Antiparallel Magnetization Configurations Research Articles

Optoelectronically controlled transistor and magnetoresistance effect in an antiferromagnetic graphene-based junction

We investigate the optoelectronic spin and spin-valley transports and magnetoresistance (MR) effect in a graphene-based junction. The results show that by modulating the direction of an electric field, the off state and the on state with fully spin-polarized currents can be realized for both parallel (P) and antiparallel (AP) magnetization configurations, because the spin-polarized directions between two antiferromagnetic (AFM) regions are opposite and consistent, respectively. Moreover, when the off-resonant circularly polarized (ORCP) light is further radiated on two AFM regions, pure spin current can be further switched into four types of fully spin-valley-polarized currents, which results in an optoelectronically controlled transistor. In particular, the conductances in the P and AP magnetization configurations are either equal or dramatically different, so the optically and electrically controlled MR effect is naturally formed that can be switched from 0 to 1. Our results suggest that graphene has a very promising potential for applications in spintronics and spin-valleytronics.

Gap engineering effects on transport and tunneling magnetoresistance properties in phosphorene ferromagnetic/normal/ferromagnetic junction

Tuning the band gap is of utmost importance for the practicality of two-dimensional materials in the semiconductor industry. In this study, we investigate the ballistic transport and the tunneling magnetoresistance (TMR) properties within a modulated gap in a ferromagnetic/normal/ferromagnetic (F/N/F) phosphorene junction. The theoretical framework is established on a Dirac-like Hamiltonian, the transfer matrix method, and the Landauer–Büttiker formalism to characterize electron behavior and obtain transmittance, conductance and TMR. Our results reveal that a reduction in gap energy leads to an enhancement of conductance for both parallel and anti-parallel magnetization configurations. In contrast, a significant reduction and redshift in TMR have been observed. Notably, the application of an electrostatic field in a gapless phosphorene F/N/F junction induces a blueshift and a slight increase in TMR. Furthermore, we found that introducing an asymmetrically applied electrostatic field in this gapless junction results in a significant reduction and redshift in TMR. Additionally, intensifying the applied magnetic field leads to a substantial increase in TMR. These findings could be useful for designing and implementing practical applications that require precise control over the TMR properties of a phosphorene F/N/F junction with a modulated gap.

Spin valve effect in CrN/GaN van der Waals heterostructures

In the pursuit of developing high-performance low-dimensional spintronic devices, two-dimensional van der Waals heterostructures comprising non-magnetic nitride and ferromagnetic materials have emerged as a crucial area of investigation. This paper proposes a novel structure that employs hexagonal two-dimensional semiconductor GaN flanked by two half-metallic two-dimensional CrN layers. The stability of the proposed structure is verified via first-principles calculations, which indicate good thermodynamic and magnetic stability. The transport characteristics of electrons in the structure are analyzed through the Band structures and density of states. Specifically, the study examines the spin polarizability of parallel and anti-parallel magnetic configurations of the CrN/GaN/CrN vdW heterostructure, and the results demonstrate a significant spin valve effect. Overall, the study establishes the potential of the CrN/GaN/CrN vdW heterostructure as a candidate for the development of innovative spintronic devices.

Giant tunnel magnetoresistance induced by thermal bias

We analyze the spin-resolved transport and, in particular, the tunnel magnetoresistance of an asymmetric ferromagnetic tunnel junction with an embedded quantum dot or molecule subject to thermal and voltage bias in the nonlinear response regime. We demonstrate that such system exhibits a giant tunnel magnetoresistance effect that can be tuned by gate and bias voltages. Large values of magnetoresistance are associated with the interplay between the Kondo correlations and the ferromagnetic-contact-induced exchange field. In particular, we show that the nonequilibrium current in the parallel and antiparallel magnetic configuration of the system changes sign at different values of the voltage and thermal bias. This gives rise to giant values of magnetoresistance, the sign of which can be controlled by the applied sources.

