High Tunnel Magnetoresistance Research Articles

Thermally Stable Perpendicular-Magnetic Tunneling Junction Spin-Valve Implementing W Bridge, Space and Cap-Layer

Perpendicular spin-transfer-torque magnetic random access memory (p-STT MRAM) has been intensively researched because of the possibility of it overcoming the scaling limitations of current dynamic random access memory (DRAM) below the 10-nm design rule1. p-STT MRAM cells consist of a selective device and a perpendicular magnetic tunneling junction (p-MTJ) spin-valve. Moreover, a lot of research has gone into improving three device parameters of these devices, i.e., the tunneling magnetoresistance (TMR) ratio, thermal stability (Δ), and switching current (JC ), at the back end of line (BEOL) temperature of 400oC3. Note that the BEOL temperature of 400oC is required in order to integrate memory cells3. Tantalum (Ta) has been used as a bridge and spacer material to ferro-couple two ferro-magnetic layers. However, so far, it has proven extremely difficult achieve a high TMR ratio in a MgO based p-MTJ spin-valve using a thickness Ta spacer at the BEOL temperature of 400oC4. In our experiment, we tried a new spacer material with a body-centered-cubic (bcc) crystal structure (i.e., tungsten). We investigated the dependency of the TMR ratio on the thickness and type of material (tantalum or tungsten) of spacers in double MgO-based p-MTJ spin-valves with a top free Co2Fe6B2 layer. Two types of double MgO based p-MTJ spin-valves with a [Co/Pt]n-based SyAF layer 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 1×10−8 torr (without breaking the vacuum; see Figs. 1a and b). The thicknesses of the Ta and W spacers were varied between 0.2 and 0.7 nm. All samples were subject to ex-situ annealing at 400oC for 30 min under a perpendicular magnetic field of 3 Tesla. The dependence of the TMR ratio on the thickness and type of material of the spacer was estimated by using current-in-plane tunneling (CIPT) at room temperature, as shown in Fig. 1c. For the Ta spacers, the TMR ratio significantly increased from ~38 to ~95% as the spacer thickness was increased from 0.20 to 0.35 nm. It remained about 98% for spacer thicknesses ranging from 0.40 to 0.53 nm. It decreased from ~90 to ~42% as the spacer thickness was further increased from 0.58 to 0.70 nm. Thus, the TMR ratio (~98%) peaked in a certain spacer thickness region (0.40~0.53 nm) for Ta. Next, regarding the samples made with W spacers, the TMR ratio significantly increased from ~85 to ~131% as the spacer thickness was increased from 0.20 to 0.30 nm. It remained at ~134% as the spacer thickness was further increased from 0.40 to 0.53 nm. The ratio slightly decreased from ~134 to ~111% for larger thicknesses ranging from 0.58 to 0.70 nm. Thus, the TMR ratio (~134.0%) also peaked in a specific spacer thickness region (0.40~0.53 nm) for W. Comparing the TMR ratios in the cases of the Ta spacer and the W spacer obviously indicates that the use of 0.20 and 0.70 nm-thick W spacers resulted in least 35% higher TMR ratios in the comparison with the use of Ta spacers. In particular, both the thinnest (0.20 nm) and thickest (0.70 nm) W spacers yielded much higher TMR ratios (~85 and ~111%) in comparison with the corresponding Ta spacers (~38 and ~42%). In our presentation, we will report the mechanism by which thickness and material of the spacer influence the TMR ratio was revealed by examining the static magnetization behavior, (100) bcc crystallinity, and depth profile of the atomic composition of the spin-valves. In addition, we will present the result of effects on W bridge/cap-layer compare with Ta bridge/cap-layer in Double MgO based 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. Reference [1] K. C. Chun et al., IEEE J. Solid-st. Circ. 48, 598 (2013) [2] K. Yamane et al., IEEE T. Magn. 49, 4335 (2013). [3] L. Thomas et al., J. Appl. Phys. 115, 172615 (2014) [4] J. Swerts et al., Appl. Phys. Lett. 106, 262407 (2015) Figure 1

