Magnetic Tunnel Junction Cells Research Articles

Magnetization Process of Stadium-Shaped Magnetic Tunnel Junction Cells for Artificial Spin Ice

Magnetization Process of Stadium-Shaped Magnetic Tunnel Junction Cells for Artificial Spin Ice

Optimizing free layer of Magnetic Tunnel Junction for true random number generator

True random number generators (TRNGs) should ideally generate lengthy chains of non-repeating, uncorrelated bit-streams that are efficient in terms of both energy and area. Current TRNG designs include source of randomness such as CMOS or non-volatile memory based devices with additional circuitry to improve the quality of randomness leading to power and area overhead. This paper addresses these issues by improving the randomness of the Spin-Orbit Torque (SOT)-Magnetic Tunnel Junction cell. Motivated by the observation that free layer thickness of Magnetic Tunnel Junction (MTJ) can be scaled to design a low-barrier device, the paper proposes a novel source of randomness called ΔSOT. This device is then used to design TRNG circuits that achieves high quality random telegraphic switching behavior without any additional circuitry making it suitable for ultra-low power applications. Evaluations show that ΔSOT-TRNG has significant reduction in energy (51%) and area (66%) compared to state-of-the-art MTJ based TRNG design. Furthermore, the work shows that the improved switching speed of the reduced barrier junction can results in 65% increase in throughput compared to MTJ based TRNG design.

SuperVAULT: Superparamagnetic Volatile Auxiliary Tamper-Proof Storage

Memory security has recently come into the spotlight as attackers have stepped up their efforts to gain illicit access to sensitive data or cause denial-of-memory-service via a variety of avenues like cold boot attacks, bus snooping, and physical probing or tampering. In this paper, we propose SuperVAULT, a novel secure storage solution for protecting secret data/keys by exploiting the superparamagnetic regime of nanomagnets. Through materials and dimensional engineering of the magnetic tunnel junction (MTJ) free layer, the energy barrier of spintransfer torque magnetoresistive random access memory (STTMRAM) cells can be designed to lie in the range of the thermal energy (kBT ). Such superparamagnetic MTJ (s-MTJ) cells, with an O(10 ns) retention time, need to be refreshed frequently. In the absence of data refresh (under attack conditions), the data they hold is thermally corrupted to a random state after an arbitrary but short amount of time. We leverage this property to devise a secure memory primitive and showcase its potential against cold boot and Boolean satisfiability (SAT) attacks. Further, the overheads for s-MTJ-based STT-MRAMs is shown to be promising for on-chip implementations.

Compact Model for Negative Capacitance Enhanced Spintronics Devices

Although spin transfer torque (STT)-based magnetic tunneling junction (MTJ) owns advantages of nonvolatility, nonlimited endurance, and fast write/read, its demand for high current density significantly casts a shadow over its future prospects. Recently, a novel three-terminal MTJ cell that combines voltage-controlled magnetic anisotropy (VCMA) effect and negative capacitance (NC) effect is proposed. Drawing support from the NC amplified VCMA effect and the three-step operation scenario, this novel MTJ cell can dramatically lower the energy consumption to fJ as well as keep high operation speed within nanoseconds. The feasibility of the proposed NC enhanced VCMA spintronics device for memory and logic application is proved by extensive physical simulation in our previous work. However, a SPICE compatible compact model of the proposed NC enhanced VCMA spintronics device is still demanded for circuit and system level evaluation. In this paper, we provide an accurate and fast compact model of NC enhanced VCMA device for both memory and logic applications.

Nonvolatile Spin-Based Radiation Hardened Retention Latch and Flip-Flop

Reduction of leakage power and vulnerability to radiation are of critical challenges in modern nanometer CMOS technologies because of scaling down requirement. Spintronic logic has been progressed to realize these demands based on magnetic-based elements properties especially magnetic tunnel junction (MTJ) such as radiation-hardened, non-volatility, and CMOS process compatibility. A novel soft-error tolerant and highly reliable latch circuit is proposed in this paper which is a promising choice for reliable and low-power architectures. Furthermore, employing the proposed shadow latch based on MTJ cells as a backup structure along with the rad-hard main latch is aimed to the realization of a nonvolatile and SEU-immune flip-flop which is applicable in next-generation CMOS circuits. Simulations of proposed circuits have been performed by SPICE tool and a 45 nm CMOS technology model to evaluate the effectiveness of topologies.

