Topological Hall effect in the A-phase of MnSi

TL;DR

This paper reports the experimental observation of a topologically quantized Hall effect in MnSi, providing direct evidence for a skyrmion lattice in the A-phase, which is a novel topological spin structure.

Contribution

It presents the first direct experimental evidence linking the topological Hall effect to a skyrmion lattice in MnSi, confirming theoretical predictions.

Findings

Observation of an additional Hall contribution in the A-phase

Identification of the contribution as topologically quantized Berry phase

Confirmation of the skyrmion lattice spin structure

Abstract

Recent small angle neutron scattering suggests, that the spin structure in the A-phase of MnSi is a so-called triple-$Q$ state, i.e., a superposition of three helices under 120 degrees. Model calculations suggest that this structure in fact is a lattice of so-called skyrmions, i.e., a lattice of topologically stable knots in the spin structure. We report a distinct additional contribution to the Hall effect in the temperature and magnetic field range of the proposed skyrmion lattice, where such a contribution is neither seen nor expected for a normal helical state. Our Hall effect measurements constitute a direct observation of a topologically quantized Berry phase that identifies the spin structure seen in neutron scattering as the proposed skyrmion lattice.