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

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.

💡 Research Summary

This paper presents a comprehensive investigation of the novel cobalt-based compound Co2Te3O8 (CTO), focusing on its crystal structure, magnetic properties, and the intriguing coexistence of static order and dynamic fluctuations.

The material crystallizes in a monoclinic structure (space group C2/c), where Co2+ ions (with spin S=3/2) form a distorted two-dimensional honeycomb lattice in the bc-plane. This lattice is built from corner-sharing Co2O10 dimer units. Bulk magnetic susceptibility and specific heat measurements reveal a clear lambda-type anomaly at TN ≈ 55 K, signaling the onset of long-range antiferromagnetic order. A large negative Curie-Weiss temperature of θ_CW = -103 K indicates dominant antiferromagnetic interactions with a moderate degree of magnetic frustration.

The core discovery of this work stems from complementary local-probe experiments. Zero-field muon spin relaxation (µSR) measurements provide unambiguous evidence for the coexistence of static and dynamic local magnetic fields below TN. While a rapidly relaxing component signifies the presence of static, ordered magnetic moments, a slowly relaxing component persists down to the lowest temperatures, indicating substantial spin dynamics that survive even within the ordered state. The relaxation rate of this dynamic component exhibits an order-parameter-like temperature dependence, peaking just below TN.

Neutron powder diffraction experiments corroborate the long-range magnetic order and determine a complex, likely non-collinear, magnetic structure. This structure is consistent with the system being an XY-like antiferromagnet, where spins are confined to a preferred plane.

Density functional theory (DFT+U) calculations support the experimental findings. They confirm the S=3/2 state of Co2+ and yield a dominant intra-dimer antiferromagnetic exchange interaction (J1). Further-neighbor interactions are also found to be antiferromagnetic, contributing to the stabilization of the observed magnetic order.

In summary, the study establishes Co2Te3O8 as a rare example of an S=3/2 honeycomb lattice antiferromagnet in a 3d transition metal system. The simultaneous observation of static long-range order via neutron diffraction and persistent spin dynamics via µSR below the ordering temperature highlights a complex magnetic ground state. This positions CTO as a promising candidate material for exploring the interplay between magnetic order and fluctuations in higher-spin honeycomb lattices, potentially bridging the understanding towards classical analogs of Kitaev spin liquid states.