  It is a common perception that the transport of a spin current in
polycrystalline metal is isotropic and independent of the polarization
direction, even though spin current is a tensorlike quantity and its
polarization direction is a key variable. We demonstrate surprising anisotropic
spin relaxation in mesoscopic polycrystalline Cu channels in nonlocal spin
valves. For directions in the substrate plane, the spin-relaxation length is
longer for spins parallel to the Cu channel than for spins perpendicular to it,
by as much as 9% at 10 K. Spin-orbit effects on the surfaces of Cu channels can
account for this anisotropic spin relaxation. The finding suggests novel
tunability of spin current, not only by its polarization direction but also by
electrostatic gating.