Tunable valley splitting in two-dimensional CrBr3/VSe2 van der Waals heterostructure under strains and electric fields

Valleytronics opens up fascinating opportunities for using the valley degree of freedom in information storage and quantum computation. Here, based on the first-principles calculations, we investigate the effects of biaxial strains and electric fields on the magnetic, electronic, and valleytronic properties of two-dimensional CrBr3/VSe2 van der Waals (vdW) heterostructure consisting of two ferromagnetic monolayers. An interlayer magnetic phase transition from parallel to antiparallel is found when a compressive strain exceeds or a tensile strain exceeds 4% is applied, while the interlayer magnetic configuration remains parallel under perpendicular electric fields. The valley splitting in the conduction bands is significantly enhanced by a compressive strain or an electric field pointing from the VSe2 to the CrBr3 layer. Specifically, a large valley splitting about 30.8 meV is obtained in the system with antiparallel interlayer magnetic configurations under a compressive strain of , which is more than three times that of pristine CrBr3/VSe2 heterostructure. Our findings provide new insights into the future valleytronic applications for two-dimensional magnetic vdW heterostructures.

Tunneling magnetoresistance calculation for double quantum dot connected in parallel shape to ferromagnetic Leads

In this paper, a theoretical model for electron transport through symmetric system consisting of two baths interferometer with one single-level quantum dot in each of its arms was considered. In this model, the dots are attached to ferromagnetic leads with parallel and antiparallel magnetic configurations. Green's function technique in this model was used. Our focus is on the Transport characteristics of conductance (G) and tunnel magnetoresistance (TMR). A special attention to the influence of an applied magnetics flux on the characteristics of conductance and tunneling magnetoresistance was paid. Concerning the study of the conductance, it was found that the effect of bonding (antibonding) states is most obvious in quantum dots at various values of the magnetic field. The change in spin-polarization value was seen to affect the increase and decrease in the conductance value. We noticed a difference in calculation of TMR in the bonding and the antibonding states, where the results show Strong dissonance in bonding state and strong attraction in antibonding state.

Magnetic domain control and its dependence on aspect ratio and thickness in Ni nanolayer patterns for nanowire spintronic devices

We investigate the aspect ratio and thickness dependence of magnetic domain formation in multiple types of ferromagnetic Ni nanolayer electrode patterns. Controlling magnetic domains is critical for spintronic devices using a group IV semiconductor, e.g. Si and Ge, nanowire as the electrodes with magnetic tunnel junction require parallel and anti-parallel magnetization configurations. Single magnetic domains are obtained in the Ni nanolayer electrode patterns on SiO2/Si substrate with an aspect ratio of 20 and a thickness of 40 nm even under the as-deposition condition, while other electrode patterns are mostly showing multiple magnetic domains. The results obtained by magnetic force microscopy also show that the magnetization switching is observed in the electrode pattern with a single magnetic domain. The results in this study show that the suitable design of the dimensions of nanolayer patterns is key to constructing a single magnetic domain in a ferromagnetic electrode for nanowire spintronic devices.

Regulating the electronic properties of the WGe2N4 monolayer by adsorption of 4d transition metal atoms towards spintronic devices.

We study the regulation of the electronic and spin transport properties of the WGe2N4 monolayer by adsorbing 4d transition metal atoms (Y-Cd) using density functional theory combined with non-equilibrium Green's function. It is found that the adsorption of transition metal atoms (except Pd, Ag and Cd atoms) can introduce a magnetic moment into the WGe2N4 monolayer. Among the transition metal atoms, the adsorption of Nb and Rh atoms transforms WGe2N4 from a semiconductor to a half-metal and a highly spin-polarized semiconductor, respectively. The half-metallic Nb-adsorbed WGe2N4 system is selected to investigate the spin transport properties, and a high magnetoresistance ratio of 107% is achieved. In both parallel and antiparallel magnetization configurations, the spin filtering efficiency reaches close to 100% in the whole bias range, and the antiparallel magnetization configuration exhibits a dual spin filtering effect with a rectification ratio of up to 104. Our study predicts that the adsorption of 4d transition metal heteroatoms is an effective method to regulate the electronic and magnetic properties of WGe2N4 towards high-performance spintronic devices.

Momentum matching induced giant magnetoresistance in two-dimensional magnetic tunnel junctions.

Giant magnetoresistance was first experimentally discovered in three-dimensional magnetic tunnel junctions (MTJs) in the late 1980s and is of great importance in nonvolatile memory applications. How to achieve a magnetoresistance as large as possible is always a central task in the study of MTJs. However, it is normally only of the order of magnitude of tens of percent in traditional MTJs. The ideal situation is the metal-insulator transition together with the magnetization reversal of one magnetic lead. In this work, we will show that this can be achieved using a two-dimensional ferromagnetic zigzag SiC nanoribbon junction based on quantum transport calculations performed with a combination of density functional theory and non-equilibrium Green's function. Specifically, with the magnetization configuration switching of the two leads from parallel to anti-parallel, the junction will change abruptly from a conducting state to an insulating state, although the two leads are always metallic, with both spin up and spin down channels crossing the Fermi level simultaneously. Extensive analysis indicates that the insulating state in the anti-parallel magnetic configuration originates not from any present mechanisms that cause full suppression of electron transmission but from momentum direction mismatching. This finding suggests a fantastic mechanism for achieving magnetoresistance or electrical switching in nanoscale devices by manipulating band dispersion.

