Spin-Wave Instability in Hexagonal Ferrites With a Preferential Plane

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Abstract
The large magnetic anisotropy field that is encountered in many planar ferrites markedly affects the rf magnetic field at which spin‐wave instability sets in. When the rf magnetic field is applied perpendicular to the dc field, spin‐wave instability can occur through the first‐or second‐order process. At ferromagnetic resonance, the first‐order process dominates if the frequency is smaller than a characteristic frequency ωc. This characteristic frequency has been calculated for ellipsoidal samples. For most shapes, the anisotropy field tends to increase ωc, thus favoring the occurrence of the first order process. In addition, the anisotropy tends to decrease the critical field at which spin‐wave instability sets in. Assuming the spin‐wave relaxation rates to be equal in the two situations compared, we find that for large anisotropy the second‐order threshold is reduced approximately in the ratio of (2/ζ)½ for relatively high frequencies; and in the ratio of (ω/ωMζ)½ for relatively low frequencies where ζ=Ha/4πM is the anisotropy field in units of the saturation magnetization. In parallel pumping, the threshold field is reduced in the ratio of 1/(1+ζ) at low biasing fields.

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