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 Ni 80 Fe 20 /Bi 2 Se 3 heterostructure. This phase separation was caused by the diffusion of Ni into Bi 2 Se 3 , forming a ternary magnetic phase of Ni:Bi 2 Se 3 . The inward diffusion of Ni led to the formation of an FeSe phase outward, transforming the original Ni 80 Fe 20 /Bi 2 Se 3 into a sandwich structure comprising FeSe/Ni:Bi 2 Se 3 /Bi 2 Se 3 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:Bi 2 Se 3 possess horizontal and perpendicular magnetic anisotropy, respectively. The overall magnetic order of the heterostructure can be easily tuned by adjusting the thickness of the Bi 2 Se 3 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.