Electric-field effect on magnetic moments in Co ultra-thin films deposited on Pt

This study investigates the electric-field effect on magnetic moments in perpendicularly magnetized Co ultra-thin films on a Pt underlayer. By applying electric fields to the Co surface through MgO/HfO2 insulating layers, we determined the change in the magnetic moment per one electrically increased electron. From systematic experiments at temperatures much below the Curie temperature, we found that the sign and the magnitude of the change depend on the nominal Co thickness tCo: the change nearly equals to −1 μB/electron for tCo ∼ 0.45 nm and more than +1 μB/electron at tCo ∼ 0.2 nm. The former value is consistent with the expectation from the celebrated Slater–Pauling curve. The latter value implies that one needs to consider some effects characteristic to Pt/Co system with extremely small tCo. The calculated band structures suggest that the intermixing between Co and Pt is related to the observed deviation from the Slater–Pauling curve with decreasing tCo.