Figure 56.

(A, B) The structure and ground states of iron-deficient Fe_3–xGeTe₂. (A) Side view of stoichiometric Fe₃GeTe₂. Fe_I (red) and Fe_II (silver) are two inequivalent Fe sites with +3 and +2 formal charges, respectively. The Fe_I–Fe_I interactions are mostly responsible for interlayer AF ordering while Fe_I-Fe_II and Fe_II–Fe_II couplings are FM. With Fe defects or doping, Fe_I–Fe_II and Fe_II–Fe_II become dominant and push interlayer ordering into FM. (B) The calculated energy differences (ΔE) between interlayer AF and FM phases as a function of hole concentration. For ΔE > 0, FM is favored (between 0.2 and 0.6 holes per formula unit). Panels (A) and (B) are adapted with permission from ref (615). Copyright 2020 American Chemical Society. (C) The layered crystal structure of MBT. The red arrow indicates the Mn sites in which antisites have been identified. (D) STM of the surface of a cleaved MBT crystal; white spots show the presence of multiple antisite point defects. Panels (C) and (D) are adapted with permission from ref (616). Copyright 2020 American Chemical Society. (E) Crystal structure representation of the (MnBi₂Te₄)_m(Bi₂Te₃)_n series, ranging from AF to FM with various compositions derived from codepositional MBE. Here, we see that with the addition of Bi₂Te₃ layers, the MnBi₂Te₄ layers cannot be coupled together and the material becomes less AF. This is a prime example of an off-stoichiometry defect. Panel (E) adapted with permission from ref (617). Copyright 2020 AIP Publishing.