Magnetic Tunnel Junction Nanopillars Research Articles

Stochastic Magnetic Actuated Random Transducer Devices Based on Perpendicular Magnetic Tunnel Junctions

True random number generators are of great interest in many computing applications such as cryptography, neuromorphic systems and Monte Carlo simulations. Here we investigate perpendicular magnetic tunnel junction nanopillars (pMTJs) activated by short duration (ns) pulses in the ballistic limit for such applications. In this limit, a pulse can transform the Boltzmann distribution of initial free layer magnetization states into randomly magnetized down or up states, i.e. a bit that is 0 or 1, easily determined by measurement of the junction's tunnel resistance. It is demonstrated that bitstreams with millions of events: 1) are very well described by the binomial distribution; 2) can be used to create a uniform distribution of 8-bit random numbers; 3) pass multiple statistical tests for true randomness, including all the National Institute of Standards tests for random number generators with only one XOR operation; and 4) can have no drift in the bit probability with time. The results presented here show that pMTJs operated in the ballistic regime can generate true random numbers at GHz bitrates, while being more robust to environmental changes, such as their operating temperature, compared to other stochastic nanomagnetic devices.

Current-driven magnetization dynamics and its correlation with magnetization configurations in perpendicularly magnetized tunnel junctions

We study spin-transfer-torque-driven magnetization dynamics of a perpendicular magnetic tunnel junction nanopillar. Through the combined investigations on spin-torque ferromagnetic resonance and microwave spectroscopy, it is found that the free layer (FL) and the weak pinned reference layer (RL) exhibit distinct dynamic behaviors. First, frequency vs field dispersion for the FL and RL shows an opposite trend as the field varies. Second, the FL can support a single spin-wave (SW) mode for both parallel and antiparallel configurations, while the RL exhibits spin-wave excitation only for the antiparallel state. Those two SW modes coexist at the antiparallel state, and their oscillation frequency exhibits a crossover phenomenon with increasing the external magnetic field, which could be helpful in the mutual synchronization of auto-oscillations for SW-based neuromorphic computing.

Compact Model of Domain Wall MTJ Driven by Spin Orbit Torque and Dzyaloshinskii–Moriya Interaction

Current-induced domain wall motion (CIDWM) shows promising prospects with low power, high density, high speed, and so on. Recent studies have demonstrated that magnetic tunnel junction (MTJ) based on CIDWM has great potential in mimicking a non-volatile artificial neuron and synapse. By combining the effect of the interfacial Dzyaloshinskii–Moriya interaction (DMI) to stabilize the Néel-type domain wall (DW) and the high-speed low-power advantages of spin orbit torque (SOT), the novel MTJ device is experimentally demonstrated with low threshold current and high propagation speed, which shows potential applications in the field of the artificial neuron and synapse. In this article, a compact model for CIDWM-MTJ based on SOT and DMI is presented. The model integrates the SOT mechanism for magnetization reversal and DW nucleation, CIDWM behaviors, and tunnel resistance theory of MTJ nanopillar. The micromagnetic simulation and the temporal evolution of the DW position are implemented by solving the Landau–Lifshitz–Gilbert (LLG) equation and the 1-D model. Based on the developed model, a hybrid CIDWM-MTJ/CMOS circuit is simulated for verification. The presented model combines the CIDWM dynamics and the MTJ magnetic dynamics, showing potential application for the simulation of all-spin artificial neural network (ANN).

A perspective on electrical generation of spin current for magnetic random access memories

Spin currents are used to write information in magnetic random access memory (MRAM) devices by switching the magnetization direction of one of the ferromagnetic electrodes of a magnetic tunnel junction (MTJ) nanopillar. Different physical mechanisms of conversion of charge current to spin current can be used in two-terminal and three-terminal device geometries. In two-terminal devices, charge-to-spin conversion occurs by spin filtering in the MTJ's ferromagnetic electrodes and present day MRAM devices operate near the theoretically expected maximum charge-to-spin conversion efficiency. In three-terminal devices, spin–orbit interactions in a channel material can also be used to generate large spin currents. In this Perspective article, we discuss charge-to-spin conversion processes that can satisfy the requirements of MRAM technology. We emphasize the need to develop channel materials with larger charge-to-spin conversion efficiency—that can equal or exceed that produced by spin filtering—and spin currents with a spin polarization component perpendicular to the channel interface. This would enable high-performance devices based on sub-20 nm diameter perpendicularly magnetized MTJ nanopillars without need of a symmetry breaking field. We also discuss MRAM characteristics essential for CMOS integration. Finally, we identify critical research needs for charge-to-spin conversion measurements and metrics that can be used to optimize device channel materials and interface properties prior to full MTJ nanopillar device fabrication and characterization.

