Domain Wall Nucleation Research Articles

Characterization of domain wall–based traps for magnetic beads separation

We characterize the magnetic behavior of an array of magnetic bead traps based on domain walls (DWs) formed in zig-zag permalloy wires patterned on a Si substrate. Using magnetic force and magneto-optical Kerr effect microscopy, we study the nucleation and annihilation of DWs for two different wire widths. Through scanning electron microscopy with polarization analysis, we analyze in detail the magnetization configuration of the DWs in the presence of a magnetic bead previously trapped by the DW stray field. Finally, we patterned the magnetic nanostructures directly on a polydimethylsiloxane (PDMS) substrate, and we show that the functionality of the device is completely maintained. These results pave the way to the integration of DW-based devices in a PDMS lab-on-a-chip system for magnetic bead separation.

Probing the magnetization reversal in epitaxial Fe/IrMn exchange biased bilayers using angle-dependent anisotropic magnetoresistance

We investigated the detailed magnetotransport properties of epitaxial Fe/IrMn exchange biased bilayers by angle-dependent anisotropic magnetoresistance measurements over a wide temperature range. Irreversible resistance jumps and smooth transitions are observed when measuring along different angles with respect to the bias at certain temperatures. The angular dependence of the switching fields shows good agreement with a domain wall (DW) nucleation model. The exchange bias, the induced uniaxial anisotropy, and the intrinsic 90° DW nucleation energy are further extracted from the angle-dependent measurements. A linear temperature dependence is observed for both the exchange bias and the induced uniaxial anisotropy, while the intrinsic 90° DW nucleation energy is independent of the temperature.

Magnetization reversal mechanisms in 35-nm diameter Fe1-xGax/Cu multilayered nanowires

In this work, magnetization reversal mechanisms in various 35 nm diameter Fe80Ga20/Cu multilayered nanowire arrays were studied by a vibrating sample magnetometer (VSM) equipped with vector coils, making it possible to monitor both x- and y-components of the sample moment during reversal. When reversal fields were applied in the low angular range (0 – 60°), all nanowire structures, irrespective of Fe80Ga20 or Cu aspect ratios, experienced reversal by nucleation and propagation of a vortex domain wall. However, when fields were applied in the high angular range of 60–90°, reversal occurred by coherent rotation. Using vector-VSM, it was further shown that in structures with pancake-like Fe80Ga20 segments, the extent to which moments in adjacent segments rotate cooperatively decreased as the Cu thickness increased.

Stochastic limits in synchronous imaging of sub-micron magnetization dynamics using scanning transmission x-ray microscopy

We demonstrate a synchronous (lock-in) technique for imaging thin-film magnetization dynamics using scanning transmission x-ray microscopy (STXM). Gated photon counting synchronized with magnetic field modulation allows image acquisition with differential contrast for high and low magnetization. We have applied this technique to 5 × 12 μm2 Ni81Fe19 ellipses with well-defined closure domains at remanence. The stochastic nature of the domain wall motion and nucleation is apparent in images recorded during cycling along successive major hysteresis loops. Synchronous imaging shows the clearest enhancement of contrast for small-amplitude domain wall motion, with a less obvious benefit at higher fields/displacements. The technique shows promise for the contrast enhancement of magnetization in dynamics in STXM.

Domain wall propagation in micrometric wires: Limits of single domain wall regime

We measured magnetic domain propagation and local domain wall(DW) nucleation in Fe-Co-rich amorphous microwires with metallic nucleus diameters from 2.8 to 18 μm. We found that manipulation of magnetoelastic energy through application of applied stresses, changing of magnetostriction constant, and variation of internal stresses through changing the microwires geometry affects DW velocity. We observed uniform or uniformly accelerated DW propagation along the microwire. The abrupt increasing of DW velocity on v(H) dependencies correlates with the location of the nucleation place of the new domain wall.

