Magnetic Random Access Memory Applications 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.

Field-free spin–orbit torque switching in ferromagnetic trilayers at sub-ns timescales

Current-induced spin torques enable the electrical control of the magnetization with low energy consumption. Conventional magnetic random access memory (MRAM) devices rely on spin-transfer torque (STT), this however limits MRAM applications because of the nanoseconds incubation delay and associated endurance issues. A potential alternative to STT is spin-orbit torque (SOT). However, for practical, high-speed SOT devices, it must satisfy three conditions simultaneously, i.e., field-free switching at short current pulses, short incubation delay, and low switching current. Here, we demonstrate field-free SOT switching at sub-ns timescales in a CoFeB/Ti/CoFeB ferromagnetic trilayer, which satisfies all three conditions. In this trilayer, the bottom magnetic layer or its interface generates spin currents with polarizations in both in-plane and out-of-plane components. The in-plane component reduces the incubation time, while the out-of-plane component realizes field-free switching at a low current. Our results offer a field-free SOT solution for energy-efficient scalable MRAM applications.

First Principle Study of Spin Tunneling Current Under Field Effect in Magnetic Tunnel Junction for Possible Application in STT-RAM

A first principle analysis of Fe/MgO/Fe magnetic tunnel junction (MTJ) having gate voltage over insulated barrier region (MgO) is presented. Because of different work functions of gate and barrier materials, transfer of density of states (DOS) in forbidden energy gap of MgO is found above Fermi energy due to Schottky effect. It is reported that diffused DOS allows high tunneling of majority channel current because of less interference of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Delta _{{1}}}$ </tex-math></inline-formula> symmetry of Bloch states with evanescent states in barrier. It shows high spin injection efficiency 0.9928, 0.9959, and 0.9871 with tunnel magneto resistance (TMR) ratios 1743.30%, 1450.06%, and 572.62% at gate voltages 1–3 V, respectively, with left and right electrodes at 0.5 V. Present work gives a basis for using MTJs as three terminal devices in magnetic random access memory (MRAM) applications.

Unveiling a Chemisorbed Crystallographically Heterogeneous Graphene/L10-FePd Interface with a Robust and Perpendicular Orbital Moment.

A crystallographically heterogeneous interface was fabricated by growing hexagonal graphene (Gr) using chemical vapor deposition (CVD) on a tetragonal FePd epitaxial film grown by magnetron sputtering. FePd was alternately arranged with Fe and Pd in the vertical direction, and the outermost surface atom was identified primarily as Fe rather than Pd. This means that FePd has a high degree of L10-ordering, and the outermost Fe bonds to the carbon of Gr at the interface. When Gr is grown by CVD, the crystal orientation of hexagonal Gr toward tetragonal L10-FePd selects an energetically stable structure based on the van der Waals (vdW) force. The atomic relationship of Gr/L10-FePd, which is an energetically stable interface, was unveiled theoretically and experimentally. The Gr armchair axis was parallel to FePd [100]L10, where Gr was under a small strain by chemical bonding. Focusing on the interatomic distance between the Gr and FePd layers, the distance was theoretically and experimentally determined to be approximately 0.2 nm. This shorter distance (≈0.2 nm) can be explained by the chemisorption-type vdW force of strong orbital hybridization, rather than the longer distance (≈0.38 nm) of the physisorption-type vdW force. Notably, depth-resolved X-ray magnetic circular dichroism analyses revealed that the orbital magnetic moment (Ml) of Fe in FePd emerged at the Gr/FePd interface (@inner FePd: Ml = 0.16 μB → @Gr/FePd interface: Ml = 0.32 μB). This interfacially enhanced Ml showed obvious anisotropy in the perpendicular direction, which contributed to interfacial perpendicular magnetic anisotropy (IPMA). Moreover, the interfacially enhanced Ml and interfacially enhanced electron density exhibited robustness. It is considered that the shortening of the interatomic distance produces a robust high electron density at the interface, resulting in a chemisorption-type vdW force and orbital hybridization. Eventually, the robust interfacial anisotropic Ml emerged at the crystallographically heterogeneous Gr/L10-FePd interface. From a practical viewpoint, IPMA is useful because it can be incorporated into the large bulk perpendicular magnetic anisotropy (PMA) of L10-FePd. A micromagnetic simulation assuming both PMA and IPMA predicted that perpendicularly magnetized magnetic tunnel junctions (p-MTJs) using Gr/L10-FePd could realize 10-year data retention in a small recording layer with a circular diameter and thickness of 10 and 2 nm, respectively. We unveiled the energetically stable atomic structure in the crystallographically heterogeneous interface, discovered the emergence of the robust IPMA, and predicted that the Gr/L10-FePd p-MTJ is significant for high-density X nm generation magnetic random-access memory (MRAM) applications.

