Perpendicular Magnetic Tunnel Junctions Research Articles

Impact of Joule heating on the stability phase diagrams of perpendicular magnetic tunnel junctions

Measured switching voltage-field diagrams of perpendicular magnetic tunnel junctions exhibit unexpected behavior at high voltages associated with significant heating of the storage layer. The boundaries deviate from the critical lines corresponding to the coercive field, which contrasts with the theoretically predicted behavior of a standard macrospin-based model. Combining recent experimental studies of the temperature dependence of spin polarization and perpendicular magnetic anisotropy, we are proposing a modified model. Our approach takes into account the Joule heating during the writing pulse, which reduces the spin polarization and the anisotropy, thereby reducing the spin torque efficiency and the coercive field during the switching. The numerical macrospin simulations based on this model are in agreement with our experimental measurements and consistent with the results derived from the linearization of Landau-Lifshitz-Gilbert equation.

Size dependence of spin-torque switching in perpendicular magnetic tunnel junctions

We simulate the spin torque-induced reversal of the magnetization in thin disks with perpendicular anisotropy at zero temperature. Disks typically smaller than 20 nm in diameter exhibit coherent reversal. A domain wall is involved in larger disks. We derive the critical diameter of this transition. Using a proper definition of the critical voltage, a macrospin model can account perfectly for the reversal dynamics when the reversal is coherent. The same critical voltage appears to match with the micromagnetics switching voltage regardless of the switching path.

Synthetic-Ferromagnet Pinning Layers Enabling Top-Pinned Magnetic Tunnel Junctions for High-Density Embedded Magnetic Random-Access Memory

Top-pinned perpendicular magnetic tunnel junctions featuring a synthetic antiferromagnet (SAF) have been extensively studied for spin-transfer-torque magnetic random-access memory (STT-MRAM). However, degraded device performance after annealing remains a problem for this design, and hinders integration with CMOS technology. This study instead uses a synthetic $f\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ (SFM) pinning layer to enable high thermal tolerance in STT-MRAM stacks. Experiment and simulation confirm functionality in 20-nm devices, including current switching. The proposed design contributes significantly to the scalability of STT-MRAM, and enables future applications requiring a top-pinned stack.

Instant-On Spin Torque in Noncollinear Magnetic Tunnel Junctions

Spin-torque-operated magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy are established charge-to-spin transducers. Their switching speed is still fundamentally limited by thermal activation, however, leading to slower switching in memory applications. In this study a canted magnetization state is engineered to overcome the incubation delay, for faster switching times. Comparison to perpendicular MTJs is performed, with direct time-resolved observation of switching dynamics in both systems. These results should facilitate optimization of MTJ speed for spintronic computing.

Structural, electronic and magnetic properties of MnxGa/Co2MnSi (x\u2009=\u20091, 3) bilayers

Directly coupled hard and soft ferromagnets were popularly used as the hybridized electrodes to enhance tunnel magnetoresistance (TMR) ratio in the perpendicular magnetic tunnel junction (pMTJ). In this paper, we employ the density functional theory (DFT) with general gradient approximation (GGA) to investigate the interfacial structure and magnetic behavior of tetragonal Heusler-type MnGa (MG)/L21-Co2MnSi (CMS) Heusler alloy bilayers with the MnGa being D022-MnGa alloy (Mn3Ga) and L10-MnGa alloy (MnGa). The MM-MS_B interface with the bridge (B) connection of MnMn termination (MM) of D022- and L10-MnGa layers to MnSi termination (MS) of CMS layers is found to be most stable in the energy point of view. Also, a strong antiferromagnetic coupling and relatively higher spin polarization can be observed in the MM-MS_B interface. Further, a remarkable potential difference to derive electrons to transfer from MG layer to CMS layer appears at the interface. These theoretical results indicate that the MG/CMS bilayers are promising candidates as coupled composites, and moreover, the D022-MG/CMS bilayer is better than L10-MG/CMS bilayer due to its larger spin polarization and built-in field at the interface.

