NMR Instabilities in Highly Magnetised Liquid Helium Solutions

Spin dynamics have been shown to be dramatically altered in anisotropic samples when the internal dipolar field dominates the external static field gradients. In recent NMR experiments on laser polarised liquids (He or Xe), spectral clustering and precession instabilities have been observed. Early analyses have suggested the generic nature of these effects, and tentatively addressed their existence in bulk systems. We report here on NMR experiments conducted in spherical samples of laser-polarised liquid helium mixtures (0.5-10% 3 He in 4 He at 1-2K). We focus on the relative influence of magnetisation density and applied field gradients on FID signals induced by 90° tipping pulses. Numerical simulations are used to study the effects of distant dipolar couplings on local and global magnetisation. A discussion of the features of FID and spin echo signals is presented.