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

We demonstrate that the layered room-temperature ferromagnet Fe7Se8 and the topological insulator Bi2Se3 form crystallographically oriented bulk composite intergrowth crystals. The morphology of the intergrowth in real space and reciprocal space is described. The basal planes of Bi2Se3 and Fe7Se8 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 Fe7Se8 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 Bi2Se3 and that a gap can be observed in the surface state dispersion. Crystals of nominal composition Bi2-xFexSe3 are shown to be bulk intergrowths of the two phases Bi2Se3 and Fe2Se3. Significant solubility of Fe in Bi2Se3 is not observed.