Field-free switching of a perpendicular magnetic tunnel junction through the interplay of spin–orbit and spin-transfer torques

Magnetization switching in magnetic tunnel junctions using spin-transfer torque and spin–orbit torque is key to the development of future spintronic memories. However, both switching mechanisms suffer from intrinsic limitations. In particular, the switching current in spin-transfer torque devices needs to be lowered, whereas an external magnetic field is required for spin–orbit torque devices to achieve deterministic switching in perpendicular magnetic tunnel junctions. Here, we experimentally demonstrate field-free switching of three-terminal perpendicular-anisotropy magnetic tunnel junction devices through the interaction between spin–orbit and spin-transfer torques. We show that the threshold current density of spin–orbit torque switching can be reduced by increasing the spin-transfer torque current density, and thus an optimal point for low-power perpendicular magnetic tunnel junction switching can be found by tuning the two current densities. Furthermore, and due to this interplay, low-power switching in two-terminal perpendicular magnetic tunnel junctions without an external magnetic field is also achieved. The interplay between spin–orbit and spin-transfer torques can be used to develop a low-power route to magnetization switching of perpendicular magnetic tunnel junctions without an external magnetic field.

Domain wall nucleation and propagation in spin-transfer torque switching with thermal effects

We conduct micro-magnetic simulations to study spin-transfer torque induced magnetization switching in perpendicular magnetic tunneling junctions. The effects of current densities and temperatures on the switching processes are studied in details. We then proposed an approach to compute the deterministic switching time by taking thermal-effect into account. The switching time is less temperature-dependent under higher current density; however, as the current density decreases, the effect of temperature on the switching time becomes more and more significant. The switching process with micro-magnetic simulations is shown to be via domain wall nucleation and propagation. The phenomena are consistent with the recent experimental found-out. We further propose a method to compute the switching time based on domain wall nucleation and propagation theory, and compare the switching time with those from macro-spin approximation. It is found the switching times from the micro-magnetic simulations are much shorter than that from the macro-spin approximations. Macro-spin approximation over-estimates the switching times due to its coherent rotation assumptions.

Impact of operating temperature on the electrical and magnetic properties of the bottom-pinned perpendicular magnetic tunnel junctions

Analogous device parameters in both the parallel (P) and anti-parallel (AP) states ensure a symmetric spin-transfer-torque magnetic random-access memory operation scheme. In this study, however, we observe an increasing asymmetry in the performance metrics with operating temperature of the bottom-pinned perpendicular magnetic tunnel junction (p-MTJ) devices. A temperature-dependent increase in the contribution of the stray field is observed in the tunneling magnetoresistance loop analysis. The switching current for P-to-AP decreases by 30% in the thermally activated switching regime by increasing the temperature from 300 K to 400 K, while it remains similar for AP-to-P. In addition, with the same temperature range, the thermal stability factor for the P state decreases 20% more than that for the AP state. We attribute those observations to the increase in the overcompensation of the stray field from the synthetic anti-ferromagnet structure. Saturation magnetization (MS) of the [Co/Pt]x-based multilayers is much less affected by temperature [MS(400 K)/MS(300 K) = 97%] compared to that of the CoFeB-based multilayers (88%). Such an impact can be more severe during the electrical switching process due to the Joule heating effect. These results suggest that, to understand and to evaluate the performance in a wide range of temperatures, it is crucial to consider the contribution of the entire magnetic components in the p-MTJ stack.

