Magnetic 2D materials

Magnetic van der Waals materials are a new addition to the growing list of 2D materials. The special feature of these new materials is that they exhibit a magnetic ground state, either ferromagnetic or antiferromagnetic, when they are thinned down to very few sheets or even one layer of materials. Another, probably more important, feature of these materials is that they can be easily produced in few layers or monolayer form using simple means such as scotch tape, which is rather uncommon among other magnetic materials like oxide magnets. Mechanical exfoliation has become the default approach for creating 2D materials from bulk crystals, especially since it is difficult to grow 2D magnetic materials from seed crystals. However, although exfoliation produces excellent samples and permits direct examination of the inherent magnetic characteristics of 2D magnets, this method requires considerable time and effort. and a first experimental demonstration. The field was expanded further with the publication of similar observations in ferromagnetism the following year. Since then, several new materials discovered and several review papers have been published. The field now includes diverse ferromagnets, antiferromagnets, and more complex orders, some with room-temperature stability such as VSe2 and MnCO3. ==Theory==

Magnetic materials have their spins aligned over a macroscopic length scale. Alignment of the spins is typically driven by exchange interaction between neighboring spins. While at absolute zero (T = 0) the alignment can always exist, thermal fluctuations misalign magnetic moments at temperatures above the Curie temperature (T_C), causing a phase transition to a non-magnetic state. Whether T_C is above the absolute zero depends heavily on the dimensions of the system. For a 3D system, the Curie temperature is always above zero, while a one-dimensional system can only be in a ferromagnetic state at T = 0 For 2D systems, the transition temperature depends on the spin dimensionality (n). CrI3. behaves like isotropic Heisenberg model. The intrinsic anisotropy in CrI3 and Fe3GeTe2 is caused by strong spin–orbit coupling, allowing them to remain magnetic down to a monolayer, while Cr2Ge2Te6 has only exhibit magnetism as a bilayer or thicker. The XY model describes the case where n = 2. In this system, there is no transition between the ordered and unordered states, but instead the system undergoes a so-called Kosterlitz–Thouless transition at finite temperature T_{KT}, where at temperatures below T_{KT} the system has quasi-long-range magnetic order. It was reported that the theoretical predictions of the XY model are consistent with those experimental observations of NiPS3. The Heisenberg model describes the case where n = 3. In this system, there is no transition between the ordered and unordered states because of the Mermin-Wagner theorem. The experimental realization of the Heisenberg model was reported using MnPS3. The above systems can be described by a generalized Heisenberg spin Hamiltonian: :H = -\frac{1}{2} \sum_{} (J \mathbf{S}_i \cdot \mathbf{S}_j + \Lambda S_j^z S_i^z) - \sum_{i} A(S_i^z)^2, Where J is the exchange coupling between spins \mathbf{S}_i and \mathbf{S}_j, and A and \Lambda are on-site and inter-site magnetic anisotropies, respectively. Setting A \rightarrow \pm\infty recovered the 2D Ising model and the XY model. (positive sign for n = 1 and negative for n = 2), while A \approx 0 and \Lambda \approx 0 recovers the Heisenberg model (n = 3). Along with the idealized models described above, the spin Hamiltonian can be used for most experimental setups, and it can also model dipole-dipole interactions by renormalization of the parameter A. However, sometimes including further neighbours or using different exchange coupling, such as antisymmetric exchange, is required. == Measuring two-dimensional magnetism==

Magnetic properties of two-dimensional materials are usually measured using Raman spectroscopy, Magneto-optic Kerr effect, Magnetic circular dichroism or Anomalous Hall effect techniques. :T_{\text{C}}(N)/T_{\text{C}}^{\text{3D}} = 1 - (C/N)^{\frac{1}{v}}, where C is a material dependent constant. For thin layers, the behavior changes to T_{\text{C}} \propto N == Tuning 2D magnetic properties ==

Owing to the weak interlayer van der Waals interactions, the magnetic properties of 2D magnets can be readily tuned through multiple external influences including applied fields, chemical doping, mechanical strain, and many more methods. Electrical modulation Applying an external electrical field through electrostatic gating or ionic-liquid gating allows carrier density, orbital occupancy, and band structure to be tuned in 2D magnets. Since magnetic exchange interactions and anisotropy depend sensitively on the electronic structure, gating can modulate the magnetic ground state and key properties such as coercivity, saturation, magnetization, and Curie temperature. produces similar modulation of magnetic hysteresis and moment while ionic-liquid gates achieve higher carrier densities that enhance ferromagnetic ordering and reorient the easy axis. Solid state protonic gating extends this strategy to materials such as Fe5GeTe2, where larger electron doping can drive transitions between ferro- and antiferro- phases. Optical field Optical excitation offers a non contact method for tuning magnetism in 2D materials by inducing photothermal effects and photo-doping that alters the electronic structure and magnetic exchange in the material. In few layered Fe3GeTe2, femtosecond laser pulses have been shown to enhance saturation magnetization, reduce coercivity, and even stabilize ferromagnetism at room temperature. This stabilization occurs through photo-generated carriers that increase the density of states and also weaken the magnetic anisotropy. Light has also been shown to drive deterministic switching of magnetic order, as CrI3 excited with circularly polarized pulses enables the selective reversal of magnetization with thresholds that depend on the pulse energy and polarization. Mechanical regulation (pressure and strain) Mechanical deformation through applied pressure or strain offers another pathway for tuning magnetism in 2D materials by modifying lattice parameters, interlayer spacing, and stacking order. These modifications all strongly influence the exchange interactions and magnetic anisotropy of the material. In CrI3, hydrostatic pressures of a few gigapascals can reversible or irreversibly alter stacking configurations leading to transitions in magnetic order. While it is seen that hydrostatic pressure can lead to a 2x increase in magnetic coupling between material layers, it is difficult to apply precise pressure on the atomically thin layers. Similar effects have been reported in strained or wrinkled Cr2Ge2Te6 films, where enhanced Curie temperatures arise from the effects of inducing strain. Chemical doping Chemical doping provides a robust means of tuning magnetic properties of 2D magnets via elemental substitutions and adjustments to stoichiometric ratios. In the Fe-Ge-Te family variations in iron content strongly affect the Curie temperature, coercivity, and magnetic ground state. This allows iron rich compositions like Fe5GeTe2 to exhibit near room temperature ferromagnetism and complex multi-stage magnetic transitions. Further tunability can be achieved by substituting the metals that comprise these 2D materials. Cobalt or nickel doping in Fe5GeTe2 can reorient magnetic anisotropy, induce antiferro- or ferro- phases, and raise the Curie temperatures above 300-400K. Non-magnetic substitutions like gallium or arsenic also modifies exchange interactions, enabling large increases in Curie temperatures and changes in magnetic anisotropy. Similar strategies apply beyond the Fe-Ge-Te system, as theoretical and experimental work shows that substituting transition metal or halogen sites in CrI3, CrGeTe3, or CrCl3 can substantially shift their magnetic transition temperatures. ==Applications==

Magnetic 2D materials can be used as a part of van der Waals heterostructures. They are layered materials consisting of different 2D materials held together by van der Waals forces. One example of such structure is a thin insulating/semiconducting layer between layers of 2D magnetic material, producing a magnetic tunnel junction. This structure can have significant spin valve effect, and thus they can have many applications in the field of spintronics. Another newly emerging direction came from the rather unexpected observation of magnetic exciton in NiPS3. ==References==