Individual Domain Walls Research Articles

Vacuum alignment and phase structure of holographic bi-layers

We study the D3/probe D5 system with two domain wall hypermultiplets. The conformal symmetry can be broken by a magnetic field, B (or running coupling), which promotes condensation of the fermions on each individual domain wall. Separation of the domain walls promotes condensation of the fermions between one wall and the other. We study the competition between these two effects showing a first order phase transition when the separation is ∼0.56λ1/4B−1/2. We identify extremal brane configurations which exhibit both condensations simultaneously but they are not the preferred ground state.

Nanoscale studies of ferroelectric domain walls as pinned elastic interfaces

Abstract The competition between elasticity and pinning of an interface in a fluctuating potential energy landscape gives rise to characteristic self-affine roughening and a complex dynamic response to applied forces. This statistical physics approach provides a general framework in which the behaviour of systems as diverse as propagating fractures, wetting lines, burning fronts or surface growth can be described. Domain walls separating regions with different polarisation orientation in ferroelectric materials are another example of pinned elastic interfaces, and can serve as a particularly useful model system. Reciprocally, a better understanding of this fundamental physics allows key parameters controlling domain switching, growth, and stability to be determined, and used to improve the performance of ferroelectric materials in applications such as memories, sensors, and actuators. In this review, we focus on piezoresponse force microscopy measurements of individual ferroelectric domain walls, allowing their static configuration and dynamic response to be accessed with nanoscale resolution over multiple orders of length scale and velocity. Combined with precise control over the applied electric field, temperature, and strain, and the ability to influence the type and density of defects present in the sample, this experimental system has allowed not only a direct demonstration of creep motion and roughening, but provides an opportunity to test less-well-understood aspects of out-of-equilibrium behaviour, and the effects of greater complexity in the structure of both the interface and the disorder landscape pinning it.

Correlation between spin structure oscillations and domain wall velocities

Magnetic sensing and logic devices based on the motion of magnetic domain walls rely on the precise and deterministic control of the position and the velocity of individual magnetic domain walls in curved nanowires. Varying domain wall velocities have been predicted to result from intrinsic effects such as oscillating domain wall spin structure transformations and extrinsic pinning due to imperfections. Here we use direct dynamic imaging of the nanoscale spin structure that allows us for the first time to directly check these predictions. We find a new regime of oscillating domain wall motion even below the Walker breakdown correlated with periodic spin structure changes. We show that the extrinsic pinning from imperfections in the nanowire only affects slow domain walls and we identify the magnetostatic energy, which scales with the domain wall velocity, as the energy reservoir for the domain wall to overcome the local pinning potential landscape.

Suppression of Walker breakdown in magnetic domain wall propagation through structural control of spin wave emission

The control of individual magnetic domain walls has potential for future spintronic memory and data processing applications. The speed and reliability of such devices are determined by the dynamic properties of the domain walls. Typically, spin precession limitations lead to Walker breakdown, limiting wall velocity resulting in low mobility. Here, we show the suppression of Walker breakdown by the careful design of small amplitude periodic nanowire structuring to match the periodicity of domain wall spin structure transformations. This opens up a channel for energy dissipation via spin wave emission, allowing a domain wall to maintain its spin structure during propagation.

Application of local transverse fields for domain wall control in ferromagnetic nanowire arrays

In ferromagnetic nanowire arrays, where each wire contains multiple domain walls, it will be necessary to select an individual domain wall (DW) to move. In the field driven DW case, the field is typically applied globally affecting all of the domain walls in the system. We present micromagnetic simulation results demonstrating selectivity and control of an individual DW in such an array of nanowires using a combination of global and locally generated magnetic fields. Arranging the orientation of the local field allows for selectivity of a specific DW and its controllable movement to a new location.

Tunable Resistivity of Individual Magnetic Domain Walls

Despite the relevance of current-induced magnetic domain wall (DW) motion for new spintronics applications, the exact details of the current-domain wall interaction are not yet understood. A property intimately related to this interaction is the intrinsic DW resistivity. Here, we investigate experimentally how the resistivity inside a DW depends on the wall width Δ, which is tuned using focused ion beam irradiation of Pt/Co/Pt strips. We observe the nucleation of individual DWs with Kerr microscopy, and measure resistance changes in real time. A 1/Δ(2) dependence of DW resistivity is found, compatible with Levy-Zhang theory. Also quantitative agreement with theory is found by taking full account of the current flowing through each individual layer inside the multilayer stack.

