Crystalline structure and magnetic properties of structurally ordered cobalt–iron alloys grown on Bi-containing topological insulators and systems with giant Rashba splitting

Abstract

Epitaxial Co₄₀Fe₄₀B₂₀ and Co₅₅Fe₄₅ (further – CoFe(B)) films were grown on Bi₂Te₃(0001) and Bi₂Se₃(0001) substrates by the laser molecular beam epitaxy (LMBE) technique at 200–400 °C. Structurally ordered ferromagnetic thin films were obtained on topological insulator surfaces for the first time. A body-centered cubic-type (bcc) crystalline structure of CoFe(B) with the (111) plane parallel to the (0001) plane of Bi₂Te₃ was observed, in contrast to the polycrystalline CoFe(B) film formed on Bi₂Se₃(0001) at RT using a high-temperature seeding layer. Using RHEED 3D reciprocal space mapping, the epitaxial relationships for the CoFe(B)/Bi₂Te₃ heterostructure were revealed. MOKE and AFM measurements showed the isotropic azimuthal in-plane behavior of the magnetization vector in CoFe(B)/Bi₂Te₃, in contrast to uniaxial magnetic anisotropy observed in CoFe(B)/Bi₂Se₃. XPS measurements showed a higher stability of CoFe(B) grown on Bi₂Te₃ towards oxidation, compared to that of CoFe(B) grown on Bi₂Se₃. XAS and XMCD measurements for both types of concerned nanostructures allowed the calculation of spin and orbital magnetic moments for Co and Fe. Additionally, we have studied the crystalline structure and magnetic properties of the CoFe(B)/BiTeI system with giant Rashba splitting.