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We observe that the diffusive spin current in a strongly interactingdegenerate Fermi gas of $^{40}$K precesses about the local magnetization. Aspredicted by Leggett and Rice, precession is observed both in the Ramsey phaseof a spin-echo sequence, and in the nonlinearity of the magnetization decay. Atunitarity, we measure a Leggett-Rice parameter $\gamma = 1.08(9)$ and a baretransverse spin diffusivity $D_0^\perp = 2.3(4)\,\hbar/m$ for a normal-stategas initialized with full polarization and at one fifth of the Fermitemperature, where $m$ is the atomic mass. One might expect $\gamma = 0$ atunitarity, where two-body scattering is purely dissipative. We observe $\gamma\rightarrow 0$ as temperature is increased towards the Fermi temperature,consistent with calculations that show the degenerate Fermi sea restores anon-zero $\gamma$. Tuning the scattering length $a$, we find that a sign changein $\gamma$ occurs in the range $0 < (k_F a)^{-1} \lesssim 1.3$, where $k_F$ isthe Fermi momentum. We discuss how $\gamma$ reveals the effective interactionstrength of the gas, such that the sign change in $\gamma$ indicates aswitching of branch, between a repulsive and an attractive Fermi gas.

Observation of the Leggett-Rice effect in a unitary Fermi gas