Optically manipulating ferromagnetism in Cr-doped topological insulators (TIs)*

Optically manipulating ferromagnetic materials has been shown to be a promising route to opto-spintronic applications. Using a combination of optically-enabled transport measurements and scanning tunneling spectroscopic (STS) measurements in the presence of circularly polarized (CP) light, we found an enhancement of magnetization in 10% Cr-doped (Bi_xSb_1-x)₂Te₃ bilayer heterostructures which consisted of a pure layer (Bi_xSb_1-x)₂Te₃ on top of a 10% Cr-doped (Bi_xSb_1-x)₂Te₃ layer. Measurements of the anomalous Hall resistance revealed an increase in R_xy and a decrease in longitudinal resistance R_xx in the bilayer magnetic TI system under CP light (wavelengths λ = 1600 ~ 1700 nm). In contrast, both R_xx and R_xy were suppressed under CP light for uniformly Cr-doped (Bi_xSb_1-x)₂Te₃. To understand the microscopic origin of these results, we performed spatially resolved STS studies on the surface state of magnetic TIs as a function of temperature and magnetic field. We further conducted optically-assisted STS studies to spatially map out the CP light-induced spectral changes to the surface state. The physical implications from correlating the spatially resolved STS under CP light with findings from macroscopic R_xx and R_xy will be discussed.