Precise Magnetic Torque Measurements on Single Crystals of Iron

A rugged balance of sufficiently high sensitivity and a rotating magnet capable of producing a field of over 3000 oersteds are used to make magnetic torque curves. The equations for the torque curve of a single crystal disk of any orientation whatsoever are derived on the assumption that the magnetic anisotropy is adequately described by a single constant. Furthermore, a method is developed for predicting the orientation from a knowledge of the angles of zero torque alone, and both the experimental and theoretical limitations of this method are discussed. It is shown experimentally that the use of cylindrical disks introduces errors that may be avoided by using ellipsoids. An experimental torque curve for a single crystal ellipsoid of 2.8 percent Si iron gave excellent agreement with the curve calculated on the basis of just the first anisotropy constant, $a_F=1.70\mathrm{}\times {10}^5$ ergs/cc. Corrections for magnetization not being parallel to applied field are briefly discussed, and it is shown that in practical cases, the magnetization cannot deviate appreciably from the plane of the specimen. The possibility is mentioned of computing a fictitious value of the second anisotropy constant because of error in interpreting experimental data. It is proposed that the anisotropy constants be obtained directly from accurate torque measurements, in order to furnish data for the computation of magnetization curves to be checked against experiment.