We report an experimental evaluation of the thermal stability factor, Δ, for magnetoresistive random access memory bits with perpendicular magnetic anisotropy and a magnetic free layer diameter of about 20 nm, using magnetic field and spin transfer torque (STT) excitations to accelerate switching. We compare the results to the values obtained by thermal-only excitation. We find that fitting the magnetic switching field distributions to a macrospin model results in identical values of Δ mean and sigma to those determined by direct thermal excitation, also known as the retention bake method. On the other hand, fitting the pulse width dependence of the STT switching voltages in the thermal activation regime can either underestimate or overestimate Δ depending on the choice of exponent ξ that defines the scaling of Δ with STT. We find the best agreement for ξ = 1.45.

We present an experimental study of the size-dependent tunneling magnetoresistance ratio (TMR) and voltage read signal in perpendicular spin transfer torque magnetoresistive random-access (MRAM) memory cells, which shows that the maximum read signal is mostly independent of the size, while TMR decreases with decreasing size. Our analysis shows that this is due to a size-dependent parasitic resistance specific to the nanofabrication process and that the intrinsic $\Delta \text{RA}$ of the cells is size-independent. As a consequence, we show that the electrical diameter of an MRAM cell can be reliably extracted down to sub-20 nm assuming that $\Delta \text{RA}$ does not depend on the cell size.

Atomic-scale spectroscopic imaging of sputtered magnetic tunnel junction structures with a thick oxygen-rich MgO barrier reveals the diffusion of iron and cobalt into the MgO barrier from CoFeB electrodes. First principles calculations are performed to (1) confirm that Fe diffusion through Mg vacancies is energetically favorable, (2) quantify the reduction of interfacial perpendicular magnetic anisotropy due to Fe diffusion into MgO, and (3) predict that the presence of Fe impurities in MgO causes an increased leakage and a tunneling magnetoresistance decrease. Through the chemical shift of the Fe L3 edge and the peak ratio Fe L3/Fe L2 measured by electron energy loss spectroscopy, we suggest that, within MgO, iron with mixed oxidation state Fe2+ and Fe3+ or higher is found in the as-grown structure, which is reduced by annealing to Fe2+. These results indicate that the stoichiometry of as-deposited MgO barrier layers plays an important role in controlling the microstructure and optimizing the performance of magnetic tunnel junctions.

Due to their interesting physical properties, myriad operational regimes, small size, and industrial fabrication maturity, magnetic tunnel junctions are uniquely suited for unlocking novel computing schemes for in-hardware neuromorphic computing. In this paper, we focus on the stochastic response of magnetic tunnel junctions, illustrating three different ways in which the probabilistic response of a device can be used to achieve useful neuromorphic computing power.

Magnetic tunnel junctions (MTJs) provide an attractive platform for implementing neural networks because of their simplicity, non-volatility, and scalability. However, in hardware realizations, device variations, write errors, and parasitic resistance degrade performance. To quantify such effects, we perform inference experiments on a 2-layer perceptron constructed from a 15 x 15 passive array of MTJs, examining classification accuracy and write fidelity. Despite imperfections, we achieve median accuracy of 95.3% with proper tuning of network parameters. The success of this tuning process shows that new metrics are needed to characterize and optimize networks reproduced in mixed signal hardware.

Spin transfer torque magnetoresistive random access memory (STT MRAM) is an alternative to SRAM [1] and Flash [2] , [3] in various embedded applications, as it can provide non-volatility concurrently with low power, high speed operation, and high endurance. In order to become a viable option for replacing DRAM as a stand-alone memory at relevant areal densities, further optimization of MRAM performance is needed, as continued technology scaling and advanced computing systems impose challenging specifications on MRAM in terms of its retention (thermal stability), read-out latencies, and write margins [4] . This talk will present our recent experimental, theoretical, and modeling results on understanding physical mechanisms that affect performance of perpendicular STT-MRAM cells under electrical, magnetic and thermal excitations. In particular, we will report experimental and modeling results that extend of our recent work on electrical self-heating in STT-MRAM [5] to smaller device size, describe our work on optimization of FL materials for improving STT efficiency and thermal stability [6] , present analytical model for calculating energy barrier for domain-wall-mediated magnetization reversal of the perpendicular FL [7] , and show our experimental results which suggest that, contrary to common understanding, fitting the magnetic-field switching probabilities P(H) to a macrospin-reversal model provides approximately correct values of perpendicular magnetic anisotropy field H k on device level.

