Domain Wall Displacement Research Articles

Enhanced current-induced domain wall motion by tuning perpendicular magnetic anisotropy

The effect of perpendicular magnetic anisotropy (PMA) on current-induced domain wall (DW) motion is investigated by micromagnetic simulations. The critical current density JC to drive DWs into periodic transformation and continuous motion by adiabatic spin transfer torque decreases with increasing PMA. Also, with optimized PMA that almost exactly compensates the demagnetizing field, the adiabatic displacement of DWs driven by currents less than JC is strongly enhanced. Since PMA can be controlled easily in magnetic multilayer films, this technique of enhancing current-induced DW motion may be practical for device applications.

Anisotropic magnetic properties and domain structure in Fe-3%Si (110) steel sheet

The understanding of the anisotropic magnetization mechanism in 3%Si steel is relevant to both the design of the electrotechnical applications of 3%Si steel and the fundamental materials science of soft magnetic materials. In the present study, the relation between the anisotropic magnetic properties and the magnetization curves of 3%Si-Fe (110) steel sheet for various directions of axial magnetization was studied taking into account the domain structure. The magnetic loss of the (110) steel sheet were measured in an applied field at an angle α to the [001] axis. The angle α was varied from 0° to 90°, in steps of 15°. In off-[001] directions, the magnetization resulted in a highly structured domain pattern and domain wall displacements, which could be related to the shape of the magnetization curves. The magnetization curves could be divided in four segments, with each segment related to a specific domain structure.

Rayleigh relation and scaling power laws in minor hysteresis loops

Seven scaling power-law relations were discovered in the minor hysteresis loops of ferromagnetic materials several years ago. These relations include universal constants that are independent of the type of material and temperature, along with coefficients that provide information about the material and lattice defects. Four universal constants and four coefficients are explained in connection with the Rayleigh relation, which is caused by two scaling laws for l(H) and p(H), where l(H) is the pinning and unpinning length in the domain wall displacement, H is the magnetic field, and p(H) is the number of domain walls involved. The dependence of the coefficients on the dislocation density, ρ, is explained to increase in proportion to the square root of ρ. This result agrees with experimental results. The force profile of the domain walls has similar structures at every point.

Indirect Control of Antiferromagnetic Domain Walls with Spin Current

The indirect controlled displacement of an antiferromagnetic domain wall by a spin current is studied by Landau-Lifshitz-Gilbert spin dynamics. The antiferromagnetic domain wall can be shifted both by a spin-polarized tunnel current of a scanning tunneling microscope or by a current driven ferromagnetic domain wall in an exchange coupled antiferromagnetic-ferromagnetic layer system. The indirect control of antiferromagnetic domain walls opens up a new and promising direction for future spin device applications based on antiferromagnetic materials.

Control of magnetic domain wall displacement using spin current in small in-plane magnetic field in Permalloy nanowires

We microscopically investigate the magnetic domain wall motion induced by current pulse application in a small in-plane magnetic field in U-shaped Permalloy wires by means of Lorentz microscopy together with simultaneous transport measurement. An in-plane magnetic field less than 7 Oe parallel to the wire direction in U-shaped geometry effectively works to impede bidirectional motion of the domain wall induced by current pulse application, i.e. to suppress the stochastic nature of the domain wall displacement. The present finding will provide practical and reliable ways of controlling and manipulating the domain wall dynamics, which are widely applicable in spintronic devices, especially when stochastic nature causes serious problems in device operation. Reliable manipulation of the magnetic state is discussed using the current-driven domain wall motion and domain nucleation in the magnetic wire device.

Dependence of Hysteresis Loop on Stress in Non-oriented Electrical Steel Sheet

What influences materials, exciting direction and flux density have on the dependence of a non-oriented (NO) electrical steel sheet on stress are discussed. The dependence on stress changes depending on flux density, because the magnetic mechanisms are domain-wall displacement at lower flux density and magnetization rotation at higher flux density. The magnetic properties of NO at low flux density are improved by tension, but deteriorated by compression because of the positive magnetostriction of easy axis ‹100›. Magnetizations are easy to rotate at high flux density. Then, those of NO, especially 50A1300, are deteriorated by tension in negative magnetostriction of hard axis ‹111›. The measured loop-width changes in the shape of a hysteresis loop due to compression which is attributed to hysteresis loss, and the effect of the steel surface in the high flux density region of the loop is reduced, and then hysteresis loss decreases. However, the loop widens with higher compression in the low-flux density region, where grain size mainly affects magnetic properties and its effect can be observed.

