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. 2024 Mar 1;9(10):11478–11483. doi: 10.1021/acsomega.3c08360

Coexistence of Ferromagnetism and Ferroelectricity in Cu-Intercalated Bilayer CrI3

Zhonghua Qian †,*, Yanbiao Wang ‡,*, Jinlian Lu §, Ziyu Wang †, Xue Rui †, Tianying Zhu †, Baopei Hua †, Guanjie Gu †, Qiyuan Peng †, Nini Guo ∥,*
PMCID: PMC10938309  PMID: 38496958

Abstract

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Design of two-dimensional (2D) multiferroic materials with two or more ferroic orders in one structure is highly desired in view of the development of next-generation electronic devices. Unfortunately, experimental or theoretical discovery of 2D intrinsic multiferroic materials is rare. Using first-principles calculation methods, we report the realization of multiferroics that couple ferromagnetism and ferroelectricity by intercalating Cu atoms in bilayer CrI3, Cux@bi-CrI3 (x = 0.03, 0.06, and 0.25). Our results show that the intercalation of Cu atoms leads to the inversion symmetry breaking of bilayer CrI3 and produces intercalation density-dependent out-of-plane electric polarization, around 18.84–90.31 pC·cm–2. Moreover, the switch barriers of Cux@bi-CrI3 in both polarization states are small, ranging from 0.31 to 0.69 eV. Furthermore, the magnetoelectric coupling properties of Cux@bi-CrI3 can be modulated via varying the metal ion intercalation density, and half-metal to semiconductor transition can be occurred by decreasing the intercalation density of metal ions. Our work paves a practical path for 2D magnetoelectron coupling devices.

Introduction

Multiferroics are defined as materials that exhibit two or more primary ferroic features in a single phase, including ferromagnetic/antiferromagnetic (FM/AFM), ferroelectric/antiferroelectric (FE/AFE), ferroelastic orders, etc.1−11 Among them, magnetoelectric materials, displaying both long-range polarity and magnetic orders, are arguably the most studied multiferroic materials. A number of materials with intrinsic multiferroicity have been synthesized experimentally and predicted theoretically, such as Sr3Co2Fe24O41,9 BiTiO3,10 CuFe2O4,11 etc., which hold promising applications for multifunctional devices for low-power, high-density, nonvolatile data storage,9 sensors,12 energy harvesting,13 actuators,14 etc. Nevertheless, most multiferroic materials face significant technological challenges in terms of downsizing and integration, prompting scientists to seek out low-dimensional multiferroic materials with an atomic thickness.

Interestingly, two-dimensional (2D) multiferroic films have been proposed theoretically and experimentally, such as CuCrP2S6,3,4 γ-AlOOH,6 AgF2,7 FeHfSe3,8 etc. Unfortunately, the variety of 2D intrinsic multiferroic materials is rather rare. Currently, numerous strategies have been proposed to achieve multiferroic materials, including doping FE compounds with magnetic transition metal (TM) ions,15−17 composing FM monolayers and FE monolayers to form 2D multiferroic heterostructures,18−24 sliding two FM monolayers to achieve electric control of magnetism,25,26 etc. In addition, the intercalation method by intercalating atoms,27−35 ions,36,37 or molecules38 has been determined to be an effective method to modulate the physical properties of 2D layered materials.27−38 For example, Tu and Wu found that inserting 3d TM atoms in layered MoS2 and Bi2Se3 could induce switchable vertical polarization as well as electrically tunable magnetism.31 Wang et al.36 and Weber et al.37 found that the Curie temperatures of 2D Fe3GeTe2 and Cr2Ge2Te6 can be enhanced as high as ∼300 K by inserting Na and organic ions. Additionally, TaS2 sheets intercalated with Fe atoms using a chemical vapor transport method show tunable magnetic order, magnetic anisotropy, magnetoresistance, etc.33−35 Despite the process, theoretical and experimental efforts on the property manipulation of 2D layered materials by intercalation are still limited.

In this work, we designed 2D multiferroic Cux@bi-CrI3 (x = 0.03, 0.06, and 0.25) by inserting Cu ions into bilayer CrI3 by density functional theory (DFT) calculations. It is found that the intercalation of Cu ions breaks the inversion symmetry of bilayer CrI3, producing a switchable out-of-plane electron polarization, around 18.84–90.31 pC·cm–2. In particular, by decreasing the intercalation density of metal ions, the electronic properties can be changed, resulting in a half-metal to semiconductor transition.