Effect of Buffer and Cap Layer on Thermally Stable Perpendicular Magnetic Anisotropy in Buffer/CoFeB/MgO/Cap Structure

The effect of buffer and cap layer on thermally stable perpendicular magnetic anisotropy (PMA) in buffer/CoFeB/MgO/cap structure was designed to detailed study. Not only the buffer layer is crucial, but the type of cap layer also affects the thermal stability of PMA. Relatively to the Ta samples, the W samples adopting W buffer or cap layer get a wider PMA thickness range for further increasing thermal stability of PMA in magnetic random access memory (MRAM) applications. And the same as the W buffer layer, the annealing temperature for W cap layer is also increased 30°C (from 270°C to 300°C). Via the detailed anomalous Hall effect (AHE) measurements, the thermal stability of PMA in buffer/CoFeB/MgO/cap was investigated. This work provides a promising way to obtain high thermal stability of PMA in CoFeB-MgO-based spintronic applications, and it is significant for designing the next-generation information storage devices.

Micromagnetic simulation of vortex core polarity in bi-component magnetic nanodisks

The nucleation and management of the vortex state in magnetic nanostructures has received a lot of interest in recent decades, with potential applications in logic networks and non-volatile magnetic random-access memory. Because of their potential use in magnetic memory, magnetic vortex structures are of considerable technical importance. Investigation of bi-component magnetic nanodisks as prospective storage systems, in both single and coupled configurations, has been performed, where the information unit is represented by vortex core polarity. The work mainly engages in magnetic nanodisks of permalloy and iron having a diameter of 200 nm and a thickness of 40 nm. The potential of adjusting the polarity of the vortex core by the application of a magnetic field using micromagnetic simulations has been illustrated. Micromagnetic simulation studies show that bi-material structures are important not just for fundamental research but also for data storage.

Probing the Spin Hall Characteristics of W/CoFeB/MgO Based Heterostructures for Spin‐Orbit Torque Based Magnetic Random Access Memory Application

AbstractIn this work, the spin Hall characteristics of W/CoFeB/MgO have been systematically studied to provide an in‐depth insight for the optimization of the SOT‐MRAM channel. The bcc and β phases of tungsten (W) are identified clearly from XRD and resistivity studies, which shows marked impact on the electrical characteristics. Moreover, the same is found have tremendous affect on W's spin Hall characteristics with a twelvefold increase of the spin Hall angle ΘSH. The evaluation of W/CoFeB interfacial spin mixing conductance using ab initio studies shows good coherency with the same evaluated experimentally, which is used to estimate interfacial spin transparency T&lt;/&gt; to accurately evaluate intrinsic spin Hall conductivity σSH of W. The findings equivocate for a strong claim on β‐W's σSH to be around −0.23 ∓ 0.03, addressing the ongoing discrepancy. Increasing boron concentration in (Co1Fe3)100−xBx and thermal annealing temperature are found to reduce ΘSH effectively which is inferred to boron diffusion toward W. Alloying of W with Ta does not necessarily enhance ΘSH, for which very optimized relative concentration and the alloy growth phase is critical. The findings can significantly contribute to the ongoing SOT‐MRAM development for achieving various means of control for improvement in the device characteristics.

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.

Impact of ambient temperature on the switching of voltage-controlled perpendicular magnetic tunnel junction

Voltage control of the magnetic anisotropy (VCMA) effect enables a voltage-mediated magnetization switching mechanism for lower-power applications. In this work, we experimentally investigate the characteristics of VCMA-induced switching and we observe a clear decrease in the critical switching voltage (Vc) at elevated temperatures. A 50% reduction in Vc is quantified when increasing the ambient temperature (T) from 300 K to 360 K. Such a T-dependence of Vc is well explained with the variations of saturation magnetization (MS), interfacial anisotropy (Ki), and VCMA coefficient (ξ). In addition, the dependences of these properties on temperature are well fitted and explained with the power law of MS(T). Our findings on the T-dependent magnetic and switching characteristics of VCMA are of technological importance for implementing VCMA in magnetic random access memory (MRAM) applications.

