Perpendicular Magnetic Tunnel Junctions Research Articles

Highly Reliable Magnetic Memory-Based Physical Unclonable Functions.

Magnetic random-access memory (MRAM), which stores information through control of the magnetization direction, offers promising features as a viable nonvolatile memory alternative, including high endurance and successful large-scale commercialization. Recently, MRAM applications have extended beyond traditional memories, finding utility in emerging computing architectures such as in-memory computing and probabilistic bits. In this work, we report highly reliable MRAM-based security devices, known as physical unclonable functions (PUFs), achieved by exploiting nanoscale perpendicular magnetic tunnel junctions (MTJs). By intentionally randomizing the magnetization direction of the antiferromagnetically coupled reference layer of the MTJs, we successfully create an MRAM-PUF. The proposed PUF shows ideal uniformity and uniqueness and, in particular, maintains performance over a wide temperature range from -40 to +150 °C. Moreover, rigorous testing with more than 1584 challenge-response pairs of 64 bits each confirms resilience against machine learning attacks. These results, combined with the merits of commercialized MRAM technology, would facilitate the implementation of MRAM-PUFs.

Electrical control of the switching layer in perpendicular magnetic tunnel junctions with atomically thin Ir dusting

The use of magnetic tunnel junction (MTJ)-based devices constitutes an important basis of modern spintronics. However, the switching layer of an MTJ is widely believed to be an unmodifiable setup, instead of a user-defined option, posing a restriction to the function of spintronic devices. In this study, we realized a reliable electrical control of the switching layer in perpendicular MTJs with 0.1 nm Ir dusting. Specifically, a voltage pulse with a higher amplitude drives the magnetization switching of the MTJ's bottom electrode, while a lower voltage amplitude switches its top electrode. We discussed the origin of this controllability and excluded the possibility of back-hopping. Given the established studies on enhancing the voltage-controlled magnetic anisotropy effect by adopting Ir, we attribute this switching behavior to the significant diffusion of Ir atoms into the top electrode, which is supported by scanning transmission electron microscopy with atomic resolution.

Memory-compatible perpendicular magnetic tunnel junctions under bi-directional strobe write pulses: A method for generating true random number bits at high speed

We propose an operation method of generating true random number bits with a perpendicular magnetic tunnel junction (pMTJ) already in use as a memory element in spin-transfer-torque driven magnetic random access memory technology. This method uses a set of regularly spaced (or intentionally irregularly spaced), minimum width, and bi-directional fast strobe-write pulses, with read operations after each write. The resulting bit-stream’s statistical properties are analyzed, and a few digital logic operation following the read is described that could significantly improve the resulting bit-streams variance and stability, insulating those from the raw variations of pMTJs.

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.

Electric-field control of nonvolatile resistance state of perpendicular magnetic tunnel junction via magnetoelectric coupling.

Magnetic tunnel junctions (MTJs) are the core elements of spintronic devices. Now, the mainstream writing operation of MTJs mainly relies on electric current with high energy dissipation, which can be greatly reduced if an electric field is used instead. In this regard, strain-mediated multiferroic heterostructure composed of MTJ and ferroelectrics are promising with the advantages of room temperature and magnetic field-free as already demonstrated by MTJ with in-plane magnetic anisotropy. However, there is no such report on the perpendicular MTJs (p-MTJs), which have been commercialized. Here, we investigate electric-field control of resistance state of MgO-based p-MTJs in multiferroic heterostructures. A remarkable and nonvolatile manipulation of resistance is demonstrated at room temperature without magnetic field assistance. Through various characterizations and micromagnetic simulation, the manipulation mechanism is uncovered. Our work provides an effective avenue for manipulating p-MTJ resistance by electric fields and is notable for high density and ultralow power spintronic devices.

Electric field control of perpendicular magnetic tunnel junctions with easy-cone magnetic anisotropic free layers.

