Anomalous hall effect and spin-orbit torques in noncollinear antiferromagnetic Mn3Ni0.35Cu0.65N thin films
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Abstract
The continued scaling of conventional charge-based electronics faces significant challenges, creating an urgent need for alternative paradigms capable of delivering lowpower, high-speed, and scalable computing technologies. Spintronics, by exploiting the spin and orbital degrees of freedom of electrons, has emerged as a promising pathway, particularly through the development of spin–orbit torque (SOT) based memory devices. In this context, noncollinear antiferromagnets (NCAFM) offer a unique materials platform, combining vanishing net magnetization with exotic spin textures that can give rise to robust electronic transport phenomena such as the anomalous Hall effect (AHE) and unconventional spin–orbit torques.
This thesis investigates the anomalous Hall effect and spin–orbit torques in the cubic antiperovskite Mn3Ni0.35Cu0.65N (Mn3NiCuN). A detailed experimental framework was established to disentangle higher-order contributions to the AHE by performing angular-dependent Hall measurements under both in-plane and out-of-plane magnetic field geometries. A nontrivial in-plane AHE was observed, confirming the presence of higher-order multipolar contributions—specifically, an octupolar moment—rather than conventional dipolar magnetization. The angular dependence of the AHE exhibited a 120° symmetry, consistent with the symmetry of the octupolar spin texture, and was further supported by phenomenological modeling that incorporated scalar spin chirality (SSC) contributions to the transport signal.
In parallel, spin–orbit torques were investigated via second harmonic Hall measurements and spin-torque ferromagnetic resonance (ST-FMR). A significant enhancement of the damping-like torque was observed near the Néel temperature. This enhancement could be attributed to two alternative mechanisms: (i) increased spin and orbital current generation from critical fluctuations of the noncollinear antiferromagnetic order, and (ii) spin currents arising from fluctuation-induced scalar spin chirality. Notably, this work presents the first experimental indication of orbital Hall effect contributions in a NCAFM, offering a new perspective on angular momentum generation in complex magnetic systems.
The findings of this thesis provide direct insight into the role of higher-order magnetic multipoles, critical spin fluctuations, and orbital transport in NCAFM. Beyond advancing fundamental understanding, these results highlight the potential of NCAFMs for developing energy-efficient, field-free spintronic devices, marking a significant step toward exploiting complex magnetic textures for future information technologies.