Maximum Tunneling Magnetoresistance Research Articles

Double MgO Based Perpendicular Magnetic-Tunnel-Junction Spin-Valve without Upper Synthetic-Anti-Ferromagnetic Multi-Layers

Recently, the semiconductor memory device has faced the difficulties of scaling down, low power consumption and high retention time. In particular, dynamic random access memory (DRAM) has been struggling with scaling limit because of reducing capacitor area. In addition, alternative memories have been studied intensively to replace DRAM. Among them, perpendicular spin transfer torque magnetic random access memory (STT-MRAM) is considered as a strong candidate because of the small scaling down, low power consumption, high read and write speed and non-volatility. STT-MRAM has the structure of 1 transistor and 1 resistance (1T-1R) called magnetic tunnel junction (MTJ). Resistance difference between parallel and antiparallel states in MTJ indicates memory sensing margin. The large difference of tunneling magneto-resistance (TMR) is required for a good memory operation. To obtain the high TMR ratio, not only perfect switching of the free layer but also hard pinning of the pinned layer is required. It is known that pinning the pinned layer is due to the exchange coupling induced by a synthetic anti-ferro-magnetic (SyAF) layer based on Ruderman Kittel Kasuya Yosida (RKKY) interaction. In our study, therefore, we investigated how to optimize the SyAF layer structure for the proper exchange coupling. MgO tunnel barrier should be b.c.c (100) crystallinity because it acts as an electrons filter which determined whether spin direction between free and pinned layers is aligned parallel or not. In particular, we propose double MgO based perpendicular magnetic-tunnel-junction spin-valve without upper synthetic-anti-ferromagnetic Multi-layers in order to obtain high TMR ratio. In addition, we investigated the different characteristics of the p-MTJ spin-valve with and without upper SyAF multi-layers. For the experiment, two structures, double MgO based p-MTJ spin-valve with and without upper SyAF multi-layers, were fabricated on a 12-inch-diameter wafer deposited with SiO2/W/TiN films in a 12-inch multi-chamber sputtering system under a high vacuum of less than 1ⅹ10-8 torr (without breaking the vacuum; see Figs. 1a and b). The thicknesses of the tungsten (W) were varied ranging from 0.18 to 0.48 nm. All samples were subject to ex-situ annealing at 350oC for 30 min under a perpendicular magnetic field of 3 Tesla. The dependence of the TMR ratio on the thickness of W bridge layer was estimated by using current-in-plane tunneling (CIPT) at room temperature as shown in Fig.1c. It shows that a TMR ratio of p-MTJ spin-valve with upper SyAF multi-layers slightly increases at the bridge layer thickness, t w up to around 0.3 nm. Then it abruptly decreases when the thickness was increased from 0.36 to 0.5 nm. On the other hand, the TMR ratio of p-MTJ spin-valve without upper SyAF multi-layer increases at the t w up to around 0.24 m. Then, the TMR ratio abruptly decreases when the thickness was increased from 0.36 to 0.4 nm. And maximum TMR ratio of p-MTJ spin-valve with upper SyAF multi-layers is 156 % while, that of p-MTJ spin-valve without upper SyAF multi-layers is 180 %. Obviously, the TMR ratio of p-MTJ spin-valve without upper SyAF multi-layers is higher 24 % than p-MTJ spin-valve wih upper SyAF multi-layers. In our presentation, we report the different characteristics of the p-MTJ spin-valve with and without upper SyAF multi-layers by examining the static magnetization behavior, MgO crystallinity, and I-V characteristic curve. In addition, we present how these properties influence the TMR ratio and mechanism of p-MTJ spin-valve. * This work was supported by a Basic Science Research Program grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (No. 2014R1A2A1A01006474) and the Brain Korea 21 PLUS Program in 2014. Figure 1

Vplyv temperatury na mahnitorezystyvni vlastyvosti strukturno nesutsil'nykh plivok Fe ta So

Magnetoresistive properties of ultrathin Fe and Co films with effective thicknesses d =3÷30 nm are studied in a temperature interval of (150–700) K. For films with d =3÷10 nm, the electrical conductivity is found to be thermally activated and the tunnel magnetoresistance effect is revealed. A decrease of the temperature down to 150 K in measurements on as-condensed films is found to result in the appearance of the magnetoresistive hysteresis and a reduction of the tunnel magnetoresistance (TMR) in a thickness interval of 10–15 nm. The maximum TMR value in the fields up to 0.7 T amounted to 1% for the as-condensed Fe films with an effective thickness of 17 nm and the Fe films with an effective thickness of 8 nm annealed at a temperature of 520 K.

