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Future development of spintronic devices urgently requires an advanced control method of spin current. Combining graphene with other various characteristic two-dimensional crystals in different ways can produce a new class of functional materials, which offers a unique platform for spintronics. A specific van der Waals heterostructure consisting of graphene and hexagonal boron nitride (h-BN) as the barrier layer in magnetic tunnel junctions (MTJs) is proposed in this paper. This heterostructure device combines the strong spin filtering effect of graphene/ferromagnet interface and the resonant tunneling effect of graphene/h-BN/graphene van der Waals heterostructure. We demonstrate an effective approach to control spin current based on the bias tunability of the spin resonance tunneling transport. Theoretical research unambiguously reveals that spin resonance appears when the electronic spectra of spin electron in two graphene layers are aligned, which results in a large magnetoresistance and negative differential resistance in the device. The efficient bias controllability of spin current, combined with the excellent magnetoresistance behavior, can be the key building blocks in future spin-based magnetic detection and high-frequency logic devices.

The capacitance of magnetic tunnel junctions (MTJs) with the tunneling magnetocapacitance (TMC) effect has drawn lots of attention but there is still a lack of research on the capacitance anisotropy. In this work, a study of the capacitance in MgO-based MTJ is performed. The negative capacitance is observed and indicates the importance of interface capacitance in the MTJ. In further research, the interface capacitance depends on the relative orientation of magnetization in the two ferromagnetic layers (FMs), exhibiting a strong anisotropy. The Debye-Frohlich model combined with the magnetization dependence of relaxation time is introduced to explain the behavior. Theory and experiment results of the relaxation time as a function of the relative angle of the two FM magnetizations are well fitted. It reveals that the TMC originates from the interface charge accumulation and spin-dependent tunneling. All the results suggest that the interface capacitance with apparent anisotropy plays an essential role in the TMC.

Due to the theoretical prediction of large tunneling magnetoresistance (TMR), graphene-based magnetic tunneling junction (MTJ) has become an important branch of high-performance spintronics device. In this paper, the non-collinear spin filtering and transport properties of MTJ with the Ni/tri-layer graphene/Ni structure were studied in detail by utilizing the non-equilibrium Green’s formalism combined with spin polarized density functional theory. The band structure of Ni–C bonding interface shows that Ni–C atomic hybridization facilitates the electronic structure consistency of graphene and nickel, which results in a perfect spin filtering effect for tri-layer graphene-based MTJ. Furthermore, our theoretical results show that the value of tunneling resistance changes with the relative magnetization angle of two ferromagnetic layers, displaying the anisotropic tunneling behavior of graphene-based MTJ. This originates from the resonant conduction states which are strongly adjusted by the relative magnetization angles. In addition, the perfect spin filtering effect is demonstrated by fitting the anisotropic conductance with the Julliere’s model. Our work may serve as guidance for researches and applications of graphene-based spintronics device.

Enhancing electric field controlled magnetism of micron-scale-thickness ferromagnetic film is of great significance to improve the performance of magnetoelectric (ME) sensors, ME tunable inductors and other ME devices. In order to achieve above purpose, multilayer preparation method was proposed and validated by experiment studies on multilayer (Cu(10 nm)/(FeGaB(100 nm)/Cu(5 nm))10) and monolayer (Cu(10 nm)/FeGaB (1000 nm)/Cu(5 nm)) samples, respectively. The results show the electric field controlled magnetism could be enhanced obviously by the multilayer preparation method. Meanwhile, the finite element simulation model was established to analyze possible causes for the improvement of electric field controlled magnetism in multilayer sample. The model revealed that multilayer preparation can reduce the in-plane demagnetizing factor and increase the internal magnetic field of the magnetic materials, thus enhancing electric field controlled magnetism. This work is conducive to the development of micron-scale-thickness films in electric field control of magnetism, which contributes to the application of ferroelectric/ferromagnetic heterostructures in ME devices.

Periodic switching of magnetic vortex core driven by alternative magnetic field, applied perpendicularly, is investigated in a curved permalloy nanodisk by using micromagnetic simulation. Modulated by the curvature, the oscillations of the out-of-plane magnetization, caused by the external magnetic field, are confined in the center region, closed to the core, which reduces the threshold switching field down to 3 mT. While, the excitation of radial spin waves after a core switching dominates the reversal process generally in flat nanodisks, making it hard to control the final core polarity. The released spin waves in the center region during the core switching are relatively weak in curved nanodisk with most of radial spin waves dissipated to the thick edge part, resulting in a stable periodic vortex core switching.

Good proton conductivity and water retention are essential for proton exchange membranes (PEMs). However, proton conductivity of existing PEMs decreases sharply at low humidity, which seriously restricts the efficient and stable operation of fuel cell system. In this study, we design a spherical bell-typed superabsorbent microsphere with imidazole groups (SBSM) using the distillation-precipitation polymerization method, and incorporate it into sulfonated poly ether ether ketone (SPEEK) matrix to enhance proton conductivity. Benefited from the hygroscopic 3D framework of the superabsorbent core and the special hollow structure of SBSM, the composite membrane remarkably raises the water retention and proton conductivity. Meanwhile, the carboxylic acid, sulfonic acid and Lewis basic imidazole generate two types of “acid-base pairs”, which serves as proton acceptors and donors to accelerate the formation of low energy paths at the membrane interfaces. The composite membrane with 15 wt% fillers exhibits the highest water retention of 15.72% and the highest proton conductivity of 0.0284 S cm−1 at room temperature and 20% relative humidity (RH). These improvements are attributed to the unique structure of the SBSMs that provides a stable aqueous environment and additional proton conduction pathways in the membrane.

Considering the nickel has an almost perfect in-plane lattice constant match with graphene, vertical Ni/graphene/Ni junctions are fabricated and characterized in this article. In order to improve the interface quality, we employ a modified fabrication process of depositing the ferromagnetic (FM) material on both sides of a suspended graphene membrane assisted with rotating the sample with a slant angle during the bottom FM film growth. The magnetoresistance (MR) ratio of about 0.1% with a low resistance-area product (RA) of 30 Ω·μm2 is obtained at room temperature and the device shows a typical metallic behavior with the linear I-V characteristic and T2-typical temperature dependence of the resistance. Interestingly, the temperature dependence of the MR displays an unusual behavior as the MR ratio reduces with the decreasing temperature, which we believe is due to the antiferromagnetic coupling between the two FM layers competing with the dipole field.