Spintronic phenomena to date have been established in ferromagnets (FMs) with collinear moments (e.g., Fe, Co, Y3Fe5O12), where all the moments can be aligned to give a large magnetization (M). In such collinear FMs, spin injection via spin Seebeck effect (SSE) is always along the out-of-plane direction (longitudinal SSE), but not in the in-plane direction (transverse SSE). In non-collinear antiferromagnets (AFs), there are magnetic moments in directions other than that along the net magnetization M, allowing general pure spin current phenomena to be established. In this work, we report the observation of vector SSE in non-collinear AF LuFeO3, where temperature gradients along both out-of-plane and in-plane direction can inject pure spin current and generate a voltage in the heavy metal via the inverse spin Hall effect. The results of longitudinal SSE (under an out-of-plane ▽zT) and the even larger transverse SSE (under an in-plane ▽yT) in LuFeO3/W are shown in the Figure. By using different metal overlayers (W, Pt and Cu/W), we show that the vector SSE is a spin current effect, where the thermovoltages are due to the magnetization from the canted moments in LuFeO3. We also ruled out the possibility of magnon Hall effect, spin Nernst effect and magnetic proximity effect in LuFeO3. One can exploit vector SSE as a vector magnetometer for detecting very small magnetization in all directions in non-collinear AF insulators. These novel results expand the realm of spintronics and enable new device architectures for AF spintronics and reveal general pure spin current phenomena beyond those in collinear magnets.

This work was supported by US NSF DMREF (1729555 and 1949701), Taiwan MOST (109-2123-M-002-002), and US DOE(DE-SC0009390).