Perpendicular Magnetic Tunnel Junctions With Low Resistance-Area Product: High Output Voltage and Bias Dependence of Magnetoresistance

We investigate the tunnel magnetoresistance (TMR) effect and its applied bias voltage dependence for perpendicular magnetic tunnel junctions (p-MTJs) with low resistance-area ( RA ) products of about 10 $\Omega\cdot \mu$ m $^{2}$ . We obtain a maximum TMR ratio of 248% among the examined devices. The bias-voltage dependence of the TMR ratio for these junctions is almost the same as that for junctions with RA products of about 10–1000 $\Omega \cdot\mu$ m $^{2}$ in the positive voltage region, while a fast drop in the TMR ratio is observed in the negative bias region. An output voltage of more than 200 mV is obtained for these p-MTJs.

Co2Fe6B2/MgO-Based Perpendicular Spin-Transfer-Torque Magnetic-Tunnel-Junction Spin-Valve without [Co/Pt]n lower Synthetic-Antiferromagnetic Layer

In recently years, the research on magnetic tunnel junctions (MTJs) using CoFeB magnetic layer and MgO tunneling barrier layer has been performed for realizing perpendicular-spin-transfer-torque magnetic-random-access-memory (p-STT-MRAM). In particular, to integrate a terra-bit-level nonvolatile memory-cell, the p-MTJ needs to achieve critical device performances such as a high tunneling magnetoresistance (TMR) ratio of ~150%, a high thermal stability (°â = E/KBT) of 74, and a low critical current density (Jo ) of 13.4 MA/cm2, where E, Ms , kB , and T are the energy barrier, the saturation magnetization, the Boltzmann constant, and the temperature, respectively.[1] In addition, to achieve CoFeB/MgO based p-MTJs grown on 12-inch TiN/W electrode, a design of [Co/Pt]n synthetic-anti-ferro-magnetic (SyAF) layer being operated at the ex-tu annealing above 350 oC has been intensively studied.[2] However, the [Co/Pt]n SyAF layer essentially has thicker a [Co/Pt]n SyAF layer since a [Co/Pt]n lower SyAF should couple antiferromagnetically with a [Co/Pt]n upper SyAF layer and couple ferromagnetically with a Co2Fe6B2 pinned layer at the same time.[3] Thicker SyAF layer has the problem of a high fabrication cost for p-MTJ spin-valves in the mass production of p-STT MRAM. Thus, we designed a Co2Fe6B2/MgO based p-MTJ spin-valve without a [Co/Pt]n lower SyAF layer, which can largely reduce the fabrication cost since the total multi-layer number (n) of [Co/Pt]n for the Co2Fe6B2/MgO-based p-MTJ spin-valve.Figures 1(a) and (b) show the spin-valve structure of a [Co/Pt]n SyAF layer with lower SyAF layer, without lower SyAF layer, respectively. The [Co/Pt]n SyAF layer without a lower SyAF layer is much thinner than that with a lower SyAF layer, as you can see in figure 1. In addition, the upper SyAF layer of p-MTJ without a lower SyAF layer is much thinner than that of p-MTJ with a lower SyAF layer because of fitting for antiferromagnetically coupling between lower and upper SyAF layer. And we studied the TMR ratio dependency on bcc nanoscale capping layer thickness of two structures by using current in-plane tunneling (CIPT) technique. In result, the maximum TMR ratios of two structures were both over 150%. In case of the p-MTJ with a [Co/Pt]n lower SyAF layer, the TMR ratio rapidly increased from 19% to 159% when the bcc nanoscale capping-layer thickness (tbcc ) increased from 0.3 to 0.6 nm. Then, it abruptly decreased from 159% to 2% when tbcc decreased from 0.6 to 0.73 nm. Also, for the p-MTJ spin-valve without a [Co/Pt]n lower SyAF layer, the TMR ratio very rapidly increased from 5% to 158% when tbcc increased from 0.3 to 0.6 nm. Then, it abruptly decreased from 158% to 2% when tbcc decreased from 0.6 to 0.73 nm.In the conference, we will present the effect of Co2Fe6B2/MgO-based perpendicular spin-transfer-torque magnetic-tunnel-junction spin-valve without [Co/Pt]n lower synthetic-antiferromagnetic layer and review the advantage of p-MTJ without lower SyAF layer. In addition, we report the physical and magnetic properties of p-MTJ by using vibrating-sampling-magnetometer (VSM), transmission-electron-microscopy (TEM). Furthermore, we review the mechanism why the magnetic properties of p-MTJs are varied by the thickness of SyAF layer. * This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.2014R1A2A1A01006474) and the Brain Korea 21 PLUS Program in 2014. Reference [1-3] [1] K. C. Chun, H. Zhao, J. D. Harms, T. H. Kim, J. P. Wang, and C. H. Kim, IEEE J. Solid-State Circuits 48, 598, 2013.[2] S. Ikeda, H. Sato, H. Honjo, E. C. I. Enobio, S. Ishikawa, M. Yamanouchi, S. Fukami, S. Kanai, F. Matsukura, T. Endoh and H. Ohno, in IEEE International Electron Devices Meeting (IEDM) IEEE, 2014, pp. 33.2.1. [3] J. G. Park, T. H. Shim, K. S. Chae, D. Y. Lee, Y. Takemura, S. E. Lee, M. S. Jeon, J. U. Baek, S. O. Park, and J.P. Hong, in IEEE International Electron Devices Meeting (IEDM) IEEE, 2014, pp. 19.2.1. Figure 1

