External In-plane Magnetic Field Research Articles

Superconductivity in twisted graphene NbSe2 heterostructures

We study the low-energy electronic structure of heterostructures formed by one sheet of graphene placed on a monolayer of ${\rm NbSe_2}$. We build a continuous low-energy effective model that takes into account the presence of a twist angle between graphene and ${\rm NbSe_2}$, and of spin-orbit coupling and superconducting pairing in ${\rm NbSe_2}$. We obtain the parameters entering the continuous model via ab-initio calculations. We show that despite the large mismatch between the graphene's and ${\rm NbSe_2}$'s lattice constants, due to the large size of the ${\rm NbSe_2}$'s Fermi pockets, there is a large range of values of twist angles for which a superconducting pairing can be induced into the graphene layer. In addition, we show that the superconducting gap induced into the graphene is extremely robust to an external in-plane magnetic field. Our results show that the size of the induced superconducting gap, and its robustness against in-plane magnetic fields, can be significantly tuned by varying the twist angle.

Ballistic rectification based on inhomogeneous magnetic stray fields

We present a ballistic rectification effect in an orthogonal four-terminal cross junction where the symmetry is broken by local magnetic fields. The input current is injected between opposing branches and the current-free branches serve as potential probes. The local magnetic field is induced by two permalloy (Py) stripes with a magnetic single-domain structure, where one end of each stripe is positioned close to the junction center. The Py stripes are oriented such that an external in-plane magnetic field can magnetize them into two different main configurations having either equally or oppositely magnetized ends. Equal magnetic ends are expected to result in a Hall-effect device, while for opposite magnetic ends, the stray field should deflect the electrons into the same output lead for both current polarities, leading to a rectifying behavior. Here, we present the proof of concept for stray-field controlled transfer characteristics. First, we show by magnetic force microscopy that both configurations are stable and the Py stripes exhibit a remanent magnetic single-domain structure. Second, we demonstrate the influence of the remanent magnetization on the low-temperature dc characteristics which are superimposed by a parasitic background. Third, we present the extracted Hall and the rectified voltage which are, respectively, linearly and parabolically dependent on the input current up to ±55 μA.

Anisotropic Flux Penetration in Superconducting Nb Films With Frozen-in In-plane Magnetic Fields

The magnetic anisotropy of superconducting niobium films of different thickness was investigated using magneto-optical imaging. A wide range of anisotropic behaviors was generated by freezing in different in-plane external magnetic fields with magnitude up to 1.12 kOe. The critical fields for the parallel configuration of the different samples were calculated, taking into account the strong deviation of the superconducting properties at small thicknesses. Above their lower in-plane critical field, the films are in the mixed state containing coplanar vortices. In structurally isotropic Nb films, when applying a perpendicular magnetic field, the frozen-in vortices were seen to guide the perpendicular ones. It is shown that the anisotropy increases with both the film thickness and the magnitude of the in-plane field. The results demonstrate that the field-induced anisotropy is a highly versatile way to manipulate vortex matter in film superconductors.

Magnetically tunable optical diffraction gratings based on a ferromagnetic liquid crystal.

Transmission optical diffraction gratings composed of periodic slices of a ferromagnetic liquid crystal and a conventional photoresist polymer are demonstrated. Dependence of diffraction efficiencies of various diffraction orders on an in-plane external magnetic field is investigated. It is shown that diffraction properties can be effectively tuned by magnetic fields as low as a few mT. The tuning mechanism is explained in the framework of a simple empirical model and also by numerical simulations based on the rigorous coupled wave analysis (RCWA). The obtained results provide a proof of principle of operation of magnetically tunable liquid crystalline diffractive optical elements applicable in contactless schemes for control of optical signals.

Itinerant surfaces with spin-orbit couplings, correlations and external magnetic fields: exact results

ABSTRACTWe analyze, in exact terms, multiband 2D itinerant correlated fermionic systems with many-body spin-orbit interactions, and in-plane external magnetic fields. Even if such systems with broad applicability in leading technologies are non-integrable, we set up an exact solution procedure for them, which is described in details. Casting the Hamiltonian in positive semidefinite form, the technique leads to the ground state, and also characterises the low lying excitation spectrum.

