Spin-spin relaxation in phase-separated 3He-4He solid mixtures

The spin-spin relaxation time T-2 in a He-3 -He-4 solid mixture with an initial concentration of 3.18 % 3He is measured during its phase separation in a temperature range of 1-250 mK. Cooling down to the region of separation was carried out by small steps (similar to 10 mK) followed by temperature stabilization for many hours. The time T-2 was measured by using the pulsed NMR technique at a frequency of 250 kHz. The spin echo method makes it possible to distinguish the contributions to magnetic relaxation from both the concentrated and the dilute separated phases. The relaxation time T-2 in the concentrated phase is found to the practically independent of temperature down to similar to 50 mK and is determined by the same He-3 - He-3 exchange interaction as in pure bulk solid He-3. It is found that the behavior of the spin echo signal at ultralow temperatures exhibits an anomaly, which may be connected with quasi-one-dimensional spin diffusion. In the dilute daughter phase the spin-spin relaxation time is inversely proportional to concentration and is described correctly by the Torrey model taking into account He-3 - He-4 tunnel exchange. The values of T-2 in this phase coincide with those for a homogenous (non-separated) mixture of same concentration.