Effect of interfacial nitrogen defects on tunnel magnetoresistance in an Fe/MgO/Fe magnetic tunnel junction

We investigated the effect of interfacial nitrogen (N) defects on tunnel magnetoresistance (TMR) in Fe/MgO/Fe magnetic tunnel junctions (MTJs) which are the basic building block of magnetoresistive random access memory. The N atoms are predicted to originate from the SiN covering for antioxidation. It was found from first-principles quantum-transport calculations that the N defects significantly worsen the TMR. This is particularly evident in the MTJ models with an additional N atom at the MgO/Fe interface, because a conduction channel appears in the antiparallel magnetization configuration due to the N defects. The TMR is directly related to the read error rate of data and the scaling of the memory cell. Therefore, the prevention of nitrogen contamination during the manufacturing processes is a prerequisite for maintaining high performance.

Conductance and shot noise in a silicene-based superconducting superlattice with two ferromagnetic electrodes

Using the scattering matrix method and the Blonder–Tinkham–Klapwijk formalism, we theoretically study the transport properties and the shot noise in a silicene-based superconducting superlattice (SCSL) with two ferromagnetic electrodes . It is found that for the antiparallel (AP) magnetization configuration, the crossed Andreev reflection (CAR) probability for spin-up electron from the K valley shows the nearly perfect transmission peak at the superconducting energy gap. With increasing the exchange splitting energy, the transmission probabilities of the elastic cotunneling and CAR processes for spin-up electron from the K valley around zero incident angle show the valley–peak–valley transitions. For the AP magnetization configuration, the local Fano factor shows the super-Poissonian value and the nonlocal Fano factor forms a Poissonian value plateau with weak oscillation within a suitable parameter regime. Compared to the local Fano factor, the nonlocal Fano factor is always sub-Poissonian for the parallel magnetization configuration, but for the AP magnetization configuration its maximum value increases and becomes super-Poissonian as the number of layers in the SCSL increases. • The crossed Andreev reflection probability shows nearly perfect transmission peak. • The transmission probabilities are symmetric about the incident angle of electron. • The super-Poissonian Fano factor is found. • The Poissonian value plateaus of the local and nonlocal Fano factors are formed. • The energy range with pure crossed Andreev reflection is robust to layer number.

Improvement of tunneling magnetoresistance and spin-valley polarization in magnetic silicene superlattices induced by structural disorder

It is well known that the unavoidable variation of the heights and widths of the barriers (structural disorder) in semiconductor superlattices and superlattices based on two-dimensional materials degrades important physical properties such as the electronic transport and the thermoelectric response. Here, we show that the structural disorder contrary to what is expected improves the magnetoresistive and spin-valleytronic properties of magnetic silicene superlattices. We reach this conclusion by studying the impact of the random variations of the width and strength of the magnetic barriers on the tunneling magnetoresistance and spin-valley polarization. The theoretical treatment is based on a Dirac-like Hamiltonian, the transfer matrix method, and the Landauer-B\"uttiker formalism to describe silicene electrons, to obtain the transmittance, and to obtain the conductance, respectively. Our results indicate that structural disorder effects improve the magnetoresistance response and the spin-valley polarization by eliminating the conductance oscillations caused by the periodic magnetic modulation as well as by differentiating the response of the conductance for the parallel and antiparallel magnetization configuration. These results could be useful in designing versatile devices with magnetoresistive and spin-valleytronic capabilities. Particularly, magnetic silicene superlattices with moderate structural disorder are more convenient than perfect or nearly perfect ones.

