pMTJ Devices Research Articles

Magnetic tunnel junction random number generators applied to dynamically tuned probability trees driven by spin orbit torque

Perpendicular magnetic tunnel junction (pMTJ)-based true-random number generators (RNGs) can consume orders of magnitude less energy per bit than CMOS pseudo-RNGs. Here, we numerically investigate with a macrospin Landau–Lifshitz-Gilbert equation solver the use of pMTJs driven by spin–orbit torque to directly sample numbers from arbitrary probability distributions with the help of a tunable probability tree. The tree operates by dynamically biasing sequences of pMTJ relaxation events, called ‘coinflips’, via an additional applied spin-transfer-torque current. Specifically, using a single, ideal pMTJ device we successfully draw integer samples on the interval [0, 255] from an exponential distribution based on p-value distribution analysis. In order to investigate device-to-device variations, the thermal stability of the pMTJs are varied based on manufactured device data. It is found that while repeatedly using a varied device inhibits ability to recover the probability distribution, the device variations average out when considering the entire set of devices as a ‘bucket’ to agnostically draw random numbers from. Further, it is noted that the device variations most significantly impact the highest level of the probability tree, with diminishing errors at lower levels. The devices are then used to draw both uniformly and exponentially distributed numbers for the Monte Carlo computation of a problem from particle transport, showing excellent data fit with the analytical solution. Finally, the devices are benchmarked against CMOS and memristor RNGs, showing faster bit generation and significantly lower energy use.

Cascadable in-memory computing based on symmetric writing and readout.

The building block of in-memory computing with spintronic devices is mainly based on the magnetic tunnel junction with perpendicular interfacial anisotropy (p-MTJ). The resulting asymmetric write and readout operations impose challenges in downscaling and direct cascadability of p-MTJ devices. Here, we propose that a previously unimplemented symmetric write and readout mechanism can be realized in perpendicular-anisotropy spin-orbit (PASO) quantum materials based on Fe3GeTe2 and WTe2. We demonstrate that field-free and deterministic reversal of the perpendicular magnetization can be achieved using unconventional charge-to-z-spin conversion. The resulting magnetic state can be readily probed with its intrinsic inverse process, i.e., z-spin-to-charge conversion. Using the PASO quantum material as a fundamental building block, we implement the functionally complete set of logic-in-memory operations and a more complex nonvolatile half-adder logic function. Our work highlights the potential of PASO quantum materials for the development of scalable energy-efficient and ultrafast spintronic computing.

Sub-ns Switching and Cryogenic-Temperature Performance of Mo-Based Perpendicular Magnetic Tunnel Junctions

Mo-based perpendicular magnetic tunnel junctions (Mo-pMTJs) can outperform mainstream Ta-pMTJs in terms of perpendicular magnetic anisotropy (PMA) and thermal tolerance. However, studies on the ultrafast switching of Mo-pMTJ devices remain limited. In addition, although pMTJ devices have potential to function as cryogenic memory cells, there has been no report on the performance of Mo-pMTJs at low temperatures until now. In this Letter, Mo-pMTJs were prepared with strong PMA and patterned into nanoscale devices. Scanning transmission electron microscopy was employed to characterize the device lateral dimension and structure integrity. Systematic probability measurements were conducted under various pulse widths and current densities. On the ultrafast timescale down to sub-ns, the switching is confirmed to enter the precessional regime. The optimization of the switching energy is discussed. Moreover, we investigate the magneto-transport properties and switching of Mo-pMTJs at low temperatures down to 2 K. The feasibility of utilizing Mo-pMTJ devices in cryogenic memory is verified through this work.

Voltage-Gate-Assisted Spin-Orbit-Torque Magnetic Random-Access Memory for High-Density and Low-Power Embedded Applications

Voltage-gate assisted spin-orbit torque (VGSOT) writing scheme combines the advantages from voltage control of magnetic anisotropy (VCMA) and spin-orbit torque (SOT) effects, enabling multiple benefits for magnetic random access memory (MRAM) applications. In this work, we give a complete description of VGSOT writing properties on perpendicular magnetic tunnel junction (pMTJ) devices, and we propose a detailed methodology for its electrical characterization. The impact of gate assistance on the SOT switching characteristics are investigated using electrical pulses down to 400ps. The VCMA coefficient ({\xi}) extracted from current switching scheme is found to be the same as that from the magnetic field switch method, which is in the order of 15fJ/Vm for the 80nm to 150nm devices. Moreover, as expected from the pure electronic VCMA effect, {\xi} is revealed to be independent of the writing speed and gate length. We observe that SOT switching current characteristics are modified linearly with gate voltage (V_g), similar as for the magnetic properties. We interpret this linear behavior as the direct modification of perpendicular magnetic anisotropy (PMA) and nucleation energy induced by VCMA. At V_g = 1V, the SOT write current is decreased by 25%, corresponding to a 45% reduction in total energy down to 30fJ/bit at 400ps speed for the 80nm devices used in this study. Further, the device-scaling criteria are proposed, and we reveal that VGSOT scheme is of great interest as it can mitigate the complex material requirements of achieving high SOT and VCMA parameters for scaled MTJs. Finally, how that VGSOT-MRAM can enable high-density arrays close to two terminal geometries, with high-speed performance and low-power operation, showing great potential for embedded memories as well as in-memory computing applications at advanced technology nodes.

