Static magnetic and ESR spectroscopic properties of the dimer-chain antiferromagnet BiCoPO5

We report a comprehensive study of the static susceptibility, high-field magnetization and high-frequency/high-magnetic field electron spin resonance (HF-ESR) spectroscopy of polycrystalline samples of the bismuth cobalt oxyphosphate ${\mathrm{BiCoPO}}₅$. This compound features a peculiar spin system that can be considered as antiferromagnetic (AFM) chains built of pairs of ferromagnetically coupled Co spins and interconnected in all three spatial directions. It was previously shown that ${\mathrm{BiCoPO}}₅$ orders antiferromagnetically at ${T}_{\mathrm{N}}\ensuremath{\approx}10\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and this order can be continuously suppressed by magnetic field towards the critical value ${\ensuremath{\mu}}₀{H}_{\mathrm{c}}\ensuremath{\approx}15$ T. In our experiments we find strongly enhanced magnetic moments and spectroscopic $g$ factors as compared to the expected spin-only values, suggesting a strong contribution of orbital magnetism for the ${\mathrm{Co}}²⁺$ ions. This is quantitatively confirmed by ab initio quantum chemical calculations. Within the AFM ordered phase, we observe a distinct field-induced magnetic phase transition. Its critical field rises to $\ensuremath{\sim}6$ T at $T\ensuremath{\ll}{T}_{\mathrm{N}}$. The HF-ESR spectra recorded at $T\ensuremath{\ll}{T}_{\mathrm{N}}$ are very rich comprising up to six resonance modes possibly of the multimagnonic nature that soften towards the critical region around 6 T. Interestingly, we find that the Co moments are not yet fully polarized at ${H}_{\mathrm{c}}$ which supports a theoretical proposal identifying ${H}_{\mathrm{c}}$ as the quantum critical point for the transition of the spin system in ${\mathrm{BiCoPO}}₅$ to the quantum disordered state at stronger fields.