Dependence Of Tunnel Magnetoresistance Research Articles

Effects of the interfacial states on the spin-dependent tunneling of Mn3Al-based magnetic tunnel junction

The study of antiferromagnets (AFMs) has become one of the hotspots of spintronics because of their strong environmental adaptability; however, their magnetic ordering structure limits their practical application. The compensated ferrimagnet Mn3Al has the characteristics of an AFM and the magnetic moment can be controlled by a magnetic field. The successful synthesis of Mn3Al, which possesses a high Curie temperature (605 K), may lead to new applications in spintronics devices. In this work, the electronic structure of free Mn3Al (001) surfaces and the interface effects of the Mn3Al electrode and the GaAs tunnel barrier in the magnetic tunnel junction (MTJ) are investigated with the non-equilibrium Green’s function method. Although redistribution of the partial density of states between the surface and bulk is observed for the two different types of Mn3Al surfaces, the half-metallicity is retained for both the surface structures. Due to the half-metallicity of the surfaces, the tunneling magnetoresistance (TMR) of the MTJ reaches up to 107%. Moreover, the TMR dependence on the GaAs barrier thickness as well as the interface is studied. Through the visual display of the interface states, we find that the interface states can promote electron tunneling in the antiparallel configuration. The high TMR can be maintained at a small barrier width owing to the weak interface states. The results are explained with the transmission coefficient in reciprocal space. This work provides some physical insight for the design and implementation of Mn3Al-based MTJs.

Interfacial Perpendicular Magnetic Anisotropy in Magnetic Tunnel Junctions Comprising CoFeB with FeNiSiB Layers

Controlling ferromagnetic thickness (t) and properties such as saturation magnetization (Ms) and effective magnetic anisotropy constant (Keff) has been regarded as critical for the performance of magnetic tunnel junctions (MTJs) with interfacial perpendicular magnetic anisotropy. Here, we report the effects of hybridizing a CoFeB layer with a FeNiSiB layer as part of a magnetic free layer structure. We deposited thin film stacks by magnetron sputtering on Si wafers with thermal oxides and carried out post-deposition heat treatment at 300 °C for 1 h in a vacuum under a magnetic field. We found that Ms and Keff could be tuned by adding a layer of amorphous FeNiSiB. While the Ms and Keff values were modified, the tunneling magnetoresistance (TMR) ratios of the MTJs were maintained, even though the CoFeB thickness was decreased by half. Moreover, an asymmetric bias voltage dependence of TMR was suppressed in the MTJs with FeNiSiB/CoFeB hybrid free layers due to improvements in the interface quality between the CoFeB/MgO interfaces.

The dependence of TMR on the barrier thickness, bias voltage and asymmetry in Fe/ZnO/Fe MTJs: A DFT study

Density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods are used to simulate spin-polarized transport properties in Fe(001)/ZnO(001)/Fe(001) magnetic tunnel junctions (MTJs) with an even number of ZnO monolayers (ML). The barrier thickness conductance, tunneling magneto-resistance (TMR) ratio, and tunneling current are investigated for parallel and antiparallel alignment of the magnetization of electrodes. The results indicate that in zero and non-zero bias voltages the slower decay of conductance in the parallel alignment configuration, compared to antiparallel alignment, leads to increased TMR ratio for thicker barriers. Additionally, it is shown that, across all thicknesses of the ZnO barrier, the TMR ratio is highly sensitive to the applied bias voltage and decreases with increasing voltage. Moreover, analysis of the 4-ML MTJ reveals that, for the same density of states (DOS) in the symmetric structure, increasing the DOS at the Fermi level of antiparallel alignment configuration in the asymmetric case results in a negative TMR ratio. The results suggest that a ZnO rock-salt type potential barrier is as suitable as the traditional MgO barrier for spintronic purposes in MTJs.

