Tunable Chern Insulator States with Coexisting Magnonic and Electronic Topology in 2D Honeycomb Kitaev Ferromagnets

TL;DR

The paper demonstrates the coexistence and simultaneous tunability of magnonic and electronic Chern insulator phases in Kitaev magnets, using MnBr3 monolayer as a prototype. A ferromagnetic honeycomb Heisenberg–Kitaev model yields a magnonic Chern insulator with a tunable sign via in-plane spin rotation, while MnBr3 also hosts an electronic Chern insulator with a quantum anomalous Hall effect that responds to the same spin orientation. The findings reveal a unified mechanism linking magnonic and electronic topology through spin reorientation, offering a platform for robust dual-quasiparticle spintronic devices and deepening the understanding of topological phenomena in 2D Kitaev magnets.

Abstract

The coexistence of topological magnons and electrons in magnetic materials presents a compelling route toward developing low-dissipation, multifunctional spintronic devices. However, material systems enabling their simultaneous realization and control remain largely unexplored. Here, we propose the coexistence and concurrent tunability of magnonic and electronic Chern insulator phases in Kitaev magnets and use MnBr$_{3}$ monolayer as a prototype. We find the significant Kitaev interaction in MnBr$_{3}$ induces the magnonic Chern insulator phase, manifesting as the magnon thermal Hall effect. Concurrently, MnBr$_{3}$ exhibits the quantum anomalous Hall effect driven by its electronic Chern insulator phase. Crucially, we demonstrate that these dual topological phases can be simultaneously controlled by reorienting the in-plane spins with an external magnetic field. Our findings not only deepen the fundamental understanding of spin excitations in Kitaev magnets but also provide a promising platform for exploring the interplay between electronic and magnonic topology.

Figures (4)

• Figure 1: (a) Schematic of coexistence and tunability of magnonic and electronic Chern insulator phases on a honeycomb lattice. The magnon wavepacket and its corresponding chiral edge state are depicted in yellow, while the electronic counterparts are shown in green. All spins are aligned in the $xy$-plane at an angle $\phi$ relative to the $x$-axis. Top-left inset plots the magnonic (electronic) Chern number as a function of $\phi$. (b) Illustration of the Heisenberg-Kitaev model on a honeycomb lattice. The Kitaev axes ($X,Y,Z$) are indicated by red, green and blue arrows, which are orthogonal to each other and form a polar angle of $\arccos(1/\sqrt{3})$ with the global $z$-axis. The model includes a bond-dependent Kitaev interaction $K$, where the coupled spin components depend on the bond direction, and a bond-independent Heisenberg interaction $J$. (c-e) Magnonic topological properties of the ferromagnetic Heisenberg-Kitaev model with spins aligned along +$x$. (c) Magnon band structures with ($J=-\frac{2}{3}~\mathrm{meV},K=-1~\mathrm{meV}$) and without ($J=-1~\mathrm{meV},K=0~\mathrm{meV}$) Kitaev interactions, labeled by $J+K$ and $J$, respectively. The inset shows the first Brillouin zone, with high-symmetry points labeled. (d) Evolution of the Wannier center for the lower magnon band as $k_y$ transverses the Brillouin zone for $K=-1~\mathrm{meV}$. The total shift of one indicates a Chern number of $C_\mathrm{mag}=-1$. (e) Magnon chiral edge states for $K=-1~\mathrm{meV}$.
• Figure 2: Atomic structure, electronic topology, and magnetic properties of MnBr3 monolayer. (a) Top and side views of the crystal structure. The unit cell is delineated by a rhombus, with Mn and Br atoms depicted as purple and brown spheres, respectively. Dashed green and red lines indicate the first- and second-nearest Mn^3+ neighbors. (b) Orbital-projected electronic band structure and (c) $k$-resolved berry curvature when spins are aligned with the $+x$ direction ($\phi=0^\circ$). The dashed lines indicate the boundary of the first Brillouin zone. (d) Electronic edge states (left panel) and anomalous Hall conductivity $\sigma_\mathrm{A}^{xy}$ (right panel). Both calculations are for $\phi=0^\circ$. (e) Spin-orientation-dependent electronic Chern number. (f) Monte Carlo simulations of magnetization (blue curve) and heat capacity (red curve) as a function of temperature.
• Figure 3: Magnonic topological properties of MnBr3 monolayer with spins aligned along the +$x$ direction. (a) Magnon band structure, with a bulk gap of 0.49 meV. The inset shows a close-up view of the gap near K$'$ point. (b) $k$-resolved Berry curvature. (c) Magnon Wannier center. (d) Magnonic edge states.
• Figure 4: Spin orientation dependence of magnonic topological properties in MnBr3. (a) Schematic of the magnon thermal Hall effect. $\Delta T$ denotes the temperature difference between the two ends. Spins are represented by purple arrows, forming an angle $\phi$ with respect to the $x$-axis. The magnon wavepacket is shaded in yellow, with a red arrow indicating its deflection. (b) Temperature dependence of $\kappa_\mathrm{TH}^{xy}$ for selected values of $\phi$. (c) Polar plot of $\kappa_\mathrm{TH}^{xy}$ as a function of temperature (radial coordinate) and spin orientation (angular coordinate), where the color scale indicates the magnitude and sign of $\kappa_\mathrm{TH}^{xy}$. Black dashed lines denote topological phase transitions. White numbers within each region are the magnonic Chern numbers. (d) Magnon band gaps near the K and K$^\prime$ points as a function of spin orientation $\phi$.