Orbital magnetism in Cd${}_2$Os${}_2$O${}_7$ 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 Cd${}_2$Os${}_2$O${}_7$, 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 $m_L/m_S=0.16\pm 0.02$, and that $m_L$ and $m_S$ are coupled in parallel ($m_L\left|\right|m_S$). These phenomena are unusual in that the expected ground state of Os${}^{5+}$(5$d^3$) 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.

Figure 1

Temperature dependence of effective magnetic susceptibility, $M/H$. Field-cooling (FC) and zero-field-cooling (ZFC) data are shown by solid and dashed curves, respectively. Inset shows the magnetization curve up to 50 T at 4.2 K.

Figure 2

(a) XMCD spectra and XAS at $L_3$ edge of Os at 5 K. (b) XMCD spectra and XAS at $L_2$ edge of Os at 5 K.

Figure 3

XMCD spectra and XMCD integrals at 10 T.

Figure 4

Schematic energy level diagram of $d^3$ state in a cubic crystal field.

Figure 5

Schematic possible diagram of an arrangement of spin and orbital magnetic moments. The induced spin and orbital magnetizations, $m_L$ and $m_S$, are along with the applied magnetic field. $M_L$ and $M_S$ are the net magnetic moments of an orbital and spin, respectively.