Observation of cluster magnetic octupole moments in antiferromagnetic Weyl semimetal Mn₃Sn nanowires using a tip-induced local temperature gradient

The antiferromagnetic Weyl semimetals such as Mn₃Sn have been attracting wide attention due to their anomalous transverse transport properties despite having barely any net magnetization. In this study, we visualized the cluster magnetic octupole moments in a (0001) oriented polycrystalline Mn₃Sn nanowire using atomic force microscopy. Our method provides magnetic images by measuring the local anomalous Nernst effect caused by a tip-contact induced temperature gradient. We first demonstrated this method on the ferromagnetic Weyl semimetal Co₂MnGa, in which magnetic domain walls were clearly visible. We applied this method to Mn₃Sn, that directly gives, after eliminating non-magnetic contributions due to the Seebeck effect, the distribution of the cluster magnetic octupole moments in Mn₃Sn with 80 nm spatial resolution. In polycrystalline Mn₃Sn nanowires, nonmonotonic anomalous Nernst effects were observed for each crystal grain with a size of approximately 100 nm. This is thought to reflect the inclination of the kagome planes. After quantitative discussions, the typical Nernst thermopower was estimated to be ~0.25 μV/K. Our results are considered to provide important information for microfabrication and accumulation of functional antiferromagnets.