Magnetoresistance In Junctions Research Articles

GMR Based Spin Valve like Magnetic Diode Exploiting Negative JMR in Heusler Alloy Thin Film/Semiconductor Heterostructures

Room temperature spin valve like magnetic diode behavior has been found in Co2MnSi/SiO2/n-Si heterostructure. The Co2MnSi (CMS) Heusler alloy film has been grown on n type Si (001) substrate with the help of Electron Beam Physical Vapour Deposition technique. We have investigated the transport properties of our heterostructure at various isothermal conditions in the temperature regime of 78-300 K. From the current (I) – voltage (V) characteristics of the junction, excellent rectifying tunnel diode like behavior have been seen near room temperature. The current (I) across the junction has been found to be increased with the application of magnetic B-field parallel to the CMS film clearly indicating negative Junction Magnetoresistance (JMR) of the heterostructure. It has been found to be negative 80% at 300 K at a magnetic field of 0.5 Tesla. When forward dc bias is applied to the heterostructure, the I-V characteristics are greatly influenced on turning on the field B = 0.5 T at 300 K, and the current reduces to the equal order of the magnitude of the reverse bias current. Hence our CMS/SiO2/n-Si heterostructure can perform in off (Ioff)/on (Ion) states with the use of zero/nonzero magnetic field like a spin valve at room temperature (300 K).

Implication of Hydrogenation in Tuning the Magnetoresistance of Graphene-Based Magnetic Junction

Hydrogenation is suggested as a preferred method for opening bandgap (BG) in graphene for use in spintronic devices. Magnetoresistance (MR) in graphene is found to increase with hydrogenation, which is in contrast to MR obtained through other methods and in pristine graphene. Hydrogenation also results in higher spin filtration (SF) with increase in bandgap in comparison to their values obtained through other methods. Bandgap opening of around ∼5.96 eV in graphene is obtained with hydrogenation. Results suggest that MR in hydrogenated structure is larger than the MR of pristine graphene. MR values of ∼99% are observed with hydrogenation in comparison to ∼90% MR in pristine structure (without hydrogenation). Spin-injection efficiency which is also a measure of spin-filtration is observed to be higher in hydrogenated graphene at around ∼90%–100% in comparison to ∼80%–90% in the pristine structure.

Spin-dependent giant junction magnetoresistance in simple Fe/p-Si(001) Schottky heretrojunction at low temperature

We report on the giant junction magnetoresistance observed at low temperature in Fe/p-Si Schottky heterojunction. The device shows good rectifying characteristics at room temperature and a dual—Schottky as well as magnetic diode—characteristics at low temperature, below 50 K. Formation of a magnetic field-dependent potential barrier due to electrical injection of spin-polarized carriers from the ferromagnetic electrode into the semiconductor is speculated to result in such large junction resistance. The magnetoresistance value is of the order of $$10^{4}\%$$ at 10 K and saturates at $$\sim 0.5\,\hbox {kOe}$$ , showing dual functionality—working as a magnetic diode as well as a magnetoresistive element.

Multistate magnetoresistance in zigzag-edge trigonal graphene magnetic junctions

Using density functional theory combined with nonequilibrium Green’s function method, spin-dependent transport through a zigzag-edge trigonal graphene molecule coupled with two ferromagnets was investigated. The results reveal that due to the intrinsic magnetism of the central graphene nanoflake, four spin configurations of the device can be achieved. By calculating the current–voltage characteristic, a three-state magnetoresistance effect is obtained companied with a large spin filtering efficiency. The intrinsic mechanism is explored as different shifts of the spin-split frontier molecular orbitals depending on the spin configurations of the device. This work indicates an intriguing prospect of graphene nanoflakes in designing novel spintronic devices, such as multistate magnetoresistance devices.

Enhanced Magnetoresistance in Chiral Molecular Junctions.

