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Magnetic Modulation and Thickness Dependence of Second Harmonic Generation in Two-Dimensional Multiferroic CuCrP2S6

Ideue Group

Two-dimensional van der Waals multiferroics are emerging material platforms for realizing unique magnetoelectric/ magneto-optical properties and also important building blocks for functional van der Waals devices. Although they provide a new opportunity of controlling multiferroic properties in the atomic layer limit, research on few-layer multiferroic crystals is limited and the effect of thickness dependent symmetries on multiferroic properties are less explored.

In this work, we studied the symmetries and magnetoelectric responses in exfoliated samples of novel van der Waals multiferroic CuCrP2S6 by optical second harmonic generation (SHG).

CuCrP2S6 is a layered material composed of the honeycomb lattice of distorted CrS6 octahedra and CuS3 triangles and pairs of P ions inside the honeycomb, and shows the structural and magnetic phase transition at TC = 190 K and TN = 32 K, respectively. At T > TC, it shows centrosymmetric crystal structure with C2/c space group (Fig. 1 (c)), in which Cu ions can move and ion conductor behaviors have been reported. With decreasing the temperature, movement of Cu ions start to freeze at T = 190 K and is completely settle down at T = 145 K, which cause the structural transition to the non-centrosymmetric phase with Pc space group (Fig. 1 (b)). Below T < TN, A-type antiferromagnetic order develops, in which spins of Cr3+ align along the a-axis (Fig. 1 (a)). Because both the spatial inversion symmetry and time-reversal symmetry are broken, CuCrP2S6 shows the magnetoelectric effect in this low-temperature phase.

Figures 1 (d)-(e) show the polarization-resolved SHG patterns at T = 5 K (Fig. 1 (d)), T = 100 K (Fig. 1 (e)), and T = 300 K (Fig. 1 (f)), respectively. At T = 300 K, small SHG signal has been observed (Fig. 1 (f)), which can be attributed to the surface or electric quadrupole terms. In the intermediate temperature region, SHG intensity along the a-axis is enhanced (Fig. 1 (e)). Since inversion symmetry is broken in this phase, electric dipole term of SHG is allowed and this additional component is generated in the SHG. In the antiferromagnetic phase, SHG intensity along the b-axis is also developed (Fig. 1 (d)), which can be explained by the magnetic dipole term. In Fig. 1 (g), we plot the temperature dependence of the SHG intensity along the polarization angle θ = 10˚ and 90˚. Here, θ = 0˚ is defined as a-axis. This temperature variation clearly indicates that SHG intensity along the a-axis (b-axis) originating from electric dipole (magnetic dipole) term reflect the structural (magnetic) phase transition in CuCrP2S6.

In order to understand the effect of magnetoelectric properties on SHG, we measured SHG under the magnetic field. Figure 2 (a) (red) shows the SHG pattern under the magnetic field (B = 1.2 T) at T = 5 K. The magnetic field is applied to the b-axis. SHG pattern changes dramatically, showing the large enhancement along the a-axis. In CuCrP2S6, the application of the magnetic field along the b-axis will generate the electric polarization along the a-axis due to the magnetoelectric effect. This electric polarization induced by the magnetoelectric effect is considered to generate a large SHG. Furthermore, the thickness dependence of the SHG induced by the magnetoelectric effect was investigated. Figure 2 (b) shows the SHG patterns under magnetic field (B = 1.2 T) along the b-axis for samples of various thicknesses. For thicker samples, the SHG pattern is mirror symmetric with respect to the a-axis. This is because that bulk sample has the glide symmetry with respect to the ac-plane. However, as the thickness is reduced, this glide symmetry is lost, resulting in modulation of the SHG pattern. These results indicate that symmetry change by thinning can modulate the magnetoelectric effect in two-dimensional van der Waals multiferroics.

In summary, we have observed magnetic modulation and its thickness dependence of SHG in CuCrP2S6. This work clarifies the unique magnetoelectric properties in CuCrP2S6 and also provide a new design principle of two dimensional multiferroics.


References
  • [1] S. Aoki, Y. Dong, Z. Wang, X. S. W. Huang, Y. M. Itahashi, N. Ogawa, T. Ideue, and Y. Iwasa, Advanced Materials 36, 2312781 (2024).
Authors
  • S. Aokia, Y. Donga, Z. Wanga, X. S. W. Huanga, Y. M. Itahashia, N. Ogawaa, T. Ideuea, and Y. Iwasaa,b
  • aThe University of Tokyo
  • bRIKEN Center of Emergent Matter Science (CEMS)