Giant Room-Temperature Topological Hall Effect in a Square-Net Ferromagnet LaMn₂Ge₂

A topological magnetic material showcases a multitude of intriguing properties resulting from the compelling interplay between topology and magnetism. These include notable phenomena such as a large anomalous Nernst effect (ANE), an anomalous Hall effect (AHE), and a topological Hall effect (THE). In most cases, topological transport phenomena are prevalent at temperatures considerably lower than room temperature, presenting a challenge for practical applications. However, the noncollinear ferromagnetic (FM) LaMn₂Ge₂, characterized by a Mn square-net lattice and a notably high Curie temperature (T_C) of approximately 325 K, defies this trend as a topological semimetal. This work observes a giant topological Hall resistivity, $$ , of ≈4.5 µΩ cm at room temperature when the angle between the applied field and the c-axis is 75°, which is significantly higher than state-of-the-art materials with noncoplanar spin structures. The single crystal neutron diffraction measurements agree with an incommensurate conical magnetic structure as the ground state. This observation suggests the enhanced spin chirality resulting from the noncoplanar spin configuration when the applied field is away from the magnetic easy axis as the origin of a large contribution to the observed THE. The findings unequivocally demonstrate that the FM LaMn₂Ge₂ holds great promise as a potential topological semimetal for spintronic applications even at room temperature.