Perpendicular Magnetic Tunnel Junctions Research Articles

Highly enhanced perpendicular magnetic anisotropic features in a CoFeB/MgO free layer via WN diffusion barrier

Ferromagnet/oxide interfaces that ensure perpendicular magnetic anisotropy (PMA) features are critical for developing spintronic technologies such as perpendicular magnetic tunnel junctions, which are the most reliable building blocks for spin transfer torque switching or spin-orbit-torque switching. As such, metal/CoFeB/MgO frames containing various transition metals have been a central component to achieve large PMA and higher annealing stability. However, metal/CoFeB/MgO frames can experience thermally activated boron and transition metal diffusion behavior during annealing at temperatures greater than 300 °C, which deteriorates PMA. In this work, we introduce the simple incorporation of tungsten nitride (WN) at the interface of Ta/CoFeB as a generic alternative approach to obtain an enhanced PMA in Ta/CoFeB/MgO frames without thermal degradation. Precise control of the thickness and composition of WN was required to ensure stable PMA even after a 425 °C annealing process, along with the achievement of a high effective magnetic anisotropy energy density of almost 10 Merg/cc.

Enhanced voltage-controlled magnetic anisotropy in magnetic tunnel junctions with an MgO/PZT/MgO tunnel barrier

Compared with current-controlled magnetization switching in a perpendicular magnetic tunnel junction (MTJ), electric field- or voltage-induced magnetization switching reduces the writing energy of the memory cell, which also results in increased memory density. In this work, an ultra-thin PZT film with high dielectric constant was integrated into the tunneling oxide layer to enhance the voltage-controlled magnetic anisotropy (VCMA) effect. The growth of MTJ stacks with an MgO/PZT/MgO tunnel barrier was performed using a combination of sputtering and atomic layer deposition techniques. The fabricated MTJs with the MgO/PZT/MgO barrier demonstrate a VCMA coefficient, which is ∼40% higher (19.8 ± 1.3 fJ/V m) than the control sample MTJs with an MgO barrier (14.3 ± 2.7 fJ/V m). The MTJs with the MgO/PZT/MgO barrier also possess a sizeable tunneling magnetoresistance (TMR) of more than 50% at room temperature, comparable to the control MTJs with an MgO barrier. The TMR and enhanced VCMA effect demonstrated simultaneously in this work make the MgO/PZT/MgO barrier-based MTJs potential candidates for future voltage-controlled, ultralow-power, and high-density magnetic random access memory devices.

Electrical-field and spin-transfer torque effects in CoFeB/MgO-based perpendicular magnetic tunnel junction

The electric-field (E) dependence of the magnetoresistance (RH) loops for top-pinned perpendicular CoFeB/MgO-based magnetic tunnel junctions (MTJs) in the presence of a spin-transfer torque (STT)-current was measured. The E effects were distinguished from the STT-current effects using a micromagnetic simulation. The coercive field (Hc) decreased and the RH loop shifted as both the positive and negative bias E increased owing to the STT current. Furthermore, E-assisted switching for an MTJ with a diameter of 20 nm, which exhibited a nearly coherent magnetization reversal, was demonstrated using micromagnetic simulation.

Study of CoFeB thickness and composition dependence in a modified CoFeB/MgO/CoFeB perpendicular magnetic tunnel junction

We studied the CoFeB thickness and composition dependence of tunneling magnetoresistance (TMR) and resistance-area product (RA) in a modified CoFeB/MgO/CoFeB perpendicular magnetic tunnel junction (MTJ), in which the bottom CoFeB is coupled to an in-plane exchange biased magnetic layer. This stack structure allows us to measure TMR and RA of the MTJs in sheet film format without patterning them, using current-in-plane-tunneling (CIPT) technique. The thickness ranges for both top and bottom CoFeB to exhibit perpendicular magnetic anisotropy are similar to what are seen in each single magnetic film stack. However, CIPT measurement revealed that there exists an optimal thickness for both top and bottom CoFeB to achieve the highest TMR value. Magnetic hysteresis loops also suggest the thickness-dependent coupling between the top and bottom CoFeB layers. We studied MTJs with two CoFeB compositions (Co40Fe40B20 and Co20Fe60B20) and found that Co20Fe60B20 MTJs give higher TMR and also wider perpendicular thickness range when used at the top layer.

