  In this study, the microscopic origins of the voltage-controlled magnetic
anisotropy (VCMA) in 3d-ferromagnetic metals are revealed. Using in-situ X-ray
fluorescence spectroscopy that provides a high quantum efficiency,
electric-field-induced changes in orbital magnetic moment and magnetic dipole
Tz terms in ultrathin Co films are demonstrated. An orbital magnetic moment
difference of 0.013{\mu}B. was generated in the presence of electric fields of
+(-)0.2 V/nm. The VCMA of Co was properly estimated by the induced change in
orbital magnetic moment, according to the perturbation theory model. The
induced change in magnetic dipole Tz term only slightly contributed to the VCMA
in 3d-ferromagnetic metals.
