Single-crystal study of the honeycomb XXZ magnet BaCo$_2$(PO$_4$)$_2$ in magnetic fields

TL;DR

This study investigates the magnetic properties and phase transitions of high-quality BaCo$_2$(PO$_4$)$_2$ single crystals, revealing long-range antiferromagnetic order, anisotropic susceptibilities, and field-induced phase transitions.

Contribution

First detailed single-crystal magnetic study of BaCo$_2$(PO$_4$)$_2$ revealing anisotropic magnetic behavior and field-induced phase transitions, highlighting similarities with related compounds.

Findings

Long-range antiferromagnetic order at T_N=3.4 K in single crystals

Strong easy-plane magnetic anisotropy observed

Field-induced transitions at H_C1≈0.11 T and H_C2≈0.3 T

Abstract

We present a study of high-quality BaCo$_2$(PO$_4$)$_2$ single crystals via magnetization, heat-capacity, thermal-expansion and magnetostriction measurements. Sharp anomalies in the thermodynamic properties at $T_N=3.4\,$K reveal a long-range antiferromagnetic order in these single-crystalline samples, which is absent in polycrystalline BaCo$_2$(PO$_4$)$_2$. The temperature dependent magnetic susceptibilities for in-plane and out-of-plane magnetic fields are strongly anisotropic and reveal a pronounced easy-plane anisotropy. A Curie-Weiss analysis implies strong orbital magnetism, as it is known from the sister compound BaCo$_2$(AsO$_4$)$_2$ that is discussed as a potential Kitaev spin-liquid material. When applying in-plane magnetic fields at low temperature, BaCo$_2$(PO$_4$)$_2$ is driven to another ordered phase at a critical field $H_{C1}\approx 0.11\,$T and then undergoes a further field-induced transition to a highly polarized paramagnetic phase at $H_{C2}\approx 0.3\,$T, which is again similar to the case of BaCo$_2$(AsO$_4$)$_2$. In addition, our lowest-temperature data reveal that the field-induced transitions in BaCo$_2$(PO$_4$)$_2$ become dominated by thermally assisted domain-wall motion.