Theory of two magnon scattering microwave relaxation and ferromagnetic resonance linewidth in magnetic thin films

A detailed analysis of the two magnon scattering contribution to the microwave relaxation and ferromagnetic resonance linewidth in isotropic and anisotropic films and disks has been made. The analysis is based on the Sparks, Loudon, and Kittel (SLK) theory for the scattering of uniform mode magnons into degenerate spin wave states for isotropic spherical samples in the presence of magnetic inhomogeneities in the form of spherical voids or pores. The SLK theory has been extended to include: (i) thin film and thick film samples magnetized in an oblique out-of-plane direction; (ii) uniaxially anisotropic materials with either easy-axis or easy-plane anisotropy and an anisotropy axis perpendicular to the disk plane; (iii) a modified density of degenerate states to account for the nonzero relaxation rate of the scattered spin waves; and (iv) two limiting cases of the scattering interaction: (a) the original SLK case where the inhomogeneities are modeled as spherical voids and the coupling to the degenerate spin waves varies with the spin wave propagation direction and (b) an isotropic scattering model where the coupling is independent of the propagation direction. The formulation is valid for thick films for which the discrete nature of the spin wave modes may be neglected. The two magnon linewidth as a function of field orientation is calculated fur three classes of material parameters corresponding to yttrium iron garnet and barium M-type and zinc Y-type hexagonal ferrites. The linewidth versus static field angle profiles show characteristic profiles which depend on the crystalline anisotropy, the sample dimensions, the nature of the scattering interaction, the inhomogeneity size, and the inhomogeneity volume fraction. These parameters, as well as the shape and evolution of the spin wave band as a function of the field angle under ferromagnetic resonance conditions, play critical roles in determining the linewidth versus angle profiles. (C) 1998 American Institute of Physics.