Magnetic domain wall conduits for single cell applications

The ability to trap, manipulate and release single cells on a surface is important both for fundamental studies of cellular processes and for the development of novel lab-on-chip miniaturized tools for biological and medical applications. In this paper we demonstrate how magnetic domain walls generated in micro- and nano-structures fabricated on a chip surface can be used to handle single yeast cells labeled with magnetic beads. In detail, first we show that the proposed approach maintains the microorganism viable, as proven by monitoring the division of labeled yeast cells trapped by domain walls over 16 hours. Moreover, we demonstrate the controlled transport and release of individual yeast cells via displacement and annihilation of individual domain walls in micro- and nano-sized magnetic structures. These results pave the way to the implementation of magnetic devices based on domain walls technology in lab-on-chip systems devoted to accurate individual cell trapping and manipulation.

Magnetization Dynamics of Amorphous Ribbons and Wires Studied by Inductance Spectroscopy.

Inductance spectroscopy is a particular formulation variant of the well known complex impedance formalism typically used for the electric characterization of dielectric, ferroelectric, and piezoelectric materials. It has been successfully exploited as a versatile tool for characterization of the magnetization dynamics in amorphous ribbons and wires by means of simple experiments involving coils for sample holding and impedance analyzer equipment. This technique affords the resolution of the magnetization processes in soft magnetic materials, in terms of reversible deformation of pinned domain walls, domain wall displacements and spin rotation, for which characteristic parameters such as the alloy initial permeability and the relaxation frequencies, indicating the dispersion of each process, can be defined. Additionally, these parameters can be correlated with chemical composition variation, size effects and induced anisotropies, leading to a more physical insight for the understanding of the frequency dependent magnetic response of amorphous alloys, which is of prime interest for the development of novel applications in the field of telecommunication and sensing technologies. In this work, a brief overview, together with recent progress on the magnetization dynamics of amorphous ribbons, wires, microwires and biphase wires, is presented and discussed for the intermediate frequency interval between 10 Hz and 13 MHz.

Magnetic Properties of Crystalline NiFe Alloy Prepared by High-Energy Ball Milling and Compacting

The structure and magnetic properties of compacted microcrystalline NiFe (81 wt% of Ni) powder is investigated. The powder of NiFe alloy prepared by ball milling of ribbon (prepared by melt spinning) remains single phase material suitable for compaction in order to prepare soft magnetic material. The bulk samples were consolidated by uniaxial compaction of the powder in vacuum. By measuring of AC and DC magnetic properties it was found out that in bulk samples the displacement of domain walls is the dominant magnetization process, while rotation of magnetization vectors prevails in powder material.

Design and performance of domain wall displacing-type field sensors using a magnetic tunnel junction and a giant magnetoresistive device

We report a new type of field sensor, which utilizes the oscillatory domain wall displacement in the ferromagnetic free layer to detect an external field. The sensor consists of a 1200 × 100 µm2 Al conducting wire placed on a spin-valve giant magnetoresistive (GMR) or magnetic tunnel junction. By transmitting an alternating current through the Al conducting wire, an ac field can be generated to oscillate the domain wall in the free layer. The oscillatory domain wall displacement reduces the influence of wall coercivity and Barkhausen effect, and enables the sensor to detect the magnetic field which is much smaller than the wall coercivity. The field sensitivities of the domain wall displacing-type GMR and tunnel magnetoresistive (TMR) sensors are 2.73 mV V−1 Oe−1 and 5.81 mV V−1 Oe−1, respectively.