Computational Methods

The DFT-based calculations were carried out by Vienna Ab initio Simulation Package (VASP)39,40 for searching the lowest-energy doping site and electronic properties of the CrI3 bilayer. The exchange-correlation energy was described by the Perdew–Burke–Ernzerhof functional41 for the generalized gradient approximation. And the DFT-D2 functional is used to account for the van der Waals interaction.42 For describing the strong electron correlation between the 3d electrons, the Hubbard U correction of Ueff was considered to describe the on-site repulsion interaction of the Cr and Cu ions. We tested the influence of Ueff values on the electronic properties of Cu0.25@AA-CrI3; as shown in Table S1 and Figure S1 in the Supporting Information, Ueff = 2.0 eV and Ueff = 3.0 eV obtain the same results. Therefore, we use Ueff = 2.0 eV for all of the calculations. The energy cutoff was set as 450 eV. The energy and force convergence thresholds for the iteration in the self-consistent field were set to 10–6 eV and 0.01 eV/Å, respectively. The vacuum layer of about 20 Å along the c-direction was adopted to avoid the interaction between two periodic units. The polarization is obtained by the modern theory of polarization based on the Berry-phase approach with dipole correction,43,44 and the climbing image nudged elastic band (cNEB)45,46 method was employed to calculate the minimum energy path and diffusion energy barriers.

Results and Discussion

As shown in Figure 1a, two types of stacking configurations for bilayer CrI3 were considered, namely, AA-stacking and AB-stacking configurations, respectively. Both configurations are found to have nearly degenerated energies with identical lattice parameters (a = 6.88 Å) and interlayer distances (d = 3.41 Å). As both AA and AB stackings may be prepared in the experiments,47 in this work, we will investigate the effect of metal insertion on the electronic properties in both AA- and AB-stacking structures. Our results show that bilayer CrI3 in both stacking configurations favors FM order, which is about 0.34 and 4.43 meV/Cr atom lower than the AFM state for AA- and AB-stacking configurations, respectively (see Table S2 in the Supporting Information). These results are consistent with previous experimental observations.30,47

Figure 1.

Figure 1

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Top and side views of the geometrical structures of AA- and AB-stacked bilayer CrI3 without (a) and with (b) Cu intercalation.

Next, we explored the possibility of intercalating Cu atoms into AA- and AB-stacking bilayer CrI3. First, the unit cell of bilayer CrI3 is selected to intercalate Cu atoms. Here, the intercalated structure is described as Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3 for AA and AB stackings, respectively, wherein the ratio of intercalated Cu atom to Cr atom is 1:4. We considered four types of intercalation configurations, denoted as Inter-I, Inter-II, Inter-III, and Inter-IV, as shown in Figure S2: (i) Inter-I—the Cu atom sits in the middle of two CrI3 layers and facing the Cr atoms in the upper and lower layers. (ii) Inter-II—the Cu atom sits on the middle of three I atoms of one CrI3 layer and facing a I atom of another CrI3 layer. (iii) Inter-III—the Cu atom is located in the middle of the two sublayers, facing the hollow site of one CrI3 layer and a Cr atom of another CrI3 layer (directly facing the hollow site in the upper and lower layers in AA stacking). (iv) Inter-IV—the Cu atom sits in the hollow site of one CrI3 layer and facing the Cr atom of another CrI3 layer (facing another hollow site of another CrI3 layer in AA stacking). Upon optimization, the Inter-VI configuration is the most favored structure for both Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3 (see Table S3). As shown in Figure 1b, the intercalated Cu atom locates in the hollow site of one CrI3 sublayer and sits in the same plane of the I atom layer. The distance of the Cu atom to the opposite CrI3 layer is about 4.10 and 4.01 Å in Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3, respectively. To characterize the energetic stability of intercalated systems, we define the intercalation energy as

graphic file with name ao3c08360_m001.jpg 1

Here, Etot and Ebilayer are the energies of the CrI3 bilayer with and without intercalation, respectively, and ECu is the energy of the single Cu atom. The calculated Eint values are −3.15 and −3.10 eV for Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3, respectively (see Table 1), which are comparable with the Fe/Co intercalated BN/MoS2 system.48 The negative energies indicate that the intercalation of Cu atoms in the CrI3 bilayer is an exothermic reaction. As shown in Figure 1b, the inversion symmetry of Cu-intercalated bilayer CrI3 in both stackings is broken, which leads to positive and negative charge centers’ spatial separation along the out-of-plane direction and results in the unequal electrostatic potential of the top and bottom CrI3 layer. Moreover, as the Cu atom is located in the hollow site of the top CrI3 sublayer (see Figure 1b), it results in a positive charge center moving up and forms an upward FE polarization (P↑) with the values of 20.01 and 90.31 pC·cm–2 in AA- and AB-stacking configurations, respectively (see Table 1).

