Orbital polarization in NiFe2O4 measured by Ni-2p x-ray magnetic circular dichroism

We report a comprehensive study of the x-ray magnetic circular dichroism (XMCD) at the Ni L2,3 edges of NiFe2O4 (trevorite), a ferrimagnetic compound which belongs to the class of strongly correlated 3d systems. The measured XMCD line shape is in good agreement with full-multiplet calculations for a 3d8 ground state including an octahedral crystal field of 10Dq= 1.2 eV. Using the XMCD sum rules, we find for the nickel an orbital to spin magnetic moment ratio of L/S=0.27 ±0.07, which means that the orbital contribution to the magnetic moment is 12±2%. The size of the error bar is not due to the neglect of band structure and hybridization, but due to Coster-Kronig transitions which give a transfer of spectral weight in the dichroism near the L2 edge. Using the sum rules for the isotropic x-ray absorption spectrum, we obtain the expectation value of the spin-orbit interaction, which can be converted into an L/S ratio of 0.34±0.11. All sum-rule results were corrected for the influence of core-valence exchange interaction, which gives rise to jj mixing between the two absorption edges. The correction is done by comparing the calculated sum-rule results with the correct ground-state values calculated as a function of crystal-field strength. The 2-eV shoulder of the L3 line shows a strong positive dichroism, which can be attributed to a spin-flip state. From the overall agreement between the experimental and theoretical results, we conclude that a localized model provides a good description for the orbital magnetization. © 1999 The American Physical Society.