Orbital magnetism in Cd₂Os₂O₇ studied by x-ray magnetic circular dichroism

X-ray magnetic circular dichroism (XMCD) at the L_2,3-edge of Os has been investigated in the antiferromagnetic phase of Cd₂Os ₂O₇, which exhibits a metal-insulator transition around 227 K. According to the sum rule, the XMCD spectra at 10 and 37 T clearly show that the ratio between the orbital magnetic moment (m_L) and spin magnetic moment (m_S) is m_L/m_S=0.16±0.02, and that m_L and m_S are coupled in parallel (m _L||m_S). These phenomena are unusual in that the expected ground state of Os5⁺(5d3) is an orbital singlet in a cubic crystal field, and m_L and m_S should be antiparallel for a less than half-filled system in accordance with Hund's third rule. It is likely that the spin-orbit coupling is important for explaining the observed orbital magnetism.