Thermodynamic signatures of a field-induced ordered intermediate phase in Na$_2$Co$_2$TeO$_6$

TL;DR

This study maps the low-temperature, high-field phase diagram of Na$_2$Co$_2$TeO$_6$, revealing multiple phase transitions and challenging the idea of a field-induced quantum spin liquid in this material.

Contribution

It provides the first comprehensive thermodynamic characterization of the field-induced phases in Na$_2$Co$_2$TeO$_6$, identifying distinct ordered states and constraining QSL scenarios.

Findings

Identified three field-induced transitions at approximately 6 T, 7.8 T, and 10.4 T.

Observed thermodynamic signatures of successive phase transitions with two intermediate phases.

Found that the intermediate phase lacks enhanced magnetic entropy, indicating an ordered state.

Abstract

The honeycomb cobaltate Na$_2$Co$_2$TeO$_6$ has recently been proposed as a candidate material for hosting field-induced quantum spin liquid (QSL) behavior. Here, we present a comprehensive thermodynamic study of its low-temperature, high-field phase diagram using magnetization, specific heat, and magnetocaloric-effect measurements down to 1 K. In zero field, we observe a weak residual moment that provides further insight into the nature of the magnetic ground state. For in-plane magnetic fields ($B \parallel a^*$), we identify three field-induced transitions at $B_{c1} \simeq 6$ T, $B_{c2} \simeq 7.8$ T, and $B_{c3} \simeq 10.4$ T. The magnetic Gr\"uneisen parameter and specific heat reveal clear thermodynamic signatures of these successive phase transitions enclosing two intermediate phases. Contrary to expectations for a field-induced QSL, the phase between $B_{c2}$ and $B_{c3}$ lacks enhanced magnetic entropy but instead shows behavior consistent with a distinct ordered state. Above $B_{c3}$, the absence of additional anomalies indicates a crossover to a conventional spin-polarized regime. Our results place stringent thermodynamic constraints on the proposed QSL scenario in Na$_2$Co$_2$TeO$_6$, calling for further microscopic investigations to establish the precise nature of the field-induced phases.