Single Magnetic Layer Research Articles

Voltage-Driven Versus Current-Driven Spin Torque in Anisotropic Tunneling Junctions

Nonequilibrium spin transport in a magnetic tunnel junction comprising a single magnetic layer in the presence of interfacial spin-orbit interaction (SOI) is studied theoretically. The interfacial SOI generates a spin torque of the form <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</b> = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">||</sub> <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</b> ×( <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</b> ×; <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</b> )+ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</b> × <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</b> , even in the absence of an external spin polarizer. For thick and large tunnel barriers, the torque reduces to the perpendicular component <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> , which can be electrically tuned by applying a voltage across the insulator. In the limit of thin and low tunnel barriers, the in-plane torque <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">||</sub> emerges, proportional to the tunneling current density. Experimental implications on magnetic devices are discussed.

Switching Windows for a Nanostructured Cell of Synthetic Ferrimagnets

The magnetization switching window of nanostructured synthetic ferrimagnets with lateral dimension of 160 nm x 80 nm under combined in-plane magnetic fields along the longitudinal and transverse directions is investigated by numerical calculation using an analytical equation for the total energy. The considered total energy equation precisely accounts for the magnetostatic energy, which is significantly large in nanostructured magnetic cells. Due to the complex magnetization alignment of synthetic ferrimagnets, a different switching criterion based on the reversibility of magnetization process is used, instead of the simple criterion frequently used for single magnetic layers. Synthetic ferrimagnets with various thickness asymmetries are considered, and switching windows are calculated both in static and dynamic conditions. The static switching windows show a smaller dependence on the thickness asymmetry than the dynamic switching windows do. The dynamic switching window at a large thickness asymmetry resembles that of a single magnetic layer. The results are discussed in terms of energy profiles that can be obtained by locating the lowest energy path linking the two stable states from the total energy surface.

Interfacial spin-orbit splitting and current-driven spin torque in anisotropic tunnel junctions

Spin transport in magnetic tunnel junctions comprising a single magnetic layer in the presence of interfacial spin-orbit interaction (SOI) is investigated theoretically. Due to the presence of interfacial SOI, a current-driven spin torque can be generated at the second order in SOI, even in the absence of an external spin polarizer. This torque possesses two components, one in plane and one perpendicular to the plane of rotation, that can induce either current-driven magnetization switching from an in-plane to out-of-plane configuration or magnetization precessions, similar to spin transfer torque in spin valves. Consequently, it appears that it is possible to control the magnetization steady state and dynamics by either varying the bias voltage or electrically modifying the SOI at the interface.

Effects of defects on the switching properties of the nanostructured cells of a single layer and a synthetic ferrimagnet

The switching properties of nanostructured magnetic cells containing defects are investigated by using micromagnetic simulation. A particular emphasis is placed on the comparison of the results for the cells of a single magnetic layer (SL) and a synthetic ferrimagnet (SyF). A different switching behavior is observed in the relatively large cell sizes; the SL cells switch incoherently, but the SyF cells switch coherently, even though the thickness asymmetry of the SyF is quite large. The formation of vortices around the defects is mainly responsible for the incoherent switching in the SL cells, while the formation of a flux-closure structure is behind the reason for the coherent rotation in the SyF cells. At a small cell, suitable for high density magnetic random access memory (MRAM), however, the switching occurs coherently in both the SL and SyF cells. These results indicate that defects, which can be introduced to the cells during nanofabrication, do not significantly affect the switching properties and also the thermal stability of the high density MRAM cells.

Focused ion beam fabrication of spintronic nanostructures: an optimization of the millingprocess

Focused ion beam (FIB) milling has been used to fabricate magnetic nanostructures(wires, squares, discs) from single magnetic layers (Co, permalloy) and spin-valve(permalloy/Cu/Co) multilayers (thicknesses 5–50 nm) prepared by ion beam sputteringdeposition. Milled surfaces of metallic thin films typically exhibit residual roughness, whichis also transferred onto the edges of the milled patterns. This can lead to domain wallpinning and influence the magnetization behaviour of the nanostructures. We haveinvestigated the milling process and the influence of the FIB parameters (incidence angle,dwell time, overlap and ion beam current) on the roughness of the milled surface. It hasbeen found that the main reasons for increased roughness are different sputter yields forvarious crystallographic orientations of the grains in polycrystalline magnetic thin films.We have found that the oblique ion beam angle, long dwell time and overlap < 1 are favourable parameters for suppression of this intrinsic roughness. Finally, we haveshown how to determine the ion dose necessary to mill through the whole thin film upto the silicon substrate from scanning electron microscopy (SEM) images only.