Tuning the Magnetic Anisotropy of Fe3O4/Pt Heterostructures Fabricated by Atomic Layer Deposition With <inline-formula> <tex-math notation="LaTeX">$In~Situ$ </tex-math> </inline-formula> Magnetic Field

Research on tuning the magnetic anisotropy of magnetic heterostructures has been receiving intensive attentions due to their intriguing tuning phenomena. In particular, tuning the magnetic anisotropy by in situ magnetic field during the fabrication process allows a remarkable control of magnetism with energy efficiency and shows significant potential applications for compact, fast, and low-power microwave and electronic devices. In this paper, in situ external out-of-plane and in-plane magnetic fields were applied during the deposition of Fe3O4 thin films on Pt substrates by atomic layer deposition at different temperatures (350 °C, 400 °C, and 450 °C). The effects of in situ magnetic field with different directions on the surface morphologies and magnetic properties of F3O4/Pt heterostructures were characterized. It was found that both the out-of-plane and in-plane external magnetic fields tuned the magnetic anisotropy of ferromagnetic Fe3O4 when deposited at 400 °C with the maximum resonance field changes of 620 and 1930 Oe along in-plane and out-of-plane directions, respectively. This paper achieves tuning the magnetic properties through in situ magnetic field and provides a possibility for spintronic devices.

Andreev reflections and magnetotransport in 2D Josephson junctions

In this paper, the subharmonic energy gap structures (SGS) and induced superconductivity, due to multiple Andreev reflections, in indium gallium arsenide (In0.75Ga0.25As) two-dimensional electron gas (2DEG) are discussed at different temperatures and magnetic fields. Strong suppression of both SGS and induced gap are observed as a function of temperature and magnetic field. The differential conductance of the Josephson junctions (JJs) as a function of external in-plane magnetic fields shows an asymmetric response to positive and negative magnetic field sweeps with a conductance maximum at close to zero. Our approach to quantum transport studies of the ballistic 2D JJs may open a new road towards scalable and integrated quantum processing and help pave the way for the development of topological circuits for the realization of the next generation of quantum processors.

Observation of spin–orbit torque-induced magnetization switching in Gd-Fe perpendicular magnetized wire with in-plane exchange bias field

We fabricated a Ta/Gd-Fe/Ta and a Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire and investigated spin–orbit torque current-induced magnetization switching in these wires. Magnetization switching in the Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire can be observed by an electric current even if the external in-plane magnetic field is not applied at all. Moreover, we successfully observed the periodical magnetization switching in the Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire in zero magnetic field. This indicates that the present wire is a promising material to realize magnetic random access memory with low power consumption.

Magnetoelectric couplings in high-density array of nanoscale Co/BiFeO3 multiferroic heterostructures

We have systematically explored the magnetoelectric (ME) coupling effect of Co/BiFeO3 multiferroic heterostructured nanodot arrays, fabricated by the anodic aluminum oxide template method. Piezoresponse hysteresis loops of these nanodots demonstrate a significant enhancement of the ME coupling effect. More intriguingly, we have realized a magnetic domain transformation from an initial single-domain state to a vortex state by applying a regional or local voltage, and the single-domain state can be recovered by using an external in-plane magnetic field. Our results will guide the invention of high-density, energy-efficient, non-volatile multifunctional ME microdevices.

Switching of Perpendicular Magnetization via ac Spin-Orbit Torque

Current-induced magnetization switching by spin-orbit torques has become of technological interest for spintronics, because it provides high-speed operation while preserving device stability. However, conventional spin-orbit-torque switching requires a rather large switching current, and an external in-plane magnetic field for deterministic switching. In this study an alternative switching scheme using a circularly polarized alternating current is investigated, which allows not only a low switching current, but also field-free switching. This achievement provides a useful means to realize energy-efficient spin-orbit-torque devices.

Thickness-dependent magnetic domain structures of Co ultra-thin film investigated by scanning transmission X-ray microscopy

Thickness-dependent magnetic domain structure of ultrathin Co wedge films (0.3 nm–1.0 nm) sandwiched by Pt layers was investigated by scanning transmission x-ray microscopy (STXM) employing X-ray magnetic circular dichroism (XMCD), utilizing elliptically polarized soft x-rays and electromagnetic fields, with a spatial resolution of 50 nm. The magnetic domain images measured at the Co L3 edge showed the evolution of the magnetic domain structures from maze-like form to the bubble-like form as the perpendicular magnetic field was applied. The asymmetric domain expansion of a 500 nm-scale bubble domain was also measured when the in-plane and perpendicular external magnetic field were applied simultaneously.

