Stress-Induced Spin Currents in Antiferromagnets Offer New Spintronic Potential.

The pursuit of efficient spintronic devices, which utilise the spin of electrons rather than just their charge, demands materials capable of generating substantial and robust pure spin currents, electrical currents comprised solely of spin angular momentum. Current methodologies often rely on spin-orbit coupling or interfaces with ferromagnetic materials, presenting limitations in both compatibility and operational stability. Researchers at Fudan University, the Chinese Academy of Sciences, and Nanjing University now propose a novel approach, leveraging mechanical stress applied to insulating antiferromagnetic materials to induce these currents, a phenomenon they term the piezospintronic effect. Yingwei Chen, Junyi Ji, Liangliang Hong, Xiangang Wan, and Hongjun Xiang detail their findings in a paper entitled ‘Generation of Pure Spin Current with Insulating Antiferromagnetic Materials’, where they present a first-principles computational method for identifying materials exhibiting significant piezospintronic responses, notably FeOOH, Cr₂O₃ and the NaMnX family (where X represents Arsenic, Bismuth, Phosphorus or Antimony).

Their work not only establishes a theoretical framework for investigating spin dipole moment related phenomena, but also suggests promising candidates for future spintronic applications.
Current spintronic devices frequently rely on materials exhibiting spin-orbit coupling or ferromagnetic interfaces, yet these approaches present challenges regarding compatibility and operational robustness. Researchers are now investigating the piezospintronic effect, a novel method for generating pure spin currents through mechanical stress applied to insulating antiferromagnetic materials, offering a potential pathway towards improved device performance. This effect differs from traditional methods by utilising mechanical deformation to induce spin currents, potentially leading to more energy-efficient devices.

The research team developed a first-principles computational method to calculate spin dipole moments and quantify the coefficients governing the piezospintronic effect, enabling accurate prediction of material performance. This ab initio approach, based on quantum mechanical principles, allows for the modelling of material behaviour without empirical parameters. They systematically classified magnetic point groups that enable this pure piezospintronic effect, establishing a theoretical framework for targeted material selection and design. Magnetic point groups define the symmetry of a material’s magnetic properties, influencing how it responds to external stimuli.

The team validated their computational method by accurately determining toroidization, a key metric for quantifying ferrotoroidicity, and confirmed its efficacy against established methods. Ferrotoroidicity describes a material’s ability to exhibit toroidal moments, analogous to electric dipole moments but related to the arrangement of magnetic spins. Calculations yielded a toroidization value of 5.98 × 10^-3 µ_B Å^-2 for LiCoPO₄, demonstrating close agreement with previously reported values.

Researchers overcame the limitations of traditional calculations, which assume localised magnetic moments, by accurately accounting for delocalised magnetic moments within their framework. Localised magnetic moments assume that magnetic properties originate from individual atoms, while delocalised moments consider the collective behaviour of electrons across the material. This improved representation enhances the applicability of the computational approach to a wider range of materials exhibiting complex magnetic behaviour.

This versatile methodology encompasses not only ferrotoroidicity but also fractional spin dipole moments and piezospintronics. The identification of FeOOH, Cr₂O₃, and NaMnX (where X represents As, Bi, P, or Sb) as exhibiting significant piezospintronic responses represents a significant step forward. These materials demonstrate a substantial capacity to generate pure spin currents when subjected to mechanical stress, paving the way for innovative device designs and applications. A spin current is a flow of spin angular momentum, distinct from conventional charge currents, and crucial for spintronic devices.

Interestingly, the analysis reveals that ionic displacement contributes dominantly to the observed piezospintronic effect, a distinction from the piezoelectric effect where electronic contributions are often more significant. Ionic displacement refers to the movement of ions within the material’s structure under stress, while the piezoelectric effect arises from the polarisation of electrons. This unique mechanism driving spin current generation provides valuable insights into the underlying physics and guides the development of optimised materials.

The study extends beyond theoretical modelling, providing a foundation for experimental verification and potential industrial applications of piezospintronic materials. By identifying specific compounds with strong responses, the research directs future efforts towards developing low-dissipation spintronic devices based on mechanically-induced spin currents. Low-dissipation devices minimise energy loss, improving efficiency and reducing operating costs.

Future work should focus on expanding the high-throughput screening to a broader range of materials, systematically investigating the relationship between crystal structure, electronic properties, and piezospintronic response. Researchers plan to conduct experimental verification of the predicted piezospintronic effects in the identified materials, alongside detailed characterisation of the generated spin currents. Further theoretical investigation into the interplay between different symmetry-breaking mechanisms and their influence on spin transport properties will also prove beneficial, enhancing our understanding of this emerging field.