The spin wave manifold (spin wave eigenfrequencies) for grain-surface-layers is obtained from the precession equation in consideration of the existence of impurity-induced and stran-induced uniaxial anisotropy field Hs. The top and bottom of the manifold for grain-surface-layers at various applied fields H are illustrated in comparison with the manifold for the grain-interior-region. With the detalled calculation, it is revealed that the off-resonance effective linewidth ΔHeff arises from the spin wave resonance excited in some grain-surface-layers and is propotional to the volume fraction of grain-surface-layers. Thus a lot of previous experiments on ΔHeff vs grain size and porosity can be understood well, and the physical meaning of ΔHeff is clarified.
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