NMR instabilities in spin-polarised liquids: He-3, He-3-He-4 mixtures and Xe-129

We report on pulsed NMR experiments performed in low magnetic field (1-2 mT ) on various spin-polarised liquid samples obtained from optically pumped g as. Thin films of pure He-3 and bulk samples of dilute He-3-He-4 mixtures a re prepared with polarisations up to 40%. Xenon is liquefied (at 166 K) wit hin a narrow U-shaped tube with polarisations of older 6%. In all these hig hly magnetised anisotropic samples, dipolar magnetic couplings have a stron g influence on spin dynamics. Spect? al clustering is obtained for small ti pping angles and usually results in precession lifetimes of several seconds . Much shorter precession lifetimes (down to 10 ms) are obtained for larger tipping angles in highly polarised samples. In all systems, 90 degrees tip ping pulses are followed by precession signals which abruptly decay after a variable delay: as long as 1 s for moderate polarisations in xenon. Use of different liquids permits us to vary the effects of dipolar couplings, qua ntum exchange and spin diffusion, and to probe their influence on the obser ved phenomena.