The influence of Fe2+ on the ferrimagnetic resonance properties of nickel ferrites was discussed. Since the ground state of Fe2+ in spinels is a singlet, the crystal field only slightly affects the motion of the spin magnetic moment of Fe2+. It was pointed out that due to the anisotropy of the exchange interaction between Fe2+ and Fe3+ or Ni2+ situated at the octahedral position, the longitudinal branch might play the major role in the relaxation process of nickel ferrites.The method used in Ref. [1] was extended to treat the strong exchange coupled system. It was confirmed that usually the expression for the line width coincides with that of the case of a weakly coupled system.Under the light of a reasonable model of anisotropic exchange interaction the experimental fact that the line width goes through a maximum at a definite temperature along all crystal directions except [001], was satisfactorily explained. The theoretical expression obtained for the anisotropy of the line width agrees with the experimental results.
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