In this paper, we present the electronic and magnetic properties of the ferromagnetic semiconductor Cr2Si2Te6 doped by charge particles by the first-principles calculations. We find that doped Cr2Si2Te6 manifests a ferromagnetic metallic phase and its Curie temperature significantly increases from 54 to 125 K, as well as the magnetic coupling changing from weak super-exchange to strong double-exchange interaction. Moreover, the magnetic easy axis in doped Cr2Si2Te6 rotates from the ⟨001⟩ direction to the ab-plane. Our results provide an easily accessible method to considerably increase the ferromagnetic transition temperature of van der Waals crystals.

Two-dimensional (2D) magnetic materials have become an intriguing topic not only because of their novel physical phenomena but also because of their potential application in magnetic storage devices, photoelectric sensors, and magneto-transport devices. Much effort has gone into the work on determining the magneto-transport properties, electronic properties, and magnetic transition temperature of FePS3,1,2 CrI3,3–7 Cr2Ge2Te6,8–11 VSe2,12–15 MnSe2,16 Fe3GeTe2,17 etc. However, the magnetic transition temperatures of these two-dimensional magnetic materials are usually very low, limiting their potential application; however, it is still challenging to find candidate materials with 2D magnetism and high magnetic transition temperature or those to elevate the Curie points of those 2D magnetic materials to room temperature by doping, pressure, or other modifications.

One of the main methods for elevating the magnetic transition temperatures of these quasi-2D magnetic materials is to intercalate charged ions into the van der Waals layers; additional mobile carriers are introduced into these van der Waals materials, establishing strong ferromagnetic coupling between local spins. For example, Deng et al.17 achieved great enhancement in the Curie temperature in Fe3GeTe2 thin films by ionic gating, which may also be used to achieve room temperature; recently, Wang et al.14 showed that through charged particle intercalation to Cr2Ge2Te6, the Curie temperature Tc showed a dramatic improvement of about three times. One naturally expects that new van der Waals compounds with a similar structure to Cr2Ge2Te6, such as Cr2Si2Te6,18,19 could also achieve much high Tc through charged ion intercalation. In comparison with Cr2Ge2Te6 and Fe3GeTe2, much less research has been performed on Cr2Si2Te6 due to the smaller magnetic transition temperature, Tc = 33 K.20,21 On the other hand, considering the fact that the availability of Si is greater than that of Ge, we think that it is worth exploring how to increase the Curie temperature of Cr2Si2Te6 for its potential extensive application.

In this paper, we propose to dope charged particles into Cr2Si2Te6 and systematically study the band structures and magnetic properties of doped Cr2Si2Te6. We find that Tc can be elevated to three times the parent phase and the easy-magnetization axis rotates from the out-of-plane direction to the in-plane direction. The rest of the paper is organized as follows: in Sec. II, we present the electronic structures and density of states (DOS) of parent and doped phases; in Secs. III and IV, we display the evolution of the Curie temperature and magnetic anisotropy upon doping; Sec. V is devoted to the conclusion.

Adopting the Vienna Ab initio Simulation Package (VASP) code with the generalized gradient approximation (GGA)+U method (the details can be seen in the  Appendix), we first study the spin-resolved band structures and atomic-resolved partial density of state (PDOS) of pristine Cr2Si2Te6 and charge doped Cr2Si2Te6. The pristine Cr2Si2Te6 displays a semiconductor behavior with an indirect bandgap of about 0.36 eV, as seen in Fig. 1(a). This bandgap, which is almost constant when U varies from 0.5 to 1.5 eV, is in agreement with that in the literature.8 The PDOS shows that Cr and Te present almost the same contribution to the density of states (DOS) and these two atoms are hybrid in the energy range of 0 to −1 eV, as seen in Fig. 1(b).

FIG. 1.

(a) Spin-resolved band structures and (b) atomic-resolved density of states of pristine Cr2Si2Te6. The red and black lines in (a) denote the spin-up and spin-down bands, respectively.

FIG. 1.

(a) Spin-resolved band structures and (b) atomic-resolved density of states of pristine Cr2Si2Te6. The red and black lines in (a) denote the spin-up and spin-down bands, respectively.

Close modal

Similar to charge intercalated Cr2Ge2Te6,14 we assume that doped Cr2Si2Te6 with charged organic molecule intercalation will expand the interlayer distance while keeping the planar atomic coordinates and crystalline symmetry unchanged, as pointed out in the  Appendix. We calculate the band structures of such charge intercalated Cr2Si2Te6 with three instances of extra electron doping per unit cell. The numerical results of the band structures of charged Cr2Si2Te6 are shown in Fig. 2(b). For comparison, we also plot the band structures of Cr2Si2Te6 with neutral intercalation as shown in Fig. 2(a). One can see that after charge particle intercalation, Cr2Si2Te6, having a semiconductor–metal transition, displays metallic behavior with two bands across the Fermi level. The atomic projection on these two bands shows that the main components arise from the Cr and Te atoms and their hybridization.

FIG. 2.

