Migration of Ni2+ Ions and Ferrimagnetic Resonance in NiFe2O4 Crystals

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Abstract
In the X‐band ferrimagnetic resonance has been observed on NiFe2O4 crystals of varying degrees of cation disorder in the lattice between 4.2 to 500 °K. This disorder was created by heating the crystals to temperatures in the range 650 to 1140 °C and then subsequently quenching them in water. This heat treatment results in an increase of the A‐site Ni2+ ions concentration. The resonance properties are shown to be very sensitive to this cation disorder. The per‐ion contribution of the A‐site Ni2+ ions to the anisotropy K1 is measured to be + 1.4 × 10−15 and + 6.6 × 10−15 erg cm−3 at 293 and 77 °K. This contribution is seen to be of the same order of magnitude as that from Co2+ ions in Fe3O4. These ions also contribute to K2. The linewidth contribution at 293 °K is of the order of 200 Oe per % of A‐site Ni2+ ions. ΔH and the g‐factor of the quenched samples show an increase at lower temperatures, a behaviour which is totally absent for the slowly cooled ones. The change in the anisotropy (ΔK1/M) brought about by quenching has been used to study the Ni2+ ion migration to the A‐sites. This process is found to consist of two different ones, one with an activation energy of 0.6 eV in the range 800 to 1000 °C and the other with 0.24 eV in the range 1000 to 1100 °C. Finally quenching from 1140 °C is shown to introduce defects in the crystal.