Magnetic tunnel junctions (MTJs) are the core element of spintronic devices. Currently, the mainstream writing operation of MTJs is based on electric current with high energy dissipation, and it can be notably reduced if an electric field is used instead. In this regard, it is promising for electric field control of MTJ in the multiferroic heterostructure composed of MTJ and ferroelectrics via strain-mediated magnetoelectric coupling. However, there are only reports on MTJs with in-plane anisotropy so far. Here, we investigate electric field control of the resistance state of MgO-based perpendicular MTJs with easy-cone anisotropic free layers through strain-mediated magnetoelectric coupling in multiferroic heterostructures. A remarkable, nonvolatile, and reversible modulation of resistance at room temperature is demonstrated. Through local reciprocal space mapping under different electric fields for Pb(Mg1/3Nb2/3)0.7Ti0.3O3 beneath the MTJ pillar, the modulation mechanism is deduced. Our work represents a crucial step toward electric field control of spintronic devices with non-in-plane magnetic anisotropy.

Spin Coupling in the Initial Stages of the Zinc‐Blende MnN Growth on the CrN (111) Surface

AbstractSpin‐polarized first‐principles calculations are carried out to study the structural, electronic, and magnetic properties of the MnN growth on the CrN (111) surface. Different magnetic configurations between Mn‐deposited atoms and the substrate are considered. The H3 site is the most favorable site for the adoption and incorporation cases and the Mn spins switch from paramagnetic to antiferromagnetic arrangement. The minimum energy pathway for the Mn diffusion is calculated, showing magnetic modifications in the substrate. A structural transition from tetragonal rock‐salt to zinc‐blende (ZB) structure as the number of MnN deposited layers increases is noticed. Thermodynamic analysis demonstrates that the growth of MnN on CrN is feasible. The density of states for an MnN layer with an extra nitrogen layer shows a half‐metallic behavior and the remaining structures exhibit a metallic character. The ZB‐MnN structures have an antiferromagnetic behavior with a metallic character. Magnetic anisotropy energies reveal a switching of easy magnetization axis, from in‐plane to out‐of‐plane with the formation of MnN on the surface. These results suggest that the CrN/ZB‐MnN system may be employed in antiferromagnetic spintronics applications as the fabrication of perpendicular magnetic tunnel junctions.

Size dependence of domain wall mediated switching dynamics of perpendicular magnetic tunnel junctions in the presence of reference layer stray field

Recent investigations on spin-transfer-torque-induced switching dynamics of perpendicular magnetic tunnel junctions (MTJ) have revealed different switching anomalies. Here, the influence of stray field from a synthetic anti-ferromagnet (SAF) based reference layer on the domain wall (DW) mediated switching of the free-layer magnet is studied via finite temperature micro-magnetic simulations for varying MTJ diameters. For larger diameters (∼80 – 120 nm), a stray field gives rise to persistent back-and-forth oscillation of the unswitched domain, delaying the switching process and causing increased write errors. For smaller diameters (∼30 – 56 nm), quasi-coherent switching occurs, as expected. For the intermediate ranges of MTJ diameters (∼60 – 70 nm), another switching mode emerges where a bubble-like feature is observed to evolve during the switching process, causing a very rapid change in magnetization. These paths are observed to originate from DWs partly in Bloch and partly in Néel configuration. We find that at the intermediate ranges of device size, the stray field becomes stronger for a given SAF configuration, and the energy difference between the Néel and Bloch configurations is also lowered. Hence, a stronger stray field could easily distort the DW propagation in the Walker breakdown regime, leading to such magnetization behavior. Our findings present interesting insights into DW-mediated switching modes in perpendicular MTJ structures caused by unoptimized stray fields from the SAF.