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.

Magnetization dynamics in the presence of pure spin currents in magnetic single and double layers in spin ballistic and diffusive regimes

In this paper we study the spin transport by using the spin-pumping effect in epitaxial magnetic single and double layer film structures. For the magnetic single layer sample we show the spin-pumping-induced interface damping increases and saturates with the Au capping layer thickness. In addition magnetic double layer structures allowed us to investigate both the spin-pump and spin-sink effects. Coupling of pure spin currents to the magnetization via spin-sink effect is studied using time-resolved magneto-optical Kerr effect. These measurements were used to study the propagation of pure spin currents across a Au spacer layer between the two ferromagnets. The propagation of spin momentum density through the Au spacer layer was well described by spin-diffusion equation, which takes into account electron momentum and spin-flip scattering. The spin-diffusion theory was integrated into modified Landau-Lifshitz equations accounting in self-consistent manner for spin-pump/sink mechanism and spin momentum density propagation. Good agreement between theory and experimental data was found.

Differential destructive interference of the circular polarization eigenmodes of scattered soft x rays at the grazing incidence in magnetic thin films

Recently, the authors found that an additional magneto-optical effect that linearly polarized soft x rays incident on a single magnetic layer on a nonmagnetic substrate can be converted to any states among the linearly s- and p- and circularly left- and right-handed polarizations by changing the grazing angle of incidence in specular reflection geometry. In this article, the authors report that the physical origin of such an effect is the differential interference of the circular polarization eigenmodes of scattered soft x rays at the grazing incidence. Totally destructive interference takes place selectively for one helicity but not for the other one at a certain grazing angle and in a specific energy region just below the absorption edges, thus leading to differential circular reflectivity. Numerical calculations using an iterative method of transmission, reflection, and propagation matrices allow us not only to verify the underlying mechanism but also to find the necessary specific conditions of photon energy and incidence angle where such a phenomenon can occur.

Fabrication and Analysis of High-Performance Integrated Solenoid Inductor With Magnetic Core

Integrated solenoid inductors with magnetic core were fabricated and analyzed. An inductance above 70 nH was achieved while keeping the coil resistance below 1 Omega and the device area below 1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> using a solenoid design with a single magnetic layer. The inductance of the magnetic inductor was more than 30 times that of the air core inductor of the identical geometry, and the quality factor of the magnetic inductor was >5. Novel inductor designs and the scalability were also examined, and an inductance density higher than 200 nH/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was obtained. The measured device properties and engineering tradeoffs were well explained by analytical models we developed.

Current-induced magnetization dynamics in single and double layer magnetic nanopillars grown by molecular beam epitaxy

Molecular beam epitaxy is used to fabricate magnetic single and double layer junctions, which are deposited in prefabricated nanostencil masks. For all Co | Cu | Co double layer junctions we observe a stable intermediate resistance state, which can be reached by current starting from the parallel configuration of the respective ferromagnetic layers. The generation of spin waves is investigated at room temperature in the frequency domain by spectrum analysis, demonstrating both in-plane and out-of-plane precessions of the magnetization of the free magnetic layer. Current-induced magnetization dynamics in magnetic single layer junctions of Cu | Cu | Cu has been investigated in magnetic fields, which are applied perpendicular to the magnetic layer. We find a hysteretic switching in the current sweeps with resistance changes significantly larger than the anisotropic magnetoresistance effect.

