Torque Measurements on Rare-Earth Doped Yttrium Iron Garnet

Abstract

This paper describes torque measurements made at 4.2 and 1.5°K on yttrium iron garnet crystals containing small rare-earth ion dopings kindly supplied by Dr. J. F. Dillon, Jr., and Dr. J. W. Nielson of the Bell Telephone Laboratories. Resonance measurements by Dillon (1959) showed that the giant anisotropy peaks observed in yttrium iron garnet at 4.2°K were due to the presence of rare-earth ion impurities and since then his results on samples containing known impurities have been interpreted by Kittel(1960) in terms of energy-level “crossovers” of the rare-earth ionin the combined crystal field and exchange field. In the present work torque measurements have been made on several crystals with different rare-earth ion impurities, but results here are confined to crystals doped with terbium (Tb 0.19%), holmium (Ho 0.19%), and dysprosium (Dy 1.0%). The crystal containing dysprosium was grown by J. L. Page in our laboratories. The torquecurves were measured in a (110) plane [also in a (100) plane for the Tb sample] in a magnetic field of 15 000 oe and the maximum torques were in the range 20–200 dyne cm. For the three crystals,the energy surface deduced from the torque curves is consistent with the resonance field behavior observed by Dillon (1959). The torque curve of the Tb crystal had more pronounced discontinuities than any other samples, and variation in the applied field between 5000 and 15 000 oe altered the magnitude but did not alter the angle at which a particular discontinuity occurred. So far,attempts to correlate the position of the discontinuities in the (110)and (100) planes which showed fewer discontinuities have proved unsuccessful with the above model. At 20°K the torque curve can be analyzed in the standard way giving values of K1 and K2 of+88 000 and −460 000 erg cm−3 respectively. The torque curve for the Ho sample at 4.2°K was steepest at the [110] and [100]directions, in agreement with the position of the resonance peaks reported by Dillon (1959). However, for the Dy crystal, no sharp discontinuities are observed and the torque curve shows a large sin8θ component. On reducing the temperature from 4.2° to 1.5°K,no change in the curve was observed. The curves for Ho and Dy were found to be quite reversible in fields of 15 000 oe.