Coexistence of static and dynamic local magnetic fields in an S = 3/2 honeycomb lattice antiferromagnet Co2Te3O8

TL;DR

This study investigates Co$_2$Te$_3$O$_8$, a $S=3/2$ honeycomb lattice antiferromagnet, using a combination of crystal structure analysis, thermodynamic measurements, neutron diffraction, muon spin relaxation, and density functional theory. The material exhibits long-range antiferromagnetic order below $T_N \approx 55$ K with an XY-like spin arrangement, while muSR reveals coexisting static and dynamic local fields and persistent spin dynamics near and below the transition. DFT calculations show dominant intra-layer AFM exchange couplings $J_1$ and $J_2$ with weaker further-neighbor and inter-layer interactions, supporting a Heisenberg-like model with moderate frustration. These results establish CTO as a platform to explore classical spin-liquid-like behavior in a spin-$3/2$ honeycomb system and motivate future single-crystal studies to further elucidate its spin dynamics and potential quantum phases.

Abstract

Two-dimensional honeycomb lattices, characterized by their low coordination numbers, provide a fertile platform for exploring various quantum phenomena due to the intricate interplay between competing magnetic interactions, spin-orbit coupling, and crystal electric fields. Beyond the widely studied Jeff= 1/2 honeycomb systems, S = 3/2 honeycomb lattices present a promising alternative route to realizing the classical spin liquid-like state within the spin-S Kitaev models. Herein, we present crystal structure, thermodynamic, neutron diffraction and muon spin relaxation (muSR) measurements, complemented by density functional theory (DFT) calculations on an unexplored 3d transition metal based compound Co2Te3O8, where Co2+ (S = 3/2) ions form a distorted honeycomb lattice in the crystallographic bc-plane without any anti-side disorder between constituent atoms. A clear lambda type anomaly around 55 K in both magnetic susceptibility and specific heat data indicates the onset of a long-range ordered state below TN= 55 K. The dominant antiferromagnetic interaction between S = 3/2 moments is evidenced by a relatively large negative Curie-Weiss temperature of -103 K derived from magnetic susceptibility data and supported by DFT calculations. The signature of long-range antiferomagnetic order state in the thermodynamic data is corroborated by neutron diffraction and muSR results. Furthermore, muSR experiments reveal the coexistence of static and dynamic local magnetic fields below TN, along with a complex magnetic structure that can be associated with XY-like antiferromagnet, as confirmed by neutron diffraction experiments.

Figures (7)

• Figure 1: (a) Rietveld refinement profile of powder X-ray diffraction data at room temperature. The solid black line represents the calculated intensity (I$_{\rm cal.}$) for the monoclinic crystal structure of Co$_{2}$Te$_{3}$O$_{8}$, fitting the observed experimental points (I$_{\rm obs.}$). Olive vertical bars indicate the positions of the Bragg reflections, while the difference between calculated and observed intensity is shown by the blue solid line. (b) Four-unit cells of Co$_{2}$Te$_{3}$O$_{8}$, where Co$^{2+}$ ions form a distorted honeycomb lattice perpendicular to the crystallographic $a$-axis. (c) Schematic shows the nearest-neighbor O$^{2-}$ ions of Co$^{2+}$ form a layer of CoO$_{6}$ distorted octahedra sandwiched between the layer of TeO$_{4}$ distorted square planar structure. (d) A single distorted honeycomb plane of Co$^{2+}$ ions perpendicular to the crystallographic $a$-axis that is composed of corner-sharing Co$_{2}$O$_{10}$ dimer.
• Figure 2: (a) Temperature dependence of magnetic susceptibility in several magnetic fields. Inset shows the derivative of magnetic susceptibility as a function of temperature in a magnetic field $\mu_{0}H$ = 1 T. The dashed vertical lines mark the temperature at which antiferromagnetic phase transition occurs. (b) Temperature dependence of inverse magnetic susceptibility in a magnetic field $\mu_{0}H$ = 1 T. The solid red line depicts the Curie-Weiss fit. (c) Isotherm magnetization as a function of magnetic field at several temperatures.
• Figure 3: Temperature dependence of specific heat in two magnetic fields where the solid line accounts for the lattice contributions due to phonons. (b) Temperature dependence of magnetic specific heat with an anomaly at $T_{\rm N}$ indicated by dashed vertical line. Well below $T_{\rm N}$, the magnon excitations are described by a $T^{3}$ behavior, as indicated by the solid line. (c) Temperature dependence of magnetic entropy change in a zero magnetic field. The pink and blue horizontal lines show the expected entropy release for high-spin ($S$ = 3/2) and low-spin ($J_{\rm eff}$ = 1/2) Co$^{2+}$ ions, respectively.
• Figure 4: (a) Neutron powder diffraction patterns for Co$_2$Te$_3$O$_8$ compound measured between 1.5 and 60 K. The intensity scale also correspond to the temperature of the measured diffraction patterns. The arrow denotes the position of the most pronounced magnetic reflections. (b) The derived temperature dependence of the marked-magnetic-peak intensity. The inset shows the refined magnetic structure. (c) The difference between the data measured at 1.5 K and 60 K compared with the calculated diffraction pattern and corresponding residual.
• Figure 5: (a) Time evolution of muon spin asymmetry at short times at several representative temperatures in zero-field. The solid red lines represent fits to the function described in the text. (b) Time evolution of muon spin asymmetry at longer times in zero-field. (c) Fourier transform of the time-dependent asymmetry depicted in (a). The arrow indicates the position of oscillating frequencies corresponding to the respective temperatures.