Twin-boundary-induced Nonrelativistic Spin Splitting Enables Spintronic Devices in BiCoO and CoO Materials

Nonrelativistic spin splitting, a phenomenon with promising applications in future spintronic devices, typically occurs only in materials possessing specific symmetry characteristics. However, a team led by Kristoffer Eggestad and Sverre M. Selbach from the Norwegian University of Science and Technology, alongside Marc Vila and Sinéad M. Griffin from Lawrence Berkeley National Lab, now demonstrates that twin boundaries actively induce this spin splitting even in materials where it would otherwise be impossible. The researchers used advanced computational modelling to reveal that these boundaries, when combined with ferromagnetic domain walls, consistently generate a form of spin splitting resembling that of unconventional magnets. This achievement establishes twin boundary engineering as a powerful and versatile strategy for both realising and controlling spin splitting across a wider range of materials, potentially paving the way for innovative spintronic technologies.

Non-saturated magnetic materials are receiving considerable attention due to their potential impact on next-generation spintronic devices. This research demonstrates that twin boundaries can induce nonrelativistic spin splitting (NRSS) in magnetic materials, even when symmetry would normally forbid it. Scientists used density functional theory and tight-binding transport calculations to investigate bismuth cobalt oxide and cobalt oxide, materials exhibiting these structural defects. The results reveal that when twin boundaries coincide with ferromagnetic domain walls, they consistently generate NRSS, a phenomenon previously observed only in a specific class of materials called altermagnets. By engineering materials with twinned states exhibiting momentum-dependent altermagnetism, scientists have shown a pathway to control these properties. Calculations on bismuth cobalt oxide reveal a relatively high energy for its 90° head-to-tail ferroelectric/ferroelastic domain wall compared to other oxides, likely due to its strong tetragonal distortion. For cobalt oxide, the 109° twin boundary is found to have the lowest energy, aligning with experimental observations.

Twin Boundaries Induce Nonrelativistic Spin Splitting

Scientists have demonstrated a new route to induce nonrelativistic spin splitting (NRSS) in magnetic materials, a phenomenon with potential applications in next-generation spintronic devices. This work reveals that twin boundaries, acting as structural defects within a material, can generate NRSS even in materials where it is otherwise forbidden by symmetry. The research team employed density functional theory and tight-binding transport calculations to investigate the behavior of bismuth cobalt oxide and layered delafossite cobalt oxide, both exhibiting twin boundaries. Results show that when twin boundaries coincide with ferromagnetic domain walls, they consistently produce NRSS, similar to that observed in altermagnets. The spin splitting arises from a specific symmetry enforced by the twin boundary, dictating the location of nodal surfaces where spin degeneracy is preserved. Specifically, the induced NRSS exhibits a symmetry equivalent to that responsible for NRSS in altermagnets, generating a momentum-dependent spin texture without requiring an intrinsically altermagnetic crystal structure.

Twin Boundaries Induce Robust Spin Splitting

This research demonstrates that twin boundaries within compensated magnetic materials can induce non-relativistic spin splitting, a phenomenon typically restricted to materials with specific symmetry characteristics. Through density functional theory and tight-binding transport calculations, scientists discovered that the presence of these boundaries consistently generates spin splitting similar to that observed in altermagnets, with nodal surfaces determined by the boundary’s underlying symmetry. Further calculations demonstrate that the strength of the spin splitting and resulting transport properties correlate with the size and density of the domains created by these boundaries. These findings establish a versatile method for controlling spin splitting through the deliberate engineering of twin boundaries, expanding the range of materials suitable for spintronic applications. The observed spin splitting remains robust even with variations in domain size, suggesting a high degree of control is achievable.