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 highfrequency/high-magnetic field electron spin resonance (HF-ESR) spectroscopy of polycrystalline samples of the bismuth cobalt oxyphosphate BiCoPO5. 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 BiCoPO5 orders antiferromagnetically at T-N approximate to 10 K and this order can be continuously suppressed by magnetic field towards the critical value mu H-0(c) approximate to 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 Co2+ 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 similar to 6 T at T << T-N. The HF-ESR spectra recorded at T << T-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-c which supports a theoretical proposal identifying H-c as the quantum critical point for the transition of the spin system in BiCoPO5 to the quantum disordered state at stronger fields.

Automated summary

A comprehensive study was conducted on the magnetic and electron spin resonance spectra of BiCoPO5, revealing larger magnetic moments and spectroscopic g factors than expected due to significant orbital contribution for Co2+ ions. A distinctive field-induced magnetic phase transition was observed within the antiferromagnetic ordered phase, with a critical field rising to around 6 T at temperatures much lower than the ordering temperature. Additionally, it was found that the Co moments were not fully polarized at the critical field, supporting the identification of this point as the quantum critical point for the transition to the quantum disordered state in BiCoPO5 at stronger magnetic fields.