Indirect magneto-ionic effect in FeSi2/Si nanocomposite induced by electrochemical lithiation and delithiation

A nanocomposite consisting of iron disilicide nanocrystals embedded in a Si matrix was prepared from industry-grade ferrosilicon by ball milling and subsequent heat treatment. By tailoring the heat treatment temperature either the metallic α-FeSi2 or the semiconducting β-FeSi2 phase could be made the dominant one, as indicated by X-ray diffraction. Magnetization curve and zero-field cooled/field cooled measurements revealed that ferromagnetic and superparamagnetic centers are present in the nanocomposites, which could be attributed to Fe-rich defective regions at the surface of the iron disilicide nanocrystals. For both nanocomposites, containing either mainly the α or β phase, we could show that the magnetization can be varied by about 40% by electrochemical lithiation and delithiation of the surrounding Si matrix, with up to 6.5% of the magnetization change beingreversible. These variations could be attributed to the formation of additional Fe-rich magnetic regions, induced by a local change of the Fe/Si fraction at the FeSi2/Si interfaces, and their subsequent partial elimination. Thus, this work demonstrates a new concept for how an "indirect magneto-ionic effect'' can be obtained in composite materials consisting of a phase prone to the electrochemical ion uptake (i.e., the Si matrix) and a magnetic phase (i.e., the FeSi2 nanocrystals).