Thin Magnetic Layer Research Articles

Bubble Wrap for Bullets: The Stability Imparted by a Thin Magnetic Layer

There has been significant recent work which examines a situation where a thin magnetic layer is `draped' over a core merging into a larger cluster; the same process also appears to be at work at a bubble rising from the cluster centre. Such a thin magnetic layer could thermally isolate the core from the cluster medium, but only if the same shear process which generates the layer does not later disrupt it. On the other hand, if the magnetized layer can stabilize against the shear instabilities, then the magnetic layer can have the additional dynamical effect of reducing the shear-driven mixing of the core's material during the merger process. These arguments could equally well to underdense cluster bubbles, which would be even more prone to disruption. While it is well known that magnetic fields can suppress instabilities, it is less clear that a thin layer can suppress instabilities on scales significantly larger than its thickness. Here we consider the stability imparted by a thin magnetized layer. Such a layer can have a significant stabilizing effect even on modes with wavelengths much larger than the thickness of the layer l; to stabilize modes ten times larger requires only that the Alfv\'en speed in the magnetized layer is comparable to the relevant destabilizing velocity -- the shear velocity in the case of pure Kelvin-Helmholtz like instability, or a typical buoyancy velocity in the case of pure Rayleigh-Taylor. We confirm our calculations with two-dimensional numerical experiments using the Athena code.

Origin of incoherent magnetic switching behavior in CoCrPt–SiO2 perpendicular magnetic recording media

Magnetic switching behavior for CoCrPt–SiO2 single layer media with oxygen contents (OCs) from 4% to 10% and recording layer thickness (tmag) from 2to27nm is investigated. Significant incoherent switching, caused by domain wall motion, and large activation volume are observed at tmag⩽6nm regardless of OC, consistent with nonuniform microstructure with less grain isolation at tmag=4nm. These results support formation of a highly exchange-coupled thin magnetic layer on top of Ru interlayer, which can explain the large difference between angular dependence of Hcr predicted by the Stoner-Wohlfarth theory and the experimental values.

Fabrication of Magnetic Tunneling Junctions with NaCl Barriers

We fabricated magnetic tunneling junctions (MTJs) with epitaxial NaCl(001) barriers grown on Fe(001). Thin magnetic layers were grown using a vacuum deposition system and junction patterns were formed using shadow masks installed in the deposition system. We confirmed that a barrier layer of NaCl(001) grows epitaxially on an Fe(001) bottom electrode layer but that the top layer of Fe is polycrystalline. The MTJs show nonlinear current–voltage characteristics and tunneling magneto-resistance (TMR) of ∼3% at room temperature. The observed TMR ratio is smaller than that expected for an epitaxial barrier with a fourfold symmetric crystal structure. Although the reason for this large discrepancy in TMR ratio between the MTJs with MgO barriers and those with NaCl barriers is unclear at this stage, novel materials such as a family of alkali halides have the potential to function as an applicable barrier for MTJs.

Layer-thickness dependence in tunneling magnetoresistance and current-driven magnetization reversal using GaMnAs-based double-barrier structures

We have investigated the layer-thickness dependence on tunneling magnetoresistance (TMR) in double-barrier structures using GaMnAs-based magnetic tunneling junctions in order to clarify the origin of low threshold current density for current-driven magnetization orientation reversal. The TMR characteristics are well explained by assuming the spin configurations due to the difference in coercive force in each magnetic layer. Using the thin middle magnetic layer, current-driven magnetization reversal in low current density and spin accumulation are realized.

Evidence of modified ferromagnetism at a buried Permalloy/CoO interface at room temperature

We have used magnetometry and resonant soft x-ray magnetic reflectometry to determine the depth-dependent charge and magnetization density on an absolute scale across a Permalloy/CoO interface above the N\'eel temperature of CoO. A thin magnetic layer of $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ forms at the interface. This layer has larger magnetization density and different temperature dependence of magnetization than Permalloy.