Easy-cone state in spin-torque diode under combined action of magnetostatics and perpendicular anisotropy

Spin-torque diodes (STDs) with interfacial perpendicular magnetic anisotropy (IPMA) in the free layer (FL) demonstrate outstanding microwave signal rectification performances. Large sensitivity values in such systems are usually associated with an easy-cone (EC) magnetic state, when the magnetization in the FL is tilted from the normal to the plane of the film. Here, we theoretically investigate the phase diagram for the EC state in an infinite FL of the magnetic tunnel junction (MTJ) considering both IPMA (of the first and of the second order) and magnetostatic interaction. We show that the increase of the magnetostatic field leads to the EC state phase expansion. For elliptical MTJ nanopillars we investigate the influence of the orientation of the nanopillar ellipticity on the obtained phase diagrams. And finally, we consider the dynamic properties and rectification efficiency of the STD under microwave current injection. Our results clarify the role of magnetostatic interaction for microwave rectification with the IPMA-based STDs and suggest approaches to the EC state effective rectification phase extension through the parameters optimization.

Spin-torque switching mechanisms of perpendicular magnetic tunnel junction nanopillars

Understanding the spin-transfer magnetization switching mechanisms of perpendicular magnetic tunnel junction nanopillars is critical to optimizing their performance in memory devices. Here, we use micromagnetics to study how the free layer's exchange constant affects its switching dynamics. Switching is shown to generally occur by (1) growth of the magnetization precession amplitude in the element center; (2) an instability in which the reversing region moves to the element edge, forming magnetic domain wall(s); and (3) the motion of the domain wall(s) across the element. For small exchange and large element diameters, step 1 leads to a droplet with a fully reversed core that experiences a drift instability (step 2). While in the opposite case (large exchange and small diameters), the central region of the element is not fully reversed before step 2 occurs. The origin of the micromagnetic structure is shown to be the free layer's non-uniform demagnetization field. More coherent, energy-efficient, and faster switching is associated with larger exchange, showing that increasing the exchange interaction strength leads to improvements in device performance.

Micromagnetic instabilities in spin-transfer switching of perpendicular magnetic tunnel junctions

Micromagnetic instabilities and non-uniform magnetization states play a significant role in spin transfer induced switching of nanometer scale magnetic elements. Here we model domain wall mediated switching dynamics in perpendicularly magnetized magnetic tunnel junction nanopillars. We show that domain wall surface tension always leads to magnetization oscillations and instabilities associated with the disk shape of the junction. A collective coordinate model is developed that captures aspects of these instabilities and illustrates their physical origin. Model results are compared to those of micromagnetic simulations. The switching dynamics is found to be very sensitive to the domain wall position and phase, which characterizes the angle of the magnetization in the disk plane. This sensitivity is reduced in the presence of spin torques and the spin current needed to displace a domain wall can be far less than the threshold current for switching from a uniformly magnetized state. A prediction of this model is conductance oscillations of increasing frequency during the switching process.

Sub-nanosecond spin-torque switching of perpendicular magnetic tunnel junction nanopillars at cryogenic temperatures

Spin-transfer magnetic random access memory devices are of significant interest for cryogenic computing systems where a persistent, fast, low-energy consuming, and nanometer scale device operating at low temperature is needed. Here, we report the low-temperature nanosecond duration spin-transfer switching characteristics of perpendicular magnetic tunnel junction (pMTJ) nanopillar devices (40–60 nm in diameter) and contrast them to their room temperature properties. Interestingly, the characteristic switching time decreases with temperature, with the largest reduction occurring between room temperature and 150 K. The switching energy increases with decreasing temperature, but still compares very favorably with other types of spin-transfer devices at 4 K, with <300 fJ required per switch. Write error rate (WER) measurements show highly reliable switching with WER ≤ 5 × 10–5 with 4 ns pulses at 4 K. Our results demonstrate the promise of pMTJ devices for cryogenic applications and show routes to further device optimization.