Thermally Assisted Spin Torque Switching in Magnetic Random Access Memory

Magnetic random access memory (MRAM) consisting of magnetic tunnel junctions (MTJs) is a fascinating candidate for high-density nonvolatile memory. An MTJ has two ferromagnetic metals called free and pinned layers separated by nonmagnetic spacer. The free layer in the MTJ shows two energetically stable states corresponding to the parallel and antiparallel alignments of the magnetizations. These magnetization alignments are distinguished by tunnel magnetoresistance (TMR) effect, i.e., the resistance at the antiparallel alignment is higher than that at the parallel alignment. The free layer is switched between these two stable states by spin torque effect [1], where the transfer of the spin angular momentum from the conducting electrons to the free layer excites the spin torque and induces the magnetization dynamics. The observation of the high TMR ratio more than 100% in CoFeB/MgO-based MTJ [2] has motivated both experimental and theoretical researchers to develop practical MRAM. The free layer should stay one of the stable states to keep information in MRAM. Unfortunately, however, the thermal fluctuation induces probabilistic switching of MTJ. Therefore, high thermal stability is required to guarantee the MRAM performance. Thermal stability of the MTJ is defined by the energy barrier of the free layer separating the two stable states divided by the thermal energy, k B T. Since the retention time of MRAM is exponentially proportional to the thermal stability, a small error in the evaluation of the thermal stability affects the estimation of the retention time significantly. Thus, an accurate evaluation of the thermal stability is necessary. The thermal stability has been evaluated by analyzing the switching probability of the free layer’s magnetization in the thermally activated region assisted by the spin torque. The fitting function of the switching probability is P=1-exp[-ft exp[-Δ(1-I/I c) b ]], where f, Δ, I, and I c are the attempt frequency, thermal stability, current, and switching current at zero temperature, respectively. The switching exponent, b, is assumed to be one [3]. In this study, we study the thermally assisted spin torque switching in an in-plane magnetized MTJ theoretically. The theoretical formula of the switching probability is derived from the Fokker-Planck equation. We find that the switching exponent is larger than one and depends on the applied electric current. The assumption that the switching exponent is one used in previous works is approximately valid only near the low current region, while relatively large current is applied in experiments to obtain the switching probability efficiently. In the high current region, the switching exponent is almost two. We also perform the numerical simulation of the Landau-Lifshitz-Gilbert (LLG) equation with stochastic torque. Figure 1(a) shows the dependence of the probability density, dP/dt, on the current pulse time for several temperatures. As shown, the probability density shows a peak at certain time. We call this time the switching time. Figure 1(b) shows the temperature dependence of the switching time on the temperature. It should be noted that the curvature of the switching time reflects the switching exponent [4]. The nonlinear dependence of the switching time is another evidence that the switching exponent is larger than one. The fact that the switching exponent is larger than one indicates that the previous works have underestimated the thermal stability. The presented formula enables us to evaluate the thermal stability with high accuracy. We also notice that the switching current at zero temperature, I c, can also be evaluated from the switching probability and our formula. We will talk the details of the theoretical analysis of the switching probability and numerical simulation. A comparison with latest experiments will also be presented. (Figure caption) Figure 1: (a) The dependence of the switching probability density on the current pulse time for several temperatures. (b) The temperature dependence of the switching time. [1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996). [2] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nat. Mater. 3, 868 (2004). S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant, and S.-H. Yang, ibid 3, 862 (2004). [3] R. H. Koch, J. A. Katine, and J. Z. Sun, Phys. Rev. Lett. 92, 088302 (2004). [4] T. Taniguchi and H. Imamura, J. Nanosci. Nanotechnol. 12, 7520 (2012). T. Taniguchi, M. Shibata, M. Marthaler, Y. Utsumi, and H. Imamura, Appl. Phys. Express 5, 063009 (2012). Figure 1

Highly Enhanced TMR Ratio and Δ for Double MgO-Based p-Mtj Spin-Valves with Top Co2Fe6B2 Free Layer By Nanoscale-Thick Iron Diffusion-Barrier