Effect of Magnetic Layer on the Microstrip-excited Rectangular Dielectric Resonator Antennas Bandwidth

A rectangular dielectric resonator antenna (DRA) fed by a microstrip line is studied. Field coupling between the feed-line and the DR is one of the main specifications of this structure, especially when an air gap exists in the actual structure. In this paper a thin magnetic layer (μr > 1) between the microstrip and the main body of the DRA is introduced. This layer significantly compensates the undesirable effect of the air gap. Then, the effect of additional layers in coupling improvement, and consequently in bandwidth enhancement, is assessed.

Electronic transport in the multilayers with very thin magnetic layers

Metallic magnetic multilayers with very thin discontinuous magnetic layers are considered. Existence of the windows (strips) in magnetic layers leads to the additional scattering mechanism for the electrons in non-magnetic spacer. Transition from antiparallel to parallel ordering of magnetic moments in nearest magnetic layers versus external magnetic field causes the change of boundary condition for the wave function of electrons in the spacer. It has to increase the magnetoresistance in such structures as compared to the multilayers without windows.

The Feasibility of$+15$Gb/in$^2$High-Density Recording With Barium-Ferrite Particulate Media and a GMR Head

The feasibility of a high-density magnetic particulate recording medium with a thin magnetic layer of evenly dispersed high-coercivity barium-ferrite particles was investigated. Thermal stability, writability, and an areal recording density of 17.5 Gb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> were confirmed for a flexible disk system using a giant magnetoresistive head

Electronic quasiparticles and evolution of Fermi level spin states in thin magnetic layers

Here we report on high-resolution photoemission of iron layers grown on a W(1 1 0) substrate. The evolution of the substrate states upon sub-monolayer adsorption of Fe atoms leads to a shift in surface state binding energy. For thicker (1 1 0) films, sharp metallic surface states are obtained. Their dispersion displays the signature of quasiparticle renormalization due to dressing with excitations. The energy scale is characteristic for the spin wave spectrum in iron, thereby giving evidence of electron–magnon coupling. Furthermore, it is found that quantum well states occur as a function of layer thickness. These modify the spin density of states at the Fermi level in the ferromagnetic film.

Spin dynamics due to spin-transfer in magnetic spin valves

We consider current-induced dynamics of a thin magnetic layer in a spin-valve structure, described by the Landau-Lifshitz-Gilbert equation extended by including the torque due to spin transfer from conduction electrons. The spin torque is calculated in terms of a macroscopic model based on the classical diffusion theory. The approach allows to analyze the current-driven magnetic switching, in particular the critical switching currents and dynamical steady states in an external magnetic field.

Fabrication of FePt/FeCo/FePt exchange-spring trilayer with very thin FeCo Layer for high-resolution MFM tips

In this work, a diffusion barrier layer is introduced for fabricating FePt/FeCo/FePt exchange-spring films with very thin soft layer. The hard magnetic properties of the films are investigated. The FePt/C/Ta/FeCo/Ta/C/FePt multilayer films are deposited on oxidized Si substrates using RF magnetron sputtering method and are annealed at 873, 923, and 973 K using rapid thermal annealing to produce the FePt L1/sub 0/ ordered hard layer. The Ta/C layers, with thickness x nm, are introduced for intermediate layers between soft and hard magnetic layers to prevent interface alloying during annealing. Results show that annealing gives hard magnetic properties to the films due to the ordering of FePt. The film with x=0 shows one-stage magnetic reversal and the H/sub c/ is about 3 kOe. The sharp peak of reversible magnetization indicates the evidence of exchange coupled soft layer with hard layer. Increasing the reversible magnetization at H/sub c/ increases the soft layer volume. Results from Auger electron spectroscopy indicate that the thin FeCo soft magnetic layer remained although the sample is annealed at high temperature. This suggest TaC particles are formed at the interface and behave as good diffusion barriers without degradation of exchange-spring magnetic property.