L10 FePd-based perpendicular magnetic tunnel junctions with 65% tunnel magnetoresistance and ultralow switching current density

L10 FePd is increasingly recognized as a potential candidate for magnetic tunnel junctions (MTJs), yet there remains room for enhancing device performance. In this work, we fabricated fully-integrated L10 FePd-based perpendicular MTJ devices and achieved a significant increase in tunnel magnetoresistance, reaching ∼65%, compared to the previous record of 25%. Notably, we observed bi-directional switching with a low switching current density of about 1.4 × 105 A/cm2, which outperforms the typical spin-transfer torque (STT) MTJ by about one order of magnitude. We propose two possible mechanisms to elucidate the switching process and associated device performance: (1) The voltage-controlled exchange coupling-driven switching of the bottom CoFeB layer; (2) The STT-driven switching of the exchange-coupled L10 FePd–CoFeB composite. While additional research is necessary, these findings may further advance the integration of L10 FePd into spintronic devices, potentially enabling low-energy memory and logic technologies.

Engineering of Voltage-Controlled Magnetic Anisotropy Magnetic Tunnel Junctions at Cryogenic Temperatures

We identified through numerical simulations the optimal condition to have a deterministic switching regime assisted by Voltage Controlled Magnetic Anisotropy (VCMA). To minimize the write energy required to reach this regime, we measure the VCMA coefficient ξ on perpendicular magnetic tunnel junctions (pMTJ) with high resistance-area product (RA) and varying thicknesses of the FeCoB storage layer and the naturally oxidized tunnel barrier. The VCMA coefficient is higher as the effective anisotropy decreases, which is the case for larger Mg and FeCoB thicknesses. The temperature dependence of ξ was shown to increase from room temperature to 5K, showing values up to 35 fJ/Vm at 10K.

Influence of Free Layer Surface Roughness on Magnetic and Electrical Properties of 300 mm CMOS-Compatible MTJ Stacks

The magnetic tunnel junction (MTJ) is a highly versatile device widely used in today’s spintronic applications such as magnetoresistive random-access memory (MRAM), magnetic sensors and prospectively as a read device in racetrack memory. Tuning the perpendicular (p-)MTJ stack to match the desired properties, such as tunnel magnetoresistance (TMR), magnetic anisotropies or coercive field of the free layer, requires careful optimization of the deposition parameters as well as precise layer thickness control. Here, the deposition of individual layers in a wedged manner across 300 mm wafers is proposed to engineer the thicknesses within the stack more efficiently. Furthermore, this technique provides detailed insights into effects related to surface roughness, magnetic anisotropy and TMR.

Probabilistic computing with voltage-controlled dynamics in magnetic tunnel junctions

Probabilistic (p-) computing is a physics-based approach to addressing computational problems which are difficult to solve by conventional von Neumann computers. A key requirement for p-computing is the realization of fast, compact, and energy-efficient probabilistic bits. Stochastic magnetic tunnel junctions (MTJs) with low energy barriers, where the relative dwell time in each state is controlled by current, have been proposed as a candidate to implement p-bits. This approach presents challenges due to the need for precise control of a small energy barrier across large numbers of MTJs, and due to the need for an analog control signal. Here we demonstrate an alternative p-bit design based on perpendicular MTJs that uses the voltage-controlled magnetic anisotropy (VCMA) effect to create the random state of a p-bit on demand. The MTJs are stable (i.e. have large energy barriers) in the absence of voltage, and VCMA-induced dynamics are used to generate random numbers in less than 10 ns/bit. We then show a compact method of implementing p-bits by using VC-MTJs without a bias current. As a demonstration of the feasibility of the proposed p-bits and high quality of the generated random numbers, we solve up to 40 bit integer factorization problems using experimental bit-streams generated by VC-MTJs. Our proposal can impact the development of p-computers, both by supporting a fully spintronic implementation of a p-bit, and alternatively, by enabling true random number generation at low cost for ultralow-power and compact p-computers implemented in complementary metal-oxide semiconductor chips.

Spiking Dynamics in Dual Free Layer Perpendicular Magnetic Tunnel Junctions.

Spintronic devices have recently attracted a lot of attention in the field of unconventional computing due to their non-volatility for short- and long-term memory, nonlinear fast response, and relatively small footprint. Here we demonstrate experimentally how voltage driven magnetization dynamics of dual free layer perpendicular magnetic tunnel junctions can emulate spiking neurons in hardware. The output spiking rate was controlled by varying the dc bias voltage across the device. The field-free operation of this two-terminal device and its robustness against an externally applied magnetic field make it a suitable candidate to mimic the neuron response in a dense neural network. The small energy consumption of the device (4-16 pJ/spike) and its scalability are important benefits for embedded applications. This compact perpendicular magnetic tunnel junction structure could finally bring spiking neural networks to sub-100 nm size elements.