Interfacial magnetism emerging from orbital hybridization is the key facilitator for practical nanoscale spintronic devices. Most devices require a capping layer, and it is widely assumed that capping-layer variations do not change the fundamental properties of magnetic films underneath. In a model Co-Fe-B/MgO system, interfacial investigations, so far, have been focused on the hybridization of Fe d and O p orbitals. However, the role of the capping layer has largely been ignored by adopting a reductionist scenario of just oxidation or charge modification. Here, we report strong modifications of interfacial magnetism in Co-Fe-B/MgO by systematically changing the Ru capping thickness using element-specific x-ray magnetic circular dichroism and x-ray absorption spectroscopy at Fe L2,3, Co L2,3, and O K edges, along with corresponding spin and orbital magnetic moment calculations and magnetometry measurements. We observe unusual spin-flip-like transitions due to capping layers and find direct evidence for systematic spin and orbital modifications, beyond just charge transformation, strikingly captured by oxygen x-ray absorption and dichroism spectra. Our result shows the importance of the capping layer and provides a complete picture of rich interfacial magnetism in the Co-Fe-B/MgO system.

We report on an experimental study of current induced switching in perpendicular magnetic random access memory (MRAM) cells with variable resistance-area products (RAs). Our results show that in addition to spin transfer torque (STT), current induced self-heating and voltage controlled magnetic anisotropy also contribute to switching and can explain the RA dependencies of switching current density and STT efficiency. Our findings suggest that thermal optimization of perpendicular MRAM cells can result in significant reduction of switching currents.

Summary form only given. Rowhammer is a known security vulnerability in recent Dynamic Random Access Memory (DRAM) devices, where repeated access to an array of memory can flip the bits in the adjacent row owing to the charge leakage/ capacitive coupling. Several studies have documented the Rowhammer effect [1, 2]and Google's project zero demonstrates two working examples of a security exploit [3]. Furthermore, many published scenarios highlight that scaled DRAM below 32nm [1]is exposed to potential hacking attacks because of the Rowhammer problem [3], owing to reduced spacings in DRAM bits. Thus, it is only natural to test any new upcoming technology for a similar problem. Though the working mechanisms of DRAM and spin transfer torque magnetic random access memory (STT-RAM) are drastically different, whether the STT-RAM is also potentially vulnerable to an analogous Rowhammer effect or not is not documented or discussed in literature. While the mechanism of failure is high enough charge leakage in DRAM, the corresponding mechanism in STT-RAM may be the lowering of the thermal barrier because of the dipolar magnetic field exerted by adjacent selected bits. The lowering of the thermal barrier in turn can increase the probability of an erroneous bit flip. To ascertain whether the effect is substantial, nearest (A bit) and next nearest neighbour bits (B bit) adjacent to an unselected bit (O bit) are simulated as shown in Fig 1. The bit diameter is 55nm and the center to center bit spacing is 200 nm. The magnetic field at the O bit is simulated, when all the A and B bits have been assumed to have the magnetization pointing in the same direction. While this configuration does not necessarily conform to the conventional idea of selecting an adjacent row of bits or even both the adjacent rows (double sided hammering), it provides a scenario where the most favorable conditions for an erroneous bit flip can be evaluated in STT-RAM. Three magnetic layers are assumed in each bit separated by appropriate non-magnetic regions corresponding to the STT-RAM stack. The top most layer corresponds to the Free layer (FL) and can be flipped while the others are held fixed. While calculating the magnetic field at the O bit, the middle layer is assumed to have a magnetization opposite to the topmost and bottom layers. The other configuration in which the topmost layers is flipped and is parallel to the middle layer is known to give a smaller magnetic field. A magnetostatic calculation is done to determine the magnetic field at the site of the FL layer of the O bit. The average of the field over the entire volume of the O bit is evaluated as a function of bit spacing using magnetostatic calculations as shown in Fig 2. For the largest bit spacing of 200 nm the simulated field is only around 7 Oe while for a 100nm bit spacing it increases to 60 Oe. The analytical calculation assumes each magnetic layer in each bit to be a point. The vector sum of all of these at the location of the O bit is shown here. The discrepancy between the simulated and the analytical results increases as the bit spacing decreases. This is because of the assumption that each layer is assumed to be a point. To assess the impact of these fields, the string method is used to evaluate the energy barrier between two magnetic states corresponding to 0 or 1 stored on the O bit. Inset of Fig 2shows that for a field of 7 Oe the change in energy barrier is less than 1 kT which is not large enough to cause an appreciable change in the bit error rate. For example based on Eqn 18 of [4] the bit error rate will only double for a 1 kT change in energy barrier assuming a retention time of 10 years and a relaxation time of 1 ns. Even for a bit spacing of 150 nm the increase in the BER will be less than an order of magnitude. However for a smaller bit spacing of 100 nm the BER can go up by 3 orders of magnitude. Therefore, as it stands rowhammer effect in STT-RAM does not appear to be appreciable at the 200 nm bit spacing. However at lower bit spacings the effect might become prominent and would require additional design rules to circumvent. As in the case of DRAM different techniques might have to be adopted to mitigate the problem.

