Characterization of Magnetic Bubble Domain Garnet Crystals by X-Ray Diffraction: Symmetry and Compositional Gradients

Abstract

At room temperature, selected pieces of Gd2.34Tb0.66Fe5O12 are magnetically uniaxial and have excellent characteristics for bubble domain devices; but x‐ray powder, precession, rotation, and Weissenberg photographs show no corresponding uniaxiality. Bond's method of accurate lattice spacing measurement shows d‐spacing gradients which can be as large as 0.065% over a distance of 1.3 mm. These gradients are large along the normal to the natural (211) faces. The lattice parameters of the two end members of this solid‐solution crystal were measured. Application of Vegard's law allows quantitative estimation of the compositional gradients due to varying rare‐earth atom ratios. X‐ray fluorescence analysis mapping showed no variation in rare‐earth atom ratios. A small piece was selected as having less than 2 ppm gradient along [211]. This was used to determine the shape of the unit cell by measuring the d spacings of eight reflections (widely separated in reciprocal space). A least‐squares refinement assuming rhombohedral symmetry and a [111] direction along the unique magnetic axis gave a rhombohedral angle of 90°−0.9 arc sec, σ=0.04 arc sec. This very small distortion could be due to compositional gradients remaining despite precautions and may not be inherent in a homogeneous crystal. The previous report of a 1/3‐min distortion was the result of an unrecognized gradient.