Low-field induced torque in potassium

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Abstract
The low-field (0.005-2T) induced torque in potassium has been examined using a new modulation method of induced torque. An oscillatory field is applied perpendicular to the main field and the direction of torque measurement. An expression for the detected oscillatory torque is presented. The amplitude of the oscillatory torque for a free-electron metal increases linearly with magnetic field at low fields, reaches a maximum at omega c tau =2 and decreases with increasing field at high fields. The predictions of the properties of induced torque presented by Bishop and Overhauser (1977, 1978) (for the case when the residual resistivity of potassium at zero field is higher in one direction) and by Lass (1972, 1976) (for a sample departing from spherical shape) are compared. It is concluded that the models are experimentally distinguishable in the low-field region omega c tau <1. A twofold torque anisotropy with peaks separated by approximately 180 degrees was observed at low fields and a fourfold pattern with peaks separated by approximately 90 degrees was dominant at higher fields. The ratio of the torques in the directions of the maximum and minimum of the twofold pattern was measured as a function of this field. This ratio was found to increase with magnetic field as predicted by the Lass theory but contrary to that predicted by Bishop and Overhauser. The zero-field limit of the ratio was in the range 1.05-1.25. The presence of the fourfold pattern at high fields depends on the precise direction of the modulation field. This is explained in terms of the theory of Lass. A reason is given why Hunt and Werner (1974) did not observe the fourfold pattern using an inductive technique. Although the Lass model explains properties of the induced torque, the samples were sufficiently spherical to rule out the direct applicability of the model.

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