Efficient Photovoltaic Current Generation at Ferroelectric Domain Walls

We elucidate the mechanism of a newly observed photovoltaic effect which occurs in ferroelectrics with periodic domain structures. Under sufficiently strong illumination, domain walls function as nanoscale generators of the photovoltaic current. The steps in the electrostatic potential function to accumulate electrons and holes on opposite sides of the walls while locally reducing the concentration of the oppositely charged carriers. As a result, the recombination rate adjacent to the walls is reduced, leading to a net diffusion current. In open circuit, photovoltages for periodically ordered domain walls are additive and voltages much larger than the band gap can be generated. The internal quantum efficiency for individual domain walls can be surprisingly high, approaching 10% for above band-gap photons. Although we have found the effect in BiFeO(3) films, it should occur in any system with a similar periodic potential.

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.

Tracking Random Walk of Individual Domain Walls in Cylindrical Nanomagnets with Resistance Noise

The stochasticity of domain-wall (DW) motion in magnetic nanowires has been probed by measuring slow fluctuations, or noise, in electrical resistance at small magnetic fields. By controlled injection of DWs into isolated cylindrical nanowires of nickel, we have been able to track the motion of the DWs between the electrical leads by discrete steps in the resistance. Closer inspection of the time dependence of noise reveals a diffusive random walk of the DWs with a universal kinetic exponent. Our experiments outline a method with which electrical resistance is able to detect the kinetic state of the DWs inside the nanowires, which can be useful in DW-based memory designs.

Domain wall anisotropic magnetoresistance in planar nanowires

The anisotropic magnetoresistance (AMR) of individual magnetic domain walls in planar nanowires has been systematically investigated as a function of the micromagnetic wall structure, width, and thickness of the nanowire. Experimentally derived thickness dependence of both the thin film resistivity and the anisotropic magnetoresistance were incorporated into the calculations. We found that the AMR value can be used to identify the wall structure and that the wall resistance is sensitive to wire width for widths less than 300 nm. Furthermore, in comparison with the detailed analysis here, a previous simplified domain wall model significantly underestimates domain wall resistance in narrower wires.

Domain wall dynamics in a two-layer uniaxial film

Numerical solution of the Slonczewski equations were used to study the motion of an individual domain wall in a two-layer uniaxial film in which the damping parameter and the gyromagnetic ratio depend on the specific layer.

Barkhausen statistics from a single domain wall in thin films studied with ballistic Hall magnetometry

The movement of a micron-size section of an individual domain wall in a uniaxial garnet film was studied using ballistic Hall micromagnetometry. The wall propagated in characteristic Barkhausen jumps, with the distribution in jump size $S$, following the power-law relation, $D(S)\ensuremath{\propto}{S}^{\ensuremath{-}\ensuremath{\tau}}$. In addition to reporting on the suitability of employing this alternative technique, we discuss the measurements taken of the scaling exponent $\ensuremath{\tau}$, for a single domain wall in a two-dimensional sample with magnetization perpendicular to the surface, and low pinning center concentration. This exponent was found to be $1.14\ifmmode\pm\else\textpm\fi{}0.05$ at both liquid helium and liquid nitrogen temperatures.

Propagation dynamics of individual domain walls inGa1−xMnxAsmicrodevices

We investigated the transport dynamics of individual magnetic domain walls by employing electrical measurements in multiterminal Ga1–xMnxAs microdevices. Domain wall propagation velocities were deduced from time-of-flight planar Hall measurements between multiple electrical probes of our samples. Domain wall motion induced by both magnetic field and electric currents was systematically investigated. Dependent on the strength of applied in-plane magnetic field, two regimes of domain wall motion, involving thermally assisted flow for low fields and viscous flow for high fields, have been identified. However our data shows no evidence of spin-current induced domain wall motion.

Intrinsic Pinning of a Ferromagnetic Domain Wall in Yttrium Iron Garnet Films with Strong Uniaxial Anisotropy

Movements of individual domain walls in a ferromagnetic garnet were studied with angstrom resolution. The measurements reveal that domain walls can be locked between adjacent crystallographic planes and propagate by distinct steps matching the lattice periodicity.

