Recent advances in two-dimensional ferromagnetism: strain-, doping-, structural- and electric field-engineering toward spintronic applications

Figures & data

Table 1. The values of transition temperatures and coercive fields for emerging 2D magnetic materials

2D material	Ground state	Transition temperature (K)	Coercive field (T)
1L-FePS3	AFM	104 [173]; 118 [174]	N/A
1L-MnSe2	FM	300 [175]	0.15 [175]
1L-VSe2	FM	330 [30]	0.11 [30]
1L-Fe3GeTe2	FM	20 [66]	0.25 [66]
2L-Cr2Ge2Te6	FM	30 [1]	N/A
1L-CrBr3	FM	34 [176]; 27 [177]	0.005 [176]; 0.008 [177]
2L-CrBr3	FM	36 [177]	0.01 [177]
2L-CrCl3	AFM	16 [177]	0.98 [177]
2L-CrI3	AFM	45 [177]	0.8 [177]; 0.7 [127]
1L-CrI3	FM	45 [2]	0.05 [2]; 0.13 [127]

Figure 1. Change in energy with respect to the magnetization angle θ_M and MAE with respect to strain for (a) CrCl₃, (b) CrBr₃ and (c) CrI₃. Reproduced with permission from [37], Copyright 2018, American Physical Society. (d), MCD image of the CrI₃ flake before (left) and after (right) applying a pressure of 1.8 GPa. (e) Magnetic field dependence of MCD at 3.5 K for two 2-layer (2 L) and two 5-layer (5 L) regions before (left) and after (right) applying pressure. Reproduced with permission from [42], Copyright 2018, Nature.

Figure 2. (a) Schematic of the four stable magnetization states when passing a large d.c. current and applying an in-plane external magnetic field. The effective spin–orbit field induced by the d.c. current and the anisotropy field are both considered. (b) The AHE resistance as a function of the in-plane external magnetic field when passing a constant d.c. current at 1.9 K. (c) Current-induced magnetization switching in the Hall bar device at 1.9 K in the presence of a constant in-plane external magnetic field. (d) Phase diagram of the magnetization state in the presence of an in-plane external magnetic field B_y and a d.c. current. Reproduced with permission from [59], Copyright 2014, Nature. (e) Conductance map [dI/dV(x,E)] along the dashed line in (f). (f) STM topography of a single H atom on graphene. (g) Comparison between DFT calculations for the local magnetic moment and the height of the occupied projected DOS (PDOS) peak. (h) Calculated magnetic moments induced by H chemisorption. Reproduced with permission from [64], Copyright 2016, Science.

Figure 3. (a) The device model based on the hole-doped 1 L-CrI₃. (b) The magnetoresistance vs. the bias voltage. The hole doping density is 0.05 e/atom (9.43 × 10¹³ cm⁻²). (c) The magnetic moment as a function of temperature under various hole doping density. Reproduced with permission from [69], Copyright 2021, Elsevier. (d) The crystal structure of monolayer GaSe. (e) Carrier density dependence of spin magnetic moment per carrier and spin-polarization energy per carrier in the out-of-plane spin-polarized ferromagnetic state. (f) Band structures along high symmetry directions at carrier density of 7 × 10¹³/cm². Reproduced with permission from [45], Copyright 2015, American Physical Society.

Figure 4. (a) An optical micrograph of the investigated device. (b) Differential tunneling conductance G as a function of B_‖ and V_b (V_g = 0 V). The color scale is blue to white to red, 6 nS to 12 nS to 19 nS. (c) Bias position of the step-like features in G as a function of B_‖. (d) Calculated magnon density of states for T = 10 K, B = 0 T (blue line), T = 10 K, B = 6.25 T (black line), and T = T_c, B = 0 T (red line). The same calculations provide T_c = 88 K. (e) Calculated changes of the position of the van Hove singularities in magnon density of states. Reproduced with permission from [90], Copyright 2018, Nature. (d) as a function of magnetic field for temperatures close to T_c. (f) Schematic illustration and optical image of the vdW heterostructure of MPS/FGT. (g) Magnetic field dependence of R_xx and R_xy in the MPS/FGT heterostructure at 10 K with a positive shift of HEB = 160 Oe at a cooling field of HFC = −10 kOe. (h) Schematic diagram of the spin polarization and magnetization at interface and bulk FGT. Reproduced with permission from [92], Copyright 2020, American Chemical Society.

