Spin currents with unusual spin orientations in noncollinear Weyl antiferromagnetic Mn3Sn

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There are intensive efforts to search for mechanisms that lead to spin-orbit torque with unusual spin orientation, particularly out-of-plane spin orientation which can efficiently switch perpendicular magnetizations. Such a phenomenon has been observed in materials with low structural symmetry, ferromagnetic materials, and antiferromagnets with noncollinear spin structures. Here, we demonstrate the observation of, in addition to out-of-plane spin orientation, spin orientation along the charge current direction in ${\mathrm{Mn}}_{3}\mathrm{Sn}$, a noncollinear antiferromagnet and Weyl semimetal. The mechanism arises from noncollinear spin structure with spin-orbit coupling and it can be viewed as spin rotation around the octupole moment, the lowest order of cluster multipole moment pertaining to the ${\mathrm{Mn}}_{3}\mathrm{Sn}$ crystal group.