In this work, we discuss the method of magnetic exchange stiffness calculation. The method is based on ab-initio electronic structure calculation by use of parallel and antiparallel magnetic domain configurations. It allows to avoid the atomic magnetic exchange interaction calculations that are more complicated, less accurate, and often not easily applicable in complex materials. The application of the developed approach was experimentally verified in the case of most common MRAM materials.

We demonstrate highly reliable STT-MRAM whose array-level write error has been eliminated by lowering the density of domain wall pinning sites in the MTJs. The core part of investigation includes the identification and quantification of domain wall pinning sites, characterization and modeling of the pinning sites, and correlation of the density of pinning sites with array-level write error rate. The experimental results show that domain wall pinning is geometrically localized and reproduced upon the repeated writing cycles. By controlling the domain wall pinning, we obtain high-density MTJ array having superior reliability without notable trailing bits of write fail.

We perform atomistic simulations of spin transfer torque switching dynamics in CoFeB/MgO/CoFeB magnetic tunnel junctions. We base our study on Slonczewski's model parametrized following the approach of Zhang, Levy, and Fert. We utilize excitation modes and the contour integral of the magnetization to perform a deeper analysis of the switching mechanism driven by spin transfer torque. Our results show a magnetization reversal driven by the combination of coherent and nonuniform excitation modes. These can be nonuniform and initiated by a coherent mode of the magnetization, or domain wall nucleated depending on the lateral size, temperature, and current density injected into the system. Larger current densities result in stronger excitation of nonuniform modes making the switching more easily subjected to thermal excitations and structural imperfections such as edge damage. Our findings agree with experimental works on spin transfer torque switching in similar CoFeB/MgO-based systems, and they suggest the presence of complex features in the magnetization dynamics. The analysis and the results presented here can help to gain a deeper understanding of spin transfer torque dynamics in nanoscale devices.

The balance between low power consumption and high efficiency in memory devices is a major limiting factor in the development of new technologies. Magnetic random access memories (MRAM) based on CoFeB/MgO magnetic tunnel junctions (MTJs) have been proposed as candidates to replace the current technology due to their non-volatility, high thermal stability and efficient operational performance. Understanding the size and temperature dependence of the energy barrier and the nature of the transition mechanism across the barrier between stable configurations is a key issue in the development of MRAM. Here we use an atomistic spin model to study the energy barrier to reversal in CoFeB/MgO nanodots to determine the effects of size, temperature and external field. We find that for practical device sizes in the 10-50 nm range the energy barrier has a complex behaviour characteristic of a transition from a coherent to domain wall driven reversal process. Such a transition region is not accessible to simple analytical estimates of the energy barrier preventing a unique theoretical calculation of the thermal stability. The atomistic simulations of the energy barrier give good agreement with experimental measurements for similar systems which are at the state of the art and can provide guidance to experiments identifying suitable materials and MTJ stacks with the desired thermal stability.

We theoretically investigate the temperature and thickness dependence of the effective Gilbert damping constant (alpha) in the Co-Fe-B/MgO system using atomistic spin dynamics. We consider a high damping constant at the interface layer and a low damping constant for the bulklike layers due to large interfacial spin-orbit coupling. We find a strong dependence of the effective Gilbert damping with the film thickness, in quantitative agreement with experimental data. The temperature dependence of the effective damping arising from thermal-spin fluctuations up to temperatures of 400 K is weak, with no apparent change over the studied temperature range. Interestingly, we find that the temperature produces a different effect: a statistical fluctuation of the Gilbert damping parameter for a given relaxation induced solely from the finite size of the system. This statistical variation of the Gilbert damping is an intrinsic effect and is important for spintronic devices operating at gigahertz frequencies, where the dynamic response must be carefully controlled.