Epitaxial wurtzite-MgZnO barrier based magnetic tunnel junctions deposited on a metallic ferromagnetic electrode

An epitaxial wurtzite (WZ) Mg0.23Zn0.77O barrier based magnetic tunnel junction (MTJ), with electrode-barrier structure of Co0.30Pt0.70 (111)/Mg0.23Zn0.77O (0001)/Co (0001), was fabricated. The good crystallinity and tunneling properties were experimentally confirmed. Electrical and magnetic investigations demonstrated its high resistance-area product of 1.05 MΩ μm2, a maximum tunneling magneto-resistance (TMR) of 35.5%, and the existence of localized states within the tunneling barrier producing TMR rapid decrease and oscillation when increasing the applied bias voltage. The TMR value almost vanished at 200 K, which was attributed to the induced moment and strong spin-orbit coupling in Pt atoms at the Co0.30Pt0.70/Mg0.23Zn0.77O interface. Owing to the ferroelectric behavior in WZ-MgZnO materials, the fabrication of WZ-MgZnO barrier based MTJs deposited on a metallic ferromagnetic electrode will open routes for electrically controllable non-volatile devices that are compatible with CMOS technology.

Effects of interface structure on spin filter tunnel junctions

In this article, the spin filter tunneling in the low biases has been investigated by considering the interfacial barrier roughness. In spite of the approaches that use an artificial periodic interface to find the effects of roughness on transport calculation, our method is based on implementing the height deviation from a mean surface as the roughness characteristics in the transfer matrix method. Moreover the spin-dependent transport properties have been calculated by using the height of a simulated interface in which the rough interface has been grown by ballistic deposition. The results show that the surface roughness suppresses the temperature dependence of spin polarization and reduces the maximum achievable tunneling magnetoresistance and spin-dependent transmission probability.