Improvement of Thermal Tolerance of CoFeB–MgO Perpendicular-Anisotropy Magnetic Tunnel Junctions by Controlling Boron Composition

We investigated annealing temperature $T_{a}$ dependence of tunnel magnetoresistance (TMR) ratio and magnetic properties for perpendicular-anisotropy (CoFe)100– X B X /MgO magnetic tunnel junctions (MTJs) with single (CoFe)100– X B X /MgO interface (s-MTJ) and double CoFeB–MgO interface (d-MTJ) structures with various boron compositions X. High TMR ratio over 100% was observed in the s-MTJ with $X= 35$ at.% after annealing at 360 °C–400 °C, whereas the s-MTJ with $X = 30$ at.% showed the degradation of TMR ratio with the increase of $T_{a}$ above 360 °C, resulting from the decrease of perpendicular anisotropy. The d-MTJ with $X = 25$ at.% maintained high TMR ratio up to $T_{a} = 400~^{\circ }\text{C}$ owing to its higher perpendicular anisotropy compared with the s-MTJ. The difference of perpendicular anisotropy between the s-MTJ and the d-MTJ can be attributed to higher interfacial anisotropy together with lower saturation magnetization of the d-MTJs. The lower saturation magnetization is attributable to two MgO layers that suppress boron diffusion from CoFeB layers, which was verified by cross-sectional line analysis using electron energy-loss spectroscopy.

Angular Dependence of Tunneling Magnetoresistance in Hybrid Fe/GaAlAs/GaMnAs Magnetic Tunnel Junctions

Tunneling magnetoresistance (TMR) phenomena in hybrid Fe/GaAlAs/GaMnAs magnetic tunnel junctions (MTJs) were investigated by rotating a magnetic field of constant strength in the film plane. When a strong field (e.g., 4000 G) is used, the magnetization in GaMnAs and Fe coherently rotates in both layers, resulting in a smooth angular dependence of TMR. In contrast, abrupt transition steps and plateaus are observed in TMR, when a weak field (below 100 G) is rotated. The behavior observed in strong fields is ascribed to tunneling anisotropic magnetoresistance, an effect that occurs when magnetizations in both magnetic layers in the MTJ are aligned parallel to each other. The tunneling behavior observed in weak fields, on the other hand, is caused by differences in relative magnetization alignments in the two layers that arise from differences in their magnetocrystalline anisotropies. The latter behavior provided the anisotropic TMR that involved with parallel and antiparallel alignments at specific crystallographic directions.

The interfacial roughness effect on spin-dependent transport in nonplanar junctions with double magnetic barriers

In this paper, we use the transfer matrix method to study the effect of roughness with nonplanar interfaces on spin-dependent transport properties in a magnetic tunnel junction, which consists of two ferromagnetic semiconductor barriers separated by a nonmagnetic (NM) layer. This trilayer is sandwiched between two NM electrodes. The roughness is modeled as a periodic corrugation interface. Based on the effective mass approximation, the spin-dependent transmission probability, and also the dependence of tunnel magnetoresistance (TMR) and electron spin polarization (SP) on the center nonmagnetic layer quantum well width are studied, theoretically, at several different temperatures. The numerical results show that the SP and TMR have an oscillatory behavior as a function of NM layer thickness and the interface roughness/islands degrade the transmission probability. Our results may be useful for the development of nanoelectronic devices.

Spin-dependent tunneling through NiFe nanoparticles

Double magnetic tunnel junctions (DMTJs) have been fabricated using alumina barriers with NiFe particles (∼1.8 nm) embedded within. The junctions exhibit spin-dependent transport properties and Coulomb blockade effects. We study differences between control samples and the DMTJs; specifically I-V characteristics and tunnel magnetoresistance (TMR) versus bias voltage characteristics. Clear differences in the systems are evident: the DMTJ with NiFe particles shows a marked peak in TMR at low bias, whereas the dependence of TMR on bias is much weaker for the control MTJ without embedded particles. Hence the TMR at low bias is enhanced by the Coulomb blockade effects.

Magnetic tunnel junctions with L1-ordered FePt alloy electrodes

Magnetic tunnel junctions (MTJs) with L10-ordered FePt alloy electrodes were prepared on MgO(110) substrates, and structure and magnetotransport properties of the MTJs have been investigated. Tunnel magnetoresistance (TMR) of 34% is observed at 77 K for a junction with the FePt alloy electrode in which the degree of chemical order S is estimated to be ∼0.7. From S dependence of TMR in the MTJs, it is suggested that the spin polarization of FePt alloy electrodes increases with chemical ordering, which is consistent with first principles band structure calculations.