The Co content and stratification effect on the magnetic properties, microstructure, and phase evolution of [NdFeBNbCu∕Co]∙n thin films

Some results concerning the magnetic and structural properties of [NdFeBNbCu∕Co]∙n thin films in view of their utilization for manufacturing permanent magnets are presented. As compared to Ta∕NdFeBNbCu∕Ta magnetic single layer, the multilayer Ta∕[NdFeBNbCu∕Co]∙3∕Ta films exhibit good hard magnetic properties and an increase in the Curie temperature due to Nb–Cu addition and stratification effect using Co as spacer layer. The influence of the additions content and the stratification effect on the microstructural and hard magnetic properties of multilayer [NdFeBNbCu∕Co]∙n thin films, when the total thickness of NdFeBNbCu layers was 540nm and the thickness of the Co spacer layers was varied successively from 0to6nm for different annealing conditions, are discussed.

Preparation, characterization and magneto-optical investigations of electrodeposited Co/Au films

Au/Co(4–8 ML)/Au single magnetic layers and Au(8 ML)/Co(4 ML)/Au(8 ML)/Co(8 ML)/Au bilayer were sequentially grown by electrodeposition on an Au(1 1 1) buffer layer electrodeposited on Si(1 1 1). The technique used in this work provides full control on the structure and the chemical composition of the different layers (no alloying) as well as on the chemistry at interfaces. scanning tunneling microscopy (STM) and atomic force microscopy (AFM) imaging and X-ray diffraction measurements show that atomically flat continuous Co(0 0 0 1) layers (4–8 ML) can be grown in epitaxy with the Au(1 1 1) substrate and that the 2 nm-thick spacer is also a continuous Au(1 1 1) layer. The Co ultrathin layers (4 and 8 ML) exhibit perpendicular magnetic anisotropy. The lateral magnetic homogeneity and magnetization reversal process have been investigated by scanning magneto-optical Kerr effect (MOKE) magnetometry and global Kerr microscopy. The correlation between magnetization switching behaviour in each layer of the Co-bilayer stack has been evidenced from in-depth sensitive MOKE measurements and microscopy. The strong coupling observed between the two Co layers is attributed to magnetostatic interaction at domain wall boundaries.

Fully epitaxial, exchange coupled SmCo5/Fe/SmCo5 trilayers

Fully epitaxial SmCo5(25 nm)/Fe/SmCo5(25 nm) trilayer films were prepared, in which the easy magnetization axis of both hard magnetic SmCo5 layers is uniquely aligned along one orientation within the film plane. The interlayer exchange coupling between the hard and soft magnetic phase is studied by hysteresis measurements in the magnetic hard and magnetic easy orientation and by dc-demagnetizing loops for different thicknesses of the soft Fe layer. The good coupling together with the optimum texture provided by the epitaxial growth lead to excellent magnetic properties. Fully coupled trilayers are achieved for 6 nm thin Fe layers, which maintain a square hysteresis with high coercivity of μ0Hc = 1.5 T and a 30% remanence enhancement over a single hard magnetic layer. For thicker Fe layers (16 nm) the exchange coupling leads to reversible rotation processes within the soft phase before the trilayer switches irreversibly at a field of 0.9 T. The still acceptable coercive field and the further improved remanence lead to a maximum energy density of 224 kJ m−3 (28 MG Oe). The specific epitaxial relation between SmCo5 and Fe with its unique orientation of the SmCo5 easy axis throughout the whole layer stack presents the essential step in the realization of thick, textured, exchange coupled multilayers with high energy density.

Current-induced switching in single ferromagenetic layer nanopillar junctions

Current-induced magnetization dynamics in asymmetric Cu∕Co∕Cu single magnetic layer nanopillars has been studied experimentally at room temperature and in low magnetic fields applied perpendicular to the thin film plane. In sub-100nm junctions produced using a nanostencil process a bistable state with two distinct resistance values is observed. Current sweeps at fixed applied fields reveal hysteretic and abrupt transitions between these two resistance states. The current induced resistance change is 0.5%, five times greater than the anisotropic magnetoresistance effect. We present an experimentally obtained low field phase diagram of current-induced magnetization dynamics in single ferromagnetic layer pillar junctions.

Effects of current on the magnetic states of permalloy nanodiscs

We demonstrate that both the vortex and the single domain magnetic configurations of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ circular nanodiscs can be achieved by the application of magnetic field or current flowing perpendicular to the disc plane. The magnetic configurations of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕\mathrm{Cu}∕{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ trilayers have been determined by the response to a small current via the giant magnetoresistance. In addition, we report detection of the vortex state in a single magnetic layer, by exploiting a magnetoresitance arising from the suppression of spin-accumulation in the vortex state. Our analysis shows that this magnetoresistance effect becomes increasingly significant in small nanostructures.