Studies of magnetization dynamics have incessantly facilitated the discovery of fundamentally novel physical phenomena, making steady headway in the development of magnetic and spintronics devices. The dynamics can be induced and detected electrically, offering new functionalities in advanced electronics at the nanoscale. However, its scattering mechanism is still disputed. Understanding the mechanism in thin films is especially important, because most spintronics devices are made from stacks of multilayers with nanometer thickness. The stacks are known to possess interfacial magnetic anisotropy, a central property for applications, whose influence on the dynamics remains unknown. Here, we investigate the impact of interfacial anisotropy by adopting CoFeB/MgO as a model system. Through systematic and complementary measurements of ferromagnetic resonance (FMR) on a series of thin films, we identify narrower FMR linewidths at higher temperatures. We explicitly rule out the temperature dependence of intrinsic damping as a possible cause, and it is also not expected from existing extrinsic scattering mechanisms for ferromagnets. We ascribe this observation to motional narrowing, an old concept so far neglected in the analyses of FMR spectra. The effect is confirmed to originate from interfacial anisotropy, impacting the practical technology of spin-based nanodevices up to room temperature.

Perpendicular Spin-Transfer Torque (STT) MRAM is a promising technology in terms of read/write speed, low power consumption and non-volatility, but there has not been a demonstration of high density manufacturability at small geometries. In this paper we present an unprecedented demonstration of a robust STT-MRAM technology designed in a 2x nm CMOS-embedded 40 Mb array. Key features are full array functionality with low BER (bit error rate), process uniformity and reliability, 10 years data retention at 125C with extended endurance to ∼ 107 cycles. All achieved with standard BEOL process temperatures. Data retention post 260°C solder reflow temperature cycle is demonstrated.

Magnetic skyrmions are nanoscale topological spin structures offering great promise for next-generation information storage technologies. The recent discovery of sub-100-nm room-temperature (RT) skyrmions in several multilayer films has triggered vigorous efforts to modulate their physical properties for their use in devices. Here we present a tunable RT skyrmion platform based on multilayer stacks of Ir/Fe/Co/Pt, which we study using X-ray microscopy, magnetic force microscopy and Hall transport techniques. By varying the ferromagnetic layer composition, we can tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of two and ten, respectively. We thus establish a platform for investigating functional sub-50-nm RT skyrmions, pointing towards the development of skyrmion-based memory devices.

First-generation MRAM, based on a field switching innovation called “Savtchenko switching,” is mass produced by Everspin in densities up to 16Mb.

A physics based switching model for perpendicular magnetic tunnel junction (p-MTJ) device is presented by combining (a) 4×4 tunneling conductance matrix derived using non-equilibrium Green's function (NEGF) formalism and (b) 4×4 ferromagnetic conductance matrix derived using 4 component spin diffusion equation. It provides qualitative as well as quantitative agreement with switching voltages in spin torque experiments and further explains the pattern of asymmetry i.e. Vc, ap-> p < Vc, p-> ap observed in most of the switching experiments and Vc, ap-> p > Vc, p-> ap observed in our measured data. Starting at the materials and phenomena level of abstraction, our model provides a unique quantitative machinery for device engineers to model p-MTJ circuits and architectures.

