Characterization of Exchange Coupling in Nanocomposite Magnets by Ferromagnetic Resonance.

Thin-film magnets, composed of nanophase dispersed Nd2Fe14B and α-Fe, were synthesized by rf magnetron sputtering onto glass substrates in an Ar atmosphere. Magnetization curves for films with different α-Fe volume fractions (VFe) were single-phase-like, suggesting a firm exchange coupling between hard and soft phases. With increasing VFe, the saturation magnetization increased linearly, while the coercivity decreased. Ferromagnetic resonance measurements were made by using a high-frequency, high-field ESR apparatus, which exhibited two absorption peaks when the field was swept with a fixed frequency. A model calculation of the resonance field was made for a simplified hard/soft two-sublattice system to analyze these results. Estimated values of the exchange-coupling constant are in good agreement with those estimated from micromagnetic analysis of the magnetization curves.