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In this study, the microscopic origin of the hydrogen effect on magnetic materials was explored through the characterization of time-dependent magnetic domain evolution. We prepared 25-nm Co30Pd70 alloy films with canted magnetic moment on SiO2/Si(001) substrates. From macroscopic Kerr hysteresis loops, considerable hydrogen-induced reduction of magnetic coercivity by a factor of 1/5 in a longitudinal direction and enhancement of magnetic remanence to saturation ratio from 60% to 100% were observed. The magnetic reversal behavior of the Co30Pd70 alloy films gradually transformed from nucleation- to domain-wall-motion dominance when H2 pressure was increased from a vacuum of 1 × 10−5 mbar to 0.8 bar. Domain size also increased considerably with H2 pressure. When H2 pressure was above 0.4 bar, the domain wall (DW) motion was clear to observe and the DW velocity was approximately 10−6–10−5 m/s. Greater hydrogen content in the Co30Pd70 alloy films promoted DW motion that was closer to the behavior of a thermally activated model. The hydrogen effects on magnetism were observed to be reversible and could have valuable future application in spintronic devices for hydrogen sensing.

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[Pd/Fe]2 multilayers were deposited on a flat MgO(001) to study the effect of hydrogen on magnetic interlayer coupling. Complex magnetic hysteresis behavior, including single, double, and triple loops, were measured as a function of the azimuthal angle in a longitudinal and transverse direction. With a combination of a 2-fold magnetic anisotropy energy (MAE) in the bottom-Fe and a 4-fold MAE in the top-Fe, the complex magnetic hysteresis behavior could be clearly explained. Two well-split hysteresis loops with almost zero Kerr remanence were measured by choosing a suitable Pd thickness and applying the magnetic field perpendicular to the easy axis of the bottom-Fe. The split double loops originated from the 90°-rotation of the top-Fe moment. On exposure to a hydrogen gas atmosphere, the separation of the two minor loops increased, indicating that Pd-hydride formation enhanced the ferromagnetic coupling between the two Fe layers. Based on these observations, we proposed that, by applying a suitable constant magnetic field, the top-Fe moment could undergo reversible 90°-rotation following hydrogen exposure. The results suggest that the Pd space layer used for mediating the magnetic interlayer coupling is sensitive to hydrogen, and therefore, the multilayer system can function as a giant magnetoresistance-type sensor suitable for hydrogen gas.

In this study, we examined the growth of Au over monolayer MoS2 flakes on SiO2/Si(001) substrates at a temperature range of 25–230 °C. Based on atomic force microscopy (AFM) images, a height reversal phenomenon was observed after deposition of 2–8 nm Au. Depending on the growth temperature and Au coverage, the height difference between the MoS2 flake and the SiO2 area reversed from a single layer height of MoS2 (∼+0.7 nm) to a range –1.0 to –3.5 nm. This indicates that, based on AFM data, the apparent height of Au/MoS2 is significantly lower than that of Au/SiO2. Scanning electron microscopy results indicate the different lateral nucleation size and shapes of Au coverage on the MoS2 and SiO2 surfaces. In addition, transmission electron microscopy images confirmed the two dimensional and three dimensional growth of Au on the MoS2 and SiO2 surfaces, respectively. The different growth modes of Au on the MoS2 and SiO2 surfaces led to significant changes in the apparent height and thus blocked electrical conduction. These results provide information about nucleation and morphology of Au coverage on MoS2/SiO2 and will be valuable for future applications.