Magnetic Logic Devices Research Articles

Strain-controlled magnetic domain wall propagation in hybrid piezoelectric/ferromagnetic structures

The control of magnetic order in nanoscale devices underpins many proposals for integrating spintronics concepts into conventional electronics. A key challenge lies in finding an energy-efficient means of control, as power dissipation remains an important factor limiting future miniaturization of integrated circuits. One promising approach involves magnetoelectric coupling in magnetostrictive/piezoelectric systems, where induced strains can bear directly on the magnetic anisotropy. While such processes have been demonstrated in several multiferroic heterostructures, the incorporation of such complex materials into practical geometries has been lacking. Here we demonstrate the possibility of generating sizeable anisotropy changes, through induced strains driven by applied electric fields, in hybrid piezoelectric/spin-valve nanowires. By combining magneto-optical Kerr effect and magnetoresistance measurements, we show that domain wall propagation fields can be doubled under locally applied strains. These results highlight the prospect of constructing low-power domain wall gates for magnetic logic devices.

Current-Induced Switching of Perpendicularly Magnetized Magnetic Layers Using Spin Torque from the Spin Hall Effect

We show that in a perpendicularly magnetized Pt/Co bilayer the spin-Hall effect (SHE) in Pt can produce a spin torque strong enough to efficiently rotate and switch the Co magnetization. We calculate the phase diagram of switching driven by this torque, finding quantitative agreement with experiments. When optimized, the SHE torque can enable memory and logic devices with similar critical currents and improved reliability compared to conventional spin-torque switching. We suggest that the SHE torque also affects current-driven magnetic domain wall motion in Pt/ferromagnet bilayers.

Damage Recovery by Reductive Chemistry after Methanol-Based Plasma Etch to Fabricate Magnetic Tunnel Junctions

The damage recovery process for magnetic tunnel junctions (MTJs) after methanol- (Me-OH) based plasma etch has been demonstrated. Me-OH and O2 plasma, which contain oxygen in the molecule, caused unavoidable modification of magnetic materials in the MTJ stack. For example, the magnetization saturation and MR ratio decreased. H2 base reductive plasma treatment was effective in recovering from this deterioration. No harmful side effects were observed in other aspects of MTJ performance such as MTJ resistance, hysteresis loop offset, and switching field. Heavier initial damage required a longer treatment time for recovery. Other types of reductive chemistry such as NH3 plasma deteriorated the MTJ when the treatment lasted more than 15 s, probably due to nitridation. The use of a highly selective Ar/Me-OH etch process along with He/H2 plasma recovery treatment is very promising for the MTJs' etch process to fabricate high-density magnetic random access memory (MRAM) and non-volatile logic devices.

Characterization of InMnSb epitaxial films for spintronics

Dilute (III, Mn)V ferromagnetic magnetic semiconductors have potential applications in spintronic devices as magnetic field sensors, spin transistors and reconfigurable logic devices. For such applications ferromagnetism at room temperature is a practical requirement. Previous work has shown that In1−xMnxSb films grown on GaAs (100) substrates by atmospheric pressure metalorganic vapour phase epitaxy were single phase and had high temperature ferromagnetism for Mn concentrations up to 2%. Here we present a study of InMnSb thin films at higher Mn concentration (10 at%) showing ferromagnetic properties at room temperature. Aberration corrected and analytical (scanning) transmission electron microscopy techniques were used to study the structure and elemental distribution of the In1−xMnxSb/GaAs system. The crystalline quality of the film and the presence of phase separation is evaluated in the context of a cluster-mediated ferromagnetism model.

Simultaneous magneto-optical Kerr effect and Sixtus-Tonks method for analyzing the shape of propagating domain walls in ultrathin magnetic wires

The controlled nucleation and propagation of magnetic domain walls in ultrathin ferromagnetic wires, such as nanowires and submicrometer wires, is extremely important for the development of new high performance magnetic domain wall logic devices. Therefore, it is equally essential to possess adequate advanced experimental investigation techniques in order to be able to achieve a comprehensive in situ analysis of as many as possible parameters related to the domain wall propagation, e.g., wall shape besides wall velocity and position. In this paper, we report on a method developed specifically for the investigation of the shape of propagating magnetic domain walls in ultrathin magnetic wires, i.e., with the diameter of the magnetic wire in the range 100-950 nm. The newly developed experimental method is based on the simultaneous use of two full-fledged experimental techniques: the magneto-optical Kerr effect for analyzing the surface effects of the passing domain wall and the Sixtus-Tonks method for the investigation of the entire moving wall. The results obtained offer essential information about the shape of the propagating magnetic domain walls, being unique to this new method.

