Flux flow in type II superconducting wires in longitudinal magnetic fields

The resistive behavior of a type II superconducting wire in the presence of a longitudinal applied magnetic field is studied. The experimental data obtained are compared with the predictions of a phenomenological model based on viscous flux flow. For appropriate surface conditions the model is in satisfactory qualitative agreement with observations. The problems involved in obtaining a numerical value for the flux flow resistivity from longitudinal field I–V data are discussed. Estimates of the flux flow resistivity are made and are shown to be in order-of-magnitude agreement with accepted values.     

Selected characteristics of plated wires from various sources

Plated wires, 2.5 and 5.0 mils in diameter, have been obtained from various sources and compared to our own 2.5 mil diameter wire made on both copper-beryllium and tungsten substrates. The wires have been tested for their digit, word, and shmoo windows, pulse output and uniformity of output, skew and magnetostriction and their uniformity, the tension and torsion stress changes in the above characteristics, the thickness of the magnetic layer, stability during accelerated aging, and their mechanical characteristics. The characteristics of the wires in the unstressed condition were all adequate for memory application. Differences in signal output and location of the operating windows could be correlated with differences in film thickness and wire diameter. Pulse signal fluctuations were found to be directly proportional to magnetostriction fluctuations. There were great differences observed in the behavior of the samples under stressed conditions. These differences were shown to arise from differences in the local fluctuations in magnetostriction, the modulus and diameter of the substrate, and differences in the metallurgical and magnetic structure of the permalloy film. Accelerated aging showed 2:1 differences in rate between some samples.     

Effect of external magnetic fields on shaped-charge operation

This paper deals with electromagnetic actions that allow one to control the shaped-charge effect at different stages of shaped-charge operation. A decrease in penetration of the shaped-charge jet is attained by production of an axial magnetic field in the shaped-charge liner immediately before shot, and production of a magnetic field in the conducting target material that is transverse to the direction of jet propagation. The considerable decrease in the penetration capability of the charge observed in the experiments is attributed to a sharp amplification of the magnetic field in the jet formation region upon liner collapse. The effects accompanying the “field pumping” prevent the normal formation of a shaped-charge jet. A model of magnetic field generation in a conducting medium subjected to high-velocity deformation with particle elongation along the magnetic flux lines is considered. X-ray photographs of shaped-charge jets formed from a “magnetized liner” are given. Theoretical calculations have shown that deformation conditions that can provide for intense magnetic field generation also arise during high-velocity jet penetration into a conducting target across the flux lines of the initial magnetic field previously produced in the target.     

Micromagnetic study of the switching fields in polycrystalline magnetic thin-film media

In this paper, we use micromagnetic simulation to study the switching field reduction and the switching field distribution in polycrystalline thin-film media. We first show that, for the coupled magnetic particles, the switching field of the high-anisotropy particle is reduced as a result of the exchange coupling with the low-anisotropy particle. The switching field reduction increases almost linearly with the anisotropy difference between the coupled particles. An optimal exchange exists that gives a maximum switching field reduction. This switching field reduction is more severe for a small applied field angle; therefore, the angular sensitivity of the switching field is reduced compared to the Stoner-Wohlfarth relation. Detailed micromagnetic simulations also show that in polycrystalline thin-film media, because of the variation and the reduction of the magnetic properties at the grain boundaries, the media coercivity can be reduced significantly. The primary reason for this switching field reduction is the exchange interaction between the grain boundary and the grain center region.     

Quasi-static crack tip fields in rate-sensitive FCC single crystals

In this work, the effects of loading rate, material rate sensitivity and constraint level on quasi-static crack tip fields in a FCC single crystal are studied. Finite element simulations are performed within a mode I, plane strain modified boundary layer framework by prescribing the two term (K − T) elastic crack tip field as remote boundary conditions. The material is assumed to obey a rate-dependent crystal plasticity theory. The orientation of the single crystal is chosen so that the crack surface coincides with the crystallographic (010) plane and the crack front lies along [10[`1]][10\overline 1] direction. Solutions corresponding to different stress intensity rates [(K)\dot]\dot{{K}}, T-stress values and strain rate exponents m are obtained. The results show that the stress levels ahead of the crack tip increase with [(K)\dot]\dot{{K}} which is accompanied by gradual shrinking of the plastic zone size. However, the nature of the shear band patterns around the crack tip is not affected by the loading rate. Further, it is found that while positive T-stress enhances the opening and hydrostatic stress levels ahead of crack tip, they are considerably reduced with imposition of negative T-stress. Also, negative T-stress promotes formation of shear bands in the forward sector ahead of the crack tip and suppresses them behind the tip.     

