Figure 30.

(a) Atomic structure of monolayer Fe₃GeTe₂. The left panel shows the view along [001]; the right panel shows the view along [010]. Bulk Fe₃GeTe₂ is a layered crystal with an interlayer vdW gap of 2.95 Å. Fe_I and Fe_II represent the two inequivalent Fe sites in the +3 and +2 oxidation states, respectively. (b) Optical image of typical few-layer flakes exfoliated on an Al₂O₃ thin film. (c) Atomic force microscopy image of the area marked by the square in (b). Mono- and few-layer flakes are clearly visible. Scale bar, 2 μm. (d) Cross-sectional profile of the Fe₃GeTe₂ flakes along the white line in (c). The steps are 0.8 nm in height, or consistent with the thickness (0.8 nm) of monolayer (1L) Fe₃GeTe₂. (e) Normalized remanent anomalous Hall resistance as a function of temperature obtained from Fe₃GeTe₂ thin-flake samples with varying numbers of layers. Arrows mark the FM transition temperature T_c. (f) Phase diagram of Fe₃GeTe₂ as layer number and temperature are varied. T_c values are determined from anomalous Hall effect, Arrott plots and RMCD are displayed in blue, red and magenta, respectively. (g) Remanent RMCD signal as a function of temperature for a sequence of selected few-layer flakes (1 L, monolayer; 2 L, bilayer; 3 L, trilayer; 4 L, four layers; 5 L, five layer). The solid lines are least-squares criticality fits of the form α(1 – T/T_c)^β. Inset: derived values of the exponent β plotted as a function of thickness. (h) Thickness-temperature phase diagram. PM denotes the region in which the flake is paramagnetic, FM1 that in which it is FM with a single domain and FM2 that in which the flake exhibits labyrinthine or stripe domains. The transition temperatures, T_c, T_c1, and T_c2, are based on the temperature-dependent RMCD or anomalous Hall effect measurements for each flake thickness. The red dashed line denotes the critical thickness at which a dimensional crossover occurs. All panels are adapted with permission from ref (12). Copyright 2018 Springer Nature.