Electric-field control of magnetic domain-wall velocity in ultrathin cobalt with perpendicular magnetization

Controlling the displacement of a magnetic domain wall is potentially useful for information processing in magnetic non-volatile memories and logic devices. A magnetic domain wall can be moved by applying an external magnetic field and/or electric current, and its velocity depends on their magnitudes. Here we show that the applying an electric field can change the velocity of a magnetic domain wall significantly. A field-effect device, consisting of a top-gate electrode, a dielectric insulator layer, and a wire-shaped ferromagnetic Co/Pt thin layer with perpendicular anisotropy, was used to observe it in a finite magnetic field. We found that the application of the electric fields in the range of ± 2-3 MV cm(-1) can change the magnetic domain wall velocity in its creep regime (10(6)-10(3) m s(-1)) by more than an order of magnitude. This significant change is due to electrical modulation of the energy barrier for the magnetic domain wall motion.

Focus on Spintronics and Spin Physics

AbstractIt is our pleasure to bring out this Focus Issue on Spintronics and Spin Physics. Spin is an important aspect of physics, ever since the concepts of intrinsic angular momentum by Samuel Goudsmit and George Uhlenbeck, and spin by Wolfgang Pauli were introduced nearly a century ago to explain the experimental observations. In the field of magnetism the electron spin plays the all‐important central role. While the study of magnetism has a long history, the function of electron spin in electron transport gained interest and importance only in the recent few decades. This is despite the fact that Mott used two spin‐current channels to explain the resistivity behavior in magnetic metals as far back as in the 1930s. Since the eighties, several terms such as magnetotransport and magnetoelectronics, have been used. However, the term spintronics has gained a considerable popularity recently. Professor Albert Fert and Professor Peter Grünberg received their Nobel Prize in 2007 for their discovery of Giant Magnetoresistance (GMR). Read sensors based on the concept of GMR in the late nineties and tunnel magnetoresistance (TMR) since the past several years, are utilized in hard‐disk drive products.The field of spintronics from the early stages of GMR further flourished with the discovery of TMR as well as spin‐torque transfer (STT) effect to switch magnetization or to move domain walls. These led to research in the areas of spin injection and manipulation in semiconductors, magnetic random access memory, magnetic logic devices, domain wall memory, and so on. As a result there are plenty of researchers all over the world pushing the frontiers in applied as well as fundamental aspects of the field.In this Focus Issue we present one Review@RRL on magnetic random access memory (MRAM). This review‐type article provides a perspective on materials with a perpendicular magnetic anisotropy. Pentalemma – a challenging situation to meet five different requirements of the MRAM is described in the paper. A review of current materials, potential candidates, and the challenges are presented.The contributed Letters cover various fields, ranging from fundamentals to device applications; papers on oxide materials discuss topics such as spin‐calorics, defect engineering of ferromagnetism in carbon doped ZnO, zinc ferrites with tunable electrical conductivity, and electronic structure of EuO spin filter tunnel contacts. On the topic of metallic films, the Letters cover FePt films for spintronics applications, Co‐doped FeSi films, and magnetic tunnel junctions with composite free layers. There are several other papers covering interactions between domain walls and magnetic vortex excitation. On the device front, the papers discuss spin‐torque oscillators and domain wall memories.We hope the Focus Issue will bring valuable knowledge for your research and stimulate more activity in the field of spintronics. We plan to have several special issues on spintronics in the future, as exciting results are appearing recurrently opening new areas of research (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Field-coupled computing: Investigating the properties of ferromagnetic nanodots

Focused ion beam irradiation on Co/Pt films is utilized to pattern ferromagnetic nanodots at medium ion dose, whereas low ion dose permits controlled modification of the coercivity. This is demonstrated experimentally and mapped to micro-magnetic simulations. Temperature measurements prove the thermal stability of films and nanodots in an application relevant temperature range. Extraordinary Hall-Effect measurements are performed on a ferromagnetic nanodot with a target size of 250nm in a Hall current device at different temperatures. This verifies the thermal stability and the read-out ability of the magnetic bistable states. Single-domain behavior, the coercivity and the switching field distribution are extracted from the hysteresis loops captured in remanence technique. These properties of ferromagnetic Co/Pt dots fulfill the demands to use the nanomagnets in field-coupled magnetic logic devices.