Low-Power and Field-Free Perpendicular Magnetic Memory Driven by Topological Insulators.

Giant spin-orbit torque (SOT) from topological insulators (TIs) has great potential for low-power SOT-driven magnetic random-access memory (SOT-MRAM). In this work, a functional 3-terminal SOT-MRAM device is demonstrated by integrating the TI [(BiSb)2 Te3 ] with perpendicular magnetic tunnel junctions (pMTJs), where the tunneling magnetoresistance is employed for the effective reading method. An ultralow switching current density of 1.5 × 105 Acm-2 is achieved in the TI-pMTJ device at room temperature, which is 1-2 orders of magnitude lower than that in conventional heavy-metals-based systems, due to the high SOT efficiency θSH = 1.16 of (BiSb)2 Te3 . Furthermore, all-electrical field-free writing is realized by the synergistic effect of a small spin-transfer torque current during the SOT. The thermal stability factor (Δ = 66) shows the high retention time (>10 years) of the TI-pMTJ device. This work sheds light to the future low-power, high-density, and high-endurance/retention magnetic memory technology based on quantum materials.

Field-Free Spin-Orbit Torque Driven Switching of Perpendicular Magnetic Tunnel Junction through Bending Current.

Current-induced spin-orbit torques (SOTs) enable fast and efficient manipulation of the magnetic state of magnetic tunnel junctions (MTJs), making them attractive for memory, in-memory computing, and logic applications. However, the requirement of the external magnetic field to achieve deterministic switching in perpendicularly magnetized SOT-MTJs limits its implementation for practical applications. Here, we introduce a field-free switching (FFS) solution for the SOT-MTJ device by shaping the SOT channel to create a "bend" in the SOT current. The resulting bend in the charge current creates a spatially nonuniform spin current, which translates into inhomogeneous SOT on an adjacent magnetic free layer enabling deterministic switching. We demonstrate FFS experimentally on scaled SOT-MTJs at nanosecond time scales. This proposed scheme is scalable, material-agnostic, and readily compatible with wafer-scale manufacturing, thus creating a pathway for developing purely current-driven SOT systems.

First-principles study on the magnetic anisotropy of the CoFe/ MgAl2O4 heterostructures

Practical applications of novel spintronic devices require heterostructures with high magnetic anisotropy (MA). Here, the MA of the CoFe/MgAl2O4 heterostructures was studied by using first-principles calculations. It was found that all the CoFe/MgAl2O4 models exhibit perpendicular magnetic anisotropy (PMA) and that the Co/MgAl2O4 interface is generally more favorable than the Fe/MgAl2O4 interface for inducing large PMA. Importantly, the CoFe/MgAl2O4 heterostructure with the Co/MgAl2O4 interface shows an interfacial PMA density of up to ∼ 0.91 mJ/m2. Moreover, the PMA of the CoFe/MgAl2O4 heterostructure is greatly enhanced compared to the corresponding bare CoFe film. Through density of state analysis combined with the layer- and orbital-hybridization-resolved MA contributions, it is found that the positive contribution from spin-orbit coupling interaction between opposite spin dz2 and dyz orbitals of the interfacial Co for CoFe/MgAl2O4 heterostructure is significantly higher than that of the surface Co for the bare CoFe film, which can well explain the enhancement of the PMA. Our calculated results extend the application of perpendicular magnetic tunnel junctions based on MgAl2O4.

The magnetoresistance acquisition system of a perpendicular magnetic tunnel junction using LabVIEW software

For accelerated productivity, and continual innovation in the field of spintronics, the electrical characterization of the magnetic tunnel junction (MTJs) is of paramount importance. This report deals with the testing of the MTJs. It focuses on the design of an experimental setup with data acquisition of the MTJs with perpendicular magnetic tunnel junctions where a high magnetic field aligned perpendicular to the film plane is required. Furthermore, devices with very small electrodes of only a few micrometers in lateral size can be tested. A computer through a LabVIEW program controls the data acquisition system.