Activation volume and interaction of metal particulate media

We have investigated the activation volume ( V ac ) and magnetostatic interaction of metal particulate (MP) media. The activation volume of MP media decreases with the decrease of physical volume ( V phy ) of metal particles. The activation volume and the ratio of V phy /V ac of advanced metal particles are 6×10 −24 m 3 and 1.5, respectively. It can be predicted that the physical volume of metal particle is about 3×10 −24 m 3 when the physical volume is equal to the activation volume. This value is agreement with the practical lower limit of physical volume of metal particle predicted by Sharrock. The negative interaction (demagnetization effect) in MP media decreases with low saturation magnetization of the metal particles, a thin magnetic layer, a high orientation of MP media, and a low packing fraction of metal particles in the MP media. The activation volume of the MP media decreased as the negative interactions decreased. In advanced MP media with low M r· t ( M r=remanent magnetization and t=thickness), the influence of interaction on the activation volume is reduced.

Exploration of the ultimate patterning potential achievable with high resolution focused ion beams

Controlled and reproducible fabrication of nano-structured materials will be one of the main industrial challenges in the next few years. We have recently proposed exploitation of the nano-structuring potential of a high resolution Focused Ion Beam Tool, to overcome basic limitations of current nano-fabrication techniques. The aim of this article is to present some new routes for material patterning, which benefit from ion-induced local property modifications or damage. In the experiments we describe hereafter an ultra-sharp pencil of 30 keV gallium ions is used to tailor the characteristics of several materials at a scale of a few nanometres. The experimental results are then compared to simulations. First, we simulate the control of collisional defects generated in a thin magnetic layer under FIB irradiation. The results explain the stable magnetic structures we have obtained experimentally. This was achieved with a low surface ion dose (1012 to 1014 ions/cm2). In addition we have explored the promising direction of “Bottom-up” or “self-organization” processes using a FIB instrument. We have defined artificial surface defects. These defects created by the impact of an 8-nm FWHM probe were used to pin the diffusion and to organize nanometre-sized gold clusters on a graphite surface.

R-modes of a neutron star with a magnetic dipole field

We study r-modes of a rotating magnetized neutron star, assuming a magnetic dipole field whose axis is aligned with the axis of rotation. We approach the problem by applying a singular perturbation theory to the oscillations of rotating stars. In this treatment, we divide the star into a thin surface magnetic layer and a non-magnetic core. We integrate linearized ideal magnetohydrodynamic (MHD) equations in the surface magnetic layer and non-magnetic oscillation equations in the core, and match the two integrations at the interface to obtain a complete solution. For a polytropic neutron star model of mass M = 1.4 M○. and radius R = 10 6 cm, the magnetic dipole field becomes effective on the modal properties of the r-modes only when the field strength Bs is much greater than 10 14 G. We also find that the damping effects caused by very short magnetic perturbations in the surface layer are not important for the r-mode instability of rapidly rotating neutron stars if the field strength Bs is smaller than 10 12 G.

Morphology and magnetic properties of ECR ion beam sputtered Co/Pt films

We report the fabrication of magnetic films using electron cyclotron resonance ion beam sputtering. The films consist of a Pt buffer layer, a thin magnetic Co layer and a Pt cap. Varying the thickness of the layers, we can tune the magnetic properties and a uniaxial perpendicular magnetic anisotropy can be obtained. Materials like amorphous silicon oxide, amorphous carbon, glass, mica and potassium bromide have been used as substrates. We have analyzed the structure of Co/Pt films on amorphous carbon by means of transmission electron microscopy. The analysis reveals that the buffer layers are polycrystalline with a (1 1 1)-texture, which increases with Pt thickness. The average grain-size is linearly correlated with the total thickness of the Pt buffer layer. While Co grows predominantly in its HCP stacking with c -axis aligned parallel to the surface normal, FCC grains with a strong (1 1 1) texture are also found. Scanning tunneling microscopy investigations reveal an average surface roughness of the Pt buffer layer of 3.0 Å . For buffer thickness ⩾ 60 Å we find similar magnetic properties for all Co films, not depending on the substrate materials. Co films of 4–8 Å thickness exhibit a perpendicular easy axis of magnetization, while a spin reorientation transition is found in a thickness range of 8–13 Å. Above 13 Å an in-plane easy axis of magnetization is obtained.