Recently, perpendicular spin-transfer-torque magnetic-random-access-memory (p-STT MRAM) has intensively been researched as a promising memory cell of overcoming the physical scaling limit of less than 10 nm in current dynamic-random-access-memory (DRAM) because of its fast write time (~10 ns), non-volatile memory operation, and low power consumption. A p-STT MRAM cell consists of a selective device and a perpendicular-magnetic-tunneling-junction (p-MTJ) spin-valve. CoFeB/MgO based p-MTJ spin-valves have been widely researched. In particular, for tera-bit-level integration, they require a high tunneling-magneto-resistance (TMR) ratio (> 150%) for a sufficient write/erase margin, high thermal stability (Δ) (> 74) for a 10-year data retention-time, and a low switching current (J c) (~1x102 MA/cm2) for low power consumption at the back end of line (BEOL) temperature of 400oC for p-STT-MRAM cells1. Recently, a Ta based double MgO-based p-MTJ spin-valve structure has been attractively proposed to achieve simultaneously a high TMR ratio and high thermal stability2. However, it has been extremely difficult to achieve a TMR ratio of >100% at 400oC (BEOL) because of Ta diffusion into the MgO tunneling barrier degrading the (100) crystallinity3. Thus, in this study, we designed a novel double MgO-based p-MTJ spin-valve with a top CoFeB free layer using W as a nanoscale-thick capping layer, spacer layer, and bridging layer by inserting a nanoscale-thick b.c.c. crystallized Fe diffusion-barrier between the W capping layer and the MgO capping layer, as shown in Fig. 1(a). First, we investigated the effect of this Fe diffusion barrier on the TMR ratio in a double MgO-based p-MTJ spin-valve with a top Co2F6B2 free layer ex-situ annealed at 400oC. Then, we examined the mechanism by which the Fe diffusion barrier enhances the TMR ratio by using current-in-plane tunneling (CIPT) at room temperature, the anisotropy energy density and thermal stability calculation by estimating the PMA magnetic moment. The TMR ratio was strongly dependent on the Fe diffusion-barrier thickness; i.e., it rapidly increased from ~131% to ~154% when the thickness increased from 0 to ~0.30 nm and it considerably decreased from ~154% to ~134% when the thickness increased from ~0.30 nm to ~0.66 nm. Thus, the TMR ratio peaked at a specific nanoscale Fe diffusion-barrier thickness, i.e., ~154% at ~0.30 nm, as shown in Fig. 1(b). Two kinds of the PMA structure, i.e., the PMA structure without a Fe diffusion barrier and with 0.30-nm thick Fe diffusion-barrier, were prepared to estimate the anisotropy energy and thermal stability. The anisotropy energy density (~0.31 erg/cm2) peaked at a specific effective thickness of the upper Co2Fe6B2 free layer (~0.67 nm) for the PMA structure without a Fe diffusion barrier; otherwise, it (~0.35 erg/cm2) peaked at a specific effective thickness of the upper Co2Fe6B2 free layer (~0.83 nm) for the PMA structure with 0.30-nm thick Fe diffusion barrier, as shown in Fig. 1(c). These result indicate that the insertion of a 0.30-nm-thick Fe diffusion barrier between the W and MgO capping layer could enhance the anisotropy energy density at a higher effective thickness of the upper Co2Fe6B2 free layer. Furthermore, thermal stability (Δ) was calculated. The Δ (33.8)peaked at a specific effective thickness of the upper Co2Fe6B2 free layer (~0.67 nm) for the PMA structure without a Fe diffusion barrier; otherwise, it (38.0) peaked at a specific effective thickness of the upper Co2Fe6B2 free layer (~0.83 nm) for the PMA structure with 0.30-nm thick Fe diffusion barrier, as shown in Fig. 3(b). These result indicate that the insertion of a 0.30-nm-thick Fe diffusion barrier between the W and MgO capping layer would increase the of the upper Co2Fe6B2 free layer approximately 15% since the ~0.30-nm thick Fe diffusion-barrier plays an excellent a role of the diffusion barrier of the W atoms ex-situ annealing at 400oC. In our presentation, we will report the mechanism by which thickness of the Fe diffusion barrier influence the TMR ratio was revealed by examining the static magnetization behavior, (100) bcc crystallinity, depth profile of the atomic composition of the spin-valves, the dead-layer thickness and thermal stability calculation, and the strain at the interface between the W capping and MgO tunneling barrier layer by theoretical calculation. * 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. Reference [1] H. Honjo et al., Symp. VLSI Tech. T160-161 (2015) [2] Y. Tomczak et al., Appl. Phys. Lett. 108, 042402 (2016) [3] Y. Takemura et al., Nanotecknology. 26, 195702 (2015) Figure 1