Axion monodromy in a model of holographic gluodynamics

The low energy field theory for N type IIA D4-branes at strong 't Hooft coupling, wrapped on a circle with antiperiodic boundary conditions for fermions, is known to have a vacuum energy which depends on the $\theta$ angle for the gauge fields, and which is a multivalued function of this angle. This gives a field-theoretic realization of "axion monodromy" for a nondynamical axion. We construct the supergravity solution dual to the field theory in the metastable state which is the adiabatic continuation of the vacuum to large values of $\theta$. We compute the energy of this state and show that it initially rises quadratically and then flattens out. We show that the glueball mass decreases with $\theta$, becoming much lower than the 5d KK scale governing the UV completion of this model. We construct two different classes of domain walls interpolating between adjacent vacua. We identify a number of instability modes -- nucleation of domain walls, bulk Casimir forces, and condensation of tachyonic winding modes in the bulk -- which indicate that the metastable branch eventually becomes unstable. Finally, we discuss two phenomena which can arise when the axion is dynamical; axion-driven inflation, and axion strings.

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.

Interlayer Thickness Effects on Magnetic Properties of X/FePtAg (X = FePt/Fe and Fe/FePt) Trilayers

A soft/hard FePt/Fe/FePtAg and Fe/FePt/FePtAg trilayers with perpendicular magnetization were prepared on a glass substrate. Inserting disordered FePt layer allowed modification of magnetic anisotropy of the Fe/FePtAg sharp interface to Fe/FePt/FePtAg stepwise interface. The out‐of‐plane coercivity field was modulated as a function of the FePt thickness because of the interface coupling attenuation between Fe and FePtAg. The coercivity was inversely proportional to the FePt thickness as evidenced by the pinning effect at the stepwise interface. Additionally, the out‐of‐plane magnetization was controlled by domain wall pinning effect. The in‐plane magnetization reversal processes occurred in two steps like in exchange spring systems and attributed to the domain wall nucleation and its propagation from Fe/FePt layer into FePtAg layer.

Influence of magnetocrystalline anisotropy on the magnetization reversal mechanism in exchange bias Co/CoO bilayers

We investigated the reversal mechanism in a Co/CoO exchange bias bilayer with a pronounced magnetocrystalline anisotropy in the ferromagnet. The anisotropy, which is induced by the growth of a highly textured Co layer, imposes a distinct reversal mechanism along the magnetically easy and hard direction. It is shown that exchange bias can be induced along both directions, despite the magnetocrystalline anisotropy. The interplay between the magnetocrystalline anisotropy and exchange bias induces a different reversal mechanism for the subsequent reversals in the two crystallographic directions. Along the hard axis, the magnetization reverses according to the reversal mechanism observed before in polycrystalline exchange bias bilayers, i.e. domain wall nucleation and motion for the first reversal and coherent rotation for the subsequent ones. Along the easy axis, domain wall motion remains the dominant reversal mechanism and magnetization rotation has only a minor contribution.

Magnetic Field Sensor Based on the Domain Wall Nucleation and Propagation

In this paper we present a magnetic field sensor based on the domain wall nucleation and propagation process in glass covered amorphous wires. The sensor utilizes the Sixtus and Tonks apparatus. A linear characteristic is reported to magnetic fields with direction opposite to the driving field, while to fields with the same direction; a monotonic, though inappropriate for sensing applications, respond is shown.

Ferroelectric Switching of Thin Langmuir-Blodgett Films of P(VDF-TrFE) Copolymer: Comparison with Liquid Crystal Switching

The switching of polarization in ferroelectric polymer films is traditionally explained in terms of nucleation and motion of domain walls although such walls have never been observed in polymer ferroelectrics. As an alternative, the Landau-Ginzburg theory of the coercive field seems to be appropriate for ultrathin (1–5 monolayer) films. Data on the switching dynamics in the ultrathin films are very scarce. In this work the experiments on the polarization switching in the 20–240 nm thick films prepared by the Langmuir-Blodgett technique had been carried out using triangular voltages of low frequency in order to separate capacitive and polarization components of the current. Then the results have been compared with the data on polarization switching in liquid crystalline, antiferroelectric Langmuir-Blodgett films. There is a striking similarity in the time dependences of repolarization current for two materials. Due to this, we tentatively suggest a switching mechanism based on the field-induced rotation of the polarization vector. The latter allows one to estimate the rotational viscosity coefficient of the ferroelectric polymer responsible for the energy dissipation.

