Junping Peng

The electronic structure and physicochemical properties of graphene are critically governed by its layer number, making high-quality multilayer graphene highly desirable for electronic and spintronic devices. However, conventional growth processes face challenges in achieving controllable synthesis of graphene with more than three layers. The main reason is that the difference in formation energy between graphene exceeding three layers and the substrate is relatively small, making it difficult to effectively distinguish between them. This paper proposes a carbon stock self-limited growth method. Using wafer-scale single-crystal Ni(111) thin films as substrates, the total dissolved carbon atoms are regulated by precisely controlling the thickness of nickel films and growth temperature, thereby achieving segregation growth of graphene with layer numbers spontaneously limited by carbon stock. Characterization results systematically confirm that controllable preparation of multi-layer graphene with tunable layers ranging from 3 to 7 and uniform thickness has been successfully realized. Further first-principles calculations reveal that carbon dissolution into the Ni(111) subsurface disrupts the substrate–graphene lattice matching, sharply decreasing the interfacial binding energy from 0.61 eV/atom to 0.41 eV/atom and thereby facilitating the formation of twisted interlayer structures. This work provides a feasible technical route for achieving the growth of twisted multilayer graphene with precise layer-number control beyond three layers, and lays a foundation of high-quality graphene materials for the development of twistronics and spintronic devices.

The superconducting/tunnel magnetoresistance (TMR) magnetic sensor, which integrates a superconducting flux concentrator with a TMR element, offers a promising pathway toward achieving high-sensitivity magnetic field detection in compact configurations. However, the design and optimization of such devices have largely relied on time-consuming numerical simulations due to the lack of an efficient analytical framework. This paper presents an analytical model for calculating the amplification factor of a superconducting flux concentrator in a superconducting/TMR composite magnetic sensor. Based on the London theory and the Greenhouse method for inductance calculation, we derive a closed-form expression for the amplification factor as a function of key geometric parameters, including the effective area, inductance, and constriction width. The model is validated through finite element simulations and experimental measurements. The results show that the analytically predicted amplification factor agrees with experimental data within a relative error of 4.09% after considering the sensor-concentrator gap. The fabricated sensor achieves an equivalent magnetic noise of 7.02 pT/√Hz at 1Hz, demonstrating its high-resolution capability. This work provides a rapid and physically insightful design tool for developing next-generation high-performance composite magnetic sensors.

The superconducting/magnetoresistive (SC/MR) composite magnetic field sensor, which combines a superconducting flux transformation amplifier (SFTA) and a MR sensing element, significantly enhances the performance of magnetic field detection, featuring high resolution, small size, and low cost. The paper begins with a detailed elucidation of the sensor's operational principle. Subsequently, it systematically reviews the development history of the sensors, focusing on key aspects such as the selection of superconducting materials, structural optimization, and integration processes with MR sensors, and summarizes the current application status of the sensor in various fields. Meanwhile, this paper explores strategies to further optimize the performance of the sensor by increasing the flux amplification factor, expanding the working magnetic field range, and suppressing noise. In response to the shortcomings of current SC/MR composite sensors, this paper finally proposes future research directions. Through continuous optimization, the magnetic field resolution of these sensors is expected to reach the sub-femotesla (sub-fT) level, thereby meeting the requirements for high-precision detection of brain magnetic signals. This review aims to provide a reference for the design and practical application of SC/MR composite sensors. By demonstrating the efficient integration path of the SFTA and MR elements, it offers important ideas and inspirations for promoting the application and development of superconducting electronics in the field of weak magnetic field detection. It also has positive significance for expanding the functionalization and integration research of superconducting materials and devices.

Physical reservoir computing using magnetic tunneling junction leverages the inherent nonlinearity of physical systems for computation, offering advantages such as low energy consumption and low hardware overhead for the time-series data preprocessing. However, parallelism and multiple-signal processing abilities remain challenges for it. In this work, we utilized the synchronized oscillations between coupling magnetic tunneling junction with its spin-transfer torque oscillators (STOs) effect as a nonlinear dynamical resource to construct the PRC unit. Unlike traditional time-multiplexing methods, this network directly processes information through the voltage amplitude and frequency dynamics of magnetic coupled STO units. The results demonstrate that this architecture achieves accuracy and memory capacity in classification tasks compared to existing solutions. Furthermore, we also used it for aeromagnetic compensation. The results show that it significantly enhances the compensation accuracy and network convergence speed. This work validates the potential of magnetic tunneling junction for high-performance parallel computing and provides an innovative path for the design of low-power neuromorphic hardware.

The orthogonal two-step annealing process is an effective strategy to linearize the response of magnetic tunnel junctions for magnetic field sensors. However, the response after the orthogonal annealing is inevitably modulated by the Neel effect from the reference layer, which results in an unexpected shift of the linear interval and a disappointing sensitivity deterioration in the weak field. Here, a non-orthogonal two-step annealing method is proposed to suppress the shift by compensating for the Neel coupling field. Experimental results show that the curve shift of junctions annealed in the non-orthogonal direction of 120° is 47.6% lower than that in the orthogonal one, with a significant sensitivity promotion in the weak field and little hysteresis increment. A simple energy minimization model is introduced to explain the results. Based on the model, the suppression of the curve shift is fulfilled with the effective field compensation for the Neel coupling field, modulated by the non-orthogonal annealing. Finally, Wheatstone bridge devices are constructed, and the bridge with non-orthogonally annealed junctions is found to have an increased sensitivity of 46.8% in the major loop along the sensing axis. Additionally, the non-orthogonal annealing method is also effective in suppressing the cross sensitivity, which is important for further application to three-axis magnetic sensors.