We conducted micromagnetic simulations to investigate magnetization switching dynamics in ferromagnet/nonmagnet bilayers driven by an in-plane current for 1) free layer with out-of-plane anisotropy, 2) free layer with in-plane anisotropy, and 3) free layer with both in-plane anisotropy and electric-field-controlled out-of-plane anisotropy. The effects of various material parameters on the switching performance are discussed. Based on the simulations, a fast and reliable switching scheme may be realized by using in-plane magnetization switching assisted by an electric-field-controlled out-of-plane magnetic anisotropy. This fast and deterministic switching does not require any external magnetic fields.

Magnetic skyrmions are robust nanoscale spin structures which have recently been discovered within several material systems. Their small size, topological stability, and ease of manipulation offer great promise for next-generation information storage devices. In the past year, several synchrotron-based experiments on multilayer films have revealed stable ~100 nm skyrmions at room temperature (RT). In order to systematically explore the viability of skyrmion-based nanoscale devices, it is timely to develop material systems offering inherent tunability of skyrmion properties. Here we describe the realisation of such a tunable RT skyrmion platform based on multilayer stacks of Ir/Fe/Co/Pt. We confirm the presence of nanoscale Neel skyrmions via established X-ray microscopy techniques, and further utilise magnetic force microscopy and Hall transport - hitherto unused for multilayer skyrmions - to investigate their evolution with magnetic field and sample composition. By varying the ferromagnetic layer composition and thickness, we demonstrate control over skyrmion size, density and ease of nucleation. We thus establish a platform for tuning and investigating properties of sub-50 nm RT skyrmions in a standard laboratory setting.

We study the electric field (EF) effect on MgO/CoFeB/Ta/CoFeB/MgO free layers by varying the thickness of the top MgO layer. The two CoFeB/MgO interfaces oppose the change in magnetic anisotropy from each other and this can be understood by considering the voltage drop as well as the efficiency of the anisotropy modulation from both interfaces. These results are proven by monitoring both coercivity and anisotropy field as a function of the applied EF. From the fit to the model, we show that the bottom CoFeB/MgO interface has a higher EF efficiency than the top interface.

The magnetic-tunnel-junction (MTJ) structure is the core of many important devices, such as magnetic recording head and STT-RAM. CoFeB/MgO/CoFeB tri-layer thin-film stack is a widely researched MTJ structure. In this tri-layer, the functional property of the MTJ, i.e. its TMR ratio, is critically dependent on the crystal orientation of the CoFe grains. In order for the desired (1 0 0) out of plane texture to develop in the CoFeB layers, B needs to be engineered to be expelled out of these CoFeB layers, and diffuse or migrate into the adjacent layers. Ta is usually used as a seed layer adjacent to the MTJ structure. In this work, we investigated the important B-migration mechanisms within this MTJ structure through a combined XPS/TOF-SIMS study. Specifically, we tried to elucidate the possible physical/chemical interactions between the B and Ta that could happen with different film stack designs. Previous works have shown that there might be two possible B-migration mechanisms. One mechanism is direct B diffusion into the adjacent Ta layer during annealing. The other B-migration mechanism is through the formation of TaBOx species, in which B could be carried out by the Ta diffusion. In particular, through studying a series of film stacks, we discussed the circumstances under which one of these B-migration mechanisms becomes dominant. Furthermore, we discussed how these B-migration mechanisms facilitated the B expulsion in a common MTJ structure.

We measure the voltage or electric field (EF) modulated change in anisotropy using two methods on the same nanometer sized device: 1) Directly using the area of the hard axis magnetization loop and 2) Indirectly using the switching field distribution method. Both methods yield similar values of efficiency. With the indirect method, the efficiency derived from the thermal stability was found to be more consistent than that from the anisotropy field. Our data also suggests that memory devices that rely solely on EF effects may benefit from larger device sizes.