Memristors: Heterogeneous Memristive Devices Enabled by Magnetic Tunnel Junction Nanopillars Surrounded by Resistive Silicon Switches (Adv. Electron. Mater. 3/2018)

Memristors: Heterogeneous Memristive Devices Enabled by Magnetic Tunnel Junction Nanopillars Surrounded by Resistive Silicon Switches (Adv. Electron. Mater. 3/2018)

Heterogeneous Memristive Devices Enabled by Magnetic Tunnel Junction Nanopillars Surrounded by Resistive Silicon Switches

AbstractEmerging nonvolatile memories (NVMs) have currently attracted great interest for their potential applications in advanced low‐power information storage and processing technologies. Conventional NVMs, such as magnetic random access memory and resistive random access memory, suffer from limitations of low tunnel magnetoresistance, low access speed, or finite endurance. NVMs with synergetic advantages are still highly desired for future computer architectures. This study reports a heterogeneous memristive device composed of a magnetic tunnel junction (MTJ) nanopillar surrounded by resistive silicon switches, named resistively enhanced MTJ (Re‐MTJ), which may be utilized for novel memristive memories, enabling new functionalities that are inaccessible for conventional NVMs. The Re‐MTJ device features a high ON/OFF ratio of >1000% and multilevel resistance behavior by combining magnetic switching together with resistive switching mechanisms. The magnetic switching originates from the MTJ, while the resistive switching is induced by a point‐switching filament process that is related to the mobile oxygen ions. Microscopic evidence of silicon aggregated as nanocrystals along the edges of the nanopillars verifies the synergetic mechanism of the heterogeneous memristive device. This device may provide new possibilities for advanced memristive memory and computing architectures, e.g., in‐memory computing and neuromorphics.

Thermal FMR Spectral Characterization of Very Low RA In-Plane MgO Magnetic Tunnel Junctions

The ability to fabricate MgO magnetic tunnel junction nanopillars with low RA and high TMR opens the door to new devices such as magnetic memories or nanooscillators. Here, we address the dynamic behavior of a CoFeB/MgO/CoFeB MTJ with a synthetic-antiferromagnetic pinned layer as a potential candidate for spin-transfer torque (STT) driven applications. The unpatterned stack presents an RA of $\sim 1.4~\Omega \mu \text{m}^{2}$ and both free layer and barrier are patterned down to a $90 \times 180$ nm2 elliptical shape. The observed high-frequency spectra displayed deviations from the uniform precession modes due to additional coupling fields. A dependence of the STT magnitude on the analyzed mode was obtained, differing from previous reports, most likely due to the very low RA employed.

Temperature dependence of spin-torque driven ferromagnetic resonance in MgO-based magnetic tunnel junction with a perpendicularly free layer

We report the temperature dependence of the spin-torque (ST) driven ferromagnetic resonance in MgO-based magnetic tunnel junction (MTJ) nanopillars with a perpendicularly free layer and an in-plane reference layer. From the evolution of the resonance frequency with magnetic field, we clearly identify the free-layer resonance mode and reference-layer mode. For the reference layer, we demonstrate a monotonic increase in resonance frequency and the effective damping with decreasing temperature, which suggests the saturated magnetization of the reference layer is dominant. However, for the free layer, the frequency and damping exhibit almost no change with temperature, indicating that the perpendicular magnetic anisotropy plays an important role in magnetization dynamics of the free layer.

Failure Analysis in Magnetic Tunnel Junction Nanopillar with Interfacial Perpendicular Magnetic Anisotropy.

