Size-Dependent Magnetism and Skyrmionic Properties in CoSi Nanoclusters

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Magnetic transition-metal silicides and germanides, crystallizing in the B20 cubic structure with broken inversion symmetry, have attracted much attention in the context of Dzyaloshinskii-Moriya interactions, helical spin structures, and magnetic skyrmions [1, 2]. Finding skyrmions in confined geometries such as nanoparticles and nanowires is particularly intriguing from the viewpoints of fundamental science and practical applications [3]. Such low-dimensional structures also often show size-induced magnetic properties as compared to corresponding bulk alloys [4]. We have used density-functional-theory (DFT) calculations and cluster-deposition experiments to investigate the size dependence of magnetic properties in equiatomic CoSi; the average particle size d of the clusters varies from about 0.6 nm to 11.6 nm. In sharp contrast to bulk CoSi, which is nonmagnetic down to liquid helium temperature, the nanoclusters order magnetically with a net low-temperature magnetic moment from 0.69 to 0.21 μB/Co as d varies from 0.6 to 11.6 nm. The fast Fourier transform (FFT) of the HRTEM image (Fig. 1a) and x-ray diffraction pattern (not shown here) of the sample having d = 11.6 nm confirm the cubic B20 structure. The corresponding DC susceptibility curves display anomalies in limited field (about 20 – 200 Oe) and temperature (100 K – 300 K) ranges (Fig. 1b), which are similar to the signature of helical or skyrmion-like spin structures. DC susceptibility and anomalous Hall resistivity data are used to determine a field-temperature magnetic phase diagram that includes a skyrmionic region and to discuss the confinement effect on skyrmions within the small diameter of CoSi nanoclusters.This research is supported by the US DOE/BES (DE-FG02-04ER46152) and performed in part in the Nebraska Nanoscale Facility and the NCMN, which are supported by the U.S. NSF (NNCI -1542182), and the Nebraska Research Initiative (NRI).