Macrospin analysis of RF excitations within fully perpendicular magnetic tunnel junctions with second order easy-axis magnetic anisotropy contribution

The conditions of field and voltage for inducing steady state excitations in fully perpendicular magnetic tunnel junctions (pMTJs), adapted for memory applications, were numerically investigated by the resolution of the Landau-Lifshitz-Gilbert equation in the macrospin approach. Both damping-like and the field-like spin transfer torque terms were taken into account in the simulations, as well as the contribution of the second order uniaxial anisotropy term (K2), which has been recently revealed in MgO-based pMTJs. An in-plane applied magnetic field balances the out of plane symmetry of the pMTJ and allows the signal detection. Using this model, we assessed the states of the free layer magnetization as a function of strength of K2 and polar θH angle of the applied field (varied from 90° to 60°). There are two stable states, with the magnetization in-plane or out of plane of the layer, and two dynamic states with self-sustained oscillations, called in-plane precession state (IPP) or out of plane precession state (OPP). The IPP mode, with oscillation frequencies up to 7 GHz, appears only for positive voltages if θH = 90°. However, it shows a more complex distribution when the field is slightly tilted out of plane. The OPP mode is excited only if K2 is considered and reaches a maximum oscillation frequency of 15 GHz. Large areas of dynamic states with high frequencies are obtained for strong values of the field-like torque and K2, when applying a slightly tilted external field toward the out of plane direction. The non-zero temperature does not modify the phase diagrams but reduces drastically the power spectral density peak amplitudes.

Influence of a composite free layer structure on thermal stability of perpendicular magnetic tunnel junction

Perpendicular magnetic tunnel junctions (pMTJs) with a CoFeB/W/CoFeB/MgO composite free layer are experimentally investigated. Magnetic anisotropy and tunneling magnetoresistance (TMR) are determined as functions of ferromagnetic (CoFeB) and capping (MgO) layer thicknesses using the ferromagnetic resonance technique. In pMTJs fabricated into 130 and 150-nm diameters, the TMR ratio > 160% and the thermal stability factor Δ > 60 are measured for the MgO capping layer thickness of tMgO ≥ 1 nm. The voltage vs. magnetic field stability diagram indicates field-free current induced switching with voltages |VS|<0.5 V.

Dependence of switching process on the perpendicular magnetic anisotropy constant in P-MTJ**Project supported by the National Natural Science Foundation of China (Grant No. 61332003) and the Natural Science Foundation of Hunan Province, China (Grant No. 2015JJ3024).

We investigate the dependence of the switching process on the perpendicular magnetic anisotropy (PMA) constant in perpendicular spin transfer torque magnetic tunnel junctions (P-MTJs) using micromagnetic simulations. It is found that the final stable states of the magnetization distribution of the free layer after switching can be divided into three different states based on different PMA constants: vortex, uniform, and steady. Different magnetic states can be attributed to a trade-off among demagnetization, exchange, and PMA energies. The generation of the vortex state is also related to the non-uniform stray field from the polarizer, and the final stable magnetization is sensitive to the PMA constant. The vortex and uniform states have different switching processes, and the switching time of the vortex state is longer than that of the uniform state due to hindrance by the vortex.

Design of Double Pinned Perpendicular Magnetic-Tunnel-Junction Spin-Valve Exhibiting Multiple Resistance States