Table 1. Intercalation Energy Eint (eV), Band Gap Eg (eV), Switch Barrier Δ (eV), and Vertical Polarization Pz (pC·cm–2) of Cu-Intercalated Bilayer CrI3.

  Eint [eV] Eg [eV] Δ [eV] Pz [pC·cm–2]
Cu0.25@AA-CrI3 –3.15 half metal 0.43 90.31
Cu0.06@AA-CrI3 –3.17 half metal 0.49 80.45
Cu0.03@AA-CrI3 –3.20 0.81 0.49 47.57
Cu0.25@AB-CrI3 –3.10 half metal 0.64 20.01
Cu0.06@AB-CrI3 –3.16 half metal 0.31 30.21
Cu0.03@AB-CrI3 –3.13 0.86 0.69 18.84
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Furthermore, to demonstrate the feasibility of the reversal of the FE polarization direction in Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3, the energy barrier using the cNEB method is explored (shown in Figure 2a,b). In the AA-stacking configuration, the Cu ion in the hollow site of the upper CrI3 sublayer forms an upward FE polarization (P↑). When adding an external electric field, the Cu ion moves straightly downward to form an Inter-III configuration without FE polarization. If the external electric field continues to increase, the Cu ion continues to move downward, ending up in the hollow site of the lower CrI3 sublayer and forming a downward FE polarization (P↓). In the AB-stacking one (see Figure 2b), the external electric field turns the Inter-IV configuration to the Inter-II configuration, then the Inter-I configuration forms the paraelectric phase (PE), and finally the Cu ion moves downward to the hollow site of the lower CrI3 sublayer and the direction of FE polarization has been switched. The switching barriers are 0.43 and 0.59 eV per Cu ion for AA- and AB-stacking configurations, respectively, which are comparable to that of reported Cr0.5MoS2.31

Figure 2.

Figure 2

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FE switching pathway of (a) Cu0.25@AA-CrI3 and (b) Cu0.25@AB-CrI3. The values denote the transition barrier between the two switchable FE states.

Figure 3 shows the band structure and projected density of states (PDOS) of Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3 in two FE states. In the case of AA-stacking configuration, Cu0.25@AA-CrI3 is FM half metal in both polarized states, in which the spin-up electrons are conducting while spin-down electrons are insulating with a band gap of 2.48 eV both in the P↑ and P↓ states. The energy differences between FM and interlayer AFM states are listed in Table S4. Similar electronic properties are found for those in the AB-stacking configuration, in which the band gap of the spin-down channel is 2.38 eV in both P↑ and P↓ states. The electronic properties of Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3 are different with that of bilayer CrI3 reported by Liu’s group,49 in which the AA-stacking and AB-stacking bilayer CrI3 are insulating with the gaps of 0.81 and 0.82 eV, respectively. The insets in Figure 3a–d are spin density plots. Clearly, the magnetic moments of the systems are mainly contributed by the Cr atom; in contrast, the intercalated Cu ions are nonmagnetic. As shown in charge density difference plots in Figure S3, the Cu ions are electron donors, which contribute charges to the I atoms from the nearest CrI3 sublayer, leading to the increase of the local magnetic moment of the Cr ions from 3.29 to 3.55 μB in AA- and AB-stacking configurations. For the CrI3 sublayer far away from the Cu ions, the local magnetic moments of Cr ions remain almost unchanged with that in bilayer CrI3, ∼3.29 μB for AA- and AB-stacking configurations.

Figure 3.

Figure 3

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Band structure (left) and PDOS (right) of (a) Cu0.25@AA-CrI3 (P↑), (b) Cu0.25@AA-CrI3 (P↓), (c) Cu0.25@AB-CrI3 (P↑), and (d) Cu0.25@AB-CrI3 (P↓), respectively. The insets are the spin density distributions.