Large tunneling magnetoresistance of a ramp-edge-type junction with a SrTiO3 barrier

Abstract The tunneling magnetoresistance (TMR) of a ramp-edge-type junction with SrTiO 3 barrier layer was studied. Samples with a structure of glass / NiO (600 A )/ Co (100 A )/ SrTiO 3 (400 A )/ SrTiO 3 (20 – 100 A )/ NiFe (100 A ) were prepared by sputtering and etched through electron cyclotron argon ion milling. Nonlinear I – V characteristics were obtained from a ramp-type tunneling junction, with dominant difference between two external magnetic fields (±100 Oe ) perpendicular to the junction edge line. Given a 40- A thick SrTiO 3 barrier, the TMR was 52.7% at a bias voltage of −50 mV . The bias voltage dependence of TMR in a ramp-type tunneling junction was similar to that of the layered TMR junction.

Bias dependence of tunneling magnetoresistance on ferromagnetic electrode thickness

We have investigated the bias dependence of the tunneling magnetoresistance (TMR) in Py/CoFe(t)/Al2O3/Py magnetic tunneling junctions with a wedge-shaped CoFe layer. The value of TMR increases slowly with CoFe thickness and saturates at t⩾2.3 nm, implying a significant bulk-like contribution in spin-dependent tunneling. The bias dependence of TMR also shows a similar trend as a function of CoFe thickness, and becomes independent of CoFe thickness at t⩾2.3 nm. The extracted tunneling characteristic length decreases slightly with increasing bias voltage due to the hot electron excitations.

Tunnel magnetoresistance in planar junctions with ferromagnetic barriers

Tunnel magnetoresistance (TMR) effect in double junctions with nonmagnetic electrodes and ferromagnetic barriers is analysed theoretically within the free-electron-like one-band model. The calculations are performed in the two limiting cases of electron tunnelling: sequential and coherent. The influence of height and thickness of the barriers and position of the Fermi level on the TMR effect is discussed. The bias voltage dependence of TMR is also analysed.

Barrier Height and Film Thickness Dependence of the TMR

We calculate the tunnel magneto-resistance (TMR) ratio by employing the Keldysh formalism. The barrier height dependence of TMR has a minimum whose origin is different from that of Slonczewski's theory. The TMR decreases and saturates with increasing film thickness. When the barrier is low and thick, the barrier height dependence of modulated density of states in the electrodes is dominant to the behavior of TMR. In this case the TMR increases with increasing bias voltage. When the barrier is high and thin, the barrier height dependence of damping factor through the insulating film is dominant to the behavior of TMR. In this case the TMR decreases greatly with increasing bias voltage. The temperature dependence is small below the temperature where electrons can hop over the barrier. As the barrier becomes high, the TMR becomes weakly temperature dependent, and the bias dependence becomes large.

Applied voltage and temperature dependence of tunneling magnetoresistance

The dependence of tunnel magnetoresistance (TMR) ratio on temperature and applied voltage has been studied for Fe/Al-oxide/Fe and 80NiFe/Al-oxide/Co junctions. Their dependence for an Fe/Al-oxide/Fe junction is greater than that for a 80NiFe/Al-oxide/Co junction. The rapid decrease of TMR ratio with increasing temperature for the Fe/Al-oxide/Fe junction below 100 K or 10 mV and small decrease below about 40 K for the 80NiFe/Al-oxide/Co junction can be explained by the spin flip scattering due to magnetic impurities.

Dependence of tunneling magnetoresistance on ferromagnetic electrode thickness and on the thickness of a Cu layer inserted at the Al2O3/CoFe interface

The dependence of the spin tunneling magnetoresistance (TMR) on the ferromagnetic electrode thickness, tCoFe, and on the thickness of a Cu layer inserted at barrier/ferromagnetic (Al2O3/CoFe) interface is reported. Junctions were fabricated through contact shadow mask or microlithography. Junction TMR increases sharply from zero to a maximum value of 16.4% (shadow masks) with increasing tCoFe from 10 to 30 Å, or to 22% (microlithography) with increasing tCoFe from 6 to 20 Å, and then varies slightly up to tCoFe of 500 Å. The initial increase of TMR with tCoFe up to 30 Å results from the spin-polarization increase in CoFe. This is supported by the magnetization measurement of (Ta 150 Å/CoFe tCoFe)×n multilayers, where the magnetization of CoFe increases from near zero to its bulk value with increase of CoFe thickness from 10 to 40 Å. The incorporation of Cu layers at the Al2O3/CoFe (top electrode) interface decreases the TMR. However, junctions with a 40 Å thick Cu layer added at the Al2O3/CoFe interface still maintain a TMR signal of 1%. The bias voltage dependence of TMR does not vary with the Cu layer thickness added at Al2O3/CoFe interface, implying that it is not significantly dependent on the barrier/ferromagnetic interface.