Covalent 2D Cr₂Te₃ ferromagnet

Figures & data

Figure 1. a, A schematic diagram of the CVD growth process. Schematic illustrations of the growth processes of Cr₂Te₃ using b, space-confined and c, conventional CVD, respectively. An optical microscope image of Cr₂Te₃ grown on mica utilizing d, the space-confined CVD and e, conventional CVD process. f, An AFM image and g, The height profile of a single Cr₂Te₃ flake. The thickness is ∼1.4 nm, corresponding to a single unit cell.

Figure 2. a, c-axis projection: unit cell of Cr₂Te₃ is shown in the red box; atoms with green halo are Cr_I atoms in the interior Cr/Vacancy layers; atoms with yellow halo are Cr_I atoms on the surface Cr/Vacancy layers. b, a-axis projection: braces show the structures used in calculations of single- to quad-layer. The Cr and Te columns are denoted by blue and brown solid columns, respectively. c, Atomically resolved HAADF-STEM image of single crystal Cr₂Te₃. Upper right inset: the reciprocal lattice obtained from an FFT of the HAADF-STEM image, with the lattice planes of Cr₂Te₃ highlighted by red arrows. Lower left inset: an enlarged image of c. d, EDS elemental mapping of the area shown by the red square in c. Green: Cr, Red: Te.

Table 1. Magnetization and MAE of 1–4 layers together with bulk Cr₂Te₃.

	1-L (FM)	2-L (FM)	3-L (FM)	4-L (FM)	Bulk (canted)
M (µB unit cell^−1)	18	30	42	54.15	19.2
M (10^3 A m^−1)	538	500	488	483	365
MAE (meV unit cell^−1)	−8.4	−9.6	−4.87	∼0	3.5
MAE (MJ m^−3)	−5.3	−4.1	−0.9	−2.6 × 10^−5	1.11

Notes: The unit cell is shown in the rhombic box in Figure 2(a). For 1–4 layers, the thickness of the unit cell is the thickness of the layers shown in Figure 2(b). For bulk Cr₂Te₃, the thickness of the unit cell corresponds to that of a bi-layer without the surface Cr atoms.

Figure 3. Competition of spin exchange, spin canting and spin-resolved total density of states (DOS). a, Schematic representation of exchange coupling in Cr₂Te₃. Competing inter-layer exchange parameters J₁₂ and J₁₃ induce canting angles ${\theta }_1$ for Cr_I and ${\theta }_2$ for Cr_III atoms. b, ${\theta }_1$ as a function of the ratio of next-nearest to nearest neighbor exchange parameters J₁₂/J₁₃; the black dot represents ${\theta }_1$ calculated from J₁₂/J₁₃ obtained using the experimental lattice constant. Spin-resolved total DOS of c, single, d, bi-, e, tri-layer and f, quad-layers Cr₂Te₃ calculated for Cr-terminated structures. Majority DOS is shown in the upper panels while minority DOS is shown in the lower panels. E_F is the Fermi energy.

Figure 4. a, Out-of-plane magnetic hysteresis loops measured at different temperatures. b, Out-of-plane and in-plane magnetic hysteresis loops acquired at 10 K. c, ZFC and FC magnetization curves. The FC magnetization of a bulk film is also included for comparison. d, Calculated reduced magnetization as a function of reduced temperature of 2D (blue) compared to its 3D counterpart (J = 3/2, black) in the mean field approximation.

Figure 5. a, AFM image and b, the corresponding height profile of a single Cr₂Te₃ crystal, which shows a thickness of 11.5 nm. The black line in a corresponds to the height profile of b. c, normalized MOKE hysteresis loops and d, temperature-dependent coercivity of the corresponding Cr₂Te₃ crystal. e, f, g, h, MOKE hysteresis loops of Cr₂Te₃ 2D crystals with different thicknesses, 25.2, 12.2, 7.8, and 2.8 nm, respectively. e–g were measured at 120 K; h was measured at 20 K. Inset: AFM images of the corresponding Cr₂Te₃ crystals.