A highly sensitive superconductor/tunnel magnetoresistance (TMR) composite magnetic sensor was designed and fabricated to satisfy the demands of detecting weak magnetic fields down to the pT range at low frequencies. First, analysis of the 1/f noise provided an optimized design scheme for the TMR unit to reduce the noise level. Then, the key structural parameters of the superconducting flux transformation amplifier (SFTA) were optimized with focus on the bonding quality between SFTA and TMR units using the finite element method to enhance the magnetic field magnification factor. An SFTA–TMR composite method with secondary alignment and multi-step bonding was proposed to fabricate the composite magnetic sensor and ensure the designed bonding gap and alignment accuracy. Experiments showed that the magnetic field sensitivity reached 5 pT/Hz^0.5 (@ 13 Hz), which is beyond the highest level of superconductor/TMR composite magnetic sensors reported to date and is a further step toward the application of weak magnetic field detection.

A superconductor/magnetoresistance (SC/MR) mixed sensor often has very high-precision detection capability, but it is only sensitive to the out-of-plane magnetic field that is perpendicular to the superconducting flux-to-field transformation amplifier (SFTA). This inherent characteristic makes it impossible for a primary SC/MR mixed sensor to measure the in-plane magnetic field, which limits its application in many fields. This paper presents a new architecture of SC/MR mixed sensors with two magnetic flux guides (MFGs) to realize in-plane magnetic field measurement, which is attributed to the combined effects of MFGs and SFTA. The finite element method simulations are used to study the magnetic flux converting effect of MFGs and magnetic field amplification effect of SFTA, which theoretically proves the feasibility of in-plane magnetic field measurement and reveals the influences of the sizes and spacing of MFGs on magnetic field magnification. Experimental results show that the SC/MR mixed sensor with MFGs can simultaneously sense the in-plane and out-of-plane magnetic fields, and the performance of the mixed sensor with MFGs is comparable to the primary one. The magnification and resolution of the mixed sensors with MFGs to the in-plane magnetic fields are, respectively, improved by 2.91 times and 2.23 times compared to those without MFGs to the out-of-plane magnetic fields. The work described in this paper is of great significance for the development of three-component weak magnetic field detection.

The strong spin filtering effect can be produced by C-Ni atomic orbital hybridization in lattice-matched graphene/Ni (111) heterostructures, which provides an ideal platform to improve the tunnel magnetoresistance (TMR) of magnetic tunnel junctions (MTJs). However, large-area, high-quality graphene/ferromagnetic epitaxial interfaces are mainly limited by the single-crystal size of the Ni (111) substrate and well-oriented graphene domains. In this work, based on the preparation of a 2-inch single-crystal Ni (111) film on an Al₂O₃ (0001) wafer, we successfully achieve the production of a full-coverage, high-quality graphene monolayer on a Ni (111) substrate with an atomically sharp interface via ambient pressure chemical vapor deposition (APCVD). The high crystallinity and strong coupling of the well-oriented epitaxial graphene/Ni (111) interface are systematically investigated and carefully demonstrated. Through the analysis of the growth model, it is shown that the oriented growth induced by the Ni (111) crystal, the optimized graphene nucleation and the subsurface carbon density jointly contribute to the resulting high-quality graphene/Ni (111) heterostructure. Our work provides a convenient approach for the controllable fabrication of a large-area homogeneous graphene/ferromagnetic interface, which would benefit interface engineering of graphene-based MTJs and future chip-level 2D spintronic applications.

The low frequency magnetic field detection ability of magnetoresistive (MR)sensor is seriously affected by 1/f noise. At present, the method to suppress the influence of low frequency noise is mainly to modulate the measured magnetic field by mechanical resonance. In this paper, a novel modulation concept employing a magnetoelectric coupling effect is proposed. A design method of modulation structure based on an equivalent magnetic circuit model (EMCM) and a single domain model of in-plane moment was established. An EMCM was established to examine the relationship between the permeability of flux modulation film (FMF) and modulation efficiency, which was further verified through a finite element simulation model (FESM). Then, the permeability modulated by the voltage of a ferroelectric/ferromagnetic (FE/FM) multiferroic heterostructure was theoretically studied. Combining these studies, the modulation structure and the material were further optimized, and a FeSiBPC/PMN-PT sample was prepared. Experimental results show that the actual magnetic susceptibility modulation ability of FeSiBPC/PMN-PT reached 150 times, and is in good agreement with the theoretical prediction. A theoretical modulation efficiency higher than 73% driven by a voltage of 10 V in FeSiBPC/PMN-PT can be obtained. These studies show a new concept for magnetoelectric coupling application, and establish a new method for magnetic field modulation with a multiferroic heterostructure.

To improve the sensitivity of the magnetic tunnel junction(MTJ)sensor, a novel architecture for a double-gap magnetic flux concentrator (MFC) was studied theoretically and experimentally in this paper. The three-dimensional finite element model of magnetic flux was established to optimize the magnetic field amplification factor, with different gaps. The simulation results indicate that the sensitivity of an MTJ sensor with a double-gap MFC can be significantly better than that of a sensor with a traditional single-gap MFC, due to the fact that the magnetic magnification sharply increases with the decrease in effective gap width. Besides this, the half-bridge MTJ sensors with the double-gap MFC were fabricated using photolithography, ion milling, evaporation, and electroplating processes. Experimental results show that the sensitivity of the MTJ sensor increased by ten times compared to the sensor without the double-gap MFC, which underlines the theoretical predictions. Furthermore, there is no significant increase in the sensor noise. The work in this paper contributes to the development of high-performance MTJ sensors.