By performing transport simulations on carefully derived Wannier tight-binding models, Janus chromium-based transition-metal dichalcogenide (TMD) monolayers are found to exhibit a spin-orbit torque (SOT) performance comparable to the most efficient two-dimensional materials, while additionally allowing for field-free perpendicular magnetization switching, due to their reduced in-plane symmetry.
Schematic representation of the field-free current induced perpendicular magnetization switching via spin-orbit torques (SOT) in magnetic chromium-based transition-metal dichalcogenide (TMD) monolayers in their Janus forms CrXTe, with X = S, Se.
The spin–orbit torque (SOT) mechanism represents an innovative method to electrically manipulate the magnetization of a magnetic material, providing remarkable energy-efficiency, writing speed, and scalability prospects, which have earned their insertion in magnetic random access memory (MRAM) applications, among other developing technologies. Although next-generation SOT-based technologies advance at a steady pace, they still face issues regarding massive density and integration. The development of SOT-MRAMs remains limited to multilayered devices, where SOT figures of merit are strongly sensitive to interface quality; while additionally, a densely packed SOT-MRAM requires electrical switching of magnets with perpendicular magnetic anisotropy (PMA), which is only achieved in conventional devices based on heavy metal/ferromagnet bilayers with the assistance of an external magnetic field. In this context, Janus transition-metal dichalcogenide (TMD) monolayers stand out as the materials of choice that can offer offer alternative paths to overcome these issues.
By concatenating ab initio and quantum transport methodologies, an exceptional SOT performance of chromium-based Janus TMD monolayers CrXTe (X = S, Se) is predicted, allowing for field-free switching of the perpendicular magnetic state. This was accomplished by deploying a robust end-to-end methodology comprising first-principles calculations, Wannier tight-binding models fully capturing reciprocal space spin textures, quantum transport simulations and critical field-free PMA switching current calculations. By comparing both the Janus and non-Janus materials under electric field, it is demonstrated that Cr-based Janus monolayers constitute an optimal SOT platform for low-energy magnetization reversal. The structural inversion symmetry breaking, inherent to Janus structures is responsible for a large SOT response generated by giant Rashba splitting, equivalent to that obtained by applying a transverse electric field of ∼100 V nm–1 in non-Janus CrTe2, completely out of experimental reach.
Altogether, it is found that magnetic chromium-based Janus TMDs offer a remarkable SOT performance originating from huge internal electric fields due to their asymmetric crystal structure, yielding a competitive switching current with the additional advantage of neither requiring assistance of external fields nor the transmission of spin current through an imperfect interface. Such results present magnetic Janus TMDs as efficient materials for designing ultimate SOT-MRAM technologies.
Team: Theory & simulation
Collaboration: ICN2 (Barcelona, Spain), CEA-Leti (Grenoble, France), CINAM (Marseille, France)
Funding: EU FLAG-ERA “MNEMOSYN”, EU Horizon 2020 “NUMERICS─H2020-MSCA-COFUND-2017”, Institut Universitaire de France (IUF), PEPR SPIN ANR-22-EXSP 0009 (SPINTHEORY)
Further reading: Field-Free Spin–Orbit Torque Switching in Janus Chromium Dichalcogenides, L. Vojáček✉, J. Medina Dueñas✉, J. Li, F. Ibrahim, A. Manchon, S. Roche, M. Chshiev, J. H. García✉, Nano Lett. 24, 11889 (2024), hal-03942584
Contact at Spintec: Mairbek CHSHIEV