SPIN RELAXATION BY SLOW BULK MOTION IN GYPSUM

Using the techniques of spin thermodynamics in solids, we extended the theory of spin relaxation by slow bulk motion proposed by J. Jeener e t al. [Adv. Magn. Reson. 14, 95 (1990)] to the more complex spectrosco pic situation prevailing in gypsum in ''2-lines'' orientation. In orde r to test the calculations, we measured the additional rate of relaxat ion due to low-amplitude, low-frequency (1 KHz) rotational vibration o f the whole crystal around various directions. In agreement with calcu lated predictions, we observed that the additional relaxation only aff ects the intermolecular spin-spin order at a rate which is temperature independent and proportional to the (average) square of the angular v elocity of bulk rotation. The measured additional rate of relaxation a lso depends strongly on the orientation of the axis of rotation, in qu antitative agreement with the calculated values.