Bulk intergrowth of a topological insulator with a room-temperature ferromagnet

We demonstrate that the layered room-temperature ferromagnet Fe ₇Se ₈ and the topological insulator Bi ₂Se ₃ form crystallographically oriented bulk composite intergrowth crystals. The morphology of the intergrowth in real space and reciprocal space is described. The basal planes of Bi ₂Se ₃ and Fe ₇Se ₈ are parallel in the micron-scale intergrowth, and hence the good cleavage inherent to the bulk phases is retained. Both phases in the intergrowth crystals display their intrinsic bulk properties: the ferromagnetism of the Fe ₇Se ₈ is anisotropic, with the magnetization easy axis in the plane of the crystals, and angle-resolved photoemission spectroscopy characterization shows that the topological surface states remain present on the Bi ₂Se ₃ and that a gap can be observed in the surface state dispersion. Crystals of nominal composition Bi _2-xFe _xSe ₃ are shown to be bulk intergrowths of the two phases Bi ₂Se ₃ and Fe ₂Se ₃. Significant solubility of Fe in Bi ₂Se ₃ is not observed.