Co20Fe60B20 thin films of various thicknesses (3 nm <= t(CFB) <= 14 nm) have been sputtered on thermally oxidized Si substrates and capped with 8 nm-thick Ir or Ru layers and then annealed at 400 degrees C. A vibrating sample magnetometer has been used to measure the CoFeB thickness dependence of the saturation magnetic moment per unit area in order to straightforwardly determine the magnetization at saturation and the magnetic dead layer thickness, found to be 1115 +/- 50 emu/cm(3) (930 +/- 50 emu/cm3) and 1.53 nm (0.76 nm) for the as-deposited CoFeB/Ir (CoFeB/Ru) sytems, respectively. These values change to 1185 +/- 50 emu/cm3 (1110 +/- 50 emu/cm(3)) and 2.93 nm (1.62 nm) for CoFeB/Ir (CoFeB/Ru) films annealed at 400 degrees C. Microstrip ferromagnetic resonance (MS-FMR) was used to investigate the Gilbert damping parameter and the magnetic anisotropy as a function of the CoFeB thickness. While the in-plane anisotropy field for the as-deposited films does not show a regular behavior versus the CoFeB thickness, a clear linear behavior of this field versus the reciprocal CoFeB effective thickness can be observed for the annealed samples. Moreover, the effective magnetization varies linearly with the inverse effective thickness of CoFeB due to the perpendicular interface anisotropy. This surface anisotropy constant, which was estimated to be 0.84 and 0.89 erg/cm(2) for the as-deposited CoFeB films capped with Ir or Ru, respectively, reinforces the perpendicular easy axis. The corresponding anisotropy constants for the annealed Ir and Ru capped films are found to be 1.07 and 0.57 erg/cm(2), respectively. Moreover, MS-FMR results revealed that the damping constant increases linearly with the reciprocal effective thickness of CoFeB, most probably due to spin pumping. The effective spin mixing conductance of the as-deposited (annealed) CoFeB/Ru and CoFeB/Ir has been estimated to be 12.93 nm(-2) (22.4 nm(-2)) and 29.3 nm(-2) (43.85 nm(-2)), respectively.

Spin-Transfer-Torque Magnetic Random Access Memory (STT-MRAM) is a memory which has bit cells made of magnetic tunnel junctions (MTJs), which comprise a storage switchable magnetic layer (“free layer”) and, typically, one thin insulating barrier and one stable magnetic layer providing reference spin polarization for read and write operations (“reference layer”). STT-MRAM may compete with conventional dynamic and static RAM on technological nodes below 22 nm, if its switching current is reduced. This goal may be achieved for MTJ, which has two insulating barriers and reference layers. Building such a double-barrier MTJ, however, faces tremendous material challenges. In this work, a new double-barrier MTJ design with a switchable reference layer is introduced. We show that its efficiency is similar to its counterpart with stable reference layers, but it is much easier to be built.

Fe/MgO-based magnetic tunnel junctions are among the most promising candidates for spintronic devices due to their high thermal stability and high tunneling magnetoresistance. Despite its apparent simplicity, the nature of the interactions between the Fe and MgO layers leads to complex finite-size effects and temperature-dependent magnetic properties which must be carefully controlled for practical applications. In this article, we investigate the electronic, structural, and magnetic properties of MgO/Fe/MgO sandwiches using first-principles calculations and atomistic spin modeling based on a fully parametrized spin Hamiltonian. We find a large contribution to the effective interfacial magnetic anisotropy from the two-ion exchange energy. Minimization of the total energy using atomistic simulations shows a surprising spin-spiral ground-state structure at the interface owing to frustrated ferromagnetic and antiferromagnetic interactions, leading to a reduced Curie temperature and strong layerwise temperature dependence of the magnetization. The different temperature dependences of the interface and bulklike layers results in an unexpected nonmonotonic temperature variation of the effective magnetic-anisotropy energy and temperature-induced spin-reorientation transition to an in-plane magnetization at low temperatures. Our results demonstrate the intrinsic physical complexity of the pure Fe/MgO interface and the role of elevated temperatures providing insight when interpreting experimental data of nanoscale magnetic tunnel junctions.

STT-MRAM cell design with dual magnetic tunnel junctions (D-MTJ) is a novel design that show a factor of ~2 in switching performance compared to conventional MRAM design. However, the disadvantage is D-MTJ tends to show lower TMR. In this presentation, we demonstrate it's possible to achieve high TMR D-MTJ cell design without compromising its performance gain. We accomplished this by thinning down the secondary MgO barrier. We observe that when thinning down the secondary barrier, the device level TMR reaches a level close to conventional MRAM design of the same free layer while still preserving its high switching performance

A rare-failure oriented optimization methodology for state-of-the-art STT-MRAM technology has been proposed. Physics-based device models and novel rare event sampling algorithms are used for massively parallel Monte Carlo simulations to identify the critical process variability sources and to evaluate the Write Error Rate (WER) at the resolution of 1E-9. New rare-failure figure-of-merits (FoMs) and design guidelines are suggested for optimizing the operation conditions of STT-MRAMs so that the energy-delay product can be minimized at satisfactory WER level.

