Spin Hall and Edelstein Effects in Novel Chiral Noncollinear Altermagnets

TL;DR

This work develops a symmetry-based framework to understand altermagnetism in noncollinear, chiral magnets and demonstrates that spin-split electronic bands and momentum-space spin textures can arise without SOC. By combining Landau theory with toy-model and first-principles analyses of Mn$_3$IrSi, the authors identify monopolar and quadrupolar altermagnetic order parameters that generate hedgehog- and quadrupole-like spin textures on the Fermi surface. These textures give rise to sizable spin Hall and Edelstein effects in the SOC-free limit, with SOC acting as a perturbation and not essential to the transport mechanisms. The findings reveal a SOC-free pathway to robust spin transport in chiral altermagnets, motivating experimental exploration and materials discovery for next-generation spintronic devices.

Abstract

Altermagnets are a newly discovered class of magnetic phases that combine the spin polarization behavior of ferromagnetic band structures with the vanishing net magnetization characteristic of antiferromagnets. Initially proposed for collinear magnets, the concept has since been extended to include certain non-collinear structures. A recent development in Landau theory for collinear altermagnets incorporates spin-space symmetries, providing a robust framework for identifying this class of materials. Here we expand on that theory to identify altermagnetic multipolar order parameters in non-collinear chiral materials. We demonstrate that the interplay between non-collinear altermagnetism and chirality allows for spatially odd multipole components, leading to non-trivial spin textures on Fermi surfaces and unexpected transport phenomena, even in the absence of SOC. This makes such chiral altermagnets fundamentally different from the well-known SOC-driven Rashba-Edelstein and spin Hall effects used for 2D spintronics. Choosing the chiral topological magnetic material Mn$_3$IrSi as a case study, we apply toy models and first-principles calculations to predict experimental signatures, such as large spin-Hall and Edelstein effects, that have not been previously observed in altermagnets. These findings pave the way for a new realm of spintronics applications based on spin-transport properties of chiral altermagnets.

Figures (3)

• Figure 1: Crystal structure of Mn$_3$IrSi and the momentum space spin texture from Landau theory a model calculation. (a) The Mn$_3$IrSi unit cell contains 12 Mn atoms, with local magnetic moments indicated. Both the crystal and magnetic structures follow chiral symmetry, and the magnetic moments are noncollinear. The bonding between nearest and second-nearest Mn atoms is labeled by dashed black lines. (b) The hedgehog and quadrupole spin textures predicted in the chiral noncollinear altermagnet. They are inversion odd: $\textbf{s(k)} = -\textbf{s(-k)}$ and inversion even: $\textbf{s(k)} = \textbf{s(-k)}$, respectively. The three-dimensional Fermi surface is taken from the minimal-band toy model of Mn$_3$IrSi at $E = E_f + 0.08$ eV. The schematic plot shown in the left panel is taken at the $k_x-k_y$ plane in grey.
• Figure 2: Band structure and spin texture of Mn$_3$IrSi from Kondo-lattice model and from first-principles. (a) and (c) are the band structure along high symmetry lines from toy model and first-principles calculation, respectively. (b) and (d) are the spin texture at $E = E_f +0.08$ eV and Fermi level ($E_f$) from first-principles and toy model calculation, respectively. The left panel shows the spin components of $<\sigma_{x/y}>$ and the right panel presents the spin components of $<\sigma_z>$. All spins are normalized for clearer visualization.
• Figure 3: Spin Hall and Edelstein effect conductivity tensor element ($\sigma_{1/0} = (\sigma^z_{xy} \pm \sigma^y_{xz})/2$) versus energy around Fermi level. (a) and (c) are spin Hall effect results from the model and projected Wannier functions of Mn$_3$IrSi, respectively. (b) and (d) are the Edelstein effect results ($\chi^s$) with the inverse scattering time $\Gamma^2= 10^{-4}$ ($t_a$) and (eV), respectively. The light dashed line in (a) and (b) are from the toy model without SOC.