Chirality-induced spin selectivity (CISS) is a recently discovered effect, whose precise microscopic origin has not yet been fully elucidated; it seems, however, clear that spin-orbit interaction plays a pivotal role. Various model Hamiltonian approaches have been proposed, suggesting a close connection between spin selectivity and filtering and helical symmetry. However, first-principles studies revealing the influence of chirality on the spin polarization are missing. To clearly demonstrate the influence of the helical conformation on the spin polarization properties, we have carried out spin-dependent Density-Functional Theory (DFT) based transport calculations for a model molecular system. It consists of α-helix and β-strand conformations of an oligo-glycine peptide, which is bonded to a nickel electrode and to a gold electrode in a two-terminal setup, similar to a molecular junction or a local probe, for example, in STM or AFM configurations. We have found that the α-helix conformation displays a spin polarization, calculated through the intrinsic magneto-resistance of the junction, about 100-1000 times larger than the linear β-strand, clearly demonstrating the crucial role played by the molecular helical geometry on the enhancement of spin polarization associated with the CISS effect.

Switchable voltage offset in a Heusler alloy-based magnetic tunnel junction

We investigate the tunneling magnetoresistance (TMR) of the magnetic tunnel junction (MTJ) comprised of the Heusler alloy (Co2MnSi) and the pinned CoFe to probe the spin polarization of Co2MnSi. From the TMR value thus obtained, we estimate 40% polarization of Co2MnSi, much less than 100% expected for the suspected half metal. Remarkably, upon applying the weak switching magnetic field to the MTJ in the parallel state, our system exhibits a finite open-circuit voltage on the order of several 10μV in the anti-parallel state. We have demonstrated that the reversal of magnetic dipolar field induced by the Heusler alloy as a free layer can generate an electromotive force of the same order and can clearly explain the observed polarity and sample size dependence of the voltage offset at zero bias current.

Effects of temperature on the magnetic tunnel junctions with periodic grating barrier

We have developed a tunneling theory to describe the temperature dependence of tunneling magnetoresistance (TMR) of the magnetic tunnel junctions (MTJs) with periodic grating barrier. Through the Patterson function approach, the theory can handle easily the influence of the lattice distortion of the barrier on the tunneling process of the electrons. The lattice distortion of the barrier is sensible to the temperature and can be quite easily weakened by the thermal relaxation of the strain, and thus the tunneling process of the electrons will be significantly altered with the variation of the temperature of the system. That is just the physical mechanism for the temperature dependence of the TMR. From it, we find two main results: 1. The decrease of TMR with rising temperature is mostly carried by a change in the antiparallel resistance (RAP), and the parallel resistance (RP) changes so little that it seems roughly constant, if compared to the RAP. 2. For the annealed MTJ, the RAP is significantly more sensitive to the strain than the RP, and for non-annealed MTJ, both the RP and RAP are not sensitive to the strain. They are both in agreement with the experiments of the MgO-based MTJs. Other relevant properties are also discussed.

Magnetoresistance of a nanostep junction based on topological insulators

Magnetoresistance of a nanostep junction based on topological insulators

Interplay of composition and anisotropy on evolution of microstructural, static and dynamic magnetic properties of CoFeB thin films on annealing

The crystallization of CoFeB electrode is essential for achieving high tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) with MgO barrier. In this study, we systematically investigated the influence of annealing on the structural and magnetic properties of CoFeB (23 nm) thin films using X-ray diffraction (XRD), magnetization (M) vs. magnetic field (H) and ferromagnetic resonance (FMR) measurements. The variations in composition and uniaxial magnetic anisotropy (UMA) of as-deposited CoFeB thin films results in a different crystalline state on annealing. It was found that the partial crystallization takes place for CoFeB compositions with high Boron content and exhibiting large UMA. This initial stage of crystallization is associated with the quick release of stress and will result in low coercivity, smaller crystallite size and a decrease in uniaxial anisotropy which will eventually lead to lower magnetic damping. However, on complete crystallization for other compositions with the formation of bcc-CoFe (110) phase, both the crystallite size and coercivity increases and cubic anisotropy emerge which results in very enhanced damping. The FMR linewidth has non-linear frequency dependence which gives direct evidence that relaxation is not exclusively governed by Gilbert damping, but it is also important to consider the contribution from two-magnon scattering (TMS). The TMS dominates the damping process in crystallized films and is mainly caused by the anisotropy dispersion created due to the grain growth. Thus, the work connects the change in microstructure and anisotropy to the magnetization dynamics, in particular, the FMR linewidth.

