Incoherent Switching Research Articles

Magnetization dynamics at finite temperature in CoFeB–MgO based MTJs

The discovery of magnetization switching via spin transfer torque (STT) in PMA-based MTJs has led to the development of next-generation magnetic memory technology with high operating speed, low power consumption and high scalability. In this work, we theoretically investigate the influence of finite size and temperature on the mechanism of magnetization switching in CoFeB–MgO based MTJ to get better understanding of STT-MRAM fundamentals and design. An atomistic model coupled with simultaneous solution of the spin accumulation is employed. The results reveal that the incoherent switching process in MTJ strongly depends on the system size and temperature. At 0 K, the coherent switching mode can only be observed in MTJs with the diameter less than 20 nm. However, at any finite temperature, incoherent magnetization switching is thermally excited. Furthermore, increasing temperature results in decreasing switching time of the magnetization. We conclude that temperature dependent properties and thermally driven reversal are important considerations for the design and development of advanced MRAM systems.

Impact of Reference-Layer Stray Field on the Write-Error Rate of Perpendicular Spin-Transfer-Torque Random-Access Memory

A finite-temperature micromagnetic study of magnetization switching and write-error rates in a perpendicular magnetic tunnel junction with and without synthetic antiferromagnetic layer (SAF) is presented. In the absence of SAF, magnetization switching is induced by domain-wall nucleation and propagation. Although the various modes of domain-wall propagation are observed to delay switching, it does not show an appreciable impact on the overall write-error-rate slopes. In the presence of the nonuniform stray field from the SAF assembly, the domain-wall-based switching modes turn on more complex magnetization dynamics that impedes the switching process. In cases where the SAF layers fail to balance each other contributing to a stronger stray field, incoherent switching modes give rise to metastable states with significantly longer lifetimes, and a dramatic change in the write-error slopes is observed. Simulation results are compared to recent experimental findings from time-domain measurements of spin-transfer-torque switching and measurements of anomalous write-error rates. These results directly prove the long-predicted relation of the SAF stray field to write-error anomaly in perpendicular spin-transfer-torque magnetic random-access memory and could be useful to solve the anomalous write-error problems.

Topologically Protected All‐Optical Memory

AbstractThe in‐memory processor has played an essential role in overcoming the von Neumann bottleneck, which arises from the partition of memory and a processing unit. Although photonic technologies have recently attracted attention for ultrafast and power‐efficient in‐memory computing, the realization of an all‐optical in‐memory processor remains a challenge. This difficulty originates from the contradiction between robustness and sensitivity in wave dynamics, requiring both noise‐immune memory states and modulation‐sensitive transitions between these states. Here, a building block that provides an all‐optical transition between topologically protected memory states is proposed. A nonlinear photonic molecule that satisfies parity‐time (PT) symmetry, revealing multiple oscillation quenching states with different degeneracies determined by PT‐symmetric phases is investigated. In terms of topology for dynamical systems, these quenching states support topologically protected dynamical trajectories suitable for stable memory states. An all‐optical bidirectional transition between these states, which allows incoherent memory switching is demonstrated. The result provides design criteria for all‐optical in‐memory processors with multilevel operations, enabling the classical‐wave counterpart of electronic memristors.

Incoherent Magnetic Switching of L1₀ FePt Grains

In this article, we present a novel micromagnetic model for L10 ordered FePt grains based on their atomic structure. In this model, each monolayer of Fe atoms is represented by a magnetic moment vector and the entire grain consists of a stack of magnetic moments coupled by magnetostatic and nearest-neighbor exchange fields. The grain model enables simulation of incoherent magnetization switching, especially near the Curie temperature. The model is applied to simulate the heat-assisted recording process in granular L10 FePt thin-film media. One of the unique findings is that grains at transition centers could have a domain wall trapped within, provided sufficient grain height. The existence of these grains, whose magnetic moments could be fractional at transition centers, resulting in significant reduction of medium noise. The noise reduction is effectively equivalent to an increase of the number of grains across the read track width. Enhancing and controlling this mechanism by material engineering could lead to significant increase of area density capability, specifically, increase of track density, without decreasing grain size.

