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By University of Tsukuba February 5, 2024

Researchers have discovered an anisotropic anomalous Hall effect in a spinel oxide thin film, a phenomenon that varies with current direction and is explained by a new model incorporating a magnetic toroidal quadrupole structure, aligning with Onsagers theorem.

The Hall effect or anomalous Hall effect occurs when electric current flows through a conductor or magnet in a magnetic field, generating voltage perpendicular to the electric and magnetic field directions. Onsagers reciprocal theorem, a fundamental theorem in materials science, states that the deflection direction of electrons remains constant, irrespective of the currents direction in the plane perpendicular to the magnetic field or magnetization.

A phenomenon contradicting this theorem has not been discovered until now. In this study, for the first time, researchers have observed an anisotropic anomalous Hall effect in a spinel oxide NiCo2O4 thin film with conical magnetic anisotropy; this characteristic depends on the currents direction.

To understand this phenomenon, researchers considered the symmetry of the experimentally observed anisotropic anomalous Hall effect from a phenomenological perspective. The findings indicated the involvement of a magnetic structure termed a clustered magnetic toroidal quadrupole.

Consequently, they proposed a physical model that explained the anisotropic anomalous Hall effect without violating Onsagers reciprocal theorem. This model successfully accounts for the coexistence of the magnetic toroidal quadrupole and ferromagnetism owing to conical magnetic anisotropy.

Reference: Quadrupole anomalous Hall effect in magnetically induced electron nematic state by Hiroki Koizumi, Yuichi Yamasaki and Hideto Yanagihara, 8 December 2023, Nature Communications. DOI: 10.1038/s41467-023-43543-1

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Pushing the Boundaries of Traditional Physics: Scientists Discover New Hall Effect - SciTechDaily