Enhancement of tunneling magnetoresistance by inserting a diffusion barrier in L1-FePd perpendicular magnetic tunnel junctions

We studied the tunnel magnetoresistance (TMR) of L10-FePd perpendicular magnetic tunnel junctions (p-MTJs) with an FePd free layer and an inserted diffusion barrier. The diffusion barriers studied here (Ta and W) were shown to enhance the TMR ratio of the p-MTJs formed using high-temperature annealing, which are necessary for the formation of high quality L10-FePd films and MgO barriers. The L10-FePd p-MTJ stack was developed with an FePd free layer with a stack of FePd/X/Co20Fe60B20, where X is the diffusion barrier, and patterned into micron-sized MTJ pillars. The addition of the diffusion barrier was found to greatly enhance the magneto-transport behavior of the L10-FePd p-MTJ pillars such that those without a diffusion barrier exhibited negligible TMR ratios (<1.0%), whereas those with a Ta (W) diffusion barrier exhibited TMR ratios of 8.0% (7.0%) at room temperature and 35.0% (46.0%) at 10 K after post-annealing at 350 °C. These results indicate that diffusion barriers could play a crucial role in realizing high TMR ratios in bulk p-MTJs such as those based on FePd and Mn-based perpendicular magnetic anisotropy materials for spintronic applications.

Magnetic tunnel junctions with an equiatomic quaternary CoFeMnSi Heusler alloy electrode

Tunnel magnetoresistance (TMR) in MgO-based magnetic tunnel junctions (MTJs) with equiatomic quaternary CoFeMnSi Heusler and CoFe alloy electrodes is studied. The epitaxial MTJ stacking structures were prepared using ultrahigh-vacuum magnetron sputtering, where the CoFeMnSi electrode has a full B2 and partial L21 ordering crystal structure. Maximum TMR ratios of 101% and 521% were observed at room temperature and 10 K, respectively, for the MTJs. The large bias voltage dependence of the TMR ratio was also observed at low temperature (LT), as similarly observed in Co2MnSi Heusler alloy-based MTJs in the past. The physical origins of this relatively large TMR ratio at LT were discussed in terms of the half-metallicity of CoFeMnSi.

Spin valve-like magnetic tunnel diode exhibiting giant positive junction magnetoresistance at low temperature in Co2MnSi/SiO2/p-Si heterostructure

The rectifying magnetic tunnel diode has been fabricated by growing Co2MnSi (CMS) Heusler alloy film carefully on a properly cleaned p-Si (100) substrate with the help of electron beam physical vapor deposition technique and its structural, electrical and magnetic properties have been experimentally investigated in details. The electronic- and magneto-transport properties at various isothermal conditions have been studied in the temperature regime of 78–300 K. The current–voltage (I–V) characteristics of the junction show an excellent rectifying magnetic tunnel diode-like behavior throughout that temperature regime. The current (I) across the junction has been found to decrease with the application of a magnetic field parallel to the plane of the CMS film clearly indicating positive junction magnetoresistance (JMR) of the heterostructure. When forward dc bias is applied to the heterostructure, the I–V characteristics are highly influenced on turning on the field B = 0.5 T at 78 K, and the forward current reduces abruptly (99.2% current reduction at 3 V) which is nearly equal to the order of the magnitude of the current observed in the reverse bias. Hence, our Co2MnSi/SiO2/p-Si heterostructure can perform in off (Ioff)/on (Ion) states with the application of non-zero/zero magnetic field like a spin valve at low temperature (78 K).

Perpendicular Magnetic Anisotropy in Heusler Alloy Films and Their Magnetoresistive Junctions.