Perpendicular spin-transfer-torque magnetic random access memory (p-STT MRAM) cells consist of a perpendicular magnetic tunnel junction and a selective device has been researched intensively because of the possibility of overcoming the scaling limitations of the current dynamic random access memory (DRAM) below the 10-nm design rule [1]. p-STT-MRAM requires high tunneling magnetoresistance (TMR) ratio greater than 150%, sufficient thermal stability (Δ=KuV/kBT) above 75 for a ten-year retention-time, and low switching current (J C0 ~ 1 MA/cm2) must be achieved for low power consumption [2-3]. However, the thermal stability is hard to maintain when scaling down in 10-nm scale of the p-STT MRAM cells for terabit integration. It requires high interfacial anisotropy K i to maintain the thermal stability requirement for 10-year retention time [4]. In this study, we designed a p-MTJ spin-valve with a double pinned structure to realize two-bit operation to secure a larger scaling limit (see Fig. 1(a)). We investigated the dependency of the number of [Co/Pt]n multilayers on the coupling strength to the pinned layers in order to control the spin-electron direction of the pinned layers. The total number of [Co/Pt]n SyAF multilayers ferro-coupled to the bottom pinned layer was reduced from 9 to 3 layers to reduce the roughness of the MgO tunneling barrier to increase the TMR ratio. Also, the number of [Co/Pt]n of the top upper SyAF multilayers was modulated so that the coercive field (H c) required to switch the spin-electron direction of the top pinned layer increased from about 0.3 kOe to 1 kOe which is sufficient to ensure the memory margin for two-bit operation, as shown in Fig. 1(b). Then, we investigated the multiple resistance states with the R-H loop of the double pinned p-MTJ spin-valve, as shown in Fig. 1(c). It showed four different resistance states depending on the spin-electron direction of the free layers and pinned layers as shown in Fig. 1(d) with a maximum TMR ratio of 166%. This implies that the double pinned p-MTJ spin-valve may be suitable for terabit integration compared to that of the single-bit operating p-STT MRAM memory cells. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.2017R1A2A1A05001285) and Brain Korea 21 PLUS Program in 2014.

A non-collinear double MgO based perpendicular magnetic tunnel junction

Double MgO based magnetic free layers are state-of-the-art solutions for providing high performance perpendicular spin-transfer torque-magnetic random access memory devices. We provide device measurements showing reduction of switching current in perpendicular magnetic tunnel junctions (p-MTJs) using non-collinear ferromagnets on the double MgO template. This structure is engineered by introducing an in-plane ferromagnetic cap, which produces in-plane stray field effects on the free layer. The non-collinear structure delivers ∼53% reduction in critical current density in STT switching without weakening the thermal stability of the devices. The advantages in device performance using non-collinearity in magnetization is sustained down to ∼20 nm MTJs. Micromagnetic simulations suggest inherent differences in the magnetization reversal process between our proposed non-collinear p-MTJ and a well-known double MgO based p-MTJ.

Electric-field-induced three-terminal pMTJ switching in the absence of an external magnetic field

Since it is undesirable to require an external magnetic field for on-chip memory applications, we investigate the use of a Rashba effective field alternatively for assisting the electric-field-induced switching operation of a three terminal perpendicular magnetic tunnel junction (pMTJ). By conducting macro-spin simulations, we show that a pMTJ with a thermal stability of 61 can be switched in 0.5 ns, consuming a switching energy of 6 fJ, and the voltage operation margin can be improved to 0.8 ns. Furthermore, the results also demonstrate that a heavy metal system that can provide a large field-like torque rather than the damping-like torque is favored for the switching.

Perpendicular magnetic tunnel junction performance under mechanical strain

In this work, we investigate effect of the mechanical stress on the performance of magnetic tunnel junctions with perpendicular magnetic anisotropy. We developed a 4-point bending setup that allows us to apply a constant stress over a large substrate area with access to electrical measurements and an external magnetic field. This setup enables us to measure key device performance parameters, such as tunnel magnetoresistance, switching current (Ic50%), and thermal stability (Δ), as a function of applied stress. We find that variations in these parameters are negligible: less than 2% over the entire measured range between the zero stress condition and the maximum stress at the point of wafer breakage.

Parallel Multilevel Cell STT-MRAMs for Optimized Area Energy and Read–Write Operations

Spin-transfer torque (STT) magnetic random access memory (MRAM) is considered as a promising candidate for the next generation embedded memory applications. The advent of a perpendicular magnetic anisotropy-based magnetic tunnel junction (PMTJ) device has made the STT-MRAM viable for low-power high-density on-chip memory architectures. However, intense research efforts are still required to optimize the single-level cell and multilevel cell (MLC) STT-MRAMs in terms of area, energy, and read–write operations at ultralow power supply voltages. In this paper, a parallel MLC (pMLC) STT-MRAM (20–50 nm) is proposed to provide smaller footprint area, energy efficient, and optimized read–write operations with very low failure probabilities at the supply voltage of 1 V. These designs utilize high- ${k}$ gate dielectric gate-all-around silicon nanowire-based vertical NMOS transistor as a switching device. The PMTJ device is modeled using Verilog-A. The HSPICE simulation results affirm the suitability of pMLC designs, e.g., 3-bit pMLC design exhibits, switching error probability much less than 10−9, and average write energy of <2 pJ with minimum cell-area at the pulsewidth of 10 ns.