Perpendicular magnetic tunnel junctions with a synthetic storage or reference layer: A new route towards Pt- and Pd-free junctions.

We report here the development of Pt and Pd-free perpendicular magnetic tunnel junctions (p-MTJ) for STT-MRAM applications. We start by studying a p-MTJ consisting of a bottom synthetic Co/Pt reference layer and a synthetic FeCoB/Ru/FeCoB storage layer covered with an MgO layer. We first investigate the evolution of RKKY coupling with Ru spacer thickness in such a storage layer. The coupling becomes antiferromagnetic above 0.5 nm and its strength decreases monotonously with increasing Ru thickness. This contrasts with the behavior of Co-based systems for which a maximum in interlayer coupling is generally observed around 0.8 nm. A thin Ta insertion below the Ru spacer considerably decreases the coupling energy, without basically changing its variation with Ru thickness. After optimization of the non-magnetic and magnetic layer thicknesses, it appears that such a FeCoB/Ru/FeCoB synthetic storage layer sandwiched between MgO barriers can be made stable enough to actually be used as hard reference layer in single or double magnetic tunnel junctions, the storage layer being now a single soft FeCoB layer. Finally, we realize Pt- or Pd-free robust perpendicular magnetic tunnel junctions, still keeping the advantage of a synthetic reference layer in terms of reduction of stray fields at small pillar sizes.

Time-resolved spin-torque switching in MgO-based perpendicularly magnetized tunnel junctions

We study ns scale spin-torque-induced switching in perpendicularly magnetized tunnel junctions (pMTJ). Although the switching voltages match with the macrospin instability threshold, the electrical signatures of the reversal indicate the presence of domain walls in junctions of various sizes. In the antiparallel (AP) to parallel (P) switching, a nucleation phase is followed by an irreversible flow of a wall through the sample at an average velocity of 40 m/s with back and forth oscillation movements indicating a Walker propagation regime. A model with a single-wall locally responding to the spin-torque reproduces the essential dynamical signatures of the reversal. The P to AP transition has a complex dynamics with dynamical back-hopping whose probability increases with voltage. We attribute this back-hopping to the instability of the nominally fixed layers.

Perpendicular magnetic anisotropy in composite MgO/CoFeB/Ta/[Co/Pd]n structures

The impact of a non-magnetic Ta spacer layer on the perpendicular magnetic anisotropy (PMA) of composite magnetic structures constituted by ultra-thin Co/Pd multilayers (MLs) and MgO/CoFeB was studied. Composite structures lacking a Ta layer present in-plane magnetic anisotropy. The strong perpendicular anisotropy observed in sole Co/Pd MLs is not sufficient to pull the magnetic moment out of the film plane, not even after annealing at 300 or 350 °C. PMA with squareness values close to unity and annealing stability up to 350 °C is observed after the insertion of an ultra-thin Ta layer. Our study demonstrates that Ta layer is essential for obtaining perpendicular magnetic axis in MgO/CoFeB/Ta/[Co/Pd]6. The exchange coupling between the MgO/CoFeB bilayer and the Co/Pd MLs is ferromagnetic with sharp switching characteristics. Perpendicular composite structures with sharp magnetization reversal and annealing stability are relevant in perpendicular CoFeB-based magnetic tunnel junctions for the development of gigabit-scale nonvolatile memory.

Magnetization switching by combining electric field and spin-transfer torque effects in a perpendicular magnetic tunnel junction.

Effective manipulation of magnetization orientation driven by electric field in a perpendicularly magnetized tunnel junction introduces technologically relevant possibility for developing low power magnetic memories. However, the bipolar orientation characteristic of toggle-like magnetization switching possesses intrinsic difficulties for practical applications. By including both the in-plane (T//) and field-like (T⊥) spin-transfer torque terms in the Landau-Lifshitz-Gilbert simulation, reliable and deterministic magnetization reversal can be achieved at a significantly reduced current density of 5×109 A/m2 under the co-action of electric field and spin-polarized current, provided that the electric-field pulse duration exceeds a certain critical value τc. The required critical τc decreases with the increase of T⊥ strength because stronger T⊥ can make the finally stabilized out-of-plane component of magnetization stay in a larger negative value. The power consumption for such kind of deterministic magnetization switching is found to be two orders of magnitude lower than that of the switching driven by current only.

