We report measurements of the transverse-spin-diffusion coefficient D& and the spin-rotation parameter Q~~for two very dilute He-He mixtures (x3=1.82X10 and 6.26X10 ) spin polarized by an 8-T magnetic field. Brute-force spin polarization up to 40% was achieved at the lowest temperature, 6 mK.We find that Q~~increases monotonically as the temperature is reduced through the Fermi temperature TF, in disagreement with the only previous experiment but in good agreement with recent theory. Unlike the earlier experiment, which measured spin echoes, the present experiments employed a spin-wave technique that avoids nonlinear excitation of the spin field. We compare our results with the recent calculations of Jeon and Mullin for spin transport in dilute gases with arbitrary polarization and degeneracy. The best fit to the data is obtained by scaling the quasiparticle interaction V(q) proposed by Ebner 0 by a modest factor, 1.07. The corresponding s-wave scattering length is a = -1.21 A. Cxood agreement is found for Q~~(T) at both concentrations and all temperatures, and for D&/Q~~(T) apart from the lower concentration at T & 20 mK. The discrepancy in Dj /Q~j at the lowest temperatures and x3 could be explained by an unanticipated polarization dependence or by modification of the spin-wave boundary condition by processes occurring at the interface between the mixture and the silica cavity wall.

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