Nuclear Relaxation in SolidHe3at Low Temperatures

Abstract

We have made measurements of the nuclear relaxation time $\tau$ in the bcc phase of solid ${\mathrm{He}}^3$ for temperatures between 0.3 and 0.04°K. The relaxation times were found to decrease when the size of the sample chamber was reduced. In the range $0.2\gtrsim T\gtrsim 0.1°$K, $\tau$ is temperature-independent and inversely proportional to the exchange parameter $J$ for our two smaller sample chambers. The relaxation is attributed to the single-phonon process occurring in the magnetically irregular region of the sample boundary; Zeeman and exchange energy in the bulk ${\mathrm{He}}^3$ then diffuse to the boundary in the time $\tau$. The diffusion, due to the exchange interaction, is temperature-independent. Below 0.1°K, $\tau$ increases mildly with temperature in the smallest chamber. The temperature dependence of $\tau$ below 0.1°K indicates a possible phonon bottleneck. An order-of-magnitude calculation shows that at about 0.2°K the rate at which the spin system puts energy into the lattice becomes comparable to the rate at which the lattice can transfer that energy to the bath. The results can be used to estimate the nuclear relaxation time in the millidegree region where the magnetic transition is expected.