Eddy-current anomaly in Fe-based nanocrystalline wires

The circular permeability, μ= μ′− jμ″, of two Fe-based nanocrystalline wires obtained by furnace annealing (S1) and by current annealing (S2) were determined from the measurements of the impedance, Z= R+ jωL, as a function of frequency ( f=0.1–100 kHz) and DC bias field ( H=0–89 kA/m). So the H-dependent low- f eddy-current anomaly factor, η( H), has been investigated. The experimental results indicate that the sample S2 by current annealing has a larger η=4 than the sample S1 by furnace annealing with η=2.8 at H=0. With increasing H, the η( H) of two samples decreases initially until about H=450 A/m, and then increases very quickly. These results have been analyzed by the helical magnetization model considering the contribution of domain wall displacements (DWD) and domain magnetization rotation (DMR).

Magnetic Dynamic Interaction in Amorphous Microwire Array

The magnetic dynamic interaction in ferromagnetic microwire arrays has been studied based on thin Co-based glass-coated amorphous microwires. The longitudinal hysteresis loops of the microwire arrays consisting of different number of wires have been measured statically with no current flowing through the arrays and dynamically with ac current flowing through the arrays. The results show that with the number of wires increases the transverse anisotropy of the arrays has been enhanced, especially when an ac current was applied to the arrays, an additional circumferential anisotropy was induced. The results are explained by a domain unification effect in the form of domain wall displacement. Moreover, the changes in the magneto-impedance effect with the number of wires in the arrays indicate that the domain unification effect is frequency dependant and related to the dimension of the arrays. The domain wall motion contributes to the magneto-impedance up to a critical frequency beyond which only domain rotation occurs. Below the critical frequency, the initial permeability of the arrays increases with the increase of the number of wires while above the critical frequency, the trend inverses. The magneto-impedance response of the microwire arrays is a consequence of the dynamic interaction of domain unification and eddy current.

Magnetic Domain Wall Pumping by Spin Transfer Torque

We show that spin transfer torque from a direct spin-polarized current applied parallel to a magnetic domain wall (DW) induces DW motion in a direction independent of the current polarity. This unidirectional response of the DW to spin torque enables DW pumping--long-range DW displacement driven by an alternating current. Our numerical simulations reveal that DW pumping can be resonantly amplified through excitation of internal degrees of freedom of the DW by the current.

Magnetic nanostructures for the manipulation of individual nanoscale particles in liquid environments (invited)

The manipulation of geometrically constrained magnetic domain walls (DWs) in nanoscale magnetic strips attracted much interest recently, with proposals for prospective memory and logic devices. Here we demonstrate that the high controllability of the motion of geometrically constrained DWs allows for the manipulation of individual nanoparticles in solution on a chip with the active control of position at the nanometer scale. Our approach exploits the fact that magnetic nanoparticles in suspension can be captured by a DW, whose position can be manipulated with nanometer scale accuracy in specifically designed magnetic nanowire structures. We hereby show that the precise control over DW nucleation, displacement, and annihilation processes in such nanostructures allows for the capture, transport, and release of magnetic nanoparticles. As magnetic nanoparticles with functionalized surfaces are widely used as molecule carriers or labels for single molecule studies, cell manipulation, and biomagnetic sensing, the accurate control over the handling of the single magnetic nanoparticle in suspension is a crucial building block for several applications in biotechnology, nanochemistry, and nanomedicine.

Effect of the thickness of Fe-3% Si crystals on the dynamics of the domain structure and magnetic losses in rotating magnetic fields

The effect of the thickness of Fe-3% Si crystals with an easy axis deviated by an angle β = 2.5° from the surface plane on the dynamics of their domain structure and magnetic losses has been studied in rotating magnetic fields. It has been established that the role of the closure domain structure in the magnetization reversal of samples decreases continuously with decreasing crystal thickness down to the thickness d = 0.10 mm, whereas the contribution of the displacements of 180o domain walls of the stripe domain structure to the change in the magnetization of the samples increases. A number of specific features of their behavior in thin samples have been revealed, such as the presence of a strong inhomogeneity of the velocities of displacement of the 180° domain walls of the stripe domain structure and the absence of a refinement of this structure up to Bm = 1.8 T. The data on the dynamics of the domain structure are used for an analysis of the variation of magnetic losses of samples in rotating magnetic fields.