(a) Atomic projected band structures of neutral intercalated Cr2Si2Te6 and (b) charge intercalated Cr2Si2Te6 with three electrons per unit cell. Red and violet lines denote the Cr and Te atoms, respectively.

FIG. 2.

(a) Atomic projected band structures of neutral intercalated Cr2Si2Te6 and (b) charge intercalated Cr2Si2Te6 with three electrons per unit cell. Red and violet lines denote the Cr and Te atoms, respectively.

Close modal

The atomic and orbital resolved PDOSs of pristine and charge intercalated Cr2Si2Te6 are shown in Fig. 3. The orbital projection is on the local coordinates of CrTe6 octahedra. In the pristine phase, one can see that the Cr and Te atoms have almost the equal contribution in the energy range of −1.0 to 0 eV below the Fermi level EF and the Cr-3d3z2–r2 orbital contributes the most to the bands in the range of −1.5 to 0 eV, compared with the other four orbitals of Cr-3d, as shown in Figs. 3(a) and 3(b). After three electrons’ intercalation, the orbital contribution significantly changes. The atomic-resolved PDOS shown in Fig. 3(c) indicates that the Cr atoms nearly double the Te atoms in the contribution to the DOS around the Fermi level. In addition, one can see that Cr-3dxz and Cr-3dyz are the main parts of the band compared with other three Cr-3d orbitals around EF.

FIG. 3.

Atomic and orbital resolved density of states (DOSs) of (a) and (b) pristine Cr2Si2Te6 and (c) and (d) charge intercalated Cr2Si2Te6.

FIG. 3.

Atomic and orbital resolved density of states (DOSs) of (a) and (b) pristine Cr2Si2Te6 and (c) and (d) charge intercalated Cr2Si2Te6.

Close modal

We now discuss the magneto-crystalline anisotropy energy (MAE). The MAE was obtained through performing a non-collinear total energy calculation including the spin–orbit coupling for two magnetization directions: one perpendicular to the quasi-2D plane i.e., ⟨001⟩ direction, and another parallel to the quasi-2D plane; here, we choose the ⟨100⟩ direction.

In pristine Cr2Si2Te6, the MAE is ΔE = E001 − E100 = −0.504 meV/Cr, indicating that the magnetization orientation prefers to be along the c-axis. This result is consistent with the experimental data available.21 By contrast, the MAE of intercalated Cr2Si2Te6 is ΔE = 0.315 meV/Cr, suggesting that the magnetization orientation tends to lie in the ab-plane.

Considering the fact that the Cr spins are interacting via Cr–Te–Cr in pristine Cr2Si2Te6, this leads to the weak super-exchange ferromagnetic coupling and the c-axis anisotropy due to the spin–orbit coupling. After electron doping through intercalation, the major DOS contribution near the Fermi level turns to planar orbitals dxz and dyz, taking into account the spin–orbit coupling between Cr ions, and these two conducting orbitals finally drive the spins from along the c-axis in pristine Cr2Si2Te6 to lying in the ab-plane in doped Cr2Si2Te6. Hence, it is the spin–orbit coupling of Cr ions as well as the orbital configuration near EF that switches the magnetic easy-axis orientation from the c-axis to the ab-plane.

Our numerical results show that both pristine and intercalated Cr2Si2Te6 are ferromagnetic; the former is a semiconductor with a magnetic moment of 3.03 µB/Cr, in agreement with earlier theoretical literature,22 while the latter is a semimetal with a magnetic moment of 3.24 µB/Cr. Furthermore, in order to investigate the influence of intercalated charged particles on magnetic transition temperature of Cr2Si2Te6, we calculated the Curie temperature Tc of Cr2Si2Te6 with a different number of doped electrons. The Curie temperature is roughly estimated from the mean-field approximation,23,24

Tc=2Δ3ZNkB,

where N is the number of Cr electrons in a unit cell of Cr2Si2Te6, and it is equal to 6; Z is the coordinate number of Cr, and it is equal to 3; and Δ is the total energy difference between the antiparallel and parallel alignments of Cr spins. In addition, the magnetic coupling parameters J can be easily obtained by Δ. By using the mean-field approach, we estimate that the magnetic transition temperature of pristine Cr2Si2Te6 is about 54.6 K, which is comparable with the experiment data, 33 K.20,21

Upon doping, the magnetic transition temperature continuously increases with the intercalation of charged particles. The magnetic coupling and transition temperature increase with doped electrons, as shown in Fig. 4. It displays a nearly linear relationship between the Curie temperatures Tc or the exchange coupling parameter J and the number of doped electrons. Moreover, the magnetic interaction between Cr in pristine Cr2Si2Te6 is acknowledged as the super-exchange type, due to the insulating nature shown in Fig. 3(a).

FIG. 4.

Dependence of the Curie temperature and exchange coupling strength on the number of doped electrons.

FIG. 4.

Dependence of the Curie temperature and exchange coupling strength on the number of doped electrons.