The role of structure on magneto-transport properties of Heusler Co2MnSi films deposited on MgO(001)

We present an experimental study identifying structural reasons that degrade spin-polarization of Co2MnSi thin films deposited on MgO(001) substrates. Through the fabrication of magnetic tunnel junctions, we measure a range of values for tunneling magneto-resistance (TMR) ratios following post-deposition annealing and epitaxial crystallization of the Heusler film. These TMR ratios reflect qualitatively the change in spin polarization of the Co2MnSi thin films. Low-temperature annealing results in low spin-polarization due to a high fraction of an amorphous phase. As annealing temperatures increase, the fraction of L21 and B2 chemically ordered phases increases, thus improving significantly the spin-polarization. However, for samples annealed at higher temperatures, significant degradation in the cubic magneto-crystalline anisotropy is observed, which we attribute to the detection of manganese diffusion into the MgO substrate. This Mn diffusion is manifested in a reduction of the value of the TMR ratio, namely, the spin polarization. Additionally, the maximum TMR ratio measured here, approximately 65% at room-temperature, is limited because the semi-coherent interface of Co2MnSi with the MgO substrate terminates with a Mn-Si layer.

MgO-Based Double Barrier Magnetic Tunnel Junctions With Synthetic Antiferromagnetic Free Layer

CoFeB/Ru/CoFeB has been used as the middle free layer in MgO-based double barrier magnetic tunnel junctions (DBMTJs). The tunneling magnetoresistance (TMR) ratio, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> (bias voltage at half maximum TMR ratio) and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> (output voltage, defined as V multiplied by TMR ratio) have been investigated as a function of annealing temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) and Ru thickness (t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ru</sub> ) in the free layer. Magnetization data reveal that the two CoFeB layers in CoFeB/Ru/CoFeB are antiferromagnetically coupled. Compared with V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> of only 0.31 V for single barrier MTJs (SBMTJs) annealed at 375°C, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> for DBMTJs is up to 0.66 V. By increasing Ta, TMR ratio first increases, reaching the highest critical value, and then decreases. The highest TMR ratio is 181% with t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ru</sub> = 0.6 nm, which is much higher than that obtained in the DBMTJs with the pure CoFeB as the free layer. The improved TMR ratio is mainly due to the relatively thorough crystallization of the CoFeB layers in the free layer. The thermal annealing has been proven to be an effective method to remove the dissimilarity of the top and bottom CoFeB/MgO interfaces. V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> in the positive and negative voltage branches follows the same trend as that of TMR ratio with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> .

Increased Perpendicular TMR in FeCoB/MgO/FeCoB Magnetic Tunnel Junctions by Seedlayer Modifications

By modifying the seedlayer in perpendicular FeCoB/MgO/FeCoB magnetic tunnel junctions (MTJs), we observe an increase in maximum tunneling magnetoresistance (TMR) from 65% up to 138%. It's found that decreasing the Ta deposition rate in Ta/Ru/Ta underlayers allows for greater annealing temperatures (up to 350 ) while still maintaining a perpendicular easy axis. An improvement is also seen at a lower temperature where both seedlayers maintain a perpendicular FeCoB easy axis indicating that the increase in TMR is not solely related to annealing at a higher temperature.

High TMR Ratio in ${\rm Co}_{2}{\rm FeSi}$ and ${\rm Fe}_{2}{\rm CoSi}$ Based Magnetic Tunnel Junctions

Magnetic tunnel junctions with Fe1+xCo2-xSi (0 < x < 1) electrodes and MgO barrier were prepared on MgO substrates by magnetron co-sputtering. Maximum tunnel magnetoresistance (TMR) ratios of 262 % at 15 K and 159 % at room temperature were observed for x=0.75. Correlations of the annealing temperature dependent atomic ordering and TMR amplitude are discussed. The high TMR for an intermediate stoichiometry is ascribed to the adjustment of the Fermi energy within a minority spin pseudo gap.

The study of transport properties in rough ferromagnetic semiconductor rough junctions

The effect of self-affine interface roughness on spin filter tunnelling as a function of interface roughness parameters and the temperature dependent barrier parameters at low biases is studied. The interface roughness is described by the k-correlation model and in terms of self-affine scaling by root-mean-square (rms) deviation Δ, correlation length ξ, and the roughness exponent H(0≤H≤1). In general, the spin-dependent transmission probability is strongly sensitive to roughness and reduced for the case of spin up and down directions in all configurations. Our numerical results show that the roughness affects spin polarization (SP) and tunnelling magnetoresistance (TMR) at different temperatures and decreases the maximum achievable TMR and SP.