Zn doping induced enhancement of multifunctional properties in NiO nanoparticles

Abstract Ni1-xZnxO nanoparticles were synthesized by wet-chemical route. XRD characterization of these samples indicated that Zn doping in NiO neither induces structural phase transformation nor any secondary phase. Rietveld refinement of XRD data indicated that the lattice parameter of NiO increases with increasing of Zn doping concentration. TEM analysis confirms the reduction of agglomeration and shape transformation from irregular to almost spherical shape up on Zn doping in NiO. Raman study combined with photoluminescence study suggested that these materials could be useful for optical applications. Magnetic characterization of Ni1-xZnxO samples indicates that all samples show weak ferromagnetism at room temperature with the persistence of unsaturation behavior even up to very high magnetic field. Our study indicated that the synthesized samples could be useful for UV emission device, magnetic random access memory and data storage applications.

Transition metal substituted Fe2P: potential candidate for MRAM application

We propose transition metal substituted Fe2P as a new promising material for spin-transfer torque magnetic random-access memory (STT-MRAM) application. Using first-principles calculations based on density functional theory and Monte Carlo simulations, we demonstrate that this material can be used as a ferromagnetic electrode in the magnetic tunnel junction (MTJ) of STT-MRAM due to its moderate perpendicular magnetic anisotropy, high ferromagnetic transition temperature, and large tunnel magnetoresistance. This work is expected to provide a basis for the development of a new class of Fe2P-based electrode materials for STT-MRAM devices.

Magnetic stray fields in nanoscale magnetic tunnel junctions

The magnetic stray field is an unavoidable consequence of ferromagnetic devices and sensors leading to a natural asymmetry in magnetic properties. Such asymmetry is particularly undesirable for magnetic random access memory applications where the free layer (FL) can exhibit bias. Using atomistic dipole–dipole calculations we numerically simulate the stray magnetic field emanating from the magnetic layers of a magnetic memory device with different geometries. We find that edge effects dominate the overall stray magnetic field in patterned devices and that a conventional synthetic antiferromagnet structure is only partially able to compensate the field at the FL position. A granular reference layer is seen to provide near-field flux closure while additional patterning defects add significant complexity to the stray field in nanoscale devices. Finally we find that the stray field from a nanoscale antiferromagnet is surprisingly non-zero arising from the imperfect cancellation of magnetic sublattices due to edge defects. Our findings provide an outline of the role of different layer structures and defects in the effective stray magnetic field in nanoscale magnetic random access memory devices and demonstrate that atomistic calculations provide a useful tool to study the stray field effects arising from a wide range of defects.

Writing magnetic memory with ultrashort light pulses

Laser pulses are the shortest stimulus known to control the magnetization of materials and to switch magnetic devices on the picosecond to femtosecond timescales. Femtosecond laser pulses have been able to trigger the fastest changes in the magnetic state of matter, and thus these pulses may lead to technologies with increased speed and energy efficiency of magnetic data storage and memory. In the past decade, materials enabling optical control of magnetism and concepts of devices employing such opto-magnetic phenomena have been shown. In this Review, we explore ultrafast all-optical switching (AOS) of magnetization as the least-dissipative and fastest method for magnetic writing. We outline the physical processes responsible for mechanisms of AOS, define the materials suitable for optical control of magnetism and test these mechanisms and materials against three important criteria of recording: speed, accompanying dissipations and scalability. In particular, we emphasize that switching magnetization with the help of light outperforms other methods in terms of the speed of the write–read magnetic recording event (less than 20 ps) and the unprecedentedly low heat load (<6 J cm−3). Finally, we outline the integration of AOS in spintronic devices and the perspective of large-scale integration towards magnetic random access memory and other memory applications with low-energy dissipations. Laser pulses can trigger fast changes in magnetic state, facilitating new magnetic data storage and memory devices. This Review outlines the mechanisms of all-optical switching and the materials suitable for the optical control of magnetism and tests these mechanisms and materials in terms of speed, accompanying dissipations and scalability. Finally, the large-scale integration of devices in memory applications with low-energy dissipations is discussed.

