Magnetic ground state of a Jeff = 1/2 based frustrated triangular lattice antiferromagnet

TL;DR

This work investigates the magnetic ground state of Ba$_4$YbReWO$_{12}$, a 4f-based triangular-lattice antiferromagnet hosting Jeff=$J_{ extnormal{eff}}=1/2$ Yb$^{3+}$ moments. Using crystal-structure analysis, magnetic susceptibility, specific heat, and muon spin relaxation, the authors establish a large crystal-field gap Δ = 278 K that isolates the $J_{ extnormal{eff}}=1/2$ ground state, and they extract a weak antiferromagnetic exchange $J_1 oughly -0.197$ K with a small next-nearest-neighbor contribution $J_2/J_1 oughly 0.15$ from low-temperature specific heat fits to a $J_1$–$J_2$ model. No long-range magnetic order or spin freezing is observed down to 56 mK (specific heat) and 43 mK (μSR); instead, a broad specific-heat maximum at 90 mK and dynamic μSR signals point to a disordered ground state with short-range spin correlations, shaped by SOC, CEF, and weak dipolar interactions. BYRWO thus serves as a promising platform to explore SOC-induced frustration and potential quantum spin-liquid–like states in a real 4f triangular-lattice magnet, with future work focusing on single-crystal studies and neutron scattering to resolve the excitation spectrum.

Abstract

The subtle interplay between competing degrees of freedom, crystal electric fields, and spin correlations can lead to exotic quantum states in 4f ion-based frustrated triangular lattice antiferromagnets. We present the crystal structure, thermodynamic and muon spin relaxation (μSR) studies of the 4f ion-based frustrated magnet Ba4YbReWO12, wherein Yb3+ ions constitute a triangular lattice. The magnetic susceptibility does not show any signature of spin freezing down to 1.9 K or long-range magnetic ordering down to 0.4 K. The low-temperature Curie-Weiss fit to the inverse magnetic susceptibility data reveals a weak antiferromagnetic exchange interaction, which is corroborated by the fit of magnetic specific heat data following the J1-J2 model with the nearest neighbor exchange interaction of J1 = -0.197 K between the Jeff = 1/2 states of the Yb3+ moments in the lowest Kramers doublet. The lowest Kramers ground state doublet is well separated from the first excited state with a gap of 278 K, as evidenced by our μSR experiments that support the realization of Jeff = 1/2 at low temperatures. The specific heat experiments do not detect a phase transition down to 56 mK. The magnetic specific heat shows a broad maximum 90 mK suggesting a disordered ground state with short range spin correlations. The associated magnetic entropy release at low temperatures is consistent with that expected for the Jeff = 1/2 state. The zero-field μSR measurements show neither the signature of spin freezing nor a phase transition, at least down to 43 mK. Our results suggest a dynamic, disordered ground state in this Jeff = 1/2 frustrated triangular lattice antiferromagnet. Ba4RReWO12 (R=rare earth) offers a viable platform to realize intriguing quantum states borne out of spin-orbit coupling and frustration

Figures (5)