Comparison of spin-wave transmission in parallel and antiparallel magnetic configurations

Parallel (P) and antiparallel (AP) configurations are widely applied in magnetic heterostructures and have significant impacts on the spin-wave transmission in magnonic devices. In the present study, a theoretical investigation was conducted into the transmission of exchange-dominated spin waves with nanoscale wavelengths in a type of heterostructure including two magnetic media, of which the magnetization state can be set to the P (AP) configuration by ferromagnetic (antiferromagnetic) interfacial exchange coupling (IEC). The boundary conditions in P and AP cases were derived, by which the transmission and reflection coefficients of spin waves were analytically given and numerically calculated. In the P configuration, a critical angle $\theta_{\textrm{c}}$ always exists and has a significant influence on the transmission. Spin waves are refracted and reflected when the incident angle $\theta_{\textrm{i}}$ is smaller than the critical angle ($\theta_{\textrm{i}} < \theta_{\textrm{c}}$), while total reflection occurs as $\theta_{\textrm{i}} \geq \theta_{\textrm{c}}$. In the AP configuration, the spin-wave polarizations of medium 1 and 2 are inverse, that is, right-handed (RH) and left-handed (LH), leading to the total reflection being independent of $\theta_{\textrm{i}}$. As demonstrated by the difference in spin-wave transmission properties between the P ($\theta_{\textrm{i}} < \theta_{\textrm{c}}$) and AP cases, there is a polarization-dependent scattering. However, as $\theta_{\textrm{i}}$ exceeds $\theta_{\textrm{c}}$, the P ($\theta_{\textrm{i}} > \theta_{\textrm{c}}$) case exhibits similarities with the AP case, where the transmitted waves are found to be evanescent in medium 2 and their decay lengths are investigated.

Einstein-de Haas Nanorotor.

We propose a nanoscale rotor embedded between two ferromagnetic electrodes that is driven by spin injection. The spin-rotation coupling allows this nanorotor to continuously receive angular momentum from an injected spin under steady current flow between ferromagnetic electrodes in an antiparallel magnetization configuration. We develop a quantum theory of this angular-momentum transfer and show that a relaxation process from a precession state into a sleeping top state is crucial for the efficient driving of the nanorotor by solving the master equation. Our work clarifies a general strategy for efficient driving of a nanorotor.

Full-Electrical Writing and Reading of Magnetization States in a Magnetic Junction with Symmetrical Structure and Antiparallel Magnetic Configuration.

Full-electrical writing and reading of magnetization states are vital for the development of next-generation spintronic devices with high density and ultralow-power consumption. Here, we proposed a method to realize the full-electrical writing and reading of magnetization states via a structural design, which only requires a symmetrical device structure and an antiparallel magnetic configuration. CrBr3, h-BN, and 1T-MnSe2 were selected to construct the device of CrBr3/h-BN/1T-MnSe2/h-BN/CrBr3, where the magnetization of two CrBr3 layers was fixed to the antiparallel state. By changing the direction and magnitude of the applied electric field, it is proved that the magnetization of 1T-MnSe2 could be reversed. Moreover, the device energies before and after the magnetization reversal are the same when the applied electric field is removed due to the structural symmetry. Meanwhile, the magnetic anisotropy energy of 1T-MnSe2 could induce an energy barrier, to guarantee the nonvolatile magnetization reversal in the present device. In addition, the tunnel magnetoresistance ratio was found up to 421%, showing a promising application to full-electrically write and read magnetization in spintronics. The present study likely promotes the development of full-electrical and ultralow-power spintronics devices.

Spin- and valley-polarized transport and magnetoresistance in asymmetric ferromagnetic WSe2 tunnel junctions

Transition metal dichalcogenides (TMDs) offer a new platform for theoretical study of two-dimensional materials and their applications, beyond graphene. Previous studies indicate that monolayers of TMDs become ferromagnetic when put in proximity to conventional ferromagnetic, leading to remarkable spin and valley transport properties when combined with the intrinsic spin-orbit coupling (SOC) inherent in these materials. In this work, we show that a magnetic tunnel junction setup consisting of an asymmetric ferromagnetic/ferromagnetic/normal ${\mathrm{WSe}}_{2}$ junction gives rise to a fully spin- and valley-polarized current for both parallel and antiparallel magnetization configurations in the presence of an off-resonance light. Due to the strong SOC of ${\mathrm{WSe}}_{2}$ together with off-resonance light, both the spin and valley polarizations are tunable and switchable. We also find that the tunneling magnetoresistance (TMR) could be tuned to 1 by off-resonance light. The relatively much stronger SOC in the conduction and valence bands of ${\mathrm{WSe}}_{2}$ compared to other compounds of the dichalcogenides family in collaboration with an off-resonance light leads to significant negative TMR at weak enough exchange fields. In appropriate parameter regimes, the TMR oscillations from negative to positive values can be tuned by the off-resonance light. Our results suggest that magnetic tunnel junctions involving ${\mathrm{WSe}}_{2}$ have very promising potential for applications in magnetic memory and spin and valleytronics devices.