For the sustainable development of spintronic devices, a half-metallic ferromagnetic film needs to be developed as a spin source with exhibiting 100% spin polarisation at its Fermi level at room temperature. One of the most promising candidates for such a film is a Heusler-alloy film, which has already been proven to achieve the half-metallicity in the bulk region of the film. The Heusler alloys have predominantly cubic crystalline structures with small magnetocrystalline anisotropy. In order to use these alloys in perpendicularly magnetised devices, which are advantageous over in-plane devices due to their scalability, lattice distortion is required by introducing atomic substitution and interfacial lattice mismatch. In this review, recent development in perpendicularly-magnetised Heusler-alloy films is overviewed and their magnetoresistive junctions are discussed. Especially, focus is given to binary Heusler alloys by replacing the second element in the ternary Heusler alloys with the third one, e.g., MnGa and MnGe, and to interfacially-induced anisotropy by attaching oxides and metals with different lattice constants to the Heusler alloys. These alloys can improve the performance of spintronic devices with higher recording capacity.

Giant tunnel magnetoresistance in polycrystalline magnetic tunnel junctions with highly textured MgAl2O4(001) based barriers

Although single-crystalline spinel (MgAl2O4)-based magnetic tunnel junctions (MTJs) are known to show a good bias voltage dependence of a tunnel magnetoresistance (TMR) ratio over MgO-based MTJs, no polycrystalline MgAl2O4-based MTJs exhibiting large TMR ratios have been grown previously due to the lack of crystallinity of the MgAl2O4 barrier. In this work, we demonstrate the growth of polycrystalline-based MTJs with large TMR ratios exceeding 240% and an improved bias voltage dependence compared to that of MgO-based MTJs. An ultra-thin CoFe/MgO seed layer on the amorphous CoFeB layer induced the growth of a highly (001)-textured MgAl2O4 barrier, which worked as a template layer for the solid epitaxy of CoFe grains during the crystallization of the CoFeB layers. High resolution scanning transmission electron microscopy shows lattice-matched epitaxy between the (001)-textured MgAl2O4 barrier and CoFe grains. This study demonstrates the industrial viability of MgAl2O4-based polycrystalline MTJs with an improved bias voltage dependence.

Layer dependent magnetoresistance of vertical MoS2 magnetic tunnel junctions.

Spin polarization of electrons through transition metal dichalcogenides (TMDs) from ferromagnetic metals (FMs) is a fascinating phenomenon in condensed matter physics. The spin polarized current makes high- and low-resistance states in FM/TMDs/FM junctions depending on magnetization alignment of FM electrodes. We have manifested vertical spin valve junctions by incorporating MoS2 layers of different thicknesses by an ultraclean fabrication method. The current-voltage (I-V) characteristics show the ohmic contact behavior, indicating that mono-, bi-, and tri-layer MoS2 work as conducting thin film. In contrast, FM/multilayer MoS2/FM junction shows non-linear I-V characteristics and the junction resistance increases as the temperature is lowered, indicating that multilayer MoS2 provides a tunneling barrier between FM electrodes. We have found that the magnetoresistance (MR) ratio increases gradually as the thickness of the MoS2 layer is increased. Our investigation will provide a guide to make an optimal choice in the development of magnetic tunnel junctions with two-dimensional layered TMDs.

Large tunneling magnetoresistance in magnetic tunneling junctions based on two-dimensional CrX3 (X = Br, I) monolayers.

Magnetic tunneling junctions (MTJs) have atomic thickness due to the use of two-dimensional (2D) materials. Combining density functional theory with non-equilibrium Green's function formalism, we systematically investigate the structural and magnetic properties of CrX3/h-BN/CrX3 (X = Br, I) MTJs, as well as their spin-dependent transport characteristics. Through calculation of the transmission spectrum, the large tunneling magnetoresistance (TMR) effect was observed in these MTJs. Moreover, their conductance based on two-dimensional materials was greatly improved over that of traditional MTJs. The transmission mechanism was analyzed using the symmetry of the orbit and the eigenstates of the transmitted electrons. We also discuss the problem of Schottky contact between different metal electrodes and devices. Our results suggest that MTJs based on two-dimensional ferromagnets are feasible.

