Ferromagnetic resonance studies of particulate media

Ferromagnetic resonance (FMR) was used to characterize particles dispersed in a nonmagnetic matrix. A sequence of media was prepared in which high-aspect-ratio CrO₂ particles were dispersed in a binder matrix with volumetric packing fractions (p) in the range 0.05 to 0.50. The resonant cavity FMR spectrometer operated at 33 GHz with the DC fields at resonance sufficient to magnetically saturate the specimens. No hysteretical effects were observed in the FMR spectra. The FMR spectra of the longitudinally oriented samples were characterized by a single response with an effective anisotropy field favoring in-plane orientation. The FMR spectra for the perpendicularly oriented samples were more complicated, with two observed resonances corresponding to effective anisotropy fields favoring perpendicular and in-plane orientations. As p was increased the response associated with the effective in-plane anisotropy field increased in strength. At p=0.44 the strengths of the two responses were comparable. In unoriented media the response associated with the effective anisotropy field favoring perpendicular orientation was also observed, but at very much smaller strengths. Thus the FMR technique simultaneously provides information regarding in-plane and perpendicular orientations of the media studied. The FMR response has been show to be sensitive to particle misorientation