NMR Instabilities in Spin-Polarised Liquids: 3He, 3He-4He Mixtures and 129Xe

We report on pulsed NMR experiments performed in low magnetic field (1–2 mT) on various spin-polarised liquid samples obtained from optically pumped gas. Thin films of pure 3He and bulk samples of dilute 3He-4He mixtures are prepared with polarisations up to 40%. Xenon is liquefied (at 166 K) within a narrow U-shaped tube with polarisations of order 6%. In all these highly magnetised anisotropic samples, dipolar magnetic couplings have a strong influence on spin dynamics. Spectral clustering is obtained for small tipping 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° tipping 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, quantum exchange and spin diffusion, and to probe their influence on the observed phenomena.