Magnetotransport in amorphous cobalt silicon and amorphous cobalt germanium thin films*

The transition-metal group IV compounds iron germanium and iron silicon show ferromagnetic properties in amorphous Fe_x(Ge,Si)_1-x films [1], and the recently shown non-trivial topology in crystalline CoSi [2] suggests a study of amorphous cobalt silicon (a-Co_xSi_1-x) and cobalt germanium (a-Co_xGe_1-x) might lead to advances in the field of topological materials. A comparative study of a-Co_xSi_1-x and a-Co_xGe_1-x thin films has been performed for 0.4<x<0.7. At low temperatures, a-Co_x(Ge,Si)_1-x shows no magnetism for x<0.6 and weak ferromagnetism above x≈0.6, well over an order of magnitude smaller than in a-Fe_x(Ge,Si)_1-x. While the Fe-based alloys show hole carriers, the Co-based alloys show electron carriers. Resistivity measurements show that ρ_xx ≥ 150µΩcm and dependents mainly on the carrier concentration, which is characteristic for amorphous metals. Hall measurements probe whether the anomalous Hall effect stems from the intrinsic (Berry curvature) mechanism, equal to the integral over occupied energy states of the density of Berry curvature, which is the sum of the spin-orbit correlations of local orbital states [1].

[1] Bouma et al, arXiv:1908.06055, 2019
[2] Takane et al, Phys.Rev.Lett. 122-076402, 2019