A Reliable, Low Power and Nonvolatile MTJ-Based Flip-Flop for Advanced Nanoelectronics

Very large-scale integrated circuit (VLSI) design faces many challenges with today’s nanometer CMOS technology, including leakage current and reliability issues. Magnetic tunnel junction (MTJ) hybrid with CMOS transistors can offer many advantages for future VLSI design such as high performance, low power consumption, easy integration with CMOS and also nonvolatility. However, MTJ-based logic circuits suffer from a reliability challenge that is the read disturbance issue. This paper proposes a new nonvolatile magnetic flip-flop (MFF) that offers a disturbance-free sensing and a low power write operation over the previous MFFs. This magnetic-based logic circuit is based on the previous two-in-one (TIO) MTJ cell that presents the aforementioned attributes. Radiation-induced single event upset, as another reliability challenge, is also taken into consideration for the MFFs and another MFF robust against radiation effects is suggested and evaluated.

Temperature dependence of magnetic properties on switching energy in magnetic tunnel junction devices with tilted magnetization

Spin transfer torque random access memory (STT-RAM) based on magnetic tunnel junction (MTJ) is a popular type of memory because of its non-volatility, small size, and nanosecond access time. The temperature effect on the magnetic properties and the initial angle between the free and pinned layer magnetizations in the MTJ cell are significant for the fast switching process in STT-RAM. In this study, the switching energy was investigated with the temperature effect on the magnetic properties at various initial angles. The results show that the saturation magnetization was decreased by increments of the initial temperature, which also resulted in the reduction of the intrinsic critical current density. Consequently, switching energy can be reduced by the increase of both the initial temperature and the initial angle. Although the initial temperature increment in the MTJ cell is interesting for the fast switching process in STT-RAM, the thermal stability factor greater than 40 and the Curie temperature were both considered for the analysis of the switching energy with magnetic properties, for stabilizing the operation of STT-RAM.

Design of Defect and Fault-Tolerant Nonvolatile Spintronic Flip-Flops

With technology down scaling, static power has become one of the biggest challenges in a system on chip . Normally off computing using nonvolatile (NV) sequential elements is a promising solution to address this challenge. Recently, many NV shadow flip-flop architectures have been introduced in which magnetic tunnel junction (MTJ) cells are employed as backup storing elements. Due to the emerging fabrication processes of magnetic layers, MTJs are more susceptible to manufacturing defects than their CMOS counterparts. Moreover, unlike memory arrays that can effectively be repaired with well-established memory repair and coding schemes, flip-flops scattered in the layout are more difficult to repair. Therefore, without effective defect and fault tolerance for NV flip-flops, the manufacturing yield will be affected severely. In this paper, we propose a fault-tolerant NV latch (FTNV-L) design, in which several MTJ cells are arranged in such a way that it is resilient to various MTJ faults. The simulation results show that our proposed FTNV-L can effectively tolerate all single MTJ faults with a considerably lower overhead than traditional approaches.

Modulating spin transfer torque switching dynamics with two orthogonal spin-polarizers by varying the cell aspect ratio

We study in-plane magnetic tunnel junctions with additional perpendicular polarizer for subnanosecond-current-induced switching memories. The spin-transfer-torque switching dynamics was studied as a function of the cell aspect ratio both experimentally and by numerical simulations using the macrospin model. We show that the anisotropy field plays a significant role in the dynamics, along with the relative amplitude of the two spin-torque contributions. This was confirmed by micromagnetic simulations. Real-time measurements of the reversal were performed with samples of low and high aspect ratio. For low aspect ratios, a precessional motion of the magnetization was observed and the effect of temperature on the precession coherence was studied. For high aspect ratios, we observed magnetization reversals in less than 1 ns for high enough current densities, the final state being controlled by the current direction in the magnetic tunnel junction cell.

Crystallization characteristics of a middle CoFeB layer in a double MgO barrier magnetic tunnel junction

The crystallization characteristics of a middle CoFeB free layer in a magnetic tunnel junction (MTJ) with double MgO barriers were investigated by tunneling magnetoresistance (TMR) measurements of patterned cells across an 8-inch wafer. The MTJ structure was designed to have two CoFeB free layers and one bottom pinned layer, separated by MgO tunnel barriers. The observed resistance showed three types of TMR curves depending on the crystallization of the middle CoFeB layer. From the analysis of TMR curves, coherent crystallization of the middle CoFeB layer with the top and bottom MgO barriers was found to occur non-uniformly: About 80% of the MTJ cells in the wafer exhibited coherent crystallization of the middle CoFeB layers with the bottom MgO tunnel barrier, while others had coherent crystallization with the top MgO tunnel barrier or both barriers. This non-uniform crystallization of the middle CoFeB layer in a double MTJ was also clearly observed in tunneling electron microscopy images. Thus, control of the crystallization of the middle CoFeB layer is important for optimizing the MTJ with double MgO barriers, and especially for the fabrication of double barrier MTJ on a large area substrate.

