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【Abstract】

In this talk, I discuss the magnetic coupling at the interface between the ferromagnetic cobalt (Co) and the antiferromagnetic van der Waals compound FePS3. The Atomic Force Microscope image clearly exhibits the surface nanostructure of the exfoliated FePS3 single crystal. There were black and white nanospots sized nearly∼50 nm in diameter and were randomly distributed on the surface with a depth and height fluctuating around ±0.7 nm, indicating that there is a monolayer defect on the surface. Further, Co-capped FePS3 areas are uniformly rough within ±0.5 nm. Interestingly, the Co/FePS3 exhibited isotropic magnetism at the surface plane, and its magnetic coercivity radically decreased by more than half when the temperature was raised from 85 K to 110–120 K, which is nearly the Néel temperature of FePS3. This observation indicates the interfacial magnetic coupling between Co thin film and FePS3 single crystal. Furthermore, the measurement of XMCD confirmed the presence of non-compensated Fe moment along the in-plane direction is parallel to the Co thin film magnetization direction. The net Fe-moment is supposed to play a significant role in mediating the magnetic coupling between the in-plane ferromagnetic Co and the perpendicular antiferromagnetic FePS3. This finding will be valuable for spintronics applications.

In the subsequent discussion, a hydrogen-sensitive FePd alloy film on Co/[Pt/Co]4/Pt multilayers with perpendicular magnetic anisotropy (PMA) was studied. Through hydrogenation, spin-reorientation transition (SRT) from a perpendicular to an in-plane direction was observed in the 2-nm-thick FePd capped multilayer. Magneto-optic Kerr effect measurements were performed in hydrogen gas at pressures ranging from a vacuum of 5 × 10−3 mbar to H2 gas at 200 mbar. By recurring change of H2 pressure reversible SRT was demonstrated. These will have valuable applications in studies on H2-sensor or H-migration controlled spintronics. Additionally, in order to enhance and stabilize the hydrogenation effect Mg was added to Pd/ferromagnetic multilayers. The bulk Mg needs a high temperature and high pressure to absorb hydrogen. With the catalytic Pd-capping layer, Mg is confirmed to absorb hydrogen at room temperature under the 1000 mbar H2 pressure. This hydrogenation effect significantly enhanced the magnetic coercivity from 25 Oe to approximately 200 Oe in an irreversible manner. After hydrogenation, the transformation of the crystalline structure from Mg to magnesium hydride (MgH2) was confirmed by XRD. This room temperature formation of highly stable MgH2 proposes promising application in spintronic devices, particularly in magnetic tunneling junctions (MTJ).

Keywords: FePS3, Ising-type antiferromagnet, Magnetic coupling, spin-reorientation transition, MOKE, XMCD