Composition dependence of magnetoresistance effect and its annealing endurance in tunnel junctions having Mn-Ga electrode with high perpendicular magnetic anisotropy

The composition dependence of the tunnel magnetoresistance (TMR) effect in Mn-Ga/MgO/CoFe magnetic tunnel junctions (MTJs) for Mn54Ga46, Mn62Ga38, and Mn71Ga29 (at. %) electrodes was investigated. An MTJ with a Mn62Ga38 electrode showed a maximum TMR ratio of 23% at 10 K and high annealing endurance up to 375 °C. The bias voltage dependence of the TMR ratio was distinct among MTJs with different Mn-Ga compositions. Here, we discuss this dependence on the basis of the difference in the Δ1 band dispersions for Mn-Ga alloys calculated by first principles.

Observation of a tunneling magnetoresistance effect in magnetic tunneling junctions with a high resistance ferromagnetic oxide Fe 2⋅5Mn 0⋅5O 4 electrode

A high resistance ferromagnetic oxide Fe 2⋅5Mn 0⋅5O 4 (FMO) property as a novel spin injector was investigated with a structure of a magnetic tunneling junction (MTJ) composed of FMO/Al–O/Ni 80Fe 20, in order to reduce an impedance mismatch problem on molecular spintronics. A tunneling magnetoresistance (TMR) effect in the MTJs was observed. The maximum TMR ratio observed in the MTJs was approximately 0.85% at room temperature (RT), and the spin-polarization of FMO was estimated to be at least 0.94% at RT.

Role of Coulomb blockade and spin-flip scattering in tunneling magnetoresistance of FeCo-Si-O nanogranular films

In this work, we report the effect of FeCo atomic fraction (0.33 &amp;lt; x &amp;lt; 0.54) and temperature on the electrical, magnetic, and tunneling magnetoresistance (TMR) properties of FeCo-Si-O granular films prepared by atom beam sputtering technique. Glancing angle x-ray diffraction and TEM studies reveal that films are amorphous in nature. The dipole–dipole interactions (particle–matrix mixing) is evident from zero-field cooled and field-cooled magnetic susceptibility measurements and the presence of oxides (mainly Fe-related) is observed by x-ray photoelectron spectroscopy analysis. The presence of Fe-oxides is responsible for the observed reduction of saturation magnetization and rapid increase in coercivity below 50 K. TMR has been observed in a wide temperature range, and a maximum TMR of −4.25% at 300 K is observed for x = 0.39 at a maximum applied field of 60 kOe. The fast decay of maximum TMR at high temperatures and lower TMR values at 300 K when compared to PFeCo2/(1+PFeCo2), where PFeCo is the spin polarization of FeCo are in accordance with a theoretical model that includes spin-flip scattering processes. The temperature dependent study of TMR effect reveals a remarkably enhanced TMR at low temperatures. The TMR value varies from −2.1% at 300 K to −14.5% at 5 K for x = 0.54 and a large MR value of −18.5% at 5 K for x = 0.39 is explained on the basis of theoretical models involving Coulomb blockade effects. Qualitatively particle–matrix mixing and the presence of Fe-oxides seems to be the source of spin-flip scattering, responsible for fast decay of TMR at high temperatures. A combination of higher order tunneling (in Coulomb blockade regime) and spin-flip scattering (high temperature regime) explains the temperature dependent TMR of these films.

Tunnelling magnetoresistance in disordered interacting nanoparticles

Tunnelling magnetoresistance (TMR) of three-dimensional disordered ferromagnetic nanoparticles has been studied based on the Monte Carlo (MC) simulations. The dipolar interaction as well as the exchange coupling between the particles are carefully considered. It is found that the maximum TMR is nearly independent of the dipolar interaction, while it decreases with an increasing exchange interaction. The numerical results also reveal that the magnetic field sensitivity of the TMR depends not only on the interactions but also on the distributions of the anisotropy axes.