Close modal

However, as seen in Fig. 3(c), intercalated Cr2Si2Te6 becomes a metal, and the exchange coupling parameter J linearly grows along with the increasing conduction electron number. According to the microscopic mechanism of the double-exchange ferromagnetism proposed by Anderson and Hasegawa,25 with the transition of pristine insulating Cr2Si2Te6 to doped metallic Cr2Si2Te6, the major DOS contribution near the Fermi level in the metallic compound originates from previously unoccupied Cr dxz and dyz orbitals, which is shown in Fig. 3(d); these conducting Cr dxz and dyz electrons generate the direct double-exchange magnetic coupling with other local spins of Cr, resulting in the increase in the magnetic coupling strength between Cr spins and finally giving rise to the enhancement in the FM Curie temperature.

We have determined the magnetic properties of pristine and electron doped Cr2Si2Te6, as well as Cr2Ge2Te6, for comparison. The results are listed in Table I; the data of Cr2Ge2Te6 are taken from the study by Wang et al.14 (a) and (b) in the upper-right corner refer to experimental and calculated data, respectively. From Table I, one can see that the Curie temperature of Cr2Si2Te6 and Cr2Ge2Te6 can be significantly lifted via intercalating charged particles into the van der Waals layers of these two compounds and their magnetic anisotropies and magnetic moments are also enhanced, indicating that charge intercalation is an effective way to improve the ferromagnetism of two-dimensional layered materials. Considering the abundance of Si element on Earth, it is more meaningful to improve the ferromagnetism in Cr2Si2Te6.

TABLE I.

Comparison of the magnetic properties of Cr2Si2Te6 and Cr2Ge2Te6.

Cr2Si2Te6-pristineCr2Si2Te6-intercalatedCr2Ge2Te6-pristineCr2Ge2Te6-intercalated
Magnetic moment (µB/Cr) 3.03 3.24 2.90a 3.07a 
MAE (meV/Cr) −0.504 0.315 −0.063b 0.591b 
Curie temperature (K) 54 125 67a 208a 
Cr2Si2Te6-pristineCr2Si2Te6-intercalatedCr2Ge2Te6-pristineCr2Ge2Te6-intercalated
Magnetic moment (µB/Cr) 3.03 3.24 2.90a 3.07a 
MAE (meV/Cr) −0.504 0.315 −0.063b 0.591b 
Curie temperature (K) 54 125 67a 208a 
a

Refers to the experimental data from Ref. 14.

b

Refers to the calculated data from Ref. 14.

In summary, we found that by doping charged particles, the ferromagnetic semiconductor pristine Cr2Si2Te6 with the weak super-exchange coupling transforms to the ferromagnetic metallic Cr2Si2Te6 with a much stronger double-exchange interaction, leading to a significant enhancement in the magnetic ordering temperature Tc. According to the mean-field theory, the estimated Curie temperature of Cr2Si2Te6 of 125 K can be achieved when three electrons per unit cell are doped, much larger than the 54 K Curie temperature in pristine Cr2Si2Te6. Besides, we showed that the out-of-plane MAE of −0.504 meV/Cr in pristine Cr2Si2Te6 is changed to the in-plane MAE of 0.315 meV/Cr after doping the charged particles, which means that the spin orientations rotate. These results demonstrate that intercalating charged particles into Cr2Si2Te6 can greatly improve its magnetic performance, providing potential application in spintronics and magnetic memory devices in the near future.

We acknowledge support from the NSFC under Grant Nos. 11474287, 11774350, and 11974354. Numerical calculations were performed at the Center for Computational Science of CASHIPS and the Hefei Advanced Computing Center.

The data that support the findings of this study are available within the article.

In this paper, the first-principles electronic structure calculations based on density functional theory (DFT) were performed using the Vienna Ab initio Simulation Package (VASP). At the beginning, we assume that the electron doping is realized by intercalated organic macromolecules; according to the assumption that intercalated cations only enlarge the vacuum distance, we adopt lattice constants a = b = 6.76 Å for pristine Cr2Si2Te6 and c = 50.66 Å, with a thickness of ∼3.469 Å for each Cr2Si2Te6 unit and an inter-slab separation of 13.417 Å, which is defined as the vacuum distance between Te atoms of the nearest-neighboring Cr2Si2Te6-slab. In addition, the planar coordinates of the atomic positions are the same as those of pristine Cr2Si2Te6.

Furthermore, we perform the GGA+U on the premise of the exchange–correlation interaction, which was treated with the generalized gradient approximation (GGA) parametrized by the Perdew–Burke–Ernzerhof formula, with U = 0.5 eV14 for pristine Cr2Si2Te6 according to the literature. After the intercalation, we adopt U = 0.2 eV for the calculation of intercalated Cr2Si2Te6 due to the screening effects among the conducting electrons in metal Cr2Si2Te6. For Brillouin zone integration, k-point meshes of 11 × 11 × 4 and 11 × 11 × 2 were used for pristine and intercalated Cr2Si2Te6, respectively. As for the calculation of MAE, we adopt a much denser k-mesh of 17 × 17 × 3 for pristine Cr2Si2Te6 and 17 × 17 × 1 for charged particle doped Cr2Si2Te6.

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