High magnetoresistance in ultra-thin two-dimensional Cr-based MXenes.

Developing effective magnetoresistance devices has become beneficial due to their extensive applications in electronics such as data storage. One challenge is to reduce the scale of devices while maintaining a high magnetoresistance. Another challenge for the applications of magnetoresistance devices involves synthesizing them with stable interfaces. Antiferromagnetic Cr-based MXenes are similar to tunneling magnetoresistance junctions, and can be synthesized in a straightforward manner. Density functional theory calculations show that the medium adhesion strength between Au electrodes and Cr-based MXene can ensure the formation of stable interfaces. Exchange coupling between Cr atoms in different atomic layers is found to decrease to 0.4 meV with increasing thickness. Magnetoresistance values are higher than 400% with 1.0 V. The results show that Cr-based MXenes are promising magnetoresistance devices with high magnetoresistance values and ultra-thin layers (about 1 nm) and exhibit tunable exchange coupling between Cr atoms through thickness control.

Asymmetric d-wave superconducting topological insulator in proximity with a magnetic order

In the framework of the Dirac–Bogoliubov–de Gennes formalism, we investigate the transport properties in the surface of a 3-dimensional topological insulator-based hybrid structure, where the ferromagnetic and superconducting orders are simultaneously induced to the surface states via the proximity effect. The superconductor gap is taken to be spin-singlet d-wave symmetry. The asymmetric role of this gap respect to the electron–hole exchange, in one hand, affects the topological insulator superconducting binding excitations and, on the other hand, gives rise to forming distinct Majorana bound states at the ferromagnet/superconductor interface. We propose a topological insulator N/F/FS junction and proceed to clarify the role of d-wave asymmetry pairing in the resulting subgap and overgap tunneling conductance. The perpendicular component of magnetizations in F and FS regions can be at the parallel and antiparallel configurations leading to capture the experimentally important magnetoresistance (MR) of junction. It is found that the zero-bias conductance is strongly sensitive to the magnitude of magnetization in FS region mzfs and orbital rotated angle α of superconductor gap. The negative MR only occurs in zero orbital rotated angle. This result can pave the way to distinguish the unconventional superconducting state in the relating topological insulator hybrid structures.

Fe concentration dependence of tunneling magnetoresistance in magnetic tunnel junctions using group-IV ferromagnetic semiconductor GeFe

Group-IV-based ferromagnetic semiconductor Ge1−xFex (GeFe) is one of the most promising materials for spin injection/detection in Si and Ge. In this paper, we demonstrate a systematic study of tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) composed of Fe/MgO/Ge1−xFex with various Fe concentrations (x = 0.065, 0.105, 0.140, and 0.175). With increasing x, the TMR ratio increases up to 1.5% when x≤ 0.105, and it decreases when x> 0.105. This is the first observation of the TMR ratio over 1% in MTJs containing a group-IV ferromagnetic semiconductor. With increasing x, while the Curie temperature of GeFe increases, the MgO surface becomes rougher, which is thought to be the cause of the upper limit of the TMR ratio. The quality of the MgO layer on GeFe is an important factor for further improvement of TMR in Fe/MgO/GeFe MTJs.

Electronic structures and magneto-transport properties of co-based Heusler alloy based magneto-resistance junctions

A direct link between band structure and the ballistic transport property of full-Heusler alloys based Co2 YZ/Al/Co2 YZ trilayers (Y = Sc, Ti, V, Cr, Mn and Fe; Z = Al, Si and Ge) has been studied by firstprinciples calculations. It is found that the transport efficiency is determined primarily by three factors related to band structure: the shape of the band crossing Fermi energy E F, the distance d of the two intersection points of Co2 YZ and Al at E F, and the absolute maximum of the energy lying in the E F-crossing band, |Emax|. The transmission coefficient distribution patterns imply that the affected factor of magneto-resistance (MR) ratio is attributed to the band features around E F. In general, an intuitively illustrated diagram is proposed to clarify the relationship between the probability of electron transition and the current magnitude.