The defects influence on domain wall propagation in bistable glass-coated microwires

We studied the domain wall (DW) dynamics of magnetically bistable amorphous glass-coated Fe74B13Si11C2 microwires. In according to our experimental results magnetic field dependences of DW velocity of studied microwires can be divided into two groups: with uniform or uniformly accelerated DW propagation along the microwire. Strong correlation between the type of the magnetic field dependence of domain wall velocity, v(H), and the distribution of the local nucleation fields has been observed.Moreover, we observed abrupt increasing of DW velocity (jump) on the magnetic field dependences of the domain wall velocity, v(H), for the both types of the v(H) dependences. At the same time usual linear increasing of the domain wall velocity with magnetic field persists below these jumps. It was found that the jump height correlates with the location of nucleation place of the new domain wall. We have measured local nucleation field distribution in all the microwires. From local nucleation field distribution we have obtained the DW nucleation locations and estimated the jump height

Controlled Manipulation of Domain Walls in Amorphous Microwires

In this paper, results on the controlled nucleation, propagation, and collision of magnetic domain walls in amorphous glass-coated Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">77.5</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7.5</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sub> microwires with positive magnetostriction are reported. The effects of nucleation fields and phase differences between the nucleation and magnetization fields, as well as of the sample dimensions on the nucleation and propagation of magnetic domain walls have been investigated. The accurate control of these parameters allows a precise selection of the collision place for domain walls propagated against each other from both ends of a sample. The precision of domain wall manipulation increases as the transverse dimensions of the microwire decrease. The controlled manipulation of magnetic domain walls in amorphous glass-coated microwires opens up new application opportunities for the amorphous microwires in domain wall logic devices.

Magnetic Properties and Microstructure of Exchange Coupled ${\rm Fe}/({\rm FePt-Ag}_{2}{\rm Se})$ Particulate Films

Multilayers Ag/[Ag2Se(0.1 nm)/FePt(1 nm)]10 were alternately deposited on a glass substrate and subsequently annealed by rapid thermal process (RTP) at 800°C for 3 minutes. After RTP, the interface between FePt and Ag2Se was intermixed to form the particulate films. The grains size of the L10 FePt decreased from 9.8 nm to 7.7 nm when the total thickness of Ag2Se intermediate layer increases to 1 nm. The Fe layers with thickness of 1 nm, 3 nm, 5 nm were deposited on FePt-Ag2Se particulate films at room temperature. The Fe/(FePt-Ag2Se) particulate film shows perpendicular magnetization. The magnetization was increased and coercivity was decreased with Fe layer thickness. When the Fe layer thickness increased up to 5nm, two-steps in-plane magnetization curve was found. The magnetization reversal was occurred by the domain wall nucleation and propagation from Fe layer into FePt-Ag2Se layer.

Low-dimensionality energy landscapes: Magnetic switching mechanisms and rates

In this paper we propose a new method for the study and visualization of dynamic processes in magnetic nanostructures, and for the accurate calculation of rates for such processes. The method is illustrated for the case of switching of a grain of an exchange-coupled recording medium, which switches through domain wall nucleation and motion, but is generalizable to other rate processes such as vortex formation and annihilation. The method involves calculating the most probable (lowest energy) switching path and projecting the motion onto that path. The motion is conveniently visualized in a two-dimensional (2D) projection parameterized by the dipole and quadrupole moment of the grain. The motion along that path can then be described by a Langevin equation, and its rate can be computed by the classic method of Kramers [4]. The rate can be evaluated numerically, or in an analytic approximation—interestingly, the analytic result for domain-wall switching is very similar to that obtained by Brown in 1963 for coherent switching, except for a factor proportional to the domain-wall volume. Thus in addition to its lower coercivity, an exchange-coupled medium has the additional advantage (over a uniform medium) of greater thermal stability, for a fixed energy barrier.

A compact model of domain wall propagation for logic and memory design

Current-driven domain wall motion is very promising for low-power, high-density, and high-speed circuits. By combining this shifting scheme with magnetic tunnel junction (MTJ) for reading and writing processes, it opens new routes for nonvolatile logic and memory applications that are crucial for the future of spintronics. This paper reports on a compact model for domain wall-MTJ-based circuit design, simulation, and evaluation. It integrates spin transfer torque mechanism for magnetization reversal and domain wall nucleation, current-driven domain wall pinning/motion behaviors, and tunnel resistance theory of MTJ nanopillar, in which the free