In situ study of oxygen and Mg effects on current-induced magnetization switching in Pt/Co bilayers in ultrahigh vacuum

We report on an in situ study of oxygen exposure and Mg dusting effects on current-induced magnetization switching in Pt/Co bilayers via controlled oxygen exposure, Co and Mg deposition, and electrical measurements in ultrahigh vacuum. We show that the oxygen exposure and Mg dusting induce changes in saturation magnetization, effective magnetic anisotropy, spin–orbit torque, and switching current density. From the correlation between these parameters and the switching current density for current-induced switching, we found that the reduction in switching current density by oxygen exposure is caused by the combined effect of enhancement in spin–orbit torque and lowering of the switching energy barrier required for incoherent switching with the latter playing a dominant role in large samples.

Coherent and incoherent switching of cavity solitons in an optically injected VCSEL

Intensity and population dynamics are studied for two different switching techniques used for creation and deletion of cavity solitons (CSs) in vertical cavity surface emitting lasers with external injection. For coherent switching, we use transient Gaussian pulse in and out of phase with the external injection respectively for switching on and off. Studies of the regimes of 10% and 20% above laser threshold revealed that Hopf instability is the cause of more switching energy required for creation of CSs in higher pumping rates. We also propose a novel method for incoherent switching using local injection of carriers in doughnut form for creation of CSs and in Gaussian shape for their deletion.

Voltage pulse induced repeated magnetization reversal in strain-mediated multiferroic nanomagnets: a size- and material-dependent micromagnetic study

Realization of complete magnetization reversal is a major challenge for electrical control of magnetism. This paper focuses on voltage pulse induced magnetization reversal which is mediated by localized strain generated in a two-phase magnetostrictive/piezoelectric multiferroic heterostructure. This paradigm takes advantage of the damping and precessional property of magnetization dynamics, of which the magnetization reversal needs precise control of the voltage pulse. We have performed a micromagnetic study of varied-size elliptical single-domain Ni and Terfenol-D nanomagnets at T = 300 K. By using the actual ground states of the nanomagnets at T = 300 K as the initial states, we obtained the key parameters of the voltage pulse which can induce repeated magnetization reversal. Then we uncovered the mechanism of the size-dependent incoherent magnetization switching of Terfenol-D nanomagnets by a quantitative analysis of the interaction between the exchange energy, demagnetization energy and stress anisotropy energy. These results lay the foundation for the application of strain-mediated magnetization switching technology, and also provide a set of guidelines for signal design and the choice of nanomagnets with proper material and size in the design of specific straintronic circuits.

Cascaded spintronic logic with low-dimensional carbon

Remarkable breakthroughs have established the functionality of graphene and carbon nanotube transistors as replacements to silicon in conventional computing structures, and numerous spintronic logic gates have been presented. However, an efficient cascaded logic structure that exploits electron spin has not yet been demonstrated. In this work, we introduce and analyse a cascaded spintronic computing system composed solely of low-dimensional carbon materials. We propose a spintronic switch based on the recent discovery of negative magnetoresistance in graphene nanoribbons, and demonstrate its feasibility through tight-binding calculations of the band structure. Covalently connected carbon nanotubes create magnetic fields through graphene nanoribbons, cascading logic gates through incoherent spintronic switching. The exceptional material properties of carbon materials permit Terahertz operation and two orders of magnitude decrease in power-delay product compared to cutting-edge microprocessors. We hope to inspire the fabrication of these cascaded logic circuits to stimulate a transformative generation of energy-efficient computing.

Strain-mediated 180° switching in CoFeB and Terfenol-D nanodots with perpendicular magnetic anisotropy

A micromagnetic and elastodynamic finite element model is used to compare the 180° out-of-plane magnetic switching behavior of CoFeB and Terfenol-D nanodots with perpendicular magnetic easy axes. The systems simulated here consist of 50 nm diameter nanodots on top of a 100 nm-thick PZT (Pby[ZrxTi1-x]O3) thin film, which is attached to a Si substrate. This allows voltage pulses to induce strain-mediated magnetic switching in a magnetic field free environment. Coherent and incoherent switching behaviors are observed in both CoFeB and Terfenol nanodots, with incoherent flipping associated with larger or faster applied switching voltages. The energy to flip a Terfenol-D memory element is an ultralow 22 aJ, which is 3–4 orders more efficient than spin-transfer-torque. Consecutive switching is also demonstrated by applying sequential 2.8 V voltage pulses to a CoFeB nanodot system with switching times as low as 0.2 ns.

