Spin-Spin Relaxation in Phase-Separated 3He-4He Solid Mixtures

The spin-spin relaxation time T₂ in a ³He-⁴He 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 (10 mK) followed by temperature stabilization for many hours. The time T₂ 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 T₂ in the concentrated phase is found to be practically independent of temperature down to 50 mK and is determined by the same ³He-³He exchange interaction as in pure bulk solid ³He. 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 taping into account ³He-⁴He tunnel exchange. The values of T₂ in this phase coincide with those for a homogeneous (non-separated) mixture of the same concentration.