The interplay of spin-orbit coupling (SOC) and magnetism gives rise to a plethora of charge-to-spin conversion phenomena that harbor great potential for spintronics applications. In addition to the spin Hall effect, magnets may exhibit a magnetic spin Hall effect (MSHE), as was recently discovered [Kimata et al., Nature 565, 627-630 (2019)]. To date, the MSHE is still awaiting its intuitive explanation. Here we relate the MSHE to the vorticity of spin currents in the Fermi sea, which explains pictorially the origin of the MSHE. For all magnetic Laue groups that allow for nonzero spin current vorticities the related tensor elements of the MSH conductivity are given. Minimal requirements for the occurrence of a MSHE are compatibility with either a magnetization or a magnetic toroidal quadrupole. This finding implies in particular that the MSHE is expected in all ferromagnets with sufficiently large SOC. To substantiate our symmetry analysis, we present various models, in particular a two-dimensional magnetized Rashba electron gas, that corroborate an interpretation by means of spin current vortices. Considering thermally induced spin transport and the magnetic spin Nernst effect in magnetic insulators, which are brought about by magnons, our findings for electron transport can be carried over to the realm of spincaloritronics, heat-to-spin conversion, and energy harvesting. I. FROM THE CONVENTIONAL TO THE MAGNETIC SPIN HALL EFFECTThe spin Hall effect (SHE) [1] and its inverse are without doubt important discoveries [2][3][4][5][6] in the field of spintronics [7,8]. They serve not only as 'working horses' for generating and detecting spin currents [9] but also as key ingredients in spin-orbit torque devices for electric magnetization switching [10][11][12]. Compared to spin-transfer torque devices [13-17], spin-orbit torque devices are faster, more robust, and consume less power upon operation [18][19][20]; for a recent review see Ref. 21. While the anomalous Hall effect (AHE) in a magnet [22] produces a transverse charge current density upon applying an electric field E, the SHE in a nonmagnet produces a transverse spin current density j γ = σ γ E (γ = x, y, z indicates the transported spin component). Mathematically, the SHE is quantified by the antisymmetric part of the spin conductivity tensor σ γ . For example, the σ z xy element comprises z-polarized spin currents in x direction as a response to an electric field in y direction.In a simple picture, the intrinsic SHE [23,24] is explained by spinning electrons that experience a spin-dependent Magnus force caused by spin-orbit coupling (SOC). It appears as if 'built-in' spin-dependent magnetic fields evoke spin-dependent Lorentz forces that result in a transverse pure spin current. The extrinsic SHE [25][26][27] is covered by Mott scattering at defects [28].Since the SHE does not rely on broken time-reversal symmetry (TRS), it is featured in nonmagnetic metals [29] or semiconductors [2]. Imposing few demands on a material's properties, a SHE can be expected in...

In experiments and applications usually the spin magnetic moment of magnons is considered. In this Paper we identify an additional degree of freedom of magnons: an orbital magnetic moment brought about by spin-orbit coupling. Our microscopic theory uncovers that spin magnetization M S and orbital magnetization M O are independent quantities. They are not necessarily collinear; thus, even when the total spin moment is compensated due to antiferromagnetism (M S = 0), M O may be nonzero. This scenario of orbital weak ferromagnetism is realized in paradigmatic kagome antiferromagnets with Dzyaloshinskii-Moriya interaction. We demonstrate that magnets exhibiting a magnonic orbital moment are omnipresent and propose transport experiments for probing it.

Our joint theoretical and computer experimental study of heat-to-spin conversion reveals that noncollinear antiferromagnetic insulators are promising materials for generating magnon spin currents upon application of a temperature gradient: they exhibit spin Seebeck and spin Nernst effects. Using Kubo theory and spin dynamics simulations, we explicitly evaluate these effects in a single kagome sheet of potassium iron jarosite, KFe 3 (OH) 6 (SO 4 ) 2 , and predict a spin Seebeck conversion factor of 0.2 µV/K at a temperature of 20 K.Introduction. Interconversion phenomena between physical quantities like sound, charge, spin, or heat [1] are cornerstones in the solid-state research for next-generation alternatives to today's CMOS technology. Two particularly active fields are those of spinelectronics (charge to spin and vice versa) [2] and spincaloritronics (heat to spin and vice versa) [3]. While the former relies on electrons, the latter may disregard electrons as fundamental carriers in favor of collective magnetic excitations, i. e., magnons, thereby circumventing Joule heating.A prominent magnonic heat-to-spin conversion phenomenon, which promises temperature control as well as wasteheat recovery, is the spin Seebeck effect (SSE) [4], comprising a spin current in a magnetic insulator as response to an applied temperature gradient. Magnons that "flow down" the gradient carry spin from the hot to the cold side of the sample. Accumulated at these ends, the spin diffuses into an adjacent heavy metal layer and gets converted into a transverse charge current by the inverse spin Hall effect [5].While the SSE is natural to ferromagnets, it does not show up in uniaxial collinear antiferromagnets, because of their spin-degenerate magnon bands. Only an external magnetic field, which causes a Zeeman splitting of the magnon bands, introduces nonzero spin Seebeck signals [6][7][8][9][10][11]. Thus, the status quo is that the heat-to-spin conversion by means of the SSE is possible in either ferromagnets (e. g., LaY 2 Fe 5 O 12 [4]) or uniaxial collinear antiferromagnets or paramagnets (e. g., MnF 2 [10] and GGG [12], respectively) in magnetic fields, or biaxial collinear antiferromagnets (e. g., NiO [13]) with nondegenerate magnon bands in zero field.An alternative to the SSE is offered by the spin Nernst effect (SNE), which describes a transverse spin current as a response to a temperature gradient in magnetic insulators. It is found both in ferromagnets [14][15][16][17], collinear antiferromagnets [17][18][19][20], and paramagnets [21]. However, its proportionality to the strength of spin-orbit coupling (SOC) renders the heat-to-spin conversion rather inefficient. Therefore, it is about time to consider spin transport in a different material class, namely in noncollinear antiferromagnetic insulators (NAIs).Herein, we show that NAIs are, in principle, materials for the generation of bulk magnon spin currents in zero magnetic field and without SOC. Taking a single kagome sheet of the NAI potassium iron jarosite KFe 3 (OH) 6 (...

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The generation of spin currents and their application to the manipulation of magnetic states is fundamental to spintronics. Of particular interest are chiral antiferromagnets that exhibit properties typical of ferromagnetic materials even though they have negligible magnetization. Here, we report the generation of a robust spin current with both in-plane and out-of-plane spin polarization in epitaxial thin films of the chiral antiferromagnet Mn3Sn in proximity to permalloy thin layers. By employing temperature-dependent spin-torque ferromagnetic resonance, we find that the chiral antiferromagnetic structure of Mn3Sn is responsible for an in-plane polarized spin current that is generated from the interior of the Mn3Sn layer and whose temperature dependence follows that of this layer’s antiferromagnetic order. On the other hand, the out-of-plane polarized spin current is unrelated to the chiral antiferromagnetic structure and is instead the result of scattering from the Mn3Sn/permalloy interface. We substantiate the later conclusion by performing studies with several other non-magnetic metals all of which are found to exhibit out-of-plane polarized spin currents arising from the spin swapping effect.