Three-Dimensional Non-Volatile Magnetic Universal Logic Gates

We present an experimental study on ultra-dense 3-D non-volatile magnetic universal logic gates with reconfigurable AND and OR functions. A four-layer magnetic structure with a net thickness of less than 30 nm is employed as a study illustration of a highly scalable magnetic logic device. In the device, the magnetic state of the top output soft layer depends on the magnetic states of the remaining three hard layers used as two input and one reset layers, respectively. To build vertically oriented magnetic devices with a gradient of the coercivity and the magnetization across the thickness, we use Co/Pd multilayers sputter-deposited via a combinatorial synthesis. Through a focused magneto-optical Kerr effect (F-MOKE) study, we relate input and output states in the 4-layer logic device to shoulders on major and minor M-H hysteresis loops. We use magnetic force microscopy (MFM) to identify magnetic logic operations.

Probing dipole coupled nanomagnets using magnetoresistance read

We experimentally demonstrated magnetoresistance (MR) read of dipole coupled nanomagnets using magnetic tunnel junctions. The MR allowed the magnetic state of individual nanomagnets to be electrically measured. The sensitivity of the read scheme enabled a systematic study regarding the nanomagnet spacing and revealed a transition in behavior. Below a spacing of 15 nm the dipole field overcomes the individual shape anisotropy and redefines the individual element easy axis along the direction transmission line. The demonstration of MR electrical read marks a significant step forward for applications such as magnetic quantum cellular automata logic devices.

Magnetologic devices fabricated by nanostencil lithography

We present magnetic quantum cellular automata (MQCA), fabricated by means ofnanostencil lithography, i.e., using a resistless shadow masking technique in ultra-highvacuum. The nanostencil tool allows the fabrication and in situ investigation of structuresusing atomic force microscopy (AFM) and magnetic force microscopy (MFM). We analyzethe error distribution within the structures to shed light on the performance and challengesof magnetic cellular logic devices. Simulations are performed to corroborate animproved concept for these devices which makes use of fourfold magnetic anisotropy.

Magnetic logic element based on an S-shaped Permalloy structure

Magnetic devices have shown the potential to be used not only as storage elements but also as nonvolatile and programmable logic devices. We present a magnetic logic device element—the S state element—that consists of a single magnetic layer. Its output can be controlled by orthogonal magnetic inputs. The reconfigurable logic element can be easily integrated with common magnetoresistive device concepts, such as spin valves or magnetic tunnel junctions. Using Permalloy as an example, we demonstrate the feasibility of magnetologic operation through micromagnetic simulations.

Giant magnetoresistance effect detection of magnetization reversal in single crystalline nanowires

The controllable and rapid magnetization reversal in nano-scale wires is fundamental to the operation of new magnetic logic and data storage devices. A lot of previous investigations for the single domain wall (DW) dynamics in nano-scale wires have been performed by soft ferromagnetic material such as polycrystalline permalloy with negligible magnetic crystalline anisotropy. In fact, it is vital to understand the DW dynamics within the crystalline anisotropy for not only the fundamental magnetism but also potential applications. The aim of this study is to present the experimental result of magnetization reversal in epitaxial single crystalline nanowires by using giant magnetoresistance effect. The DW at the edge of the wire, and the switching field strongly depends on the crystalline anisotropy.

Magnetic oxide nanowires with strain-controlled uniaxial magnetic anisotropy direction

While magnetic nanowires generally have a preferential magnetization direction along the wire axis to minimize magnetostatic energy, it is shown here for epitaxial magnetic oxide nanowires that substrate-induced strain can be used to tailor the magnetic easy axis in any direction. La0.67Sr0.33MnO3 (LSMO) nanowires were prepared by pulsed laser deposition of LSMO thin films on NdGaO3 (NGO) substrates of two different orientations [NGO(110)o and NGO(010)o], followed by patterning into arrays of nanowires by laser interference lithography. The uniaxial compressive strain from the substrate induces a strong uniaxial magnetic anisotropy in the LSMO that dominates the anisotropy. Hence, one obtains LSMO nanowires having a magnetic easy axis that can lie in any direction, including perpendicular to the wire axis. In marked contrast, similar nanowires on SrTiO3(001) substrates without significant uniaxial strain exhibit the usual preferential magnetization direction along the wire axis, as dictated by magnetostatic shape anisotropy. The tunable magnetic anisotropy direction is a useful feature for applications of magnetic nanowires in magnetic memory, sensor, and logic devices.

