQUID measu rements show that the saturation moment does not depend on the thickne ss, it is concluded that the thickness dependence of the effective mag netization M(eff) is caused by a second-order uniaxial anisotropy aris ing mainly from the broken symmetry of the crystal field at surfaces a nd near the edges of interfacial dislocations.