Quasi-static and dynamic switching of exchange biased micron-sized TMR junctions

We study the quasi-static and dynamical switching of magnetic tunnel junction patterned in micron-sized cells with integrated field pulse line. The tunnel junctions are CoFe/AlO/CoFe with an exchange biasing layer of MnIr. Quasi-static characterizations have been used to determine anisotropy, coercive as well as exchange bias fields. Dynamic switching measurements are done by applying fast-rising magnetic field pulses (178 ps–10 ns) along the hard axis of the junction with a quasi-static easy-axis applied field. We identify the field conditions leading to no-switching, to direct-writing and to toggle switching. We identify these field conditions up to the precessional limit, and construct the experimental dynamical astroïd. The magnetization trajectories leading to direct-writing and to toggle switching are well described by macrospin simulations.     

Nonuniform electric fields in pulse heated wires

The influence of nonuniform electric fields on the measurement of the electrical conductivity in pulse-heated wires is studied analytically. Two causes for nonuniformity are considered: switching-on of an external voltage source (transient skin effect) and temperature-dependent change of the electrical conductivity. This problem usually has two strictly different time scales: a short one, on which the transient skin effect takes place, and a long one, on which heat conduction happens. Here, the short time scale is considered.Paper presented at the Third Workshop on Subsecond Thermophysics, September 17–18, 1992, Graz, Austria.     

Faster magnetic walls in rough wires

In some magnetic devices that have been proposed, the information is transmitted along a magnetic wire of submicrometre width by domain wall (DW) motion. The speed of the device is obviously linked to the DW velocity, and measured values up to 1 km x s(-1) have been reported in moderate fields. Although such velocities were already reached in orthoferrite crystal films with a high anisotropy, the surprise came from their observation in the low-anisotropy permalloy. We have studied, by numerical simulation, the DW propagation in such samples, and observed a very counter-intuitive behaviour. For perfect samples (no edge roughness), the calculated velocity increased with field up to a threshold, beyond which it abruptly decreased--a well-known phenomenon. However, for rough strip edges, the velocity breakdown was found to be suppressed. We explain this phenomenon, and propose that roughness should rather be engineered than avoided when fabricating nanostructures for DW propagation.     

AC loss characteristics of YBCO conductors carrying transport currents in external AC magnetic fields

In this work we present dependence of AC losses in a YBCO tape conductor carrying an AC transport current in an external AC magnetic field on the angle between the direction of the external field and the tape face of the conductor. The losses were measured by a calorimetric method whose validity was proven by simultaneous electric and calorimetric measurements of the losses in the same part of the conductor. The experiment was conducted at 77 K. The measured data show that the AC losses are dominated by the normal component of the external magnetic field to the tape surface. It is also shown that the AC losses sharply drop when the external magnetic field is parallel to the tape surface, which is due to extremely high aspect ratio of the YBCO conductor.     

Calculations of external bit fields

The amplitude and distribution of the external fields emanating from recorded transitions are of great interest to technologists in the field of information storage and retrieval and, more particularly, to those studying alternate readout techniques to the conventional inductive readout method. Such areas include the magnetic transfer process, Hall probe techniques, and the magneto-optic and Lorentz beam readout techniques. In this paper we used a harmonic analysis simulation of the non-linear digital recording process, and calculated the amplitude and distribution of external bit fields for a wide range of media magnetic properties and thicknesses, and as a function of bit density and spacing above the surface of the media. Justification for using the harmonic analysis model is based upon the good agreement obtained with the reported spatial behavior of the external bit fields measured directly by a scanning electron microscope. In addition, calculations based on simpler models of a line, step, and linear transition show that the harmonic model is a superior first-order approximation of the nonlinear nature of digital magnetic recording.     

Pulse switching in thin magnetic films

The main results of the investigations of pulse switching in thin magnetic films with uniaxial anisotropy which have been obtained in Moscow State University are reviewed. Simultaneous investigation of integral switching properties, inner effective field, and dynamic domains produced during pulse switching has increased our understanding of bi-directional incoherent rotation mechanism and found new peculiarities of the pulse switching by domain boundary propagation. A new variety of incoherent rotation which manifests itself at strong fields has been found. It has been also found that the curve representing the pulse field dependence of an inverse switching time in the general case consists of five distinct regions. The relation between these regions and the switching mechanisms are discussed.     

