Magnetization Plateaus in the Spin-Orbit Coupled Bilayer Triangular Lattice Antiferromagnet Rb2Co2(SeO3)3

TL;DR

This work investigates magnetization plateaus in the spin-orbit–coupled bilayer triangular-lattice antiferromagnet Rb2Co2(SeO3)3, revealing a cascade of plateaus at $M/M_s = 1/3$, 1/2, 2/3, and 5/6 with a weak $1/6$ feature and no zero-field gap. By combining ultralow-temperature measurements up to 60 T with a two-stage theory, the authors show that a bond-operator treatment of interlayer dimers within a bilayer XXZ framework must include spin-orbit–driven, bond-dependent anisotropy to generate finite plateau slopes; near saturation, a projected triangular pseudospin model with modest further-neighbor interactions reproduces the high-field plateaus. The findings indicate that anisotropic exchange arising from spin-orbit coupling is essential to stabilize the full plateau hierarchy, a mechanism absent in purely $U(1)$-symmetric interpretations. The work thus provides a unified picture linking interlayer dimerization, triplon condensation, and pseudospin dynamics, with implications for Co-based triangular bilayers and frustrated quantum magnets in general.

Abstract

Geometric frustration among competing spin exchanges can give rise to novel quantum phases by enhancing fluctuations that drive magnetic systems beyond the classical regime. We investigate the frustrated array of strongly correlated spin dimers in the bilayer triangular lattice antiferromagnet \rcs{} under applied magnetic fields. A cascade of magnetization plateaus appears at \(M/M_s = 1/3, 1/2, 2/3,\) and \(5/6\), together with a weak anomalous feature near \(M/M_s = 1/6\), in fields up to 60 T. Concurrent changes in magneto-dielectric response follows the plateau boundaries. The finite slope of each plateau and the absence of a zero-field gap in our ultralow-temperature ac susceptibility down to 20 mK indicate broken \(U(1)\) spin-rotation symmetry. A minimal bilayer-dimer model treated with bond operator representation reproduces the low-field sequence only when \(U(1)\) symmetry is explicitly lifted by spin-orbit-driven, bond-dependent anisotropy. Near saturation, a projected triangular pseudospin model accounts for the high-field plateaus with modest further-neighbor interactions. These results demonstrate that anisotropic exchange arising from spin-orbit-coupled moments is essential for stabilizing the full plateau hierarchy in \rcs{}, a mechanism overlooked in previous interpretations of Co-based triangular bilayers.

Figures (7)

• Figure 1: Temperature dependent dc susceptibility $\chi$ at various magnetic fields along $c$-axis.
• Figure 2: (a) Magnetization measured in pulsed magnetic field at various temperatures. All the curves begin at zero, but vertical offsets were applied for clarity. The Van Vleck shown in the figure was subtracted from the data. (b) The derivatives of magnetization with respect to the magnetic field (d$M$/d$H$). The red arrow indicates the small slope changes around 1/6 plateau consistent with the inset of (a).
• Figure 3: Temperature dependent ac susceptibility measured at various external dc fields. The black arrows indicate anomalies.
• Figure 4: Capacitance as a function of pulsed magnetic field at various temperatures.
• Figure 5: Lattice structure of $\mathrm{Rb_2Co_2(SeO_3)_3}$. Magnetically active $\mathrm{Co^{2+}}$ ions occupy the vertices; the interlayer spacing satisfies $d_0\!\ll\!d_{1,2,3}$.