Fabrication and Reliability Analysis of Nanoscale Magnetic Tunnel Junctions

Magnetic tunnel junction with perpendicular magnetic anisotropy (p-MTJ) is the core component in spintronics-based memory, logic and sense devices, the synthetic anti-ferromagnetic (SAF) has been used to fix the reference layer and cancel the stray field in free layer and few works report the SAF-based reliability analysis of p-MTJ nanopillars. Here, we developed a complete set of method for single p-MTJ devices using CMOS compatible process, and manufactured 80-nm diameter p-MTJs with lower performance degrade. The results of our p-MTJs also revealed that anomalous transitions in major resistance versus external magnetic field ([Formula: see text]) curves can be assisted with the uncompensated SAF flipping after nanofabrication, even though well magnetostatic matched during film stack design. We have also given a reliability analysis of magnetization states in anomalous major [Formula: see text] curves and implied that SAF parameters must be designed for etched nanopillars.

A low temperature functioning CoFeB/MgO-based perpendicular magnetic tunnel junction for cryogenic nonvolatile random access memory

We investigated the low temperature performance of CoFeB/MgO-based perpendicular magnetic tunnel junctions (pMTJs) by characterizing their quasi-static switching voltage, high speed pulse write error rate, and endurance down to 9 K. pMTJ devices exhibited high magnetoresistance (>120%) and reliable (error rate < 10−4) bi-directional switching with 2–200 ns voltage pulses. The endurance of the devices at 9 K surpassed that at 300 K by three orders of magnitude under the same write conditions, functioning for more than 1012 cycles with 10 ns write pulses. The switching voltage at 9 K was observed to increase by 33% to 93%, depending on the pulse duration, compared to that at 350 K. Ferromagnetic resonance and magnetization measurements on blanket pMTJ film stacks suggest that the increased switching voltage is associated with an increase in the energy barrier of the free layer with decreasing temperature. Our work demonstrates that CoFeB/MgO-based pMTJs have great potential to enable cryogenic magnetic random access memory and that their exchange stiffness, magnetization, and effective magnetic anisotropy can be further optimized to lower operating power and improve endurance.

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.

Novel metallization processes for sub-100 nm magnetic tunnel junction devices

In this work, a novel method consisting of side-wall passivation and a top-metal reveal process is proposed for spintronics devices based on magnetic tunnel junctions (MTJs). The method can efficiently protect ferromagnetic metals and magnesium oxide, which are the key materials of MTJs, and effectively establish electrical contact with the interconnect metals for perpendicular MTJs (p-MTJs). The scheme consists of the passivation of MTJs with dual dielectrics – silicon nitride and silicon oxide – followed by planarization and selective wet etching. The proposed integration scheme was successfully demonstrated with 80-nm-diameter p-MTJ devices.

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.

Self-Aligned Contacting Processes for the 80 nm p-MTJ Device Fabrication by Wet Approach

In this work, a novel process integration scheme for p-MTJ devices’ passivation and contacting was proposed. The method can efficiently protect the ferromagnetic metals and the magnesium oxide which are the key building block of p-MTJs, and effectively make electrical contact with the interconnect metals for p-MTJs. The scheme consists of passivation of p-MTJs with dual dielectrics - silicon nitride and silicon oxide, followed by planarization and selective wet etch. The proposed integration scheme was successfully demonstrated with 80 nm size p-MTJ devices.

3D Cross-Point Spin Transfer Torque Magnetic Random Access Memory

We explore a 3D cross-point spin transfer torque magnetic random access memory (STT-MRAM) array based on the integration of a perpendicular magnetic tunneling junction (pMTJ) with a matching two-terminal selector. The integrated two-terminal device provides a unique opportunity for a high density, low cost stackable storage class memory that can achieve a fast operation speed, long data retention, low bit error rate (BER) and high endurance. 55[Formula: see text]nm size pillar shaped pMTJ and selector devices have been fabricated and characterized. The selector is compatible with pMTJ whether it is in the high or low resistance state. The pMTJ can be RESET and SET after the selector turns on. We model the dynamic switching of the coupled pMTJ and selector devices. Our model shows the importance of the optimal matching of pMTJ magnetic properties with selector resistive properties to achieve high performance.

Perpendicular magnetic tunneling junction switching dynamic modes, extreme events, and performance scaling

The performance of the state-of-the-art perpendicular magnetic tunneling junction (pMTJ) device is fundamentally determined by the physics of material “extreme events.” A dynamic mode approach is used to study “extreme events” of stochastic nonlinear magnetization switching, including magnetic interactions and non-uniform magnetization dynamics. Our theory and experiment show that the magnetization switching “extreme events” are well characterized by the dynamic modes of interacting magnetic systems. The dynamic modes provide a clear understanding of the physical processes of the magnetization switching “extreme events.” We predict markedly different pMTJ scaling behaviors for spin transfer torque, spin-orbit-interaction torque, and thermal fluctuations at different operation speeds and bit error rate conditions. Understanding these scaling behaviors is critical for existing and emerging pMTJ device applications.