Negative magnetoresistance in thin superconducting films with parallel orientation of current and magnetic field

Thin superconducting films can exhibit negative magnetoresistance when an in-plane external magnetic field is aligned parallelwith the transport current. We explain this effect as due to appearance of parallel vortices in the plain of the film at the first critical magnetic field Hc1 which leads to an enhancement of the superconducting properties and impedes the motion of the current induced perpendicular vortices. Our theoretical results are based on a numerical solution of the time-dependent and stationary 3D Ginzburg-Landau equations.

Deterministic Current Induced Magnetic Switching Without External Field using Giant Spin Hall Effect of \u03b2-W

Giant spin Hall effect (GSHE) has received significant attention for its potential in future spintronic applications. Spin current via GSHE-based thin films provides an effective and promising means to manipulate magnetization. However, an external in-plane magnetic field is required to consistently switch the perpendicular magnetic moment. We present an approach to realize field-free deterministic perpendicular magnetic switching with a new structure of FM/NM/FM. Our method takes advantage of the large spin Hall angle of transition metal β-W, so that the critical switching current density is only on the order of 106A/cm2 in the absence of magnetic field.

Study of Nucleation/Annihilation Process and Vortices Characteristics in Co/Py Rectangular Bilayers

Vortices are topological structures, which are characterized by circular organization of magnetic moments and by chirality. In this paper, we investigate the impact of film lateral dimensions and thicknesses on magnetization reversibility by applying an in-plane external magnetic field on Co(tCo)/Py (3 nm) bilayers where the Py is the permalloy, a ferromagnetic alloy of nickel and iron (Ni80Fe20). The study is achieved by using appropriate micromagnetic simulations. We also show that the vortices number and their sizes can be controlled by playing on the lateral dimensions and the thickness of the bilayer.

Self-Adaptive Write Circuit for Magnetic Tunneling Junction Memory With Voltage-Controlled Magnetic Anisotropy Effect

Currently, commercial semiconductor-based memories face the problem of static energy consumption caused by leakage currents. Magnetoelectric random access memory (MeRAM), as a type of nonvolatile memory, provides a solution to this problem and can be used to replace the entire memory hierarchy as a universal memory. In contrast to spin transfer torque RAM (STT-RAM), MeRAM uses voltage controlled magnetic anisotropy (VCMA) effect to manipulate the switching of magnetic tunnel junctions (MTJs). MeRAM or VCMA-MTJ technology is more energy efficient and less likely to break down than STT-RAM. However, VCMA-MTJ suffers from serious write problems caused by variations in the VCMA coefficient, the external in-plane magnetic field, and the CMOS process. In this paper, a compact and SPICE-compatible VCMA-MTJ model is proposed. Then, a self-adaptive write circuit is proposed in which the write process is robust to variations in the VCMA coefficient and the external in-plane magnetic field. Finally, two practical methods are proposed to make our proposed self-adaptive write circuit also robust to 3 $\sigma$ CMOS process variations, as demonstrated by Monte Carlo simulations. The proposed self-adaptive write circuit can assist in the design of next-generation high-speed, low-power MeRAM chips.

Magnetic Domain Patterns in Bilayered Ribbons Studied by Magnetic Force Microscopy and Magneto-Optical Kerr Microscopy.

The magnetic domain patterns of amorphous bilayered FeSiB/FeNbSiB and FeNbCuSiB/CoSiB ribbons are observed and analysed using the magneto-optical Kerr microscopy (MOKM) and magnetic force microscopy (MFM). Both microscopic techniques are highly sensitive to the sample surface; possibility of Kerr microscopy to visualize the domains separately in both layers is achieved by focusing the laser spot on the ribbon cross section. Wide curved domains as well as fine fingerprint domains were detected at the surface of ribbons due to presence of local stresses coming from the preparation process. With respect to high lateral resolution of MFM and its out-of-plane magnetization sensitivity, the perpendicularly magnetized crossed stripe domain patterns can be selected as well. Coiling of the ribbons on the half-round-end sample holder is often used to induce and control the magnetic anisotropy of these alloys. Changes in the magnetic domain structure at the outer-coiled surface and its dependence on the sign of magnetostriction coefficient are discussed in detail. Finally, the MFM images in the presence of external in-plane magnetic field up to ±40 kA/m are shown.

