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