Anomalous van Vleck paramagnetism induced by the spin–orbit coupling in Ba3CoSb2O9

We investigated magnetic moment behaviors of a frustrated antiferromagnet Ba 3 CoSb 2 O 9 using the field-dependent full-multiplet CoO 6 cluster model calculations with physical quantities determined from the Co L 2,3 -edge X-ray absorption spectroscopy (XAS) analysis. The XAS analysis shows that the Co 3 d spin–orbit coupling is essential to determine the ground state, which becomes Kramer’s doublet states, the so-called J eff = 1/2 states, split by the spin–orbit coupling from the conventional high spin S = 3/2 4 T 1 (t 2g 5 e g 2 ) states. The calculation yields a 2.17 μ B ground state magnetic moment consistent with the reported ~ 1.93 μ B extrapolated from the high field M – H magnetization curve. Our field-dependent model calculations successfully well reproduce the field-dependent magnetization M ( H ) in the high field region (> 30 T). The spin–orbit coupling turns out to originate an anomalously large van Vleck paramagnetism well explaining the reported susceptibility.