Giant voltage manipulation of MgO-based magnetic tunnel junctions via localized anisotropic strain: A potential pathway to ultra-energy-efficient memory technology

Voltage control of magnetization via strain in piezoelectric/magnetostrictive systems is a promising mechanism to implement energy-efficient straintronic memory devices. Here, we demonstrate giant voltage manipulation of MgO magnetic tunnel junctions (MTJ) on a Pb(Mg1/3Nb2/3)0.7Ti0.3O3 piezoelectric substrate with (001) orientation. It is found that the magnetic easy axis, switching field, and the tunnel magnetoresistance (TMR) of the MTJ can be efficiently controlled by strain from the underlying piezoelectric layer upon the application of a gate voltage. Repeatable voltage controlled MTJ toggling between high/low-resistance states is demonstrated. More importantly, instead of relying on the intrinsic anisotropy of the piezoelectric substrate to generate the required strain, we utilize anisotropic strain produced using a local gating scheme, which is scalable and amenable to practical memory applications. Additionally, the adoption of crystalline MgO-based MTJ on piezoelectric layer lends itself to high TMR in the strain-mediated MRAM devices.

Understanding the Spin Transport in H2O-Adsorbed Graphene-Based Magnetic Tunnel Junction

First-principles calculations of spin-dependent electronic transport properties of a magnetic tunnel junction consisting of a H2O (water)-adsorbed graphene nanosheet sandwiched between two CrO2 half-metallic ferromagnetic (HMF) electrodes is reported. H2O adsorption on graphene opens a bandgap in the graphene nanosheet which makes it more suitable for use as a tunnel barrier in magnetic tunnel junctions. It was found that H2O adsorption suppresses transmission probabilities for a spin-down channel in the case of parallel configuration (PC) and also suppresses transmission in antiparallel configuration (APC) for both spin-up and spin-down channels which result in higher tunnel magnetoresistance (TMR) at higher bias voltages in these structures. HMF electrodes were found suitable to achieve the perfect spin filtration effect and high TMR. I-V characteristics for both parallel and antiparallel magnetization states of junction are calculated. A high value of TMR ∼100 % is obtained at all bias voltages in the range of 0 to 1.2 V. High TMR suggests its usefulness in spin valves and other spintronics-based applications.

The Physics of Spin-Transfer Torque Switching in Magnetic Tunneling Junctions in Sub-10 nm Size Range

The spin-transfer torque magnetic tunneling junction (MTJ) technology may pave a way to a universal memory paradigm. MTJ devices with perpendicular magnetic anisotropy have the potential to have high thermal stability, high tunneling magnetoresistance, and low critical current for energy-efficient current-induced magnetization switching. Using devices fabricated through focused ion beam etching with Ga- and Ne-ion beams, this paper aimed to understand the size dependence of the current/voltage characteristics in the sub-10 nm range. The switching current density drastically dropped around 1 MA/cm $^{\mathrm { {2}}}$ as the device size was reduced below 10 nm. A stability of over 22 kT measured for a 5 nm device indicated a significantly reduced spin relaxation time.

Influence of the Reference Layer Composition on the Back-End-of-Line Compatibility of Co/Ni-Based Perpendicular Magnetic Tunnel Junction Stacks

