EC-07 Colossal anisotropic absorption of spin currents induced by chirality

Spin currents are essential for the development of modern spintronics concepts (1) (2).Via spin pumping, the flow of spin current across the FM/NM interface can be probed by examining the phenomenological Gilbert damping factor, α, and it constitutes one of the key parameters describing the dynamics of FM materials and the spin-related characteristics of an adjacent NM. The enhanced damping Δα is typically determined by the material parameters for a prepared FM/NM interface and is isotropic regardless of the spin polarization orientations (Fig. 1A). The chiral induced spin selectivity (CISS) effect, where the structural chirality of a material determines the preference for the transmission of electrons with one spin orientation over that of the other, is emerging as a design principle for creating next-generation spintronic devices (3). Recent work suggests that chiral matter displays large effective SOC, and manifests efficient ‘spin filtering’ when a charge current passes along the chiral axis of materials. According to the generalized Landau-Lifshitz-Gilbert equation, the Gilbert damping factor is treated as a tensor, implying that control over the rotational or orientational anisotropy could play a crucial role in directing spin current (Fig.1 B) (4). The combination of structural chirality and enhanced SOC presents an exciting opportunity to engineer the anisotropy of the Gilbert damping factor through the CISS effect. Here, we find that the Gilbert damping in a chiral cobalt oxide film exhibits a maximum (minimum) value when the direction of the spin polarization is parallel (perpendicular) to the chiral axis, whereas an achiral analogue of the cobalt oxide film shows isotropic damping. By rotating the magnetization orientation of the FM from the in-plane to the out-of-plane direction, a twofold symmetry of the anisotropic damping manifests and displays a maximum-minimum ratio of ten times. This effect is attributed to the strong spin-flip process arising from chirality-induced band spin-splitting, which yields a reciprocal large spin transmission probability along the chiral axis. These studies reveal a pathway for tailoring anisotropic non-local Gilbert damping through structural chirality.References: 1. A. Hoffmann, S. D. Bader, Opportunities at the Frontiers of Spintronics. Phys.Rev. Applied 4, 047001 (2015). 2. J. Puebla, J. Kim, K. Kondou, Y. Otani, Spintronic devices for energy-efficient data storage and energy harvesting. Comm. Mater. 1, 24 (2020). 3. R. Naaman, Y. Paltiel, D. H. Waldeck, Chiral molecules and the electron spin. Nat. Rev. Chem. 3, 250-260 (2019). 4. R. Sun, Z. Wang, B. P. Bloom, A. H. Comstock, C. Yang, A. McConnell, C. Clever, M. Molitoris, D. Lamont, Z.-H. Cheng et al. Colossal anisotropic absorption of spin currents induced by chirality. Sci. Adv. 10, eadn3240 (2024)