Structural and magnetic properties of electrodeposited Cobalt nanowire arrays

Ordered magnetic nanowires have tremendous potential in future magnetic storage and high frequency magnetic logic devices. Here, we present the fabrication of ordered arrays of Cobalt nanowires by electrodeposition through porous polycarbonate membranes. Vertically and horizontally aligned nanowires were produced in presence of an external bias field during post deposition etching of the polycarbonate membrane. Structural and compositional analyses have been carried out to establish the material and structural purity. The magneto-optical Kerr effect was employed to measure the magnetic hysteresis loops for the nanowires assembled in the substrate plane. A good magneto-optical signal to noise ratio is observed with clean ferromagnetic hysteresis loops. The loops measured with external magnetic field applied parallel and perpendicular to the axis of the nanowires show a clear difference in the shape and the coercive field, indicating the effect of shape anisotropy in these samples. Micromagnetic simulations were performed to understand the experimental results and to obtain insight to the magnetization reversal mechanism in magnetic nanowires.

Magnetic logic devices composed of permalloy dots

Magnetic logic devices have been investigated by micromagnetics simulation and experiment. The simulation shows that the magnetic logic devices composed of 4 elliptical permalloy dots perform both the NAND and NOR logic operations. The experiments indicate that the distance between adjacent dots must be shorter than 50 nm and 70 nm along the long axis and the short axis of the dots, respectively. The micro-fabricated test devices show the above logic operations.

Topological Frustration in Graphene Nanoflakes: Magnetic Order and Spin Logic Devices

Magnetic order in graphene-related structures can arise from size effects or from topological frustration. We introduce a rigorous classification scheme for the types of finite graphene structures (nanoflakes) which lead to large net spin or to antiferromagnetic coupling between groups of electron spins. Based on this scheme, we propose specific examples of structures that can serve as the fundamental (NOR and NAND) logic gates for the design of high-density ultrafast spintronic devices. We demonstrate, using ab initio electronic structure calculations, that these gates can in principle operate at room temperature with very low and correctable error rates.

MFM studies of interlayer exchange coupling in Co/Ru/Co films: Effect of Ru layer thickness

Antiferromagnetically coupled magnetic thin films are promising candidates for the design of new magnetic storage and logic devices. The ability to control the interlayer thickness, therefore the magnetic reversal response, of exchange-coupled magnetic layers is of paramount importance in nanotechnology, especially in magnetic sensing element design and applications. In this work, magnetic force microscopy (MFM) with improved sensitivity and high spatial resolution probes was used to obtain a more detailed view of magnetization reversal behavior and domain evolution in the indirect exchange-coupled trilayer system: Co/Ru/Co. The effect of the variable Ru interlayer thickness on the exchange coupling and thus the magnetic domain structure during the ferromagnetic (FM)/antiferromagnetic (AF) coupling transition in Co/Ru/Co films is well demonstrated. The MFM images display a distinct signature of AF coupling for the films with Ru thickness of 0.4 nm. MFM has proven to be an effective tool for detecting FM/AF interlayer coupling and exploring magnetic domain structures in exchange-coupled layered thin films.

Substantial reduction in the depinning field of vortex domain walls triggered by spin-transfer induced resonance

An influence of spin-transfer-induced resonant excitation of a vortex magnetic domain wall on magnetization reversal process was investigated in Permalloy nanowires. Sizable reduction of the depinning field was observed under an application of radio frequency current when the frequency coincided with resonance of the vortex core. The resonant frequency was found to increase by shrinking the wire width. These results lead the authors to conclude that the spin-transfer-induced resonant excitation assists the depinning of the vortex domain walls. This provides an important insight for the realization of novel magnetic memory and logic devices based on current-driven domain wall manipulation.

Controlled depinning of domain walls in a ferromagnetic ring circuit

The authors have investigated dynamics of paired domain walls in a ferromagnetic ring circuit, driven by a variable pulsed magnetic field. The magnetic reversal accompanied by the propagations of the domain walls depends not only on the amplitude but also on the rise time of the applied pulsed magnetic field. The threshold of the rise time is found comparable to the propagation time of a domain wall along a half-path of the ring. This finding may be useful for the high speed operation of the magnetic logic devices based on the domain wall motion.