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.

The effect of light-irradiated area on the spin dependent photocurrent in zigzag graphene nanoribbon junctions

In this work, we study the photogalvanic effect of a zigzag graphene nanoribbon junction with a centro-symmetrical structure which consists of 8 zigzag chains by density functional calculations. Specifically, we focus on the cases where the irradiated region is just part of the central region and located at different positions, with an aim to see how the spin dependent photocurrents will change and whether pure spin current can be obtained. It is found that the magnitude of the spin-dependent photocurrents increases with a gradual increase of the irradiated region and pure spin current is achieved when and only when the entire central region is irradiated. In addition, we studied the additive effect in this device to see that if we divide the central region into two parts, whether the sum of the spin current generated by irradiating the two parts individually is equal to that produced when the entire central region is irradiated. It is found that the sum of the spin currents produced by irradiating the two parts individually is smaller than that obtained by irradiating the whole central region, which means that the rule of "1 + 2 = 3" does not hold and the coupling effect between the two parts is important in photocurrent generation.

Giant tunneling electroresistance in a 2D bilayer-In2Se3-based out-of-plane ferroelectric tunnel junction

Ferroelectric tunnel junctions (FTJs) have great potential in nonvolatile memory devices and have been extensively studied in recent years. Compared with conventional FTJs based on perovskite-type oxide materials as the barrier layer, two-dimensional (2D) van der Waals ferroelectric materials are advantageous in improving the performance of FTJs and achieving miniaturization of FTJ devices due to the features such as atomic thickness and ideal interfaces. In this work, we present a 2D out-of-plane ferroelectric tunnel junction (FTJ) constructed using graphene and bilayer-In₂Se₃. Using density functional calculations combined with the nonequilibrium Green's function technique, we investigate the electron transport properties in the graphene/bilayer-In₂Se₃ (BIS) vdW FTJ. Our calculations show that the FTJ we constructed can be switched from ferroelectric to antiferroelectric by changing the relative dipole arrangement of the BIS to form multiple nonvolatile resistance states. Since the charge transfer between the layers varies for the four different polarization states, the TER ratios range from 10³% to 10¹⁰%. The giant tunneling electroresistance and multiple resistance states in the 2D BIS-based FTJ suggest that it has great potential for application in nanoscale nonvolatile ferroelectric memory devices.

Bipolar spin-filtering and giant magnetoresistance effect in spin-semiconducting zigzag graphene nanoribbons

Spintronics is one of the main topics in condensed matter physics, in which half-metallicity and giant magnetoresistance are two important objects to achieve. In this work, we study the spin dependent transport properties of zigzag graphene nanoribbons (ZGNR) with asymmetric edge hydrogenation and different magnetic configurations using the non-equilibrium Green's function method combined with density functional calculations. Our results show that when the magnetic configurations of the electrodes change from parallel to antiparallel, the currents in the tunnel junction change substantially, resulting in a high conductance state and a low conductance state, with the tunnel magnetoresistance (TMR) ratio larger than 1 × 10⁵% achieved. In addition, in the parallel magnetic configurations, an ideal bipolar spin filtering effect is observed, making it flexible to switch the spin polarity of current by reversing the bias direction. All these features originate from the spin semiconducting behavior of the asymmetrically hydrogenated ZGNRs. The findings suggest that asymmetric edge hydrogenation provides an important way to construct multi-functional spintronic devices with ZGNRs.

Realizing pure spin current by the photogalvanic effect in armchair graphene nanoribbons with nano-constriction engineering

We propose nano-constriction engineering of armchair graphene nanoribbons (AGNRs) to construct photoelectric nanodevices aiming to generate pure spin currents through the photogalvanic effect (PGE) using first-principles calculations. Two devices with different symmetries were designed, one by introducing only one isosceles zigzag triangle defect on the lower edge of the central region ('D1') and the other by two symmetrically distributed isosceles zigzag triangle defects on the two edges ('D2'). The results show that pure spin current without accompanying charge current can be generated in both junctions, but with a big difference that pure spin current can be generated only at special polarization angles θ = 0°, 90° and 180° in device D1, while it can be generated at any polarization angle in D2. The robustness in D2 is attributed to the spatial inversion symmetry in geometry and the inversion antisymmetry of spin density. These findings suggest that local magnetism engineering provides a reliable method for generating robust pure spin currents with the PGE in nonmagnetic systems, especially opening up new possibilities for the application of AGNRs in spintronics.