Magnetic tunnel junction nanopillar with interfacial perpendicular magnetic anisotropy (PMA-MTJ) becomes a promising candidate to build up spin transfer torque magnetic random access memory (STT-MRAM) for the next generation of non-volatile memory as it features low spin transfer switching current, fast speed, high scalability, and easy integration into conventional complementary metal oxide semiconductor (CMOS) circuits. However, this device suffers from a number of failure issues, such as large process variation and tunneling barrier breakdown. The large process variation is an intrinsic issue for PMA-MTJ as it is based on the interfacial effects between ultra-thin films with few layers of atoms; the tunneling barrier breakdown is due to the requirement of an ultra-thin tunneling barrier (e.g., <1 nm) to reduce the resistance area for the spin transfer torque switching in the nanopillar. These failure issues limit the research and development of STT-MRAM to widely achieve commercial products. In this paper, we give a full analysis of failure mechanisms for PMA-MTJ and present some eventual solutions from device fabrication to system level integration to optimize the failure issues.

Impact of MgO Thickness on the Performance of Spin-Transfer Torque Nano-Oscillators

A magnetic tunnel junction (MTJ) stack incorporating an MgO wedge was deposited over a 200 mm wafer and nanofabricated into circular nanopillars with a diameter of 200 nm. Due to the variable MgO thickness, we could obtain MTJ nanopillars with resistance $\times $ area (RA) ranging from less than $1~\Omega \mu \text{m}^{2}$ up to $15~\Omega \mu \text{m}^{2}$ and tunnel magnetoresistance ratios up to $\sim 100$ %. It was observed that the ferromagnetic coupling ( $H_{F})$ displayed by the transfer curves increases for smaller values of RA. This variation is attributed to the orange-peel coupling, which is caused by the MgO roughness, with a larger impact for thinner barriers. The RF output of the nanopillars caused by the spin-transfer torque-induced magnetic precession of the free layer was measured as a function of bias current and external magnetic fields for nanopillars with different RA values. Results concerning typical devices in the thin MgO region ( $1.3~\Omega \mu \text{m}^{2})$ and in a thicker MgO region ( $2.7~\Omega \mu \text{m}^{2})$ are shown to illustrate the dependence found. It was observed that the higher output power ( $\sim 300$ nW) and the smaller linewidth ( $\sim 90$ MHz) could be obtained with the higher RA region. These results suggest that there must be an optimum RA value for the production of nano-oscillator devices, which depends on the tradeoffs between the need for large breakdown voltages and strong spin polarization of the bias current (favored for larger RA values) and between the need of endurance under large current densities (favored for smaller RA values). Provided the breakdown voltage of the MgO barrrier can be made large enough, nanopillars based on MTJ stacks with an intermediate RA can provide a solution for the production of large output power spin-transfer nano-oscillators.

Effect of MgO Barrier Thickness with Tilted Magnetization on Temperature Increase in Magnetic Tunnel Junction Devices during Current-Induced Magnetization Switching

Recently, there has been a growing interest in the thermal stability in magnetic tunnel junction (MTJ) devices with an aspect of the temperature increment during current-induced magnetization switching (CIMS) process. In this work, the temperature increment is explored with factors of the tile of the initial magnetization direction in free layer, θ0, and the MgO layer thickness for different pulse durations, tp. The results show that the highest temperature in MTJ nanopillar is significant at the θ0 of1°-5° and the pulse duration tp &lt; 0.4 ns. Moreover, the temperature results with decreasing the MgO layer thickness are not considerable difference at θ0 of1°-5° for the same tp.

Magnetic phase transitions in Ta/CoFeB/MgO multilayers

We study thin films and magnetic tunnel junction nanopillars based on Ta/Co20Fe60B20/MgO multilayers by electrical transport and magnetometry measurements. These measurements suggest that an ultrathin magnetic oxide layer forms at the Co20Fe60B20/MgO interface. At approximately 160 K, the oxide undergoes a phase transition from an insulating antiferromagnet at low temperatures to a conductive weak ferromagnet at high temperatures. This interfacial magnetic oxide is expected to have significant impact on the magnetic properties of CoFeB-based multilayers used in spin torque memories.

Temperature dependence of microwave oscillations in magnetic tunnel junctions with a perpendicularly magnetized free layer

We experimentally study the temperature dependence of the spin-transfer-torque-induced microwave oscillations in MgO-based magnetic tunnel junction nanopillars with a perpendicularly magnetized free layer. We demonstrate that the oscillation frequency increases rapidly with decreasing temperature, which is mainly ascribed to the temperature dependence of both the saturation magnetization and the perpendicular magnetic anisotropy. We also find that a strong temperature dependence of the output power while a nonmonotonic temperature dependence of spectral linewidth are maintained for a constant dc bias in measured temperature range. Possible mechanisms leading to the different dependences of oscillation frequency, output power, and linewidth are discussed.

