Sputter Deposited Ni-Based Intrinsic Magnetic Topological Insulator Thin Films

Three-dimensional topological insulators (TIs) exhibit unique properties such as topologically protected Dirac surface states (TSS) that enable metallic surface states while the bulk remains insulating. These TIs, including Bi_1-xSb_x alloys and hexagonal X₂Q₃ compounds (X = Bi, Sb, Bi_1-xSb_x; Q = Se, Te), also demonstrate the quantum anomalous Hall effect (QAHE). The QAHE occurs when a gap is created in the TSS by introducing perpendicular magnetic order and breaking time reversal symmetry, resulting in spin polarized, chiral edge currents while maintaining an insulating bulk state. Magnetic order can be introduced through compositional doping, stoichiometric intrinsic magnetic TI (MTI), or proximity-induced magnetization (PIM) methods. Although the QAHE has been observed in experiments for each of these methods, the temperatures involved are currently impractical for technology, being in the milliKelvin range. A phenomenological trend is that the magnetic ordering temperature (Curie temperature, T_C, or Néel temperature, T_N) is typically at least ten times higher than the observed QAHE temperature. In our previous work, we studied the interfacial magnetic phases of sputtered Bi₂Te₃/Ni₈₀Fe₂₀ and confirmed with selected area diffraction the existence of the NiBi₂Te₄ phase. We found a relatively high T_N of 63 K; thus, this phase is very promising to exhibit a high QAH state. Here we expand our work on Ni-based MTI materials through sputtering single-phase Ni-doped Bi₂Te₃ films, an intrinsic MTI. We present crystallographic structure characterization and temperature dependent magnetometry. To further understand the depth-dependent nuclear and magnetic structure we performed polarized neutron reflectometry.