Spin-Seebeck Signatures of Spin Chirality in Kagome Antiferromagnets

TL;DR

This work demonstrates that the ground-state vector spin chirality ${oldsymbol{K}}_ ext{v}$ in kagome non-collinear antiferromagnets can be probed by the spin Seebeck effect at a kagome AFM/NM interface. By deriving bosonic Bogoliubov-de Gennes magnon Hamiltonians for the two opposite chiral states and applying linear response theory, the authors show that the $(-)$-chiral state hosts a near-gapless magnon branch, leading to a finite in-plane SSE component $I_{s,y}^-$ and a larger out-of-plane component $I_{s,z}^-$ compared to the $(+)$ state. These differences manifest as distinct inverse spin Hall signals, with predicted voltages in a Pt/NM layer and an estimated 81% disparity in AISHE signals between chiralities, enabling real-time chirality switching detection. The results establish SSE as a powerful tool to access chirality-driven magnon transport in frustrated 2D magnets and point to practical experimental routes for chirality readout. $D$, $K$, and the threshold $D/K=1/(4\\sqrt{3})$ govern the chirality and magnon-band topology, while interfacial exchange and spin-diffusion parameters control the magnitude of the pumped spin currents.

Abstract

Non-collinear antiferromagnets (NCAFMs) are appealing for antiferromagnetic spintronics, as they combine the advantages of collinear antiferromagnets with novel emergent phenomena stemming from their complex spin structures. These phenomena are often associated with the intrinsic spin chirality, which characterizes the handedness of the ground-state spin configuration. Non-collinear antiferromagnets (NCAFMs) are appealing for antiferromagnetic spintronics, as they combine the advantages of collinear antiferromagnets with novel emergent phenomena stemming from their complex spin structures. These phenomena are often associated with the intrinsic spin chirality, which characterizes the handedness of the ground-state spin configuration. Here, we investigate a kagome NCAFM interfaced with a normal metal and demonstrate that the ground-state vector spin chirality can be probed through measurements of the spin Seebeck effect (SSE). Starting from a microscopic spin Hamiltonian, we derive the corresponding bosonic Bogoliubov-de Gennes Hamiltonians for the two chiral configurations. Using linear response theory, we obtain a general expression for the spin current thermally pumped into the normal metal by the SSE. We show that a sizable in-plane spin current emerges exclusively in the negative-chiral state, providing a direct signature for real-time detection of chirality switching in kagome NCAFMs. In addition, we find a field-dependent out-of-plane spin current whose magnitude differs between the two chiralities by about 81%, reflecting their distinct magnon band structures.

Figures (3)

• Figure 1: (color online). A schematic representation of a kagome NCAFM monolayer in a ground state with a. positive and b. negative chirality. The easy axes are denoted with dashed black arrows, the spin directions with red arrows, and the boundaries of the unit cell are outlined with dashed grey lines, respectively.
• Figure 2: (color online). Magnon energy bands along high-symmetry lines in the first Brillouin zone for a. the $(+)$-chiral ground state and b. the $(-)$-chiral ground state. The inset diagrams highlight the band structures near the $\Gamma$ point along the $x$-axis. We have used the parameter values $\sqrt{2}a=3.785$ Å, spin $S=1$, $\gamma=1.76\cdot 10^{11}$ 1/Ts, $J=39.94~\mathrm{meV}/\hbar^2$, $K=5~\mathrm{meV}/\hbar^2$, $K_{\bot}=0$, $D = K/4\sqrt{3}$, and $B = 5$ T.
• Figure 3: (color online). a. Schematic of a 2D kagome AFM/3D NM heterostructure (NM in cyan). The SSE-driven spin current (blue arrow) yields an ISHE-induced electric field (green arrow) in the NM. b. The $z$-component of the spin current in the positive (red) and negative (blue) chiral states, normalized by the constant $y$-component of the spin current in the negative chiral state (green), as a function of the applied out-of-plane magnetic field $B$. In both cases, $I_{\mathrm{s},x}=0$. Results are shown for $T_\mathrm{AF}=300$ K, $T_\mathrm{N}=299$ K, $\tau_s=50$ fs, $\lambda=5.21$ nm, and the AFM parameters as specified in Sec. \ref{['Sec2']}.