Ultra-low switching energy and scaling in electric-field-controlled nanoscale magnetic tunnel junctions with high resistance-area product

We report electric-field-induced switching with write energies down to 6 fJ/bit for switching times of 0.5 ns, in nanoscale perpendicular magnetic tunnel junctions (MTJs) with high resistance-area product and diameters down to 50 nm. The ultra-low switching energy is made possible by a thick MgO barrier that ensures negligible spin-transfer torque contributions, along with a reduction of the Ohmic dissipation. We find that the switching voltage and time are insensitive to the junction diameter for high-resistance MTJs, a result accounted for by a macrospin model of purely voltage-induced switching. The measured performance enables integration with same-size CMOS transistors in compact memory and logic integrated circuits.

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.

Underlayer Effect on Perpendicular Magnetic Anisotropy in Co20Fe60B20/MgO Films

Perpendicular Magnetic Tunneling Junctions (pMTJs) with Ta\CoFeB\MgO have been extensively studied in recent years. However, the effects of the underlayer on the formation of the CoFeB perpendicular magnetic anisotropy (PMA) are still not well understood. Here we report the results of our systematic use of a wide range of elements (Ti, V, Cr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt and Au) encompassed by columns IVA, VA, VIA, VIIA and VIIIA of the periodic table as the underlayer in a underlayer\Co20Fe60B20\MgO stack. Our goals were to survey more elements which could conceivably create a PMA in CoFeB and thereby to explore the mechanisms enabling these underlayers to enhance or create the PMA. We found underlayer elements having both an outer shell of 4d electrons (Zr, Nb Mo, and Pd) and 5d electrons (Hf, Ta, W, Re, Ir, and Pt) resulted in the development of a PMA in the MgO-capped Co20Fe60B20. Hybridization between the 3d electrons of the Fe or Co (in the Co20Fe60B20) at the interface with the 4d or 5d electrons of the underlayer is thought to be the cause of the PMA development.

Low current writing perpendicular magnetic random access memory with high thermal stability

Spin reorientation temperature (TSR) is investigated in both layers, including Co/Ni multilayer and composite layer consisting of Co/Pt multilayer with CoFeB layer. The TSR of both layers can be adjusted in a wide range, and the low TSR is obtained in both layers. It is also suggested a writing method based on spin transfer torque (STT) with spin reorientation, for low current writing perpendicular magnetic random memory with high thermal stability. This method is to reorient the magnetization from a perpendicular to an in-plane state, which can be caused by Joule heating the MTJ with thermal barriers above TSR by the injected current pulse. This polarized current can also produce the STT to push the magnetization toward a deterministic perpendicular direction from an in-plane state, when decreasing the magnitude of current pulse. Since the perpendicular anisotropy dominates magnetization switching during cooling, the critical current density is determined by the Joule heating for realization of spin reorientation in this writing method. Macrospin modeling shows the critical current density is 2×106A/cm2, which is two orders magnitude lower than that of conventional STT switching. This writing method can be applied to the perpendicular magnetic tunnel junction comprised of Co/Ni multilayer or composite layer.

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.

Perpendicular magnetic tunnel junction with enhanced anisotropy obtained by utilizing an Ir/Co interface

A highly scalable perpendicularly magnetized storage layer of a spin-torque-switching magnetic random-access memory (STT-MRAM) was developed. This storage layer attains a perpendicular magnetic anisotropy (PMA) of above 0.9 erg/cm2 at a thickness of 2 nm. Such high PMA is suitable for pushing STT-MRAM technology beyond the 20 nm node. The key was to realize dual interfacial PMA at both the Ir/Co and FeB/MgO interfaces in the united structure of the storage layer. While a high PMA was retained, a high magnetoresistance ratio (100%) and a low resistance–area product (3.0 Ω µm2) were also achieved.