Spin-transfer torque magnetic random access memory (STT-MRAM) is currently explored to challenge the dynamic random access memory and embedded memory applications. Perpendicular magnetic tunnel junction (p-MTJ) stacks used in the STT-MRAM must be compatible with CMOS back-end-of-line processing, such as high perpendicular magnetic anisotropy, high tunnel magnetoresistance (TMR), and intact resistance area (RA) product at temperatures, as high as 400°C. In this paper, we deposited a bottom-pinned Co/Ni-based p-MTJ stack with a trilayered Co/spacer/CoFeB as a reference layer and a CoFeB film as the storage layer. By replacing the standard Ta spacer material with CoFeBTa, the thickness of the spacer layer can be increased from 4 to 8 Å without TMR and RA penalty, which is beneficial to process controllability. Moreover, the TMR increases for an 8 Å CoFeB polarizer from 125% to 145% for a similar RA product after a 300°C 30 min anneal. In addition, the thermal budget of the p-MTJ stack containing a CoFeBTa spacer was improved. The p-MTJ stack with a 12 Å CoFeBTa reference layer spacer can even withstand annealing at 400°C for 90 min and can retain a TMR value of 100% at RA 10 Ω · μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , while TMR dropped below 40% for the p-MTJ stack containing a pure Ta spacer layer.

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.

Magnetic tunnel junctions with a rock-salt-type Mg1−xTixO barrier for low resistance area product

Ti-doped MgO (MTO) barriers were examined for polycrystalline magnetic tunnel junctions (MTJs) in combination with CoFeB ferromagnetic electrodes. The high tunneling magnetoresistance (TMR) ratio up to 240% and 160% was observed for the MTJs with Mg0.95Ti0.05O and Mg0.9Ti0.1O barriers after annealing at 450 °C. This high TMR ratio implies the presence of coherent tunneling. For a given thickness, MTJs with the MTO barriers were confirmed to have lower resistance-area product (RA) compared to those with the MgO barriers, suggesting the intrinsically lower barrier height of the MTO barriers. The MTO-based MTJs exhibit higher TMR ratio than those of the MgO-based MTJs for the RA range lower than 5 Ω μm2 as a consequence of thicker barriers and better wettability. This work has demonstrated the potential of the MTO barriers for low-RA MTJs.

Effect of coupling ability between a synthetic antiferromagnetic layer and pinned layer on a bridging layer of Ta, Ti, and Pt in perpendicular-magnetic tunnel junctions

By fabricating CoFeB/MgO/CoFeB-based perpendicular-magnetic tunnel junction (p-MTJ) spin-valves stacked with a [Co/Pd]n-SyAF layer based on a TiN bottom electrode on a 12 inch Si wafer (001) substrate, we investigated how the bridging layers of Ta, Ti, and Pt and their thickness variation affected the tunneling magneto-resistance (TMR) ratio of Co2Fe6B2 pinned-layer behavior in magnetic-tunnel-junctions. TMR ratios for Ta, Ti, and Pt bridging layers were observed to be 64.1, 70.2, and 29.5%, respectively. It was confirmed by high resolution transmission electron microscopy (HR-TEM) that this difference resulted from CoFeB/MgO/CoFeB MTJ layers with Ta and Ti bridging layers being textured well with a bcc (100) structure, indicating that Ta and Ti bridging layers bridged SyAF fcc (111) and MTJ bcc (100). On the other hand, the MTJ layer with Pt bridging layer was incorrectly textured, indicating that a Pt bridging layer is unsuitable to bridge SyAF fcc (111) and MTJ bcc (100) due to Pt being diffused into the CoFeB pinned-layer. In addition, perpendicular magnetic anisotropy (PMA) behavior of the CoFeB pinned-layer was found to depend strongly on a bridging layer thickness; higher TMRs of Ta and Ti were observed at the optimal bridging layers’ thickness, which enable the realization of PMAs of the pinned-layer and ferro-coupling of the pinned-layer with the lower-SyAF layer. Among the three bridging materials (Ta, Ti, and Pt), we observed that Ti showed the highest TMR ratio and widest thickness range for a high TMR ratio, indicating that a higher TMR ratio is needed to obtain the best deposition process margin.

Understanding the effect of vacancy defects on spin transport in CrO2–graphene–CrO2 magnetic tunnel junction

In this paper, we investigate the effect of vacancy defects on spin transport in graphene-based magnetic tunnel junction (MTJ). An increase in conductance was seen in vacancy-defected MTJ structure which is in contrast to the results reported in past where a decrease in conductance with vacancy was observed for graphene sheets. This increase in conductance may be due to the use of CrO2 half-metallic ferromagnet (HMF) electrodes instead of metallic or ferromagnet (FM) electrodes. Furthermore, high tunnel magnetoresistance (TMR) [Formula: see text]99% and perfect spin filtration was obtained for both vacancy-defected and no-defect (pristine) MTJ structures. The TMR in vacancy-defected structure is seen to decrease by 6.2% and by 13% at bias voltages of 1.2[Formula: see text]V and 1.4[Formula: see text]V, when compared to TMR in no-defect MTJ structure.

