Ferromagnetic resonance in polycrystalline ferrites with large anisotropy—I: General theory and application to cubic materials with a negative anisotropy constant

Abstract

If the anistropy field is much larger than the single-crystal line width and the saturation magnetization, the shape of the resonance line is essentially determined by the spreading of resonance frequencies for different grains due to crystalline anisotropy. More precisely, if w( H) dH is proportional to the number of grains that have their resonance in the range of applied d.c. field between H and ( H) dH, the absorption versus field curve should be a smeared-out image of the distribution function w( H). The distribution function has characteristic singularities arising from the stationary points of the resonance field versus orientation surface. The behavior of w( H) in the vicinity of the singularities is calculated for first-order cubic anisotropy, and w( H) is obtained by interpolation. For small anisotropy fields, the calculated line shape has a single peak which corresponds to grains in which a [110] direction is aligned with the d.c. field. For larger anisotropy fields (γH a/lΩ> 0.5) , a secondary peak occurs. It corresponds to grains in which an easy axis is aligned with the d.c. field. The theory accounts for secondary resonance peaks at low fields observed in ferrimagnetics near the compensation point.