An integrated tunable isolator based on NiZn film fabricated by spin-spray plating

An innovative type of tunable isolator with a planar comb-like microstrip transmission line, which generate circular polarization magnetic field, has been realized with polycrystalline NiZn ferrite thick films fabricated by spin-spray plating (SSP) process with thickness of 10μm. The phase compositions, microstructure, magnetic hysteresis loop, and ferromagnetic resonance (FMR) linewidth of NiZn ferrite thick films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and electron spin resonance (ESR) spectrometer, respectively. The NiZn ferrite thick films possess 4800Gauss saturation magnetization and 190Oe FMR linewidth measured at X-band. With an in-plane dc magnetic bias perpendicular to the comb-like microstrip transmission line, the transmission direction of left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP) were proved to be opposite. The non-reciprocal ferromagnetic resonance absorption leads to 11.6dB isolation and 5.78dB insertion loss at 17.57GHz with magnetic bias field of 3.5kOe. Furthermore, with external in-plane magnetic fields range from 0.5kOe to 3.5kOe, the central frequency was tuned from 5.63GHz to 17.57GHz. The state-of-the-art tunable isolator with a planar comb-like microstrip transmission line exhibit a great potential to be applied in different microwave components and radar system.

Novel Magnetic Tunneling Junction Memory Cell With Negative Capacitance-Amplified Voltage-Controlled Magnetic Anisotropy Effect

The high current density required by magnetic tunneling junction (MTJ) switching driven by the spin transfer torque (STT) effect leads to large power consumption and severe reliability issues, hindering the timetable for STT magnetic random access memory to mass market. By utilizing the voltage-controlled magnetic anisotropy (VCMA) effect, the MTJ can be switched by the voltage effect and is postulated to achieve ultralow power (fJ). However, the VCMA coefficient measured in experiments cannot meet the requirement for MTJ with dimensions below 100 nm. And an external in-plane magnetic field usually is demanded for precessional VCMA switching. Here, in this paper, a novel approach for the amplification of the VCMA effect, which borrows ideas from negative capacitance, is proposed. The feasibility of the proposal is proved by physical simulation and in-depth analysis. Since the amplified VCMA effect, the external magnetic field can be eliminated. A three-terminal novel MTJ memory cell is designed with which both low power and high speed can be achieved.

Rashba induced spin multistability of the intersubband optical absorption in asymmetric coupled quantum wells

We study the multistable behavior of the intersubband optical absorption for InSb-based tunnel-coupled quantum wells. We consider four sublevels coming from the splitting of the two deepest levels due to the inversion asymmetry of the structure (Rashba effect), and a weak external in-plane magnetic field (Zeeman effect). Photoexcitation with an intense terahertz pump produces the redistribution of nonequilibrium electrons among the four spin sublevels. The redistribution produces a photoinduced self-consistent potential, giving rise to the renormalization of energy distance between sublevels. Depending on total electron concentration, magnetic field intensity, and pumping efficiency, we find different multistable behaviors in the intersubband optical absorption spectrum. Based on the matrix density, we describe the electron redistribution by means of a system of balance equations for electron concentrations.

Role of Perpendicular Anisotropy in the Zigzag Domain Wall Structure in Thin Magnetic Films

We investigate apparently zigzag domain walls in ultrathin magnetic films and calculate the total energy in zero applied field considering anisotropies in the second-order approximation. We discuss the change in zigzag wall morphology and scale as a function of the dipolar length and the total energy depend on film parameters exclusively anisotropy constants, crystallographic orientation and film thickness for a given dipolar length. We explain also the role of the second-order anisotropy in the total energy and the dynamics response of the in-plane magnetization for a given dipolar length. We use some constants in connection with experimental data reported for amorphous TbCo films grown in external in-plane magnetic field by high-frequency ion sputtering and for MnAs on GaAs films.