AlOx barrier growth in magnetic tunnel junctions for sensor applications

Magnetic tunnel junction (MTJ) research has been focused on MgO-based crystalline structures due to high tunnel magnetoresistance (TMR), despite requiring a more severe process control than previous generations of MTJ stacks based on amorphous barriers (e.g. AlOx). In this work, we study the electrical transport properties in AlOx barriers in MTJ sensors fabricated using Ion beam sputtering and remote plasma oxidation. Amorphous barriers were prepared from oxidation of thin Al films, deposited in single step barrier (SSB-Al 1nm/oxidation) or double step barrier (DSB-Al 0.5nm/oxidation/Al 0.5nm/oxidation) structures. We show tunable resistance-area products (RxA) ranging from ≈10Ωμm2 (suited for nano devices) up to ≈100kΩμm2 (suited for large area sensors) with TMR above 30%. For all geometries studied, the structures have a coercivity free linear response and require none or one annealing step. This makes them very competitive for all industrial applications where the TMR level is not the dominant specification to meet.

The zero-moment half metal: How could it change spin electronics?

The Heusler compound Mn2RuxGa (MRG) may well be the first compensated half metal. Here, the structural, magnetic and transport properties of thin films of MRG are discussed. There is evidence of half-metallicity up to x = 0.7, and compensation of the two Mn sublattice moments is observed at specific compositions and temperatures, leading to a zero-moment half metal. There are potential benefits for using such films with perpendicular anisotropy for spin-torque magnetic tunnel junctions and oscillators, such as low critical current, high tunnel magnetoresistance ratio, insensitivity to external fields and resonance frequency in the THz range.

Tunneling magnetoresistance in junctions composed of ferromagnets and time-reversal invariant topological superconductors

Tunneling magnetoresistance between two ferrromagnets is an issue of fundamental importance in spintronics. In this work, we show that tunneling magnetoresistance can also emerge in junctions composed of ferromagnets and time-reversal invariant topological superconductors without spin-rotation symmetry. Here the physical origin is that when the spin-polarization direction of an injected electron from the ferromagnet lies in the same plane of the spin-polarization direction of Majorana zero modes, the electron will undergo a perfect spin-equal Andreev reflection, while injected electrons with other spin-polarization directions will be partially Andreev reflected and partially normal reflected, which consequently has a lower conductance, and therefore, the magnetoresistance effect emerges. Compared to conventional magnetic tunnel junctions, an unprecedented advantage of the junctions studied here is that arbitrary high tunneling magnetoresistance can be obtained even when the magnetization of the ferromagnets are weak and the insulating tunneling barriers are featureless. Our findings provide a new fascinating mechanism to obtain high tunneling magnetoresistance.

Termination layer compensated tunnelling magnetoresistance in ferrimagnetic Heusler compounds with high perpendicular magnetic anisotropy.

Although high-tunnelling spin polarization has been observed in soft, ferromagnetic, and predicted for hard, ferrimagnetic Heusler materials, there has been no experimental observation to date of high-tunnelling magnetoresistance in the latter. Here we report the preparation of highly textured, polycrystalline Mn3Ge films on amorphous substrates, with very high magnetic anisotropy fields exceeding 7 T, making them technologically relevant. However, the small and negative tunnelling magnetoresistance that we find is attributed to predominant tunnelling from the lower moment Mn–Ge termination layers that are oppositely magnetized to the higher moment Mn–Mn layers. The net spin polarization of the current reflects the different proportions of the two distinct termination layers and their associated tunnelling matrix elements that result from inevitable atomic scale roughness. We show that by engineering the spin polarization of the two termination layers to be of the same sign, even though these layers are oppositely magnetized, high-tunnelling magnetoresistance is possible.