• Figure 1: (a) Two-phase Rietveld refinement of the XRD pattern of the polycrystalline sample of Ba$_4$YbReWO$_{12}$, recorded at room temperature, indicates that it crystallizes in trigonal crystal structure with $R\Bar{3}m$ space group. The solid orange circles represent the experimental data, the black line is the calculated pattern, the green vertical bars are the Bragg's positions and the blue line indicates the difference between the experimental and simulated curve. Minor impurity observed at 2$\theta$$=$ 29.9$^\circ$ is refined by including 2.3 % Yb$_2$O$_3$ phase, indicated by violet vertical bars. (b) Schematic of one unit cell of BYRWO generated by VESTA. The YbO$_6$ octahedra (violet) are connected to BaO$_{12}$ polyhedra (green) and (Re/W)O$_6$ octahedra (grey). (c) Layers of Yb$^{3+}$ triangles are stacked along the c-axis.
• Figure 2: (a) Temperature dependence of magnetic susceptibility of BYRWO obtained in various magnetic fields down to 0.4 K. The inset shows the absence of bifurcation of zero field-cooled (ZFC) and field-cooled (FC) susceptibilities recorded at 100 Oe. (b) The Curie-Weiss fits to the inverse susceptibilities. The solid orange line represents the high-temperature CW fit and the solid pink line is the low-temperature CW fit. The inset shows the variation of $\theta_\textnormal{CW}$ by changing the upper limit of the fit with the lower limit being fixed at 4 K and 5 K.
• Figure 3: (a) Magnetization isotherms obtained at various temperatures. The solid red lines depict the fits of the Brillouin function. The solid dark yellow line represents the linear fit to the high field magnetization in order to extract Van Vleck contribution from the slope. (b) Left: Zoomed panel of the magnetic isotherms below 1 K, where small plateau like behavior is observed, Right:Variation of $dM/d(\mu_0H)$ with the field.
• Figure 4: (a) The specific heat at 0 T measured in the range 0.056 K to 200 K. The red solid line represents the $\beta T^3$ fit up to 14 K in order to estimate the lattice specific heat at low temperatures. Bottom inset: the zero field specific heat reveals a transition at 2.2 K, that is due to residual Yb$_2$O$_3$. The pink line represents the nuclear Schottky ($C_\text{N}$) contribution. In the top inset, the curve with red solid spheres denotes a simulated curve in the absence of Yb$_2$O$_3$, by matching the specific heat values below and above the transition. It represents an estimated $C_{m}$ after subtracting the contribution of Yb$_2$O$_3$. (b) Fit of the magnetic specific heat data to the triangular lattice $J_1-J_2$ model, where $J_1$ and $J_2$ represent the nearest and next-nearest neighbor intraplanar interactions, respectively, as depicted in the schematic triangular lattice in the inset. The blue spheres denote the experimental data and the lines correspond to various values of $J_2/J_1$ with $J_1=-0.197$ K. The inset shows the goodness of fit $\chi^2$ for various values of $J_2/J_1$. (c) The lattice contribution subtracted specific heat at low temperatures. The red solid lines represent the two-level Schottky fit. Inset: The evolution of the Zeeman splitting with magnetic field. The obtained g from the slope of the linear fit (purple solid line) is in agreement with the $J_\textnormal{eff}$ = 1/2 nature of the Yb$^{3+}$ moments. (d) Temperature evolution of magnetic entropy in applied magnetic fields. The cyan dashed line that saturates at $\approx 2.65$ J/mole.K, represents the estimated entropy contribution by subtracting the contribution of Yb$_2$O$_3$. The dotted red line corresponds to entropy Rln2, which is expected for $J_\textnormal{eff}$ = 1/2.
• Figure 5: (a) Time evolution of muon spin polarization at selected representative temperatures in zero field. (b) Temperature dependence of the muon spin relaxation rate ($\lambda$) where a solid line denotes the thermally activated behavior of crystal electric field excitations. The inset depicts the $\mu$SR relaxation rate at low temperature, highlighting the onset of spin correlations below 1 K, ruling out spin freezing down to 43 mK within the experimental time window. (c) Temperature dependence of the stretched exponent. (d) Time evolution of the muon spin polarization in several longitudinal magnetic fields at 36 mK. (e) Muon spin relaxation rate ($\lambda_{\rm LF}$) as a function of LF. The solid blue line represents a phenomenological model given by $\lambda_{\rm LF} (H_{\rm LF}) = \lambda_{\rm LF_{\rm 2D}}(H_{\rm LF}) + \lambda_{\rm 0D}$, which accounts for two-dimensional diffusive spin excitations and zero-dimensional localized spin excitations. (f) Stretched exponent ($\beta_{\rm LF}$) as a function of longitudinal magnetic fields.