Large perpendicular magnetic anisotropy in epitaxial Fe/MgAl2O4(001) heterostructures

We investigated the perpendicular magnetic anisotropy (PMA) and related properties of epitaxial Fe (0.7 nm)/MgAl2O4(001) heterostructures prepared by electron beam evaporation. Using an optimized structure, we obtained a large PMA energy of ∼1 MJ/m3 at room temperature, which is comparable to that in ultrathin-Fe/MgO(001) heterostructures. Both the PMA energy and saturation magnetization showed a weak temperature dependence, ensuring a wide working temperature range in applications. The effective magnetic damping constant of the 0.7 nm Fe layer was found to be ∼0.02 using the time-resolved magneto-optical Kerr effect. This study demonstrated the suitability of the Fe/MgAl2O4 heterostructure for use in perpendicular magnetic tunnel junctions, as well as good agreement with theoretical predictions.

Ion irradiation-induced easy-cone anisotropy in double-MgO free layers for perpendicular magnetic tunnel junctions

Ion irradiation-induced easy-cone anisotropy in double-MgO free layers for perpendicular magnetic tunnel junctions

Ion irradiation-induced easy-cone anisotropy in double-MgO free layers for perpendicular magnetic tunnel junctions

We have used the ferromagnetic resonance in the X-band (9.37 GHz) to investigate the effect of 400 keV Ar+ irradiation on the perpendicular magnetic anisotropy (PMA) and Gilbert damping parameter, α, of double-MgO free layers designed for application in perpendicular magnetic tunnel junctions. The samples comprised a MgO/Fe72Co8B20/X(0.2 nm)/Fe72Co8B20/MgO layer stack, where X stands for an ultrathin Ta or W spacer. Samples with two different total FeCoB layer thicknesses, tFCB = 3.0 nm and tFCB = 2.6 nm, were irradiated with ion fluences ranging from 1012 cm−2 to 1016 cm−2. The effective first-order PMA field, BK1, decreased nearly linearly with the logarithm of the fluence for both FeCoB thicknesses and spacer elements. The decrease in BK1, which is likely caused by an ion-induced intermixing at the FeCoB/MgO interfaces, resulted in a reorientation of the magnetization of the free layers with tFCB = 2.6 nm, initially exhibiting a perpendicular easy-axis anisotropy. For intermediate fluences, 1013 cm−2 and 1014 cm−2, easy-cone states with different cone angles could be induced in the free layer with a W spacer. Importantly, no corresponding increase in the Gilbert damping was observed. This study shows that ion irradiation can be used to tune the easy-cone anisotropy in perpendicular magnetic tunnel junctions, which is interesting for spintronic applications such as spin-torque magnetic memory devices, oscillators, and sensors.

Ultrahigh Tunneling-Magnetoresistance Ratios in Nitride-Based Perpendicular Magnetic Tunnel Junctions from First Principles

Worldwide research on devices for tomorrow's spintronic logic and storage continues apace. The authors use $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations to investigate the electronic structures, magnetic properties, and tunneling magnetoresistance (TMR) effect of magnetic tunnel junctions based on the ferromagnetic nitrides ${M}_{4}$N ($M$ = Fe, Co, Ni). The case of iron presents a half-metallic state, and an ultrahigh TMR ratio greater than 24,000%---strong motivation for experimentalists.