Single-crystal study of the honeycomb XXZ magnet BaCo2 ( PO4 ) 2 in magnetic fields

We present a study of high-quality $\mathrm{Ba}{\mathrm{Co}}_{2}{(\mathrm{P}{\mathrm{O}}_{4})}_{2}$ single crystals via magnetization, heat-capacity, thermal-expansion, and magnetostriction measurements. Sharp anomalies in the thermodynamic properties at ${T}_{N}=3.4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ reveal a long-range antiferromagnetic order in these single-crystalline samples, which is absent in polycrystalline $\mathrm{Ba}{\mathrm{Co}}_{2}{(\mathrm{P}{\mathrm{O}}_{4})}_{2}$. 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 $\mathrm{Ba}{\mathrm{Co}}_{2}{(\mathrm{As}{\mathrm{O}}_{4})}_{2}$ that is discussed as a potential Kitaev spin-liquid material. When applying in-plane magnetic fields at low temperature, $\mathrm{Ba}{\mathrm{Co}}_{2}{(\mathrm{P}{\mathrm{O}}_{4})}_{2}$ is driven to another ordered phase at a critical field ${\ensuremath{\mu}}_{0}{H}_{C1}\ensuremath{\approx}0.11\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ and then undergoes a further field-induced transition to a highly polarized paramagnetic phase at ${\ensuremath{\mu}}_{0}{H}_{C2}\ensuremath{\approx}0.3\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, which is again similar to the case of $\mathrm{Ba}{\mathrm{Co}}_{2}{(\mathrm{As}{\mathrm{O}}_{4})}_{2}$. In addition, our lowest-temperature data reveal that the field-induced transitions in $\mathrm{Ba}{\mathrm{Co}}_{2}{(\mathrm{P}{\mathrm{O}}_{4})}_{2}$ become dominated by thermally assisted domain-wall motion.