Internal Magnetoelastic Coupling Constant of Nickel Ferrite

Abstract

The coupling of the spin system to ionic displacements in a ferromagnetic crystal can cause displacements of the ions which do not result in external strains. A search was made for such magnetostrictive displacements of the oxygen ions in nickel ferrite by looking for a change in the intensity of the 442 x-ray diffraction line. The sample was a disk-shaped single crystal of nickel ferrite cut so that the face of the sample was a (221) plane. The intensity of the line when the magnetization was perpendicular to the disk was compared with the intensity when the magnetization was essentially in the plane of the disk near a [111] (easy) direction; these intensities were found to be equal to within 0.5%. The equilibrium ionic displacements were expressed in terms of the internal magnetoelastic coupling constants, assuming plausible modes of displacement of the oxygen ions. Changes in the structure factor of the 442 line resulting from these displacements were calculated. From these results and our upper limit for the fractional difference in the x-ray intensity, we find that Bintγ≤1.6×108 and Bintε≤0.8×108, where Bintγ and Bintε are phenomenological magnetoelastic coupling constants appropriate to oxygen displacements belonging to the Γγ representation and Γε representation, respectively. Similar measurements on cobalt ferrite have been initiated, and changes in the intensity of the x-ray diffraction line have been observed as the magnetization direction was changed. Preliminary results yield values of Bint at least ten times as large as in nickel ferrite.