Relaxation Processes for Ferromagnetic Resonance in Thin Films

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Abstract
Ferromagnetic‐resonance linewidth measurements have been made for two hundred Ni–Fe alloy films (77% Ni) 150–3200 Å thick at frequencies from 1–9 Gc/sec with the static field in the film plane. To avoid dispersion effects samples with the smallest linewidth (ΔHmin) were selected for each thickness. For film thickness less than a frequency‐dependent critical thickness Dω, ΔHmin is independent of film thickness. For thicker films where D > Dω, ΔHmin increases linearly with film thickness. The observed Dω values (about 1000 Å) are in good agreement with predictions based on magnon scattering involving spin waves degenerate with the uniform mode. Because of the magnetostatic mode modification of the spin‐wave dispersion relation for thin films there are no spin‐wave states degenerate with the uniform mode for D < Dω and magnon scattering is not allowed. The applicability of two different magnon processes, s‐d exchange and two‐magnon scattering, is discussed. Neither mechanism provides a completely satisfactory explanation of the data. The present data indicate: (1) For D > Dω, the linewidth is a linear function of film thickness and a magnon process is important. (2) For D < Dω, a magnon process is not important. (3) The anisotropy dispersion makes a contribution to ΔH, which is significant and may mask the above effects if the dispersion is large.