Spin Transport in Dilute, Spin-Polarized Solutions of Helium-Three in Helium-Four

We have investigated ^3He spin diffusion in two dilute solutions of ^3 He in ^4He, with atomic fraction x^3=1.82times 10^{-3} and 6.26 times 10 ^{-4}, spin-polarized by an 8 T field. We do not find evidence that the diffusion coefficient for spins transverse to the average magnetization (D _|) declines or saturates at temperatures down to 0.20T_{rm F}, contrary to previous experiment (Gully and Mullin 1984) but in accord with current theory. We have compared our measurements with the latest theory of Jeon and Mullin (1991); our data is mostly in good agreement with their theory if a slightly modified version of a ^3 He-^3He interaction due to Ebner (1967) is used. The congruence between data and theory supports the conclusion that the s-wave approximation to the interaction is not useful for transport calculations even for these rather dilute solutions. There may be an one unresolved discrepancy: our diffusion constant for the lowest concentration at the lowest temperatures is 25% higher than theory predicts. This could be due to a polarization dependance for D_| or to a modification of the boundary condition by a bound ^3He state, or possibly due to errors in the theoretical calculation.