Chirality-Induced Noncollinear Magnetization and Asymmetric Domain-Wall Propagation in Hydrogenated CoPd Thin Films.

Array-patterned CoPd-based heterostructures are created through e-beam lithography and plasma pretreatment that induces oxidation with depth gradient in the CoPd alloy films, breaking the central symmetry of the structure. Effects on the magnetic properties of the follow-up hydrogenation of the thin film are observed via magneto-optic Kerr effect microscopy. The system exhibits a strong vertical and lateral antiferromagnetic coupling in the perpendicular component between the areas with and without plasma pretreatment, and asymmetric domain-wall propagation in the plasma-pretreated areas during magnetization reversal. These phenomena exhibit evident magnetic chirality and can be interpreted with the Ruderman-Kittel-Kasuya-Yosida coupling and the Dzyaloshinskii-Moriya interaction (DMI). The sample processing demonstrated in this study allows easy incorporation of lithography techniques that can define areas with or without DMI to create intricate magnetic patterns on the sample, which provides an avenue toward more sophisticated control of canted spin textures in future spintronic devices.

Twisted light induced magnetic anisotropy changes in an interlayer exchange coupling system.

All-optical switching of magnetic materials is a potential method for realizing high-efficiency and high-speed data writing in spintronics devices. The current method, which utilizes two circular helicities of light to manipulate magnetic domains, is based on femtosecond pulsed lasers. In this study, we demonstrate a new all-optical switching method using a continuous-wave Laguerre-Gaussian beam (twisted light), which allows photons to carry orbital angular momentum with discrete levels, lℏ, to modify the magnetic anisotropy of an interlayer exchange coupling system. The easy axis of the heterojunction Pt(5 nm)/Co(1.2 nm)/Ru(1.4 nm)/Co(0.4 nm)/Pt(5 nm) on a SiO2/Si substrate dramatically changed after illuminating it with a laser beam carrying a sufficient quantum number of orbital angular momentum. Based on a simple numerical calculation, we deduced that the interaction between the dynamical phase rotation of the electric field and the metal surface could generate an in-plane circular current loop that consequently induces a perpendicular stray field to change the magnetic anisotropy. This finding paves the way for developments in the field of magnetic-based spintronics using light with orbital angular momentum.

Magnetic patterning through graphene protection against oxidation and interlayer diffusion.

Graphene (Gr) has been demonstrated to protect metallic thin films against oxidation. Based on this idea, we propose a new method to fabricate microstructured magnetic domains using patterned single-layer Gr. In the first experiment, single-layer Gr was transferred onto a CoPd alloy film pregrown on a SiO2/Si(001) substrate. Subsequently, the single-layer Gr was patterned through electron beam lithography followed by oxygen plasma etching to expose selective micron-sized areas of CoPd. The exposed areas of CoPd were more easily oxidized compared to the areas protected by Gr, which is found to result in significant magnetic contrast between the protected and surface-oxidized areas of CoPd. In the second experiment, a lithographically-patterned Gr layer was placed between the Fe and CoPd layers to block interlayer diffusion area-selectively during sample annealing. Magnetic contrast is observed to be established between the Pd/Fe/Gr/CoPd and Pd/Fe/CoPd areas, leading to a magnetic structure that matches the pattern of the lithographed Gr. These observations demonstrate that Gr patterning is a simple and powerful method for magnetic patterning, which can be applied in the fabrication of future data-storage and spintronic devices.

Hydrogen-mediated magnetic domain formation and domain wall motion in Co30Pd70 alloy films.

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.

Reversible 90-Degree Rotation of Fe Magnetic Moment Using Hydrogen.

[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.