Practical and Theoretical Investigations of a Rotating Coilless Actuator Using the Inverse Magnetostrictive Effect

In this paper, we present a novel rotating actuator based on the interaction of a permanent magnet with a magnetostrictive soft ribbon submitted to mechanical stress. The direction and magnitude of the stress generates a directional magnetic anisotropy (easy axis) in the soft ribbon. A nonzero angle between the polarizing magnet and the induced easy axis results in a torque between the two: hence the magnet being free to rotate aligns itself with the easy axis. A demonstrator of this principle was made using a Nd-Fe-B magnet above a disk cut from an iron-based amorphous ribbon glued between two unidirectional piezoelectric actuators. The analytical model takes into account the magnetic domains and assumes a magnetization mechanism of both coherent rotation and domain-wall displacement, with a uniaxial anisotropy induced by the applied stress.

Effect of Surface Microstructure on Magnetization Process in Fe$_{80.5}$Cu$_{1.5}$Si$_{4}$B$_{14}$ Nanocrystalline Alloys

The magnetization processes of the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">80.5</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.5</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> nanocrystalline alloy were observed by Kerr effect. In the specimen annealed with heating rate of 0.3°C/s (NA), the pinning of domain walls was observed. Owing to such pinning effect in the NA specimen, high remanent magnetic flux density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Br</i> was obtained, and the hysteresis was observed in the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B-H</i> loop in the region of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> > 1.5 T. Whereas, the smooth domain wall displacement was observed in the specimen annealed with the heating rate of 3°C/s (HA). The coarser grains near the surface observed in the NA specimen are considered to be the pinning site of the domain walls. In the HA specimen, grain growth in the heating process of annealing is suppressed.

Direct observation of high velocity current induced domain wall motion

We study fast vortex wall propagation in Permalloy wires induced by 3 ns short current pulses with sub 100 ps rise time using high resolution magnetic imaging at zero field. We find a constant domain wall displacement after each current pulse as well as current induced domain wall structure changes, even at these very short timescales. The domain wall velocities are found to be above 100 m/s and independent of the domain wall spin structure. Comparison to experiments with longer pulses points to the pulse shape as the origin of the high velocities.

Stress Dependence of Magnetic Properties in Non-oriented Electrical Steel Sheets

This paper discusses the stress dependence of magnetizing properties in non-oriented electrical steel sheets (NO). Compression stresses deteriorate the magnetic properties at low flux density, but improve them at high flux density, where magnetization rotations are induced. Tensile stresses reduce magnetizing fields at low flux density, but increase them at high flux density. The stress dependence of NO is affected by the exciting direction, as the rolling direction of NO is easy to magnetize and sensitive to stresses. High-grade types of NO (50A290, etc), which have high Si content and large crystalline grains, are more sensitive to stresses than low grades (50A1300, etc), and the magnetizing fields of high grades at high compression are higher than those of low grades. This stress dependence may depend on magnetization behaviors (domain wall displacement and magnetization rotation) and magnetostriction. Domain wall displacement mainly occurs at lower flux density, and in this case tensile stresses improve magnetic properties and compression deteriorate them because magnetic domains orient the magnetization to <100> and λ100 is positive. However, magnetizations rotate to exciting directions at high flux densities, with the result that the <111> component of magnetization is induced, and in this case compression improves the magnetizing properties, because λ111 is negative, whereas tensile stress deteriorates them. Therefore, stress dependences may be modeled by magnetization behaviors and magnetostriction, and analysis of the magnetic properties using this model of stress dependence may improve NO steels and their applications.

Numerical Analysis of DWDD Disk with Control Layer

Scattering characteristics of a domain wall displacement detection (DWDD) disk with a control layer were investigated by finite-difference time-domain (FDTD) analysis. DWDD is one of the high-density storage technologies of magneto-optical (MO) disks and the control layer is used to suppress ghost signals due to a rear process. The effects of the control layer on the scattering characteristics were studied.