Electric-field-assisted switching in magnetic tunnel junctions

The advent of spin transfer torque effect accommodates site-specific switching of magnetic nanostructures by current alone without magnetic field. However, the critical current density required for usual spin torque switching remains stubbornly high around 10(6)-10(7) A cm(-2). It would be fundamentally transformative if an electric field through a voltage could assist or accomplish the switching of ferromagnets. Here we report electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities. These results represent a crucial step towards ultralow energy switching in magnetic tunnel junctions, and open a new avenue for exploring other voltage-controlled spintronic devices.     

Magnetic domain walls in thin molybdenum permalloy films observed under external magnetic fields by electron microscopy

Dynamic observations of domain walls have been made using the Lorentz Microscopy technique. The aim of the investigations was to study the influence of magnetic fields on the domain wall stability and also to study the influence of structural features such as grain boundaries, twins' inclusions and second-phase particles on the initial permeability. Most of the thin foils used had thicknesses less than 100 nm hence the specimens could be referred to as thin films. Cross-tie walls which were noted to occur in materials with high permeability, i.e. materials of low crystal anisotropy,K _i, were observed at intermediate thicknesses of the thin foils. Both the cross-tie length and period are dependent upon film thickness, each becoming shorter with increasing thickness. The influence of the applied field on the cross-tie wall stability depends on both the magnitude and direction of the applied field. Secondly, it was observed experimentally that besides the transition from the Néel wall to the Bloch wall, transitions also occur as a function of the applied WA Structura! features such as grain boundaries, twins, inclusions, which may take many forms as holes or cracks, etc., had a very strong pinning effect on the domain wall motion, leading to a reduction in permeability.     

Synthesis of magnetic fields

The paper deals with some problems of magnetic fields synthesis, depending on determination of the current density distribution, which generates the required magnetic field in the investigated region. Such problems can be reduced to the linear, or nonlinear Fredholm integral equations of the first kind, or to the set of these equations. Fredholm integral equation of the first kind belongs to the class of the ill-posed problems, and for its solving the method of regularisation has been used. In the paper there are given some useful results of synthesis of magnetic fields in few practical configurations.     

Moment switching in nanotube magnetic force probes

Magnetic images of high density vertically recorded media using metal-coated carbon nanotube tips exhibit a doubling of the spatial frequency under some conditions (Deng et al 2004 Appl. Phys. Lett. 85 6263). Here we demonstrate that this spatial frequency doubling is due to the switching of the moment direction of the nanotube tip. This results in a signal which is proportional to the absolute value of the signal normally observed in MFM. Our modeling indicates that a significant fraction of the tip volume is involved in the observed switching, and that it should be possible to image high bit densities with nanotube magnetic force sensors.     

The switching criterion in perpendicular magnetic recording

This paper presents a theoretical analysis of the effect of the "switching criterion", the level at which self-consistency is assumed in calculations on the perpendicular magnetic recording process. It can be proven that in a perpendicular recording configuration with an ideal keeper layer and a recording layer with a rectangular hysteresis loop, the switching criterion in stand-still recording situations is immaterial, because self-consistency is reached at all depth levels simultaneously. If either the keeper layer is absent, or the recording layer's hysteresis loop is sheared, it is shown that the higher the level at which self-consistency is assumed, the sharper the stand-still recorded transitions will be.     

Apparatus for calorimetric measurements of losses in MgB2 superconductors exposed to alternating currents and external magnetic fields

Inexpensive superconducting wires with low AC losses would open up for a large superconductor market in AC electrical power applications. One candidate for this market is the MgB₂ conductor. In the development of an AC optimized superconductor, high-quality measurements of the AC losses under application-like conditions must be available. This article describes an apparatus built for this purpose. The measurement method is calorimetric. The temperature increase of the superconductor sample is measured and compared to the temperature increase due to a heater with known heat input. The system is designed for measurements of losses due to magnetic fields combined with transport currents. Results from tests verifying the capabilities of the system are given, as well as from initial AC loss measurements on a tape-shaped MgB₂ superconductor.