A theory of spin wave parametric instability is developed that simultaneously allows for cubic anisotropy and first- and second-order uniaxial anisotropies. The contributions of various anisotropies to a dispersion relation and first-and second-order nonlinearity coefficients are considered. A technique for studying the parametric instability of oscillations in magnetostatic resonators that relies on measuring their Q factors is proposed. Measuring data are used to determine the relaxation frequencies of parametrically excited spin waves in yttrium iron garnet films at frequencies close to 1 GHz. It is shown that a nonlinearity in the bias dependence of the spin wave frequencies should be allowed for in studying the relaxation parameters of ferrites.
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