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 et al. [Adv. Magn. Reson. 14, 95 (1990)] to the more complex spectroscopic situation prevailing in gypsum in ''2-lines'' orientation. In order to test the calculations, we measured the additional rate of relaxation due to low-amplitude, low-frequency (1 KHz) rotational vibration of the whole crystal around various directions. In agreement with calculated predictions, we observed that the additional relaxation only affects the intermolecular spin-spin order at a rate which is temperature independent and proportional to the (average) square of the angular velocity of bulk rotation. The measured additional rate of relaxation also depends strongly on the orientation of the axis of rotation, in quantitative agreement with the calculated values.