Co2MnSi Heusler alloy as an enhancing layer of perpendicular magnetic anisotropy for MgO-based magnetic tunnel junctions with L10 ordered FePd

Ultra-thin Co2MnSi Heusler alloy improves perpendicular magnetic anisotropy of FePd in an MgO-based magnetic tunnel junction after annealing it just once at a temperature of as low as 400 °C. Co2MnSi as thin as 1.0 nm inserted between MgO and FePd facilitated phase-transformation of 3-nm-thick FePd to ordered L10 and led a change in magnetic anisotropy to perpendicular-to-the-plane. To make it even better, FePd also helped the phase-transformation of Co2MnSi to ordered B2 known to have high spin polarization, which makes the L10 FePd/B2 Co2MnSi bilayer promising for perpendicular-magnetic tunnel junction and improving both thermal stability and tunnel magnetoresistance.

Thermally Robust Perpendicular STT-MRAM Free Layer Films Through Capping Layer Engineering

In order to enable the spin-transfer torque (STT) magnetoresistive random access memory technologies, it is essential to control the thermal budget, perpendicular magnetic anisotropy, and magnetic damping parameter of perpendicular magnetic tunnel junction (pMTJ) thin films. This paper demonstrates the enhancement of pMTJ free layer (FL) properties through selective engineering of capping materials placed between a CoFeB FL and the top electrode. By introducing a capping layer of Mg or MgO, thermal budget and tunneling magnetoresistance (TMR) ratio are significantly improved over the conventional FL schemes due to reduced atomic intermixing at interfaces. In full-stack pMTJ films, thin Mg above the FL enables the TMR above 170% after 400 °C annealing. Capping via MgO increases the interface anisotropy, allowing for the use of thicker CoFeB FLs with magnetic damping constants as low as 0.003. We discuss the implications of these capping layer improvements and suggest that Mg and MgO show the potential for optimizing the FLs with high-thermal budget and good STT efficiency.

Competing Anisotropy-Tunneling Correlation of the CoFeB/MgO Perpendicular Magnetic Tunnel Junction: An Electronic Approach.

We intensively investigate the physical principles regulating the tunneling magneto-resistance (TMR) and perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO magnetic tunnel junction (MTJ) by means of angle-resolved x-ray magnetic spectroscopy. The angle-resolved capability was easily achieved, and it provided greater sensitivity to symmetry-related d-band occupation compared to traditional x-ray spectroscopy. This added degree of freedom successfully solved the unclear mechanism of this MTJ system renowned for controllable PMA and excellent TMR. As a surprising discovery, these two physical characteristics interact in a competing manner because of opposite band-filling preference in space-correlated symmetry of the 3d-orbital. An overlooked but harmful superparamagnetic phase resulting from magnetic inhomogeneity was also observed. This important finding reveals that simultaneously achieving fast switching and a high tunneling efficiency at an ultimate level is improbable for this MTJ system owing to its fundamental limit in physics. We suggest that the development of independent TMR and PMA mechanisms is critical towards a complementary relationship between the two physical characteristics, as well as the realization of superior performance, of this perpendicular MTJ. Furthermore, this study provides an easy approach to evaluate the futurity of any emerging spintronic candidates by electronically examining the relationship between their magnetic anisotropy and transport.

ChemInform Abstract: Tetragonal Heusler‐Like Mn‐Ga Alloys Based Perpendicular Magnetic Tunnel Junctions

AbstractReview: 105 refs.

A Portable Dynamic Switching Model for Perpendicular Magnetic Tunnel Junctions Considering Both Thermal and Process Variations

A Portable Dynamic Switching Model for Perpendicular Magnetic Tunnel Junctions Considering Both Thermal and Process Variations

A Multilevel Cell for STT-MRAM Realized by Capping Layer Adjustment

A multilevel cell (MLC) allows gigabit high-density integration of a spin-transfer torque magnetic random-access memory (STT-MRAM). In this paper, we present an MLC structure based on MgO/CoFeB interfacial perpendicular magnetic anisotropy (PMA) and capping layer adjustment. The effect of capping layer toward interfacial PMA modulation has been investigated by first-principles calculations, which is consistent with the previous experimental results. Two methods have been proposed to realize this STT-MRAM MLC: 1) capping layer thickness and 2) material composition design. Using a physics-based compact model of perpendicular magnetic tunnel junctions, resistance–current curves with four-level resistance states are demonstrated, for which the threshold switching currents with clear separations prove the feasibility of this concept.