Subject Number : S-17-MS-2015
Support Type : Common use
Proposal Title (English) : Interfacial spin and orbital magnetic moments and their anisotropies in Pd studied by x-ray magnetic circular dichroism in Co/Pd multilayers
Username (English) :　J. Okabayashi　
Affiliation (English) : The University of Tokyo

1. Summary
Ultrathin Co/Pd multilayers are artificial nanomaterials that exhibit perpendicular magnetic anisotropy (PMA), due to the spin-orbit interactions, achieved through interfacial chemical bonding. Extensive efforts have been made for studying interfaces of ultra-thin magnetic multilayers and nanostructures. Studies on Co atoms performed using X-ray magnetic circular dichroism (XMCD) have suggested the enhancement of orbital magnetic moments at the interfacial Co that is adjacent to Pd. It has been reported that the PMA emerges due to the cooperative effects between spin moments in 3d TMs and large spin-orbit interactions in the non-magnetic 4d or 5d TMs. In this study, we study the anisotropy of the orbital magnetic moments of both Co and Pd elements using one specific experiment, due to the challenges in detection of the induced magnetic moments, of Pd in particular.

2. Experimental
We prepared the sample of Co/Pd multilayered structures: Co (0.69 nm)/Pd (1.62 nm) for PMA with stacking five periods on the Si substrates. Sample surface was sputtered by Ar ions before the XMCD measurements in order to remove the oxygen contamination. We performed XMCD experiments at BL4B, UVSOR, Institute of Molecular Science. Total electron yield mode by directly detecting the sample current was adopted. A magnetic field of ±5 T was applied along the direction of the incident polarized soft x-ray.

3. Results and Discussion
We successfully observed clear XMCD signals in Pd M-edges after the removal of surface contamination. Although the XAS line shapes overlap with those of O K-edge absorption, clear XMCD signals induced by the proximity with Co layers are observed. The Pd M-edge XMCD line shapes are almost identical, which suggests isotropic orbital moments in Pd, which do not directly contribute to the PMA. On the other hand, clear Co L-edge XAS and XMCD with angular dependence reveal the enhancement of orbital moments in the surface normal direction because of PMA. Then, we discuss the quadrupole-like contribution of the interfacial Pd layer. Our Pd XMCD results indicate that the Pd orbital moments induced at the interface are isotropic. Note that the magnetic dipole term mT is an order of magnitude smaller than the orbital moments, i.e., less than 0.005 B, comparable with the detectable limits. Therefore, the relatively large spin-orbit coupling constant and small Pd exchange splitting contribute to the appearance of PMA by means of quadrupole-like interactions through the interfacial proximity effects

4. Others
The author acknowledges the collaboration with Prof. Yoshio Miura (Kyoto Tech. U) and Prof. Hiro Munekata (Tokyo Tech.). We also acknowledge to Prof. T. Yokoyama, Dr. Y. Takagi, and Dr. Y. Uemura for the technical supports at BL4B.

5. Publication/Presentation
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6. Patent
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