Compared with free-standing bilayer CrI3, the Fermi levels of Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3 in both polarization states are upshifted and cross the Fermi level of the spin-up channel due to the electron transfer occurring from the Cu atom to CrI3 sublayer. As seen from the PDOS plots, the valence band maximum (VBM) of Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3 is mainly contributed by the CrI3 sublayer close to the intercalated Cu ion, and the conduction band minimum (CBM) is mainly from the CrI3 sublayer far away from the Cu atom. Therefore, the built-in electric field induced by the intercalated Cu atom can tune the electronic and magnetic properties of the CrI3 bilayer.

Furthermore, the influence of the intercalation density of Cu ions on the electronic and magnetic properties of bilayer CrI3 is explored. Here, we constructed the intercalated structures with larger supercells of bilayer CrI3 (2 × 2 and 3 × 3 supercells), which have a lower metal intercalation density, namely, Cu0.06@AA(AB)-CrI3 and Cu0.03@AA(AB)-CrI. Compared with Cu0.25@AA(AB)-CrI3, the distance of the Cu atom to the opposite CrI3 layer is smaller, around 0.15/0.20 and 0.18/0.19 Å in Cu0.06@AA/AB-CrI3 and Cu0.03@AA/AB-CrI3, respectively. Moreover, the polarization of the intercalated systems weakens with the decrease of the intercalation density (see Table 1). The polarization values of Cu0.06@AA-CrI6 and Cu0.06@AB-CrI3 are 80.45 and 47.57 pC·cm–2, respectively, while those of Cu0.03@AA-CrI3 and Cu0.03@AB-CrI3 are reduced to 30.21 and 18.84 pC·cm–2, respectively. The switching barriers are 0.49/0.44 and 0.49/0.47 eV for Cu0.06@AA/AB-CrI3 and Cu0.03@AA/AB-CrI3, respectively (see Figures S4 and S5). Moreover, the Fermi level decreases with the decrease in intercalated Cu metal concentration. As shown in the PDOS plots in Figure 4, Cu0.06@AA/AB-CrI3 are half metals with small electronic states crossing the Fermi level, which are turned to be semiconductors for Cu0.03@AA/AB-CrI3 in both stackings, and the band gaps are 0.55 and 0.66 eV for Cu0.06@AA/AB-CrI3, respectively. Our results show that the electronic property of Cux@AA/AB-CrI3 can be effectively modulated by changing the intercalated Cu atom concentration.

Figure 4.

Figure 4

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PDOS of (a) Cu0.06@AA-CrI3, (b) Cu0.03@AA-CrI3, (c) Cu0.06@AB-CrI3, and (d) Cu0.03@AB-CrI3 in switchable polarization states, respectively.

Conclusions

By using first-principles calculations, we systematically studied the magnetoelectronic coupling properties of Cu-intercalated bilayer CrI3 in both AA and AB stackings, Cux@AA/AB-CrI3 (x = 0.03, 0.06, and 0.25). Our results show that the intercalant can induce a spontaneous inversion symmetry breaking, and the polarization values of Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3 are 20.01 and 90.31 pC·cm–2, respectively. The switching barriers are 0.43 and 0.59 eV per Cu ion in Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3, respectively. Moreover, the electronic properties of Cux@AA/AB-CrI3 are found to be sensitive to the metal ion density. By decreasing the intercalation concentration, a half-metal to semiconductor transition occurs. Our study provides a promising way to design 2D multiferroics.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (41975062).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c08360.

  • Influence of Ueff values on the electronic properties of Cu0.25@AA-CrI3; band structures of (a) Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3; energy difference of bilayer CrI3 in AA and AB stackings; four configurations with different interpolation positions in two-stacking bilayer CrI3; energy difference of four configurations with different interpolation positions in two-stacking bilayer CrI3; energy difference between FM and interlayer AFM states of Cux@AA-CrI3; top and side views of the charge density difference plots of Cu0.25@AA-CrI3 and Cu0.25@AB-CrI3; FE switching pathway of Cu0.06@AA-CrI3 and Cu0.06@AB-CrI3; FE switching pathway of Cu0.03@AA-CrI3 and Cu0.03@AB-CrI3 (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c08360_si_001.pdf (1.1MB, pdf)

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