Magnetically Induced Ordering in Nickel and Magnesium Ferrous Ferrites

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Abstract
Microwave measurements have been made at 9.5 GHz in the temperature range of 4.2°–30°K on a series of spinel single crystals with nominal compositions (M1−xFex2+Fe23+)O4; M is nickel or magnesium and x assumes values between 0.16 and 0.61. All samples developed a noncubic contribution to the anisotropy when cooled from above 30° to 4.2°K in the presence of a saturating magnetic field, the annealing field. The dependence of the fields for resonance at 4.2°K upon the orientation of the annealing field in the (110) plane is described by an induced uniaxial anisotropy energy of the form Ea=K′Σi cos2τi cos2τ′i, where τi and τi′ are the angles between the magnetization and the three cube-edge directions at the measuring and annealing temperature, respectively. The temperature dependence of the fields for resonance as the samples are warmed from 4.2°K indicates that there is an additional temperature dependent term in the anisotropy. For the compositions investigated the temperature Ta, above which all anneal effects vanish increases with increasing ferrous iron concentration from 12° to 24°K. Also, the magnitude of the induced anisotropy increases in a nonlinear manner with increasing ferrous ion concentration. The induced anisotropy is attributed to a magnetic ordering of distortions associated with the ferrous ions. The nonlinear increase of the induced anisotropy and the increase of Ta with increasing ferrous ion concentration are interpreted in terms of an interaction between these active centers.

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