Power consumption is the main limitation in the development of new high performance random access memory for portable electronic devices. Magnetic RAM (MRAM) with CoFeB/MgO based magnetic tunnel junctions (MTJs) is a promising candidate for reducing the power consumption given its nonvolatile nature while achieving high performance. The dynamic properties and switching mechanisms of MTJs are critical to understanding device operation and to enable scaling of devices below 30 nm in diameter. Here we show that the magnetic reversal mechanism is incoherent and that the switching is thermally nucleated at device operating temperatures. Moreover, we find an intrinsic thermal switching field distribution arising on the sub-nanosecond time-scale even in the absence of size and anisotropy distributions or material defects. These features represent the characteristic signature of the dynamic properties in MTJs and give an intrinsic limit to reversal reliability in small magnetic nanodevices.

The magnetic properties of 20 nm thick sputtered CoFeB thin films of different compositions have been investigated and compared to those of traditional 3D ferromagnetic materials (Co, Fe and Ni). A vibrating sample magnetometer has been used to measure their magnetization at saturation (Ms). Their dynamic magnetic properties were studied using microstrip line ferromagnetic resonances, as well as Brillouin light scattering (BLS) techniques. The effective magnetizations and gyromagnetic factors are firstly measured from resonance spectra obtained for in-plane and perpendicular applied fields. The angular dependence of the resonance field then allows us to derive parameters describing the in-plane magnetic anisotropy, which is found to result from a superposition of small uniaxial and fourfold terms. Frequency and angular dependencies of the ferromagnetic resonance linewidth have been used to determine the Gilbert damping coefficient for each sample. The perpendicular surface standing modes, observed in BLS spectra, allow the evaluation of the exchange stiffness constant, which is found to vary linearly with Ms (for CoFeB), in agreement with the simple model presented here. Finally, the thickness dependence of the Gilbert damping parameter of Co15Fe45B40 revealed a contribution due to spin pumping leading to a spin mixing conductance of 31 nm−2. The BLS measurements on the 5 nm thick Co15Fe45B40 film evidenced a large frequency asymmetry attributed to the interfacial Dzyaloshinskii–Moriya interaction (DMI) originating from the interface with Pt. The surface DMI constant has been estimated to be  =  −0.33 pJ m−1.

We report on a new source of in-plane anisotropy in nanomagnets due to the presence of Dzyaloshinskii-Moriya interaction (DMI). This anisotropy depends on the shape of the magnet, and is orthogonal to the demagnetization shape anisotropy. This effect originates from the DMI energy reduction due to an out-of-plane tilt of the spins at edges oriented perpendicular to the magnetization. Our investigation combining experimental, numerical and analytical results demonstrate that this energy reduction can compensate the demagnetization shape anisotropy energy in magnets of elongated shape, provided that their volumes is small enough and thus that their magnetization is quasi-uniform.

In this paper, a review of the developments in MRAM technology over the past 20 years is presented. The various MRAM generations are described with a particular focus on spin-transfer torque MRAM (STT-MRAM) which is currently receiving the greatest attention. The working principles of these various MRAM generations, the status of their developments, and demonstrations of working circuits, including already commercialized MRAM products, are discussed.

MgO bilayer systems emphasizing the influence of the iron layer thickness on the geometry, the electronic structure and the magnetic properties. Our calculations ensure the unconstrained structural relaxation at scalar relativistic level for various numbers of iron layers placed on the magnesium oxide substrate. Our results show that due to the formation of the interface the electronic structure of the interface iron atoms is significantly modified involving charge transfer within the iron subsystem. In addition, we find that the magnetic anisotropy energy increases from 1.9 mJ m-2 for 3 Fe layers up to 3.0 mJ m-2 for 11 Fe layers.

We present a systematic first-principles study of Fe | MgO bilayer systems emphasizing the influence of the iron layer thickness on the geometry, the electronic structure and the magnetic properties. Our calculations ensure the unconstrained structural relaxation at scalar relativistic level for various numbers of iron layers placed on the magnesium oxide substrate. Our results show that due to the formation of the interface the electronic structure of the interface iron atoms is significantly modified involving charge transfer within the iron subsystem. In addition, we find that the magnetic anisotropy energy increases from 1.9 mJ m(-2) for 3 Fe layers up to 3.0 mJ m(-2) for 11 Fe layers.

In order to increase the thermal stability of a magnetic random access memory (MRAM) cell, materials with high spin-orbit interaction are often introduced in the storage layer. As a side effect, a strong Dzyaloshinskii-Moriya interaction (DMI) may arise in such systems. Here we investigate the impact of DMI on the magnetic cell performance, using micromagnetic simulations. We find that DMI strongly promotes non-uniform magnetization states and non-uniform switching modes of the magnetic layer. It appears to be detrimental for both the thermal stability of the cell and its switching current, leading to considerable deterioration of the cell performance even for a moderate DMI amplitude.