A Quasi-Analytical Model for Energy-Delay-Reliability Tradeoff Studies During Write Operations in a Perpendicular STT-RAM Cell

One of the biggest challenges that the current spin-transfer-torque-based random access memory (STT-RAM) industry faces is maintaining high thermal stability while trying to switch within a given voltage pulse and energy cost. In this paper, we present a physics-based analytical model that uses a modified Simmons tunneling expression to capture the spin-dependent tunneling in a magnetic tunnel junction (MTJ). Coupled with an analytical derivation of the critical switching current based on the Landau–Lifshitz–Gilbert equation and the write error rate derived from a solution to the Fokker–Planck equation, this model provides us a quick estimate of the energy-delay-reliability tradeoffs in perpendicular STT-RAM devices due to thermal fluctuations. In other words, the model provides a simple way to calculate the energy consumed during write operation that ensures a certain error rate and delay time while being numerically far less intensive than a full-fledged stochastic calculation. We calculate the worst case energy consumption during antiparallel (AP)-to-parallel (P) and P-to-AP switchings and quantify how increasing the anisotropy field <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$H_{K}$</tex></formula> and lowering the saturation magnetization <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$M_{S}$</tex></formula> can significantly reduce the energy consumption. A case study on how manufacturing variations of the MTJ cell can affect the energy consumption and delay is also reported.

Current-induced switching of exchange bias in nano-scaled magnetic tunnel junctions with a synthetic antiferromagnetic pinned layer

This report investigates the current-induced switching of exchange bias without an external magnetic field in nano-scaled magnetic tunnel junction (MTJ) cells. An MTJ stack film was patterned into an ellipse with dimensions of 120 nm × 270 nm by using standard electron beam lithography in combination with ion beam etching. A spin-polarized current pulse with a duration of 100 ns was used to switch the exchange bias direction of the synthetic antiferromagnetic (SAF) pinned layer. It is worth noting that the MTJ cell was initialized in a high resistance state before applying the current pulse. For the application of both positive and negative current pulses, the resistance can be switched from the high (antiparallel) state to the low (parallel) one at 2.95 and −2.80 mA, respectively. After the current-induced switching, it was found that the magnetoresistance curve is reversed relative to the one before the current-induced switching. Predominantly, this behavior is independent of the polarity of the current pulse. As a result, it is shown that the exchange bias in the SAF pinned layer changes its direction with a sufficient supply of current pulse.

A High-Reliability, Low-Power Magnetic Full Adder

Recently, ultra-low power circuits based on logic-in magnetic tunnel junction (MTJ) memory structure have been studied thanks to its non-volatility, infinite endurance, high access speed, and easy integration with CMOS process. However, this type of circuit suffers from low reliability both in memory cell and sensing amplifier circuits, which greatly limits its practical applications for logic computation. In this paper, we present a new design of magnetic full adder (MFA) to overcome this issue based on the thermally assisted switching (TAS) MTJ cell and pre-charge sensing amplifier (PCSA) circuit. By using CMOS 65 nm design kit and a precise TAS-MTJ model, mixed simulations have been performed to demonstrate its high reliability keeping low power and small die area.

Coupling strength with off-axial external field in magnetic tunnel junction cells

A series of hysteresis loops with off-axial external magnetic fields have been measured to identify magnetization reversal properties as well as coupling effects in magnetic tunnel junctions (MTJs). MTJ films, consisting of a synthetic artificial antiferromagnetic pinned layer of CoFeB/Ru/CoFe/IrMn, were patterned into an elliptical cell array with sizes of 200 × 300 nm and 500 × 750 nm. The hysteresis loops were measured using a vibrating sample magnetometer with various directions of the external magnetic field; the angle θ between the external magnetic field (H→E) and the biasing field of the pinned layer (H→B) was varied from 0° to 90°. The two significant coupling effects, ascribed to exchange bias between CoFe/IrMn and Ruderman–Kittel–Kasuya–Yosida–like coupling in CoFeB/Ru/CoFe, can be observed in these hysteresis loops. The angular dependence of the hysteresis loops is employed to characterize the variation of these coupling effects in extended and patterned MTJ films.

Effect of resistance-area product on spin-transfer switching in MgO-based magnetic tunnel junction memory cells

We use ultrafast current-induced switching measurements to study spin-transfer switching performance metrics, such as write energy per bit (EW) and switching current density (Jc), as a function of resistance-area product (RA) (hence MgO thickness) in magnetic tunnel junction cells used for magnetoresistive random access memory (MRAM). EW increases with RA, while Jc decreases with increasing RA for both switching directions. The results are discussed in terms of RA optimization for low write energy and current drive capability (hence density) of the MRAM cells. Switching times &amp;lt;2 ns and write energies &amp;lt;0.3 pJ are demonstrated for 135 nm×65 nm CoFeB/MgO/CoFeB devices.