Patterning nanostructures to study magnetization processes

Lithography techniques such as electron-beam lithography and focused-ion-beam milling are widely used to fabricate structures with dimensions well below 1 µm. These techniques have been used to produce planar magnetic structures with sub-micrometer dimensions and well controlled geometry. This has allowed the study of basic magnetic behaviour and the development of structures with potential for applications in magnetic recording and magnetic logic devices. The techniques of electron beam lithography and focused-ion-beam milling for the fabrication of magnetic nanostructures are outlined here. These techniques have been used to fabricate ribbon-like planar nanowires to study the behaviour of the individual magnetic domain walls which mediate the reversal process in such elongated structures. These methods allow the production of structures in which the location of domain wall formation and position can be controlled, allowing separation and study of the domain wall nucleation and propagation processes. Domain wall injection and domain wall propagation behaviour are investigated and shown to be stochastic processes.

Ferromagnetic domain wall on nanometer scale

Movements of individual domain walls in a ferromagnetic garnet were studied with angstrom resolution. The measurements reveal that domain walls can be locked between adjacent crystallographic planes and propagate by distinct steps matching the lattice periodicity.

Electrical transport across an individual magnetic domain wall in (Ga,Mn)As microdevices

Recent studies demonstrate that an individual magnetic domain wall (DW) can be trapped and reproducibly positioned within multiterminal (Ga,Mn)As microdevices. The electrical resistance obtained from such measurements is found to be measurably altered by the presence of this single entity. To elucidate these observations we develop a simple model for the electrical potential distribution along a multiterminal device in the presence of a single DW. This is employed to calculate the effect of a single DW upon the longitudinal and transverse resistance. The model provides very good agreement with experimental observations, and serves to highlight important deviations from simple theory. We show that measurements of transverse resistance along the channel permits establishing the position and the shape of the DW contained within it. An experimental scheme is developed that enables unambiguous extraction of the intrinsic DW resistivity. This permits the intrinsic contribution to be differentiated from resistivities originating from the bulk and from magnetic anisotropy - effects that are generally manifested as large backgrounds in the experiments.

Negative intrinsic resistivity of an individual domain wall in epitaxial (Ga,Mn)As microdevices.

Magnetic domains, and the boundaries that separate them (domain walls, DWs), play a central role in the science of magnetism. Understanding and controlling domains is important for many technological applications in spintronics, and may lead to new devices. Although theoretical efforts have elucidated several mechanisms underlying the resistance of a single DW, various experiments report conflicting results, even for the overall sign of the DW resistance. The question of whether an individual DW gives rise to an increase or decrease of the resistance therefore remains open. Here we report an approach to DW studies in a class of ferromagnetic semiconductors (as opposed to metals) that offer promise for spintronics. These experiments involve microdevices patterned from monocrystalline (Ga,Mn)As epitaxial layers. The giant planar Hall effect that we previously observed in this material enables direct, real-time observation of the propagation of an individual magnetic DW along multiprobe devices. We apply steady and pulsed magnetic fields, to trap and carefully position an individual DW within each separate device studied. This protocol reproducibly enables high-resolution magnetoresistance measurements across an individual wall. We consistently observe negative intrinsic DW resistance that scales with channel width. This appears to originate from sizeable quantum corrections to the magnetoresistance.

Electron transport and magnetoresistance in ferromagnetic nanostructures

Recent studies of very narrow Ni wires have shown that the resistance of a wire containing domain walls can be smaller than for a sample without walls; i.e., the domain wall resistance is effectively negative. In this paper we investigate the corresponding behavior of permalloy wires. For these wires the resistance decreases markedly during magnetization reversal. We show that this decrease is due to a combination of coherent rotation and a negative domain wall resistance. We are able to separate these two effects, and extract an estimate for the resistance of an individual domain wall.

METALLIC AND SEMICONDUCTOR HALL MICROPROBES FOR WIDE TEMPERATURE RANGE APPLICATIONS

We report on mesoscopic Hall sensors allowing accurate magnetization studies of submicron or nanometer-sized samples over a wide temperature range. Even at 300 K, the probes can reliably resolve local changes in dc field of ≈ 1 G with spatial resolution of ≈ 1 μm, which corresponds to a flux sensitivity of less than 0.1 ϕ0 (ϕ0=h/e is a flux quantum). The resolution increases 100 times at temperatures below 80 K. The capabilities of new micromagnetometers are demonstrated by measuring nm-scale movements of individual domain walls in a ferromagnet.