Discovery of two-dimensional (2D) magnetism in van der Waals (vdW) materials have unfurled diverse range of possibilities for development of novel spintronics, multiferroic and quantum computing devices using atomically thin materials [1]. Among the vast pool of vdW materials, CrGeTe₃ and CrX₃ (X = Cl, Br, I) are Mott insulators with a charge gap, thus facilitating suitable platforms to exploit both charge and spin degrees of freedom. At low temperature, these insulators share a common layered rhombohedral R3 -crystal structure held together by weak vdW forces along the c-axis. Each single layer consists of a honeycomb network of edge sharing octahedra formed by a central Cr atom bonded to six ligand atoms (Te or X), as illustrated in Fig. 1(a) for CrGeTe₃.

Fig. 1. (a) Single layer of CrGeTe₃ illustrating the honeycomb network of edge sharing CrTe₆ octahedra. In case of CrX₃, the place of Ge-Ge dimers remains vacant. (b) Crystalline electric field splitting of Cr 3d orbitals into t_2g - and e_g -manifolds. ∆ is the energy difference between t_2g- and e_g levels. J^Te_His the Hund’s coupling energy at Te site. (c) Schematic picture of indirect superexchange FM interaction between Cr e_g-orbitals via two Te p-orbitals. t_pd(t_p'd')is the virtual hopping between e_g(e'_g) and p(p') orbitals. θ is the geometrical Cr-Te-Cr bond angle.

The crystalline field effect ensuing from this octahedra splits the Cr-3d orbitals into t_2g - and e_g-manifolds [Fig.1(b)]. The onsite Coulomb repulsion localizes the t_2g-electrons driving the system into an insulating state significantly well above T_C. Although, a direct antiferromagnetic exchange interaction exists between t_2g-electrons, thermal fluctuation inherent to 2D suppress the long-range magnetic order. The spin orbit coupling (SOC) emanating through the covalent bond between ligand p and Cr-e_g orbital generates the magnetocrystalline anisotropy energy to counteract the thermal fluctuation. Below TC, the superexchange interaction between Cr-e_g electrons via two different ligand p-orbitals, as schematically portrayed in Fig.1(c), benefits from the distortion of CrTe₆ octahedra and the Hund’s energy gain at the ligand Te (or X) site to stabilize the FM order. The correlation between t_2g-electrons also move up the ligand p bands close to Fermi level, thus opening a band gap between the Cr d conduction band and the ligand p valence band indicating a charge transfer type Mott behaviour.

However, one drawback for the practical technological application is the low Curie temperature, T_C, mostly below liquid nitrogen temperature. For exploitation of these attributes, it is essential to realize this novel FM state at or close to room temperature. Recently, T_C of CrGeTe₃ has been raised around 200 K either by intercalating large organic ions into single crystals [2] or using sophisticated process like electrochemical doping to field effect transistor devices [3]. Although, these results are promising, such filling control methods lead to strong charge inhomogeneity as well as structural and chemical defects, including creation of magnetic impurity, unfavourable for device application.

On the other hand, application of pressure is advantageous, which not only controls the bandwidth but also the spin exchange pathways via subtle modification of bond length and angle between atoms avoiding the complication of disorder. Here, for the first-time we show the evidence of nearly room temperature FM in CrGeTe₃ bulk single crystals with application of pressure [4]. Using highly sensitive dc magnetic susceptibility and resistivity measurement under high quality hydrostatic pressure, we found that T_C of CrGeTe₃ exceeds 250 K above 9.1 GPa. In Fig. 2, we present the pressure-temperature phase diagram of CrGeTe₃ which uncovers a rare example of bandwidth-controlled insulator-metal transition (IMT) in a vdW materials. Moreover, the remarkable absence of accompanied structural transition and spin crossover involving magnetic to non-magnetic states across IMT, clearly indicates the pressure driven changes of electronic property are purely electronic in origin. Furthermore, these findings show that electronic property of CrGeTe₃ can be switched much more responsively using external perturbation, such as strain, doping compared to other vdW insulators.

Fig. 2. Pressure temperature phase diagram of CrGeTe₃. Color scale represents the magnitude of ρ_ab. θ_CW and T^χ_C respectively are the Curie-Weiss temperature and Curie temperature, estimated from magnetic susceptibility. T^ρ_C and TFL are Curie temperature and Fermi liquid temperature determined from ρ_ab.

References

• [1] C. Gong et al., Nature, 546, 265 (2017).
• [2] N. Wang et al., J. Am. Chem. Soc, 141, 17166 (2019).
• [3] I. A. Verzhbitskiy et al., Nat. Electron., 3, 460 (2020).
• [4] Dilip Bhoi, J. Gouchi, N. Hiraoka, Y. Zhang, N. Ogita, T. Hasegawa, K. Kitagawa, H. Takagi, K. H. Kim, and Y. Uwatoko, Phys. Rev. Lett. 127, 217203 (2021)

Authors

• D. Bhoi, J. Gouchi, N. Hiraoka^a, Y. Zhang^b, N. Ogita^c, T. Hasegawa^c, K. Kitagawa^a, H. Takagi^a,d,e, K. H. Kim^f,g, and Y. Uwatoko
• ^aThe University of Tokyo
• ^bSoutheast University
• ^cHiroshima University
• ^dInstitute for Functional Matter and Quantum Technologies, University of Stuttgart
• ^eMax Planck Institute for Solid State Research
• ^fCeNSCMR, Seoul National University
• ^gInstitute of Applied Physics, Seoul National University