Magnetism and magnetotransport in amorphous cobalt-germanium and amorphous cobalt-silicon thin films*

Amorphous Fe_xGe_1-x and Fe_xSi_1-x exhibit ferromagnetic ordering for x≥0.4 in thin films and have an enormous intrinsic anomalous Hall effect (AHE), spin polarization, and anomalous Hall angle. While crystalline B20 phase FeGe is magnetic up to room temperature, showing a skyrmion phase, and B20 FeSi is an antiferromagnetic insulator, often described as a Kondo insulator, B20 phase CoSi is a paramagnetic semimetal hosting exotic topological states [1]. Spin-orbit coupling plays a significant role in all of these alloys and compounds. Amorphous Co_xGe_1-x and Co_xSi_1-x thin films have been grown over the range 0.4≤x≤0.7; both alloys show an onset of ferromagnetic ordering not until x≥0.6, despite density-functional theory results which suggest ferromagnetism for as low as x=0.45. Magnetotransport measurements show that the resistivity depends on x, relatively independent of the specific elements. In contrast to the iron-based alloys which have hole carriers and negative magnetoresistance (MR), the cobalt-based alloys are dominated by electron carriers and have positive MR. Magnetotransport to explore possible exotic topological states and the AHE at high x related to the density of Berry curvature in amorphous materials will be discussed.

[1] DOI: 10.1103/PhysRevLett.122.076402