The Electrical Characterization of Magnetic Tunneling Junction Cells Using Conductive Atomic Force Microscopy with an External Magnetic Field Generator

We examined the tunneling current behaviors of magnetic tunneling junction (MTJ) cells utilizing conductive atomic force microscopy (AFM) interfaced with an external magnetic field generator. By introducing current through coils, a magnetic field was generated and then controlled by a current feedback circuit. This enabled the characterization of the tunneling current under various magnetic fields. The current-voltage (I-V) property was measured using a contact mode AFM with a metal coated conducting cantilever at a specific magnetic field intensity. The obtained magnetoresistance (MR) ratios of the MTJ cells were about 21% with no variation seen from the different sized MTJ cells; the value of resistance <TEX>$\times$</TEX> area (RA) were 8.5 K-12.5 K <TEX>$({\Omega}{\mu}m^2)$</TEX>. Since scanning probe microscopy (SPM) performs an I-V behavior analysis of ultra small size without an extra electrode, we believe that this novel characterization method utilizing an SPM will give a great benefit in characterizing MTJ cells. This novel method gives us the possibility to measure the electrical properties of ultra small MTJ cells, namely below <TEX>$0.1\;{\mu}m\;{\times}\;0.1\;{\mu}m$</TEX>.

Temperature dependence of magnetoresistance oscillations due to Coulomb staircases in a nanometer vertical ferromagnetic tunnel junction

Magnetic random access memory is considered to be a promising candidate for a future nonvolatile memory. The size of the magnetic tunnel junction (MTJ) cell has been gradually decreasing and has reached the sub-100 nm level. As the size of the MTJ cell approaches the 10 nm level, we must consider the effects of the interaction between the spin-dependent tunneling and the Coulomb blockade, such as magneto-Coulomb oscillation. Spin-dependent tunneling in a nanometer ferromagnetic junction has been investigated in Coulomb blockade regime. The current-bias voltage (I-V) characteristics of NiO/Co/NiO/Co double junctions with a 30 nm contact hole fabricated by an electron beam direct process have been measured in magnetic fields. The temperature dependence of the I-V curve shows clear Coulomb staircases at temperatures below 50 K. At 12 K, the Coulomb threshold of the I-V curve depends on the magnetization configuration, which leads to a tunnel magnetoresistance (TMR) of over 500% around the Coulomb threshold. Although the origin of such a large TMR has not been well proven, spin accumulation in a nanometer metal island is considered. The bias dependence of TMR shows oscillatory behavior due to Coulomb staircases, and at higher temperatures, the TMR oscillations disappear. The TMR oscillation results from the enhancement of TMR at the steps of the Coulomb staircase associated with discrete charging effects and spin accumulations of a nanometer metal island.

Study of the Relationship Between the Scalability of MTJs and Switching Field by Using Scanning Probe Microscope

In order to use the magnetic tunnel junction (MTJ) as a component for memory applications, we need to analyze it and to obtain a well de ned switching eld, in spite of variations in the sample size and position. We studied the switching eld variation in terms of the sizes of sub-micron MTJ cells by measuring the I-V characteristics of the MTJ cells and the hysteresis in the I-H loop, which was obtained by measuring the tunneling current for a sweeping magnetic eld.

Back-hopping after spin torque transfer induced magnetization switching in magnetic tunneling junction cells

In some cases such as junctions with low magnetic thermal activation energy, the magnetization of the free layer in MgO-based magnetic tunnel junctions (MTJs) can back hop to its original direction after successful spin torque induced switching. The back-hopping is observed in both current directions corresponding to parallel-to-antiparallel and antiparallel-to-parallel switchings. For bias voltage pulses with increasing pulse width, the threshold voltage for back-hopping appears to decrease together with spin-torque switching and junction breakdown thresholds, but its rate of decrease is less. Increasing the anisotropy field Hk by increasing the MTJ aspect ratio can raise the threshold voltage of back-hopping significantly.

Optimizing Write Current and Write Power Consumption in Magnetoresistive Random Access Memories by Using Generalized Asteroid Curves

Theoretical expressions for minimizing write current and write power consumption in magnetoresistive random access memories (MRAMs) are presented. The expressions are derived based on a generalized asteroid curve which approximately represents the minimum switching magnetic fields of magnetic tunnel junction (MTJ) memory cells recently developed for smaller write currents and better cell selectivity. The generalized asteroid curve has a shape parameter to fit the curve to the switching characteristics of MTJ cells. The derived expressions contain the shape parameter as well as other cell and array structural parameters, and thus give theoretical limits of the write current and power from physical points of view including MTJ shape.