Hall Effect and Topological Phase Transition in Exchange-Coupled Nanomagnets

Abstract

The real-space Berry phase associated with conduction carriers has recently attracted much attention from the viewpoints of fundamental science and spin-electronics applications. The noncoplanar spin textures responsible for the Berry phase usually appear during the magnetization reversal, often occur in systems with Dzyaloshinskii-Moriya interactions (DMI) due to, broken inversion symmetry, and yield skyrmions in specific temperature and magnetic-field ranges [1, 2]. One key aim in skyrmion research is to realize small feature sizes, which lead to a pronounced topological Hall effect (THE), at high temperatures. Confined geometries such as nanoparticles or clusters are potential systems for this purpose because the competing exchange and magnetocrystalline anisotropy of nanoparticles are expected to cause substantially noncoplanar noncollinear spin structures even in the absence of DMI. Here, we use experiments and micromagnetic simulations to investigate spin textures in exchange-coupled Co nanoclusters produced by inert-gas condensation-type cluster deposition. The nanoclusters have an average size of 14 nm and exhibit a significant topological Hall effect (THE), as shown in the experimental Hall data of Fig. 1(a). The micromagnetic THE simulations, Fig. 1(b), predict the skyrmion number Q by integration over the skyrmion density and are in good agreement with the experimental data. THE is caused by the noncoplanar noncollinear spin structure of the Co nanoclusters, visualized in Fig. 1(c). A characteristic feature of both experimental THE and theoretical prediction (skyrmion number Q) is a drastic change over a relatively small field range, which we interpret as a topological phase transition analogous to the Lifshitz transition in metals.This work is supported by NSF-DMREF (No. 1729288), NSF-EQUATE (OIA-2044049), the NU Collaborative Initiative, and NCMN,  Fig. 1. Topological Hall effect (THE) in Co nanoclusters: (a) measured THE at 300 K and 10 K, (b) calculated Q as a function magnetic field and (c) simulated spin structure. The insets in (b) show the simulated spin textures at different fields. Red and blue colors represent spin-up and spin-down magnetization directions, respectively.