Ultrafast field-free magnetization switching using bi-directional spin Hall current and antiferromagnetic interlayer exchange

We present an approach to realize ultrafast field-free spin-orbit torque (SOT) switching of a ferromagnetic layer with perpendicular magnetic anisotropy. In this scheme, the heavy metal layer serves as both the spin Hall channel and the interlayer exchange spacer. The magnetization switching of the adjacent ferromagnetic layer is driven by magnetic inertia, exhibiting intriguing and rich switching behavior under current pulses of different amplitudes and durations. Based on the macrospin simulation, we investigated the switching behavior of the composite-free layer, i.e., two antiferromagnetically coupled ferromagnetic layers, and demonstrated the possibility of field-free switching within tens of picoseconds. Our results show that the positive sign product of field-like SOT and damping-like SOT and the strong antiferromagnetic exchange between the two coupled ferromagnetic layers are critical to realize the deterministic switching. This work is expected to stimulate further experimental efforts for designing high performance magnetic random-access memory application.

Stability of point defects near MgO grain boundaries in FeCoB/MgO/FeCoB magnetic tunnel junctions

Magnetic tunnel junctions employing FeCoB as the ferromagnet and MgO as a spacer layer exhibit high performance and are attractive for magnetic random access memory applications. Upon postdeposition annealing, B is observed to diffuse out of the FeCoB layers inducing crystallization of FeCo. It is known that a large proportion of B escapes into the adjacent tantalum underlayer. While diffusion of B into bulk MgO is known to be unfavorable, it is possible that B could diffuse into grain boundaries (GBs) in the polycrystalline MgO layer, affecting its electronic properties. In this paper, density functional theory is used to investigate the stability and electronic properties of oxygen vacancy and B interstitial defects at MgO GBs. We show that both types of defects exhibit increased stability at the GBs, and we introduce electronic states in the gap that could negatively impact performance. These predictions are consistent with recent experimental results, and we discuss further means to confirm the results experimentally using techniques such as x-ray or ultraviolet photoelectron spectroscopy.

The effect of HfO2 on the magnetic anisotropy, electrical structure and microstructure of CoFeB/MgO films

Metal oxides are used in magnetic tunnel junctions to improve the magnetic properties and thermostability of materials in high-performance applications. In this study, an ultrathin Hf film is deposited on a Ta buffer layer. Moreover, subsequent annealing at 350 °C leads to the oxidation of Hf into HfO2, as confirmed by X-ray photoelectron spectrometry (XPS). The effect of HfO2 on magnetic anisotropy is evaluated in Ta/Hf/CoFeB/MgO/Ta multilayer films. Easy-axis magnetisation transition from in-plane to out-of-plane direction is observed at an annealing temperature of 200 °C. Moreover, the sample annealed at a temperature of 350 °C exhibits clear perpendicular magnetic anisotropy, which satisfies industry requirements in terms of annealing temperature for magnetic random access memory applications. XPS, positron annihilation spectroscopy (PAS), X-ray reflectivity (XRR) and atomic force microscopy (AFM) are used to study film composition, defects, interface width and surface roughness in Ta/Hf/CoFeB/MgO/Ta multilayers. XPS results demonstrate partial suppression of Ta atom diffusion and enhanced formation of Fe (Co)oxides in CoFeB/MgO. The results of PAS analysis suggest that the density of film defects decreases after annealing at 350 °C. XRR and AFM results reveal that annealing at 350 °C produces a smoother CoFeB/MgO interface. Together, these factors lead to improved magnetic properties and thermostability in a Ta/Hf/CoFeB/MgO/Ta film.

Spin transfer torque switching in exchange-coupled amorphous GdFeCo/TbFe bilayers for thermally assisted MRAM application

Exchange-coupled amorphous GdFeCo/TbFe memory layers in giant magneto-resistance (GMR) devices for spin transfer torque (STT) switching have been studied, and temperature dependence of the critical current density of the GMR devices was measured to discuss the effect of exchange-coupled bilayers as a memory layer of the thermally assisted magnetic random access memory (MRAM). The GMR devices having amorphous GdFeCo and TbFe memory bilayers with various thicknesses were prepared by magnetron sputtering and subsequent micro-fabrication processes. A pulsed current was applied to the GMR devices in order to investigate the spin transfer torque (STT) switching. The maximum magneto-resistance (MR) ratio was around 0.15%, and the coercivity of the memory bilayer increased with the TbFe thickness and decreased with elevating temperature. The critical current densities Jc to switch the memory bilayer with structure of Gd21.4 (Fe90Co10)78.6(9 nm)/Tb16Fe84 (1 nm) as low as 2.2 × 107 A cm−2 was obtained. The Jc reduced with increasing the temperature and was found to scale with the effective anisotropy Keff of GdFeCo/TbFe bilayer, which is believed to be suitable for the application of thermally assisted STT-MRAM.