Anisotropy enhanced dual magnetic layer media design for high-density perpendicular recording

An anisotropy enhanced dual magnetic layer structure is proposed to improve the recording performance of perpendicular media. The initial layer is made of low noise Co-Cr-Pt-O layer and the top layer is made of higher anisotropy Co-Cr-Pt-B layer. A higher nucleation field than the single layer structures is obtained for the dual magnetic layer media. By optimizing the layer structure and properties, better signal-to-noise and lower adjacent track erasure can be achieved for the dual magnetic layer design over single magnetic layer media.

Switching field distribution in magnetic tunnel junctions with a synthetic antiferromagnetic free layer

By replacing the traditional single magnetic free layer in magnetic random access memory (MRAM) with a synthetic antiferromagnetic (SAF) layer composed of two antiferromagnetically coupled layers, the possibility of the kink generation has been suppressed to a minimum level and the normalized free-layer shift has been reduced from 15% to less than 5%. These phenomena are thought to be from the reduction of the effective thickness and magnetic moment by introducing a SAF free layer. Since the SAF free layer decreases the magnetostatic interaction with the pinned layer, free-layer shift is also decreased. A SAF free layer forms a closed magnetic loop between two antiferromagnetically coupled layers. Therefore, it increases a tendency to become a single domain and its switching field distribution is enhanced. With the optimized SAF free layer, array quality factor [AQF, σ(Hc)∕Hc] is increased to more than 10 and switching window could be obtained which is the area to be selectively switched in the memory cell array. When adopted in a 64-kb MRAM, it is confirmed that cells can be selectively switched without the disturbance to the other cells.

Bipolar high-field excitations inCo∕Cu∕Conanopillars

Current-induced magnetic excitations in Co/Cu/Co bilayer nanopillars ($\sim$50 nm in diameter) have been studied experimentally at low temperatures for large applied fields perpendicular to the layers. At sufficiently high current densities excitations, which lead to a decrease in differential resistance, are observed for both current polarities. Such bipolar excitations are not expected in a single domain model of spin-transfer. We propose that at high current densities strong asymmetries in the longitudinal spin accumulation cause spin-wave instabilities transverse to the current direction in bilayer samples, similar to those we have reported for single magnetic layer junctions.

Soft x-ray resonant magneto-optical Kerr effect as a depth-sensitive probe of magnetic heterogeneity: A simulation approach

We report a noticeable depth sensitivity of soft x-ray resonant magneto-optical Kerr effect able to resolve depth-varying magnetic heterostructures in ultrathin multilayer films. For various models of depth-varying magnetization orientations in an ultrathin Co layer of realistic complex layered structures, we have calculated the Kerr rotation, ellipticity, intensity spectra versus grazing incidence angle ϕ, and their hysteresis loops at different values of ϕ for various photon energies hν’s near the Co resonance regions. It is found from the simulation results that the Kerr effect has a much improved depth sensitivity and that its sensitivity varies remarkably with ϕ and hν in the vicinity of the resonance regions. These properties originate from a rich variety of wave interference effects superimposed with noticeable features of the refractive and absorptive optical effects near the resonance regions. Consequently, these allow us to resolve depth-varying magnetizations and their reversals varying with depth in a single magnetic layer and allow us to distinguish interface magnetism from the bulk properties in multilayer films. In this paper, the depth sensitivity of the Kerr effect with an atomic-scale resolution is demonstrated and discussed in details in several manners with the help of model simulations for various depth-varying spin configurations.

Characteristics of thin film inductors using magnetic multilayered films with ceramic intermediate layers

Circular spiral thin film inductors were fabricated using as-deposited magnetic multilayered films with ceramic intermediate layers, and a thin film inductor with a single magnetic layer was also fabricated for comparison. The magnetic multilayered films have good soft magnetic properties and high resistivity thus can decrease the eddy current loss. Employing the magnetic multilayered films, we got a higher quality factor (>30) and better inductor characteristics as compared with the inductor with a single magnetic layer, such as a high operating frequency (>80 MHz) without decreasing the inductance.