Figure 5. (a) Side view and (b) six sublattices of the calculated crystalline structures for graphene on top of a six bilayer EuO film. (c) Total density of states of p_z orbitals of graphene. (d) Band structures for graphene on EuO with graphene shifted by different distances. Reproduced with permission from [100], Copyright 2013, American Physical Society. (e) Top panel: a false-colored device image taken by a scanning electron microscope. Bottom panel: a schematic drawing of the Zeeman splitting of the Dirac cone in graphene and the spin-up hole-like and spin-down electron-like carriers at the charge neutrality point. (f) Top panel: Non-local resistance R_nl as a function of gate voltage V_g under dierent µ_0H for a CVD-graphene/EuS device at temperature T. Bottom panel: Comparison of R_nl,D versus µ_0H curves for the graphene device before (pristine) and after EuS deposition. (g) Top panel: Comparison of the temperature dependence of R_nl,D and of M of the graphene/EuS heterostructure. Bottom panel: Comparison of the normalized non-local resistance and longitudinal resistance in graphene/EuS. (h) Top panel: Field dependence of R_nl,D in graphene/EuS versus that in graphene/AlO_x. Bottom panel: Quantitative estimation of the Zeeman splitting energy E_z in the presence of EuS. Reproduced with permission from [101], Copyright 2016, Nature.

Figure 6. (a) Conductance as a function of gate voltage V_g measured in a trilayer FGT device. Data were obtained at T = 330 K. (b, c) R_xy as a function of external magnetic field recorded at representative gate voltages, obtained at T = 10 K (b) and T = 240 K (c). (d) Phase diagram of the trilayer FGT sample as the gate voltage and temperature are varied. (e) Coercive field as a function of the gate voltage. (f) R_xy of a four-layer FGT flake under a gate voltage of V_g = 2.1 V. (g) Remanent Hall resistance R_xy as a function of temperature. Reproduced with permission from [66], Copyright 2018, Nature. (h) A schematic side view of a dual-gate bilayer CrI₃ field-effect device. (i) Doping density–magnetic field phase diagram at 4 K. (j) MCD versus magnetic field at 4 K at representative gate voltages. Reproduced with permission from [127], Copyright 2018, Nature.

Figure 7. (a) Conductances $G_P^{min}$ (▽), $G_P^{maj}$ (Δ) and $G_{AP}^{\sigma }$ (×) of a Ni/Gr_n/Ni junction as a function of the number of graphene layers n for ideal junctions. (b) Majority and minority spin band structures (green) of a single graphene layer absorbed upon a 13 layer (111) Ni slab for a BC configuration with d = 3.3 Å, and an AC configuration with d = 2.0 Å. Reproduced with permission from [134], Copyright 2007, American Physical Society. (c) Magnetic states of bilayer CrI₃ with different external magnetic fields. (d) Schematic illustration of a 2D spin-filter MTJ with bilayer CrI₃ sandwiched between graphene contact. (e) Tunneling current of a bilayer CrI₃ sf-MTJ at selected magnetic fields. Reproduced with permission from [142], Copyright 2018, Science.

Figure 8. (a) Diagram of a proposed 2D XOR spin logic gate, where A, B and M are ferromagnetic electrodes on top of a spin transport channel. I_s and I_out denote the injection and detection currents, respectively. (b) I_out measured as a function of H. Reproduced with permission from [160], Copyright 2016, American Physical Society. (c) Design of the reprogrammable magnetologic gate for a universal NAND operation. Reproduced with permission from [162], Copyright 2017, Nature.

Figure 9. (a) A schematic representation of the HMM-SGS junction for parallel orientation of the magnetization directions of the electrodes and the corresponding current–voltage (I–V) curves. (b) The same as (a) for the antiparallel orientation of the magnetization directions of the electrodes. Reproduced with permission from [168], Copyright 2020, American Physical Society. (c) Schematic representation of the magnetic tunnel transistor. (d) Band diagram of the MTT under flatband condition, (e) the OFF-state, and (f) the ON-state. Reproduced with permission from [169], Copyright 2019, American Chemical Society.

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