Design Optimization and Analysis of Multicontext STT-MTJ/CMOS Logic Circuits

High power issues have become the main drawbacks of CMOS logic circuits as technology node shrinks below 45 nm. Emerging spintronics nanodevices-based hybrid logic-in-memory architecture has recently been investigated to overcome these issues. Among them, spin-transfer-torque-based magnetic tunnel junction (STT-MTJ) nanopillar is one of the most promising spintronics nanodevices thanks to its nonvolatility, fast access speed, and 3-D integration with CMOS technology. However, hybrid STTMTJ/CMOS logic faces severe reliability issues in ultradeep submicron technology nodes (e.g., 28 nm) due to the increasing process variations and reduced supply voltage. This paper presents architecture designs and comparative study of multicontext hybrid STT-MTJ/CMOS logic structures with a particular focus on reliability investigation. Their merits and shortcomings are demonstrated depending on the addressed applications. Finally, some design considerations and strategies are also presented to further optimize their reliability performance. Transient and Monte Carlo statistical analyses are performed by using an industrial CMOS 28-nm design kit and a physics-based STT-MTJ nanopillar compact model to exhibit their functionalities and effectiveness.

Thermal and spin-transfer-torque excitation of precessional modes in magnetic tunnel junction nanopillars with symmetric interfaces and a thick free layer

The microwave emission of magnetic tunnel junction nanopillars with symmetric CoFeB/MgO/CoFeB interfaces and a 9-nm-thick free layer was studied as a magnetic field was applied in plane at different angles ${\ensuremath{\phi}}_{H}$ with respect to the easy axis. As ${\ensuremath{\phi}}_{H}$ was increased, a more complicated mode spectrum containing modes of larger amplitude was observed. The character of the different modes was deduced by screening many devices fabricated from the same wafer, observing the variation of frequency both between different junctions and with ${\ensuremath{\phi}}_{H}$, and through comparison with micromagnetic simulation. Within selected junctions at certain ${\ensuremath{\phi}}_{H}$ values, the dependence of the frequency, amplitude, and linewidth of the lowest-frequency edge-localized mode upon the applied field and current can be interpreted in terms of combined thermal and spin-transfer-torque (STT) excitation. Fitting to a simple analytical model yielded values for the in-plane and out-of-plane spin STT. The out-of-plane STT was found to contain terms with both a linear and a quadratic dependence on the bias current. The linear term was often found to dominate, as expected for a tunnel junction with asymmetric rather than symmetric interfaces, suggesting that the fuller structure of the free and reference layers should be taken into account. The amplitude of the in-plane STT was somewhat larger than expected, suggesting that the dominant edge mode occupies only part of the area of the free layer but is subject to a proportionately larger fraction of the torque exerted upon the free layer as a whole. This might be expected when the resonant modes of the free layer interact and the lowest-frequency mode dominates.

Micromagnetic and magneto-transport simulations of nanodevices based on MgO tunnel junctions for memory and sensing applications

This work provides a systematic simulation study of magnetic tunnel junction (MTJ) nanodevices behavior, consisting of a multilayered stack incorporating an in-plane CoFeB free layer and a synthetic antiferromagnetic CoFe-based pinned layer, and including exchange and interlayer couplings. A finite element tool is used to simulate both the magnetic and magneto-transport behaviors of these MTJ nanopillars with distinct geometries, namely circles with diameter ranging from 20nm up to 250nm and ellipses with aspect ratios of 1/2, 1/3 and 1/5, corresponding to sizes from 20×40nm2 up to 50×250nm2. This study envisages two clear applications for nanopillars: memory and sensor devices. We address the impact of the nanopillar size on the coercivity and saturation field, as figures of merit for device performance. In particular a competitive sensitivity of 0.15%/Oe is envisaged for sensors with a size of 50×100nm2. Our results provide a validation of this simulation method as a expedite tool to assist the nanofabrication process.