記録再生シミュレーションによるキーパー媒体の熱安定性評価

A read/write simulation based on the Poisson equation by a finite element method was performed on keepered media (KM) with a thin soft magnetic layer (keeper layer) under a recording layer and a thin-film head. An extendedn Stoner-Wohlfarth (SW) model was used for the recording layers, in witch the switching rate of each SW particle at finite temperatures is taken into account using the Neel-Arrhenius law and the master equation.Thermal stability of the medium was estimated by the recorded magnetizations and the corresponding output voltage waveforms for 423 kBPI calculated at 300 K as a function of time t after the recording. Although the output voltage of the KM was slightly less than that of the conventional medium due to magnetic flux circulation through the keeper layer, the output voltage reduction rate due to thermal fluctuation decreased remarkably with a thin keeper layer.

Giant Magnetoimpedance Effect in Surface Modified CoFeMoSiBAmorphous Ribbons

Thin magnetic Fe layers in thickness of 10–240 nm were deposited ontoa wheel surface of CoFeMoSiB amorphous ribbons to check our concept ofa new type of heterogeneous magnetoimpedance materials formed by twodifferent magnetic parts. The presence of an additional iron layermodifies the magnetoimpedance response of the composite material andleads to increase of the magnetoimpedance ratio from 330 to 345% at afrequency of 3.5 MHz. Two possible mechanisms are discussed forexplanation to the observed behaviour. Modification of the surfaceproperties of the amorphous ribbons may have certain potential fortechnological applications.

High anisotropy CoCrPt(B) media for perpendicular magnetic recording

CoCrPt(B) media with high magnetic anisotropy have been fabricated at a thin magnetic layer thickness (10 nm) and a thin interlayer thickness (4 nm). The hard magnetic properties of the CoCrPt enable addition of boron, which aids to magnetically decouple the grains. These media are thermally stable and have an enhanced signal-to-noise ratio.

Near-field imaging of ultrathin magnetic films with in-plane magnetization.

A new approach to near-field magneto-optical imaging was developed capable of visualization of in-plane magnetization of ultrathin magnetic structures. The approach relies on the magneto-optical effect specific for thin magnetic layers and employs near-field transmission measurements of longitudinal and/or transverse magneto-optical effect arising from the presence of thin film interfaces. The near-field magneto-optical contrast of in-plane domain structure of ultrathin Co film has been demonstrated in different polarization configurations.

Imaging magnetic domain structures with soft X-ray microscopy

The current achievements in magnetic transmission soft X-ray microscopy will be reviewed. The magnetic contrast is given by X-ray magnetic circular dichroism (X-MCD), i.e., the dependence of the absorption coefficient of circularly polarized X rays on the projection of the magnetization in a ferromagnetic system onto the photon propagation direction. X-MCD contrast can reach, e.g., at L2,3 edges in transition metals, large values up to 50%. Combined with a soft X-ray microscope where Fresnel zone plates acting as optical elements provide a lateral resolution down at 25 nm, it allows for imaging magnetic microstructures. Specific features of this photon-based technique are the recording of images in varying external magnetic fields, an inherent chemical specificity, a high sensitivity to thin magnetic layers, due to the large contrast, and the possibility to distinguish between in-plane and out-of plane contributions. In this report, recent results obtained with the XM-1 microscope at the ALS (Berkeley/CA) demonstrate the broad applicability of this novel experimental technique to both fundamental and technological relevant issues in nanomagnetism. The future potential will be briefly outlined.