Surface ferromagnetism in a chiral topological semimetal CoSi

TL;DR

This study demonstrates that surface ferromagnetism in CoSi is caused by RKKY interactions between distorted bonds at the surface, distinguished through magnetization measurements on oxidized versus fresh samples.

Contribution

It provides experimental evidence linking surface ferromagnetism in CoSi to RKKY interactions, clarifying the mechanism behind the phenomenon.

Findings

Fresh CoSi surfaces exhibit strong ferromagnetism.

Oxidized CoSi samples do not show significant ferromagnetism.

Magnetization behavior indicates surface state-induced interactions.

Abstract

Despite the chiral topological semimetal CoSi is known as bulk diamagnetic, it shows unusual surface ferromagnetism of debatable origin. The ferromagnetic ordering has been attributed to the distorted bonds, the superlattice of ordered vacancies, or even to topological surface textures due to the spin polarization in the neighboring Fermi arcs. We experimentally compare magnetization reversal curves for initially oxidized CoSi single crystals and cleaved samples with a fresh, oxide-free surface. While the oxidized CoSi samples do not show sizable ferromagnetism, the fresh CoSi surface gives a strong ferromagnetic response, which is accompanied by the pronounced modulation of the angle dependence of magnetization, as it can be expected for easy and hard axes in a ferromagnet. In addition to the first order reversal curves analysis, this observation allows us to distinguish between different mechanisms of the ferromagnetic ordering in CoSi single crystals. We conclude that the surface states-induced RKKY interaction between distorted bonds near the sample surface is responsible for the strong ferromagnetic multi-domain behavior for freshly cleaved samples.