February 23, 2024, phys rev. lett.**Published the research of Professor Ren Wei's research group at Shanghai University**, entitled "Spin-Chirality-Driven Multiferroicity in Van der Waals Monolayers".
Multiferroic materials, i.e., materials that show both ferropolarization and magnetic ordering, allow, on the one hand, the study of fundamentally novel fundamental physical phenomena, and on the other hand, their application to promising functional devices. As a result, this type of material has attracted more and more attention in recent decades. In particular, type II multiferroic systems exhibit ferroelectricity driven by complex non-centrically symmetric spin configurations and arise from various microscopic mechanisms. It is worth noting that the control of ferropolarization can be achieved when applying a magnetic field that modulates the magnetic sequence. Type II multiferroic materials exhibit strong magnetoelectric coupling in nature, which has been extensively studied. The vast majority of multiferroic compounds are oxides, which limits the understanding of the role of the chemical properties of anions in the final polyferroic and magnetoelectric properties.
Achieving magnetoelectric coupling of two-dimensional layered compounds is a long-term goal. Recently reported bulk phase mNi2, CUBR2, and fewlayer Nii2 all have helical magnetic sequences and exhibit ferropolarization. On the other hand, the 120-degree magnetic structure and related magnetoelectric coupling in transition metal halides have not been fully studied, and the possible microscopic mechanisms of driving ferroelectric polarization remain to be studied.
In this study, driven by the expected contribution of two-dimensional multiferroic systems with strong magnetoelectric coupling to the development of multifunctional nanodevices, the authors propose VX2 (x = ci, br, i) monolayers as a new class of spin-chiral-driven van der Waals multiferroic materials with spin helix planes located on (001) and (100) crystal planes. The 120-degree magnetic structure of the frustration in the triangular lattice induces ferroelectric polarization perpendicular to the plane of the spin helix, the sign of which is changed by a change in spin chirality. It can be seen that under an applied electric field perpendicular to a single layer, one magnetic chirality can be stabilized over the other, allowing for long-sought electrical control of spin textures.
In addition, it was found that polarization does not simply depend on the strength of the atom's spin-orbit coupling. This unexpected feature is due in part to the strong spin-lattice interaction in transition metal halides, which allows even weakly spin-orbit coupled systems to exhibit considerable spin-driven polarization. In addition, the spin configuration of the volume counterpart with interlayer antiferromagnetic interactions shows the same spin chirality, so it is also able to exhibit spin-induced polarization. In fact, such compounds exhibit considerable distortion of spin-driven structures, which facilitates the study of multifunctional spin-electrical-lattice coupling.
Fig. 1 (a) Structural side and top views of the transition metal halide; (b-c) Spin arrangement of a 120-degree structure with (001) and (100) helical planes.
Fig. 2 (a) Three single-layer double potential wells; (b) Magnetic (100)-AFM configurations of ferroelectric bistability in clockwise and counterclockwise helical sequences, exhibiting opposite spin chiral arrangement.
Fig. 3 Effect of lattice size on magnetoelectric coupling, including dipoles induced by the (001)- and (100)-afm configurations.
[**Link].liu, c., ren, w. &picozzi, s. spin-chirality-driven multiferroicity in van der waals monolayers. phys. rev. lett., 2024, 132, 086802.
[Other relevant literature].[1] kurumaji, t., seki, s., ishiwata, s. et al. magnetic-field induced competition of two multiferroic orders in a triangular-lattice helimagnet mni2. phys. rev. lett., 2011, 106, 167206.
2] song, q., occhialini, c.a., ergeçen, e. et al. evidence for a single-layer van der waals multiferroic. nature, 2022, 602, 601–605.
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