Temperature and Bias-Assisted Transport Properties of LSMO/AlO/CoFeB Magnetic Tunnel Junction

We have observed the transport properties in magnetic tunnel junction with the multilayered structure: (50 nm)/Al-O (1 nm)/CoFeB (5 nm)/Ta (10 nm)/Ru (10 nm). The maximum normal tunneling magnetoresistance (TMR) ratio was found to be 12.4% at a temperature range around 100 K, and the maximum inverse TMR ratio was observed to be 3% at a temperature of 5 K . The inverse TMR decreases with increasing temperature until it reaches a temperature range of around 50 K, whereby it switches to the normal TMR behavior. The appearance of inverse TMR at low temperatures is attributed to the presence of minority-spin density of states (DOS) of half-metal around the Fermi level present at the half-metal-insulator interface. The vanishing of inverse TMR and increase of normal TMR above certain temperature range is due to no contribution from the minority-spin DOS in the tunneling process. These results were found to be reproducible and the TMR ratio of more than 3% and 11% was obtained at room temperature before and after annealing at a temperature of 235 K for an annealing time of one hour, respectively.

The influence of interfacial roughness on the coherence of structure and magnetic couplingacross barriers in Fe/MgO multilayers

Single-crystal Fe/MgO multilayers are model systems in which to study magnetic tunneljunctions. We find that the interfacial roughness leads to the loss of coherence of the crystalstructure. For thick MgO layers ferromagnetic (FM) ordering is found using polarizedneutron reflectivity (PNR). For thin MgO layers magnetization measurements reveal thepresence of antiferromagnetic (AF) interactions, but no long-range AF order is foundusing PNR. After cycling in a hysteresis loop, FM correlations are found at thecoercive point, and this will limit the maximum tunnelling magnetoresistance.

Perpendicular magnetic tunnel junctions using Co-based multilayers

CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) free and reference layers composed of Co/M (where M=Pd or Ni) multilayers have been optimized for high PMA and high tunneling magnetoresistance (TMR). The effects of Co thickness, Pd thickness, and the number of Co/Pd bilayers on the anisotropy and coercivity of the [Co/Pd]n multilayer films have been studied for both free and reference layers. The damping parameter α of CoFeB capped multilayers was determined using broadband ferromagnetic resonance. The transport properties of the patterned MTJ stacks were measured from 10 to 400 K. A maximum TMR of 10% at 10 K (5%–10% at 300 K) was obtained for these perpendicular MTJs, regardless of whether or not they were magnetically annealed for MgO–CoFeB crystallization. This indicates that the fcc-bcc-fcc transitions from the fcc multilayers to the bcc CoFeB/MgO/CoFeB do not promote the “giant MgO TMR effect” caused by symmetry filtering.

CoFeB Inserted Perpendicular Magnetic Tunnel Junctions with CoFe/Pd Multilayers for High Tunnel Magnetoresistance Ratio

The effect of Co20Fe60B20 insertion on tunnel magnetoresistance (TMR) properties of MgO barrier magnetic tunnel junctions (MTJs) was studied with Co90Fe10/Pd multilayer electrodes showing perpendicular anisotropy. The TMR ratio, which depended on the sputtering power density of the CoFeB layer, reached 43% at 1.77 W/cm2 in MTJs with a 1.8-nm-thick CoFeB layer inserted on both sides of the MgO barrier. With increasing CoFeB layer thickness, the optimal annealing temperature (Ta) realizing the maximum TMR ratio increased along with the TMR ratio. The MTJs with 3.0-nm-thick CoFeB deposited at 1.77 W/cm2 showed a TMR ratio of 91% at Ta = 250 °C, where the perpendicular magnetic anisotropy is maintained.

Coherent Transport in High Performance Double Magnetic Junctions

Spin-dependent transport is considered in double magnetic tunnel junctions of two types F-I-F-I-F and F-I-N-I-F with thin insulating (I) barriers between the ferromagnetic (F) or non-magnetic (N) electrodes. Using the Landauer formalism within single-band tight-binding dynamics for atomically perfect epitaxial multilayers, the dependencies of tunnel magnetoresistance (TMR) on the on-site atomic energies epsilonI or epsilonN and number nI of atomic planes in the respective layers are obtained. It is found that the maximum TMR corresponds to the "shallow band" regime with respect to epsilonN and a series of peaks appears with respect to nI, essentially enhancing the performance of a simple F-I-F junction. These results suggest the material and structural choices for optimum TMR devices of advanced compositions.