Conditioning Circuits for Nanoscale Perpendicular Spin Transfer Torque Magnetic Tunnel Junctions as Magnetic Sensors

This article demonstrates a new type of magnetic sensor using a perpendicular spin transfer torque magnetic tunnel junction (MTJ). The sensing element has a cylindrical shape of 50 nm in diameter and is to our knowledge among the smallest magnetic sensor ever reported. This article describes the principle of operation of the sensing element and the associated signal processing electronics, which delivers a signal proportional to the external magnetic field. Experimental results are detailed and compared to the state-of-the-art commercially available integrated magnetic sensors as well as published magnetoresistive sensors based on MTJs with comparable size. The measured sensitivity of the developed sensor is 1.28 V/T, and its dynamic range reaches 80 mT. The measured noise level is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$21.8\,\mu \text{T}/\surd $ </tex-math></inline-formula> Hz. Two different operating principles of the proposed sensor are described and compared, one based on a time-to-digital converter and one based on a pulsewidth-modulated (PWM) signal. Both methods require only standard microelectronics components, which are suitable for monolithic integration of the sensing element with its conditioning electronics. Subsequent improvements of the sensing element as well as conditioning electronics are required to further lower the noise level. The sensing element and its conditioning electronics are compatible with fabrication processes already used in magnetic random access memory fabrication. This opens the way to mass production and addresses various markets, such as consumer electronics, automotive, industrial sensing, physics experiments, or medical devices.

Enhancement of Magnetic and Electric Transport Performance of Perpendicular Spin-Orbit Torque Magnetic Tunnel Junction by Stop-on-MgO Etching Process

We investigate the magnetic and electric transport performance of the spin-orbit torque magnetic tunnel junction (SOT-MTJ) with perpendicular magnetic anisotropy (PMA) by stop-on-MgO etching process. This process etches away the hard mask (HM), synthetic antiferromagnetic layer (SAF) and reference layer (RL) while keeping the MgO barrier layer and free layer (FL) retained. Compared with the SOT-MTJ without stop-on-MgO etching process, the SOT-MTJ with stop-on-MgO etching process exhibits nearly zero offset field ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${H}_{\text {off}}{)}$ </tex-math></inline-formula> and lower critical switching current density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${J}_{c}{)}$ </tex-math></inline-formula> with higher thermal stability factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta {)}$ </tex-math></inline-formula> . Although the magnetic property of the FL underneath the MgO is retained, the switching speed can still reach as fast as 1 ns. Furthermore, the SOT-MTJ with stop-on-MgO etching process presents much more concentrated distribution of the statistical coercivity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${H}_{c}{)}$ </tex-math></inline-formula> , tunneling magnetoresistance (TMR), <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${H}_{\text {off}}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta $ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${J}_{c}$ </tex-math></inline-formula> . The resistance-based MTJ yield can raise up to 100%, which is advantageous in the high-speed and large-scale memory application.

Tunnel magnetoresistance exceeding 100% in magnetic tunnel junctions using Mn-based tetragonal alloy electrodes with perpendicular magnetic anisotropy

We studied MgO barrier magnetic tunnel junctions (MTJs) comprising perpendicularly magnetized MnGa and FeCoB electrodes. In those perpendicular (p-) MTJs, we utilized thin metastable bcc CoMn alloys as an interlayer between MnGa and MgO to enhance the tunnel magnetoresistance (TMR) effect. Moreover, we investigated the effect of a thin Mg interlayer between MnGa and CoMn. Owing to the interlayer engineering, we achieved a TMR ratio over 100%, the highest value observed for p-MTJs with a tetragonal MnGa electrode. Our study contributes to the further development of spintronic devices using p-MTJs with various Mn-based tetragonal alloy electrodes.