Huge tunneling magnetoresistance in magnetic tunnel junction with Heusler alloy Co2MnSi electrodes

Magnetic tunnel junction with a large tunneling magnetoresistance has attracted great attention due to its importance in the spintronics applications. By performing extensive density functional theory calculations combined with the nonequilibrium Green’s function method, we explore the spin-dependent transport properties of a magnetic tunnel junction, in which a non-polar SrTiO3 barrier layer is sandwiched between two Heusler alloy Co2MnSi electrodes. Theoretical results clearly reveal that the near perfect spin-filtering effect appears in the parallel magnetization configuration. The transmission coefficient in the parallel magnetization configuration at the Fermi level is several orders of magnitude larger than that in the antiparallel magnetization configuration, resulting in a huge tunneling magnetoresistance (i.e. &amp;gt; 106), which originates from the coherent spin-polarized tunneling, due to the half-metallic nature of Co2MnSi electrodes and the significant spin-polarization of the interfacial Ti 3d orbital.

Full range modulation of giant magnetoresistance in graphene–like zigzag nanoribbons via dual edge disorders

The effects of dual edge disorders on the giant magnetoresistance RM in graphene-like zigzag nanoribbons of width number n (n-ZNRs) are studied systematically using the tight-binding model with non-equilibrium Green's functions. Manipulation of RM in the full range from −100% to 100% can be realized if the disorder potentials on the lower and upper edges have the same strength but opposite signs. This occurs because the spin-up and spin-down conductance may vanish together not only in the parallel and but also in the antiparallel magnetic configuration, benefiting from the symmetry of the energy band structure in honeycomb lattice. The disorder potential ratio and positions can also effectively modulate RM as well as the ZNR width and edge magnetization. Distinguished patterns of RM behavior are observed in n-ZNRs of even and odd n, respectively. The patterns are expected insensitive to the width of the domain wall and remain qualitatively similar as soon as the relative positions of the dual disorders hold fixed.

Tunneling magnetoresistance and spin-valley polarization in magnetic silicene superlattices

We study the tunneling magnetoresistance and the spin-valley transport in silicene-based magnetic superlattices. The superlattice profile is obtained by the periodic modulation of the exchange field induced by ferromagnetic electrodes. The silicene band gap in the superlattice structure is also modulated through an external perpendicular electric field. The concept of parallel and antiparallel magnetization configurations of single magnetic junctions is extended to the periodic case by switching the magnetization orientation of the adjacent magnetic barriers in parallel and antiparallel fashion. The transfer matrix method and the Landauer-B\"uttiker formalism are used to obtain the transmission and transport properties, respectively. We find an oscillating conductance once the periodic modulation is incorporated. By tuning the external perpendicular electric field a conductance gap is obtained for the antiparallel configuration, which results in an enhancement of the tunneling magnetoresistance with respect to single magnetic junctions. In the case of the spin-valley polarization it is not possible to obtain two well-defined polarization states by simply switching the magnetization orientation, as in single magnetic junctions, due to the equivalence of the spin-valley conductance components. However, by inducing structural asymmetry in the width of barriers-wells, two well-defined polarization states can be reached. Moreover, an additional enhancement of the tunneling magnetoresistance is induced by the structural asymmetry. Our findings indicate that magnetic periodic modulation can be an option to improve the tunneling magnetoresistance and the spin-valley polarization of silicene-based structures.

Large magnetoresistance in a Co/MoS2/graphene/MoS2/Co magnetic tunnel junction

We demonstrate a large magnetoresistance (MR) in a $\mathrm{Co}/\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{Co}$ magnetic tunnel junction by means of ab initio transport calculations. A Co electrode turns out to be an excellent spin injector for a $\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_{2}$ barrier. The transmission spectrum, current-voltage characteristics, spin injection efficiency, and magnetoresistance are calculated for the modeled device at various bias voltages in the parallel and antiparallel magnetic configurations. A remarkable change in the transmission spectrum and a subsequent change in total current through the junction have been observed, when the relative magnetic orientations of the electrodes are altered. The huge change in current due to the change in the relative magnetic orientation of the Co electrodes produces a high magnetoresistance up to $1270%$. The obtained values of the device parameters clearly indicate that a $\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_{2}$ heterostructure would be an excellent compound for highly efficient spin-valve device applications.