High tunneling magnetoresistance ratio in perpendicular magnetic tunnel junctions using Fe-based Heusler alloys

Heulser alloys Fe2Cr1−xCoxSi (FCCS) with different Co compositions x have been predicted to have high spin polarization. High perpendicular magnetic anisotropy (PMA) has been observed in ultra-thin FCCS films with magnetic anisotropy energy density up to 2.3 × 106 erg/cm3. The perpendicular magnetic tunnel junctions (p-MTJs) using FCCS films with different Co compositions x as the bottom electrode have been fabricated and the post-annealing effects have been investigated in details. An attractive tunneling magnetoresistance ratio as high as 51.3% is achieved for p-MTJs using Fe2CrSi (FCS) as the bottom electrode. The thermal stability Δ can be as high as 70 for 40 nm dimension devices using FCS, which is high enough to endure a retention time of over 10 years. Therefore, Heusler alloy FCS is a promising PMA candidate for p-MTJ application.

Brillouin zone spin filtering mechanism of enhanced tunneling magnetoresistance and correlation effects in aCo(0001)/h -BN/Co(0001)magnetic tunnel junction

The 'Brillouin zone spin filtering' mechanism of enhanced tunneling magnetoresistance (TMR) is described for magnetic tunnel junctions (MTJ) and studied on an example of the MTJ with hcp Co electrodes and hexagonal BN (h-BN) spacer. Our calculations based on local density approximation of density functional theory (LDA-DFT) for Co(0001)/h-BN/Co(0001) MTJ predict high TMR in this device due to Brillouin zone filtering mechanism. Owning to the specific complex band structure of the h-BN the spin-dependent tunneling conductance of the system is ultra-sensitive to small variations of the Fermi energy position inside the BN band gap. Doping of the BN and, consequentially, changing the Fermi energy position could lead to variation of the TMR by several orders of magnitude. We show also that taking into account correlation effects on beyond DFT level is required to accurately describe position of the Fermi level and thus transport propertied of the system. Our study suggests that new MTJ based on hcp Co-Pt or Co-Pd disordered alloy electrodes and p-doped hexagonal BN spacer is a promising candidate for the spin-transfer torque magnetoresistive random-access memory (STT-MRAM).

Origin of interfacial perpendicular magnetic anisotropy in MgO/CoFe/metallic capping layer structures.

Spin-transfer-torque magnetic random access memory (STT-MRAM) attracts extensive attentions due to its non-volatility, high density and low power consumption. The core device in STT-MRAM is CoFeB/MgO-based magnetic tunnel junction (MTJ), which possesses a high tunnel magnetoresistance ratio as well as a large value of perpendicular magnetic anisotropy (PMA). It has been experimentally proven that a capping layer coating on CoFeB layer is essential to obtain a strong PMA. However, the physical mechanism of such effect remains unclear. In this paper, we investigate the origin of the PMA in MgO/CoFe/metallic capping layer structures by using a first-principles computation scheme. The trend of PMA variation with different capping materials agrees well with experimental results. We find that interfacial PMA in the three-layer structures comes from both the MgO/CoFe and CoFe/capping layer interfaces, which can be analyzed separately. Furthermore, the PMAs in the CoFe/capping layer interfaces are analyzed through resolving the magnetic anisotropy energy by layer and orbital. The variation of PMA with different capping materials is attributed to the different hybridizations of both d and p orbitals via spin-orbit coupling. This work can significantly benefit the research and development of nanoscale STT-MRAM.

First-principles study of spin transport in $$\hbox {CrO}_{2}\hbox {-SiCNT-CrO}_{2}$$ CrO 2 -SiCNT-CrO 2 magnetic tunnel junction

We report the spin transport in silicon carbide nanotube based magnetic tunnel junction with half-metallic-ferromagnet $$\hbox {CrO}_{2}$$CrO2 as electrodes. The simulations based on first principles suggest high tunnel magnetoresistance $$\sim $$~100 % at zero bias, TMR remains high at higher bias voltages. I---V characteristics show that spin current in parallel magnetic configuration is much higher than the spin current in anti-parallel configuration. Perfect spin-filtration effect is obtained for this structure. Transmission coefficients are also calculated to understand the spin and bias dependent non-equilibrium transport properties.

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