Magnetic interactions in anisotropic Nd-Dy-Fe-Co-B/α-Fe multilayer magnets

The magnetic properties and the possible interaction mechanisms of anisotropic soft- and hard-magnetic multilayers have been investigated by altering the thickness of different kinds of spacer layers. The metal Ta and the insulating oxides MgO, Cr2O3 have been chosen as spacer layers to investigate the characteristics of the interactions between soft- and hard-magnetic layers in the anisotropic Nd-Dy-Fe-Co-B/α-Fe multilayer system. The dipolar and exchange interaction between hard and soft phases are evaluated with the help of the first order reversal curve method. The onset of the nucleation field and the magnetization reversal by domain wall movement are also evident from the first-order-reversal-curve measurements. Reversible/irreversible distributions reveal the natures of the soft- and hard-magnetic components. Incoherent switching fields are observed and the calculations show the semiquantitative contributions of hard and soft components to the system. An antiferromagnetic spacer layer will weaken the interaction between ferromagnetic layers and the effective interaction length decreases. As a consequence, the dipolar magnetostatic interaction may play an important role in the long-range interaction in anisotropic multilayer magnets.

Dynamic Error in Strain-Induced Magnetization Reversal of Nanomagnets Due to Incoherent Switching and Formation of Metastable States: A Size-Dependent Study

Modulation of stress anisotropy of magnetostrictive nanomagnets with strain offers an extremely energy-efficient method of magnetization reversal. The reversal process, however, is often incoherent and hence error-prone in the presence of thermal noise at room temperature. Occurrence of incoherent metastable states in the potential landscape of the nanomagnet can further exacerbate the error. Stochastic micromagnetic simulations at room temperature are used to understand and calculate energy dissipations and switching error probabilities in this important magnetization switching methodology. We find that these quantities have an intriguing dependence on nanomagnet size: small nanomagnets perform better owing to the fact that they are more resilient to the formation of metastable states and magnetization dynamics in them is more coherent. However, for a fixed stress anisotropy energy density, smaller nanomagnets will also have poorer resilience against thermal instability. Thus, the challenge in straintronics is to maximize the stress anisotropy energy density by developing materials and processes that yield the largest magnetostriction.

Incoherent excitation and switching of spin states in exciton-polariton condensates

We investigate, theoretically and numerically, the spin dynamics of a two-component exciton-polariton condensate created and sustained by non-resonant spin-polarized optical pumping of a semiconductor microcavity. Using the open-dissipative mean-field model, we show that the existence of well defined phase-locked steady states of the condensate may lead to efficient switching and control of spin (polarization) states with a non-resonant excitation. Spatially inhomogeneous pulsed excitations can cause symmetry breaking in the pseudo-spin structure of the condensate and lead to formation of non-trivial spin textures. Our model is universally applicable to two weakly coupled polariton condensates, and therefore can also describe the behaviour of condensate populations and phases in 'double-well' type potentials.

Effects of magnetocrystalline anisotropy and magnetization saturation on the mechanically induced switching in nanomagnets

The effects of magnetocrystalline anisotropy (Ku) and magnetization saturation (Ms) on the mechanically induced switching in nanomagnets are studied using a constraint-free phase field model, which allows explicit magneto-mechanical coupling and strictly constant magnetization magnitude. The effects of Ku and Ms on the transition boundary between the coherent and incoherent switching modes are presented in terms of the nanomagnet geometry. It is found that Ms rather than Ku can affect the transition boundary between the two switching modes. In the coherent mode, there exists a critical strain (εc) to induce a deterministic 90° switching. By using the dynamic nature and overrun behavior of the magnetization, a deterministic 180° switching can occur if the mechanical strain is removed once the magnetization rotates to the largest achievable angle (ϑ1m). For 90° switching, increasing Ku can enhance both εc and ϑ1m, whereas Ms incurs no noticeable changes. For 180° switching, the switching time (ts) increases with Ms linearly, but initially decreases with increasing Ku and then saturates. The results for ts suggest that moderate Ku and Ms are advisable to simultaneously obtain relatively low εc, quick switching, high storage density, and high magnetization-state stability in nanomagnets. This work provides insight on tuning mechanically assisted nanomagnet-based logic and memory devices through Ms and Ku.

Calculation of energy-barrier lowering by incoherent switching in spin-transfer torque magnetoresistive random-access memory

To make a useful spin-transfer torque magnetoresistive random-access memory (STT-MRAM) device, it is necessary to be able to calculate switching rates, which determine the error rates of the device. In a single-macrospin model, one can use a Fokker-Planck equation to obtain a low-current thermally activated rate ∝exp(−Eeff/kBT). Here, the effective energy barrier Eeff scales with the single-macrospin energy barrier KV, where K is the effective anisotropy energy density and V the volume. A long-standing paradox in this field is that the actual energy barrier appears to be much smaller than this. It has been suggested that incoherent motions may lower the barrier, but this has proved difficult to quantify. In the present paper, we show that the coherent precession has a magnetostatic instability, which allows quantitative estimation of the energy barrier and may resolve the paradox.

