Gyrotropic Magnetic Effect (GME)

The gyrotropic magnetic effect is the low-frequency limit of optical gyrotropy in metals and semimetals coupled to chiral spin textures. It is the subject of "Gyrotropic Magnetic Effect in Metallic Chiral Magnets" by Nisarga Paul, Takamori Park, Jung Hoon Han and Leon Balents, Phys. Rev. Lett. 135, 246704, published 12 December 2025 (doi:10.1103/vxtm-kgrx).

Mechanism

In these systems the chiral spin texture, which lacks inversion symmetry, imprints itself upon the electronic structure through Hund's coupling, leading to novel low-frequency optical activity. The authors use perturbation theory and numerical diagonalization of both relativistic and nonrelativistic models of conduction electrons coupled to spin textures, analyzing single-q and multi-q textures. Analytical expressions for the rotatory power are derived in terms of universal scaling functions, and estimates based on realistic material parameters reveal an experimentally viable range of values.

Orbital and spin contributions

The GME arises from the orbital and spin magnetic moments of conduction electrons:

• The orbital part is closely tied to Berry curvature and plays a significant role in relativistic metals, but not in nonrelativistic metals where there is no inherent Berry curvature.
• The spin contribution can be significant in nonrelativistic metals with a large Fermi energy.

The authors propose the GME as a sensitive probe of magnetic chirality and symmetry breaking in metallic chiral magnets.

Orbital magnetic displacement current

A companion result is reported by Koushik Ghorai, Sankar Sarkar and Amit Agarwal, "Intrinsic Gyrotropic Magnetic Current of Orbital Origin" (arXiv:2601.04787). In gyrotropic crystals an oscillating magnetic field induces a charge response known as the gyrotropic magnetic current. Its conventional origin is attributed to magnetic-field-modified band energy and a shift in the Fermi surface; a recent study identified an additional spin-driven magnetic displacement contribution. Ghorai et al. complete the picture by identifying the orbital counterpart of the magnetic displacement current, using a density-matrix formulation incorporating both minimal coupling and spin-Zeeman interactions. They uncover an orbital contribution to the wavepacket velocity arising physically from the time variation of the magnetic-field-induced charge polarization. In the low-frequency transport regime this mechanism becomes purely intrinsic.

The effect is illustrated in the PT-symmetric antiferromagnet CuMnAs, where the intrinsic gyrotropic magnetic current reverses sign upon Néel vector reversal, establishing it as a direct probe of antiferromagnetic order in CuMnAs and other PT-symmetric antiferromagnets.

Relation to gyrotropic order

The GME is distinct from, but related to, the spontaneous Gyrotropy observed in 1T-TiSe₂, where electrons organize into a macroscopically chiral state in an originally achiral lattice. Both concern chirality in electronic systems; the GME concerns the optical and transport response of metals coupled to chiral spin textures, while gyrotropic order concerns the spontaneous formation of the chiral electronic state itself.