MnGa-based fully perpendicular magnetic tunnel junctions with ultrathin Co2MnSi interlayers

Because tetragonal structured MnGa alloy has intrinsic (not interface induced) giant perpendicular magnetic anisotropy (PMA), ultra-low damping constant and high spin polarization, it is predicted to be a kind of suitable magnetic electrode candidate in the perpendicular magnetic tunnel junction (p-MTJ) for high density spin transfer torque magnetic random access memory (STT-MRAM) applications. However, p-MTJs with both bottom and top MnGa electrodes have not been achieved yet, since high quality perpendicular magnetic MnGa films can hardly be obtained on the MgO barrier due to large lattice mismatch and surface energy difference between them. Here, a MnGa-based fully p-MTJ with the structure of MnGa/Co2MnSi/MgO/Co2MnSi/MnGa is investigated. As a result, the multilayer is with high crystalline quality, and both the top and bottom MnGa electrodes show well PMA. Meanwhile, a distinct tunneling magnetoresistance (TMR) ratio of 65% at 10 K is achieved. Ultrathin Co2MnSi films are used to optimize the interface quality between MnGa and MgO barrier. A strong antiferromagnetic coupling in MnGa/Co2MnSi bilayer is confirmed with the interfacial exchange coupling constant of −5erg/cm2. This work proposes a novel p-MTJ structure for the future STT-MRAM progress.

Co/Ni]-CoFeB hybrid free layer stack materials for high density magnetic random access memory applications

Alternative free layer materials with high perpendicular anisotropy are researched to provide spin-transfer-torque magnetic random access memory stacks' sufficient thermal stability at critical dimensions of 20 nm and below. We demonstrate a high tunnel magetoresistance (TMR) MgO-based magnetic tunnel junction stack with a hybrid free layer design made of a [Co/Ni] multilayer and CoFeB. The seed material on which the [Co/Ni] multilayer is deposited determines its switching characteristics. When deposited on a Pt seed layer, soft magnetic switching behavior with high squareness is obtained. When deposited on a NiCr seed, the perpendicular anisotropy remains high, but the squareness is low and coercivity exceeds 1000 Oe. Interdiffusion of the seed material with the [Co/Ni] multilayers is found to be responsible for the different switching characteristics. In optimized stacks, a TMR of 165% and low resistance-area (RA) product of 7.0 Ω μm2 are attained for free layers with an effective perpendicular magnetic anisotropy energy of 1.25 erg/cm2, which suggests that the hybrid free layer materials may be a viable candidate for high density magnetic random access memory applications.

Enhanced interlayer exchange coupling in antiferromagnetically coupled ultrathin (Co70Fe30/Pd) multilayers

We report the studies of magnetization reversal and magnetic interlayer coupling in synthetic antiferromagnetic (SAF) [Pd/Co70Fe30]9/Ru(tRu)/Pd(tPd)/[Co70Fe30/Pd]9 structure as functions of inserted Pd layer (tPd) and Ru layer (tRu) thicknesses. We found the exchange coupling field (Hex) and perpendicular magnetic anisotropy (PMA) can be controlled by both the tPd and tRu, The Hex shows a Ruderman-Kittel-Kasuya-Yosida-type oscillatory decay dependence on tRu and a maximum interlayer coupling strength Jex = 0.522 erg/cm2 is achieved at tPd + tRu ≈ 0.8 nm in the as-deposited sample. As it is known that a high post-annealing stability of SAF structure is required for magnetic random access memory applications, the dependence of Hex and PMA on the post-annealing temperature (Ta) is also investigated. We found that both high PMA of the top Co70Fe30/Pd multilayer is maintained and Hex is enhanced with increasing Ta up to 350 °C for tRu &amp;gt; 0.7 nm in our SAF structure.