Magnetic Switching Behavior of Granular Perpendicular Magnetic Media With Different Lateral and Vertical Exchange Coupling

The magnetic switching behavior was measured for granular oxide media containing an exchange control layer (ECL). Investigations of the switching behavior show that the initial minor loop slope represents the degree of incoherent switching in the film. Multilayered granular oxide media with different inter-granular coupling in both lateral and vertical directions, is prepared by varying the oxide grain isolation and ECL thickness. The switching behavior of well isolated oxide grains strongly depends on the thermal stability of the switching segments through the ECL layer. The oxide grains with higher thermal stability vertically decouple at thinner ECL, resulting in a stronger incoherency of switching. In addition, the increase of lateral exchange coupling between oxide grains reduces the vertical decoupling at given ECL condition. For media with a smaller cluster size, the ECL magnetization and thickness has a more pronounced effect on the switching, indicating that fabrication of well isolated grains is important to optimize the media switching mode.

Characteristic field angular dependence of magnetization switching assisted by spin wave excitation

We found the characteristic dependence of magnetization switching assisted by spin wave excitation on the magnetic field angle (θ) for exchange-coupled L10-FePt/Permalloy bilayers, where θ was the angle from the easy magnetization axis of L10-FePt. Without the spin wave being excited, an incoherent switching occurred through the domain wall motion. The frequency for the switching field (Hsw) reduction under the spin wave excitation did not depend on θ. This characteristic dependence is attributable to less variation in the resonance frequencies of the spin waves against θ. We also increased the efficiency of the Hsw reduction further with θ.

Effects of the transverse bias field on the dynamic magnetization switching behavior

The dynamic magnetization switching behavior of a nanostructured magnetic thin film under in-plane magnetic fields is well described by a switching phase diagram composed of two critical curves, one constructed at static and the other at dynamic conditions. The coherent magnetization switching occurs at high magnetic fields above the static critical curve while the incoherent magnetization switching occurs in the region enclosed by the static and the dynamic critical curves. In the incoherent switching region, an abnormal behavior of alternating non-switching/switching that cannot be explained by using the conventional switching phase diagram occurs under a strong transverse bias field. Analytically obtained energy profiles are successfully used to explain this abnormal switching behavior, and the key factor is identified to be the energy difference between the initial state and the saddle point. More extensive and systematic results for the switching phase diagram indicate that the location of the saddle point with respect to the initial state is another important factor affecting the abnormal alternating non-switching/switching behavior.

ChemInform Abstract: Spin‐Transfer Magnetization Switching in Ordered Alloy‐Based Nanopillar Devices

AbstractReview: 55 refs.

An alternate switching/non-switching behavior of a nanostructured magnetic thin film under sub-Stoner-Wohlfarth switching fields

Dynamic magnetization switching can occur under sub-Stoner-Wohlfarth switching fields, and the switching behavior is well described by the switching phase diagram, showing the regions of non-switching, coherent, and incoherent switching. However, in the incoherent switching region, an alternate switching/non-switching behavior is observed under a strong bias field in the transverse direction, invalidating the usual switching phase diagram. This abnormal switching behavior is explained by a damped oscillation motion of the magnetization, which is restricted by the energy well at a given condition.

Spin-transfer magnetization switching in ordered alloy-based nanopillar devices

This paper reviews spin-transfer magnetization switching in ordered alloy-based nanopillar devices. L10-ordered FePt was used for one of the earliest demonstrations of spin-transfer switching in perpendicularly magnetized systems. The behaviour of magnetization switching deviates from the predictions based on a macro-spin model, suggesting incoherent magnetization switching in the system with a large perpendicular magnetic anisotropy. The effect of a 90° spin injector on spin-transfer switching was also examined using L10-ordered FePt. Full-Heusler alloys are in another fascinating material class for spin-transfer switching because of their high-spin polarization of conduction electrons and possible small magnetization damping. A B2-ordered Co2FeAl0.5Si0.5-based device showed a low intrinsic critical current density of 9.3 × 106 A cm−2 for spin-transfer switching as well as a relatively large current-perpendicular-to-plane giant-magnetoresistance (CPP-GMR) up to ∼9%. The specific physical properties of ordered alloys may be useful for fundamental studies and applications in spin-transfer switching.