Resonance relaxation in Mn xFe yO 4 by slow relaxing manganic and ferrous ions

Abstract

Ferrimagnetic resonance linewidth measurements are reported between 1.5–400°K at 9.3 Gc s and 16.8 Gc s for single crystals of composition Mn x Fe y O 4 with x = 1.321, 1.003, 0.989 and 0.954. All the samples show a maximum in the linewidth at ~ 20°K, whilst the sample x = 0.954 shows an additional maximum at 260°K. The low temperature maxima are described by the ‘slow relaxing ion’ mechanism of relaxation with δ ~ 7.5 cm −1 where δ is the splitting between the ground and first excited states of the relaxing ion. Comparison of the data with the compositions indicates that this ion is Mn 3+. Moreover, the value of δ and the anisotropy in δ are in good agreement with the values deduced for the Mn 3+ ion by krupička et al. from Palmer's magnetocrystalline anisotropy data of Mn x Fe y O 4 with x > 1. Values of the relaxation time, τ, of the manganic ion are also deduced. The high temperature linewidth maximum is associated with Fe 2+. The measurements do not agree with the formulation by Heeger, Blocker III and Chosh of the ‘fast relaxing ion’ mechanism of relaxation. However, the measurements can be fitted by the ‘fast relaxing ion’ mechanism as formulated by de Gennes, Kittel and Portis, and also by the ‘slow relaxing ion’ mechanism. The ‘fast’ mechanism requires δ ⪆ 100cm −1 and τ ⪅5 × 10 −14 sec; the ‘slow’ mechanism requires δ ~ 270 cm −1 and τ $ ̌ 4 × 10 -12 sec. These values of τ suggest that the ‘slow relaxing ion’ mechanism is the more plausible explanation, but a more definite conclusion is not possible on the basis of the measurements and present knowledge of the energy levels of Fe 2+ ions in the spinel lattice. The above interpretations are in some respects at variance with an earlier interpretation of line-widths in manganese ferrite by H eeger et al.