CoFe₂O₄ Nanoparticle-Integrated Spin-Valve Thin Films Prepared by Interfacial Self-Assembly

We report the fabrication of nanoparticle-integrated spin-valve system and investigate its magnetic properties and magnetotransport behaviors. Using a modified interfacial self-assembly method, chemically synthesized CoFe₂O₄ nanoparticles were assembled as a Langmuir film on liquid/air interface. This film was further deposited on the sputtered thin films of bottom-pinned spin valve without additional treatment. The nanoparticle-assembled film with multilayer structure exhibits uniform and compact surfaces. Magnetization and magnetoresistance study show that the integrated nanoparticles give rise to a reduced interlayer coupling field and an increased magnetoresistance (MR) ratio in the spin valve. By analyzing the magnetic interaction between the nanoparticles and the spin valve, it is inferred that the magnetic stray field induced by the single-domain magnetic nanoparticles reduces the external magnetic field on the free layer, leading to the change of free-layer magnetization and the attenuation of interlayer coupling. The decrease of this ferromagnetic-type interlayer coupling resulted in a more favorable antiparallel magnetization configuration, manifested by the enhancement of MR ratio. This work demonstrates the integration of self-assembled nanoparticles with exchange-biased thin films, and the results suggest that nanoparticle integration can be employed as an alternative route to modulate the magnetization switching and magnetoresistance of spin valves

Observation of Room-Temperature Magnetoresistance in Monolayer MoS₂ by Ferromagnetic Gating

Room-temperature magnetoresistance (MR) effect is observed in heterostructures of wafer-scale MoS₂ layers and ferromagnetic dielectric CoFe₂O₄ (CFO) thin films. Through the ferromagnetic gating, an MR ratio of −12.7% is experimentally achieved in monolayer MoS₂ under 90 kOe magnetic field at room temperature (RT). The observed MR ratio is much higher than that in previously reported nonmagnetic metal coupled with ferromagnetic insulator, which generally exhibited MR ratio of less than 1%. The enhanced MR is attributed to the spin accumulation at the heterostructure interface and spin injection to the MoS₂ layers by the strong spin–orbit coupling effect. The injected spin can contribute to the spin current and give rise to the MR by changing the resistance of MoS₂ layers. Furthermore, the MR effect decreases as the thickness of MoS₂ increases, and the MR ratio becomes negligible in MoS₂ with thickness more than 10 layers. Besides, it is interesting to find a magnetic field direction dependent spin Hall magnetoresistance that stems from a combination of the spin Hall and the inverse spin Hall effects. Our research provides an insight into exploring RT MR in monolayer materials, which should be helpful for developing ultrathin magnetic storage devices in the atomically thin limit

Light-Triggered Reversible Tuning of Second-Harmonic Generation in a Photoactive Plasmonic Molecular Nanocavity

The ultrasmall mode volume and ultralarge local field enhancement of compact plasmonic nanocavities have been widely explored to amplify a variety of optical phenomena at the nanoscale. Other than passively generating near-field enhancements, dynamic tuning of their intensity and associated nonlinear optical processes such as second-harmonic generation (SHG) play vital roles in the field of active nanophotonics. Here we apply a host–guest molecular complex to construct a photoswitchable molecule-sandwiched metallic particle-on-film nanocavity (MPoFN) and demonstrate both light-controlled linear and nonlinear optical tuning. Under alternating illumination of ultraviolet (UV) and visible light, the photoactive plasmonic molecular nanocavity shows reversible switching of both surface-enhanced Raman scattering (SERS) and plasmon resonance. Surprisingly, we observe more significant modulation of SHG from this photoactive MPoFN, which can be explained qualitatively by the quantum conductivity theory (QCT). Our study could pave the way for developing miniaturized integrated optical circuits for ultrafast all-optical information processing and communication

Atomic-Scale Mechanism on Nucleation and Growth of Mo₂C Nanoparticles Revealed by in Situ Transmission Electron Microscopy

With a similar electronic structure as that of platinum, molybdenum carbide (Mo₂C) holds significant potential as a high performance catalyst across many chemical reactions. Empirically, the precise control of particle size, shape, and surface nature during synthesis largely determines the catalytic performance of nanoparticles, giving rise to the need of clarifying the underlying growth characteristics in the nucleation and growth of Mo₂C. However, the high-temperature annealing involved during the growth of carbides makes it difficult to directly observe and understand the nucleation and growth processes. Here, we report on the use of advanced in situ transmission electron microscopy with atomic resolution to reveal a three-stage mechanism during the growth of Mo₂C nanoparticles over a wide temperature range: initial nucleation via a mechanism consistent with spinodal decomposition, subsequent particle coalescence and monomer attachment, and final surface faceting to well-defined particles with minimum surface energy. These microscopic observations made under a heating atmosphere offer new perspectives toward the design of carbide-based catalysts, as well as the tuning of their catalytic performances

Nonlithographic Fabrication of Crystalline Silicon Nanodots on Graphene

We report a nonlithographic fabrication method to grow uniform and large-scale crystalline silicon (Si) nanodot (c-SiNDs) arrays on single-layer graphene by an ultrathin anodic porous alumina template and Ni-induced Si crystallization technique. The lateral height of the template can be as thin as 160 nm and the crystallization of Si can be achieved at a low temperature of 400 °C. The effects of c-SiNDs on graphene were studied by Raman spectroscopy. Furthermore, the c-SiNDs/graphene based field effect transistors were demonstrated