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

We demonstrate that the layered room-temperature ferromagnet Fe sub(7)Se sub(8) and the topological insulator Bi sub(2)Se sub(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 sub(2)Se sub(3) and Fe sub(7)Se sub(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 sub(7) Se sub(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 sub(2)Se sub(3) and that a gap can be observed in the surface state dispersion. Crystals of nominal composition Bi sub(2-x) Fe sub(x)Se sub(3) are shown to be bulk intergrowths of the two phases Bi sub(2) Se sub(3) and Fe sub(2) Se sub(3). Significant solubility of Fe in Bi sub(2) Se sub(3) is not observed.