Perpendicular magnetic tunnel junction (p-MTJ) spin-valves designed with a top Co2Fe6B2 free layer and a nanoscale-thick tungsten bridging and capping layer

Recently, p-MTJ spin-valves with top Co2Fe6B2 free layers have been widely studied. They potentially experience crystallinity degradation in the face-centered-cubic (fcc) structure of the MgO tunneling barrier as a result of the diffusion of Ta atoms and a significant amount of Fe atoms from the Co2Fe6B2 free and pinned layers. A new design of the p-MTJ spin-valve using a nanoscale-thick W bridging and capping layer demonstrated the absence of significant crystallinity degradation in the fcc structure and showed a limited diffusion of Fe atoms, thus achieving a TMR ratio of ~143% at an ex-situ annealing temperature of 400 °C. Coating quantum tunneling devices with thin metallic bridges may lead to magnetic memory cells smaller than 20 nanometres. Dynamic-random-access memory (DRAM) devices are popular because of their simple one-transistor, one-capacitor construction, but they are tricky to produce at nanoscale dimensions. A promising way to construct alternative DRAM devices based on perpendicular magnetic tunnel junctions (p-MTJ) has now been outlined by Jea-Gun Park and colleagues from Hanyang University in South Korea. Their device, which sandwiches a magnesium oxide tunnel barrier between layers of magnetic cobalt—iron—boron films, uses external magnetic fields to switch between on and off memory states. Normally, such p-MTJ devices suffer from degraded magnesium oxide crystallinity and deleterious diffusion of iron atoms. However, the team found that nanoscale-thin layers of tungsten placed under and above the magnetic films prevented premature device breakdown. New design of the p-MTJ spin-valve using W bridging and capping layer demonstrated no significant crystallinity degradation and no considerable Fe atoms diffusion, thus achieving the TMR ratio of ~143% at an ex-situ annealing temperature of 400 oC. A better crystallinity of the MgO layer for W case than Ta case would be related to less diffusion amount of metal atoms and lattice strain in MgO layer. In addition, higher Fe and Co atom concentrations of Co2Fe6B2 free and pinned layers for W case than Ta case would be associated with less solubilities in W layers than Ta layers.

Tunneling magnetoresistance of perpendicular magnetic tunnel junction using L1 FePt electrodes on MgO/CrRu/TiN under-layers

A perpendicular magnetic tunnel junction (pMTJ) device was fabricated using L10 ordered FePt electrodes, which were deposited on MgO(8 nm)/CrRu(10 nm)/TiN(4 nm) under-layers. It was found that the MgO/CrRu/TiN under-layer helps lower the required FePt deposition temperature to below 400 °C, and provides a well-ordered bottom L10 FePt electrode with root-mean-square (RMS) surface roughness close to 0.4 nm. Magnetoresistance (MR) ratio and resistance-area (RA) were measured at room temperature by the current-in-plane tunneling (CIPT) method from a lithographically unpatterned PMTJ sample and 138% and 6.4 kΩ μm2 were obtained, respectively. A PMTJ test pattern, with a junction size of 80 × 40 μm2, was also fabricated and showed a MR ratio and RA product of 108% and 4 ∼ 6 kΩ μm2, respectively, in good agreement with the CIPT measurements.

Design and fabrication of a perpendicular magnetic tunnel junction based nonvolatile programmable switch achieving 40% less area using shared-control transistor structure.

A compact nonvolatile programmable switch (NVPS) using 90 nm CMOS technology together with perpendicular magnetic tunnel junction (p-MTJ) devices is fabricated for zero-standby-power field-programmable gate array. Because routing information does not change once it is programmed into an NVPS, high-speed read and write accesses are not required and a write-control transistor can be shared among all the NVPSs, which greatly simplifies structure of the NVPS. In fact, the effective area of the proposed NVPS is reduced by 40% compared to that of a conventional MTJ-based NVPS. The instant on/off behavior without external nonvolatile memory access is also demonstrated using the fabricated test chip.

Comparison of the interfacial structure between MgO and Al–O oxidation layers for perpendicular magnetic tunnel junction

We discuss the interfacial structure of MgO and Al–O barrier layers and influence on the magnetic properties of perpendicular magnetic tunnel junction (pMTJ) devices. The pMTJs layer structures analyzed were Si-wafer∕Pt∕Gd(FeCo)∕FeCo∕MgO (AlO)∕FeCo∕Tb(FeCo)∕Pt. The deposit of all pMTJs structures was carried out by rf and dc magnetron sputtering systems. Transmission electron microscopy (TEM) clearly showed that the interfacial structure of FeCo∕MgO or AlO∕FeCo in the pMTJs was very smooth and uniform. Hysteresis loops obtained by an alternating gradient magnetrometer (AGM) for the different oxide barrier layers of pMTJ structures showed that the Al–O layer performs better than the MgO layer. An additional discussion on the oxide layer thickness in the TEM and AGM measurements is also presented.