Heterostructured ferromagnet-topological insulator with dual-phase magnetic properties

The introduction of ferromagnetism at the surface of a topological insulator (TI) produces fascinating spin-charge phenomena. It has been assumed that these fascinating effects are associated with a homogeneous ferromagnetic (FM) layer possessing a single type of magnetic phase. However, we obtained phase separation within the FM layer of a Ni₈₀Fe₂₀/Bi₂Se₃ heterostructure. This phase separation was caused by the diffusion of Ni into Bi₂Se₃, forming a ternary magnetic phase of Ni:Bi₂Se₃. The inward diffusion of Ni led to the formation of an FeSe phase outward, transforming the original Ni₈₀Fe₂₀/Bi₂Se₃ into a sandwich structure comprising FeSe/Ni:Bi₂Se₃/Bi₂Se₃ with dual-phase magnetic characteristics similar to that driven by the proximity effect. Such a phenomenon might have been overlooked in previous studies with a strong focus on the proximity effect. X-ray magnetic spectroscopy revealed that FeSe and Ni:Bi₂Se₃ possess horizontal and perpendicular magnetic anisotropy, respectively. The overall magnetic order of the heterostructure can be easily tuned by adjusting the thickness of the Bi₂Se₃ as it compromises the magnetic orders of the two magnetic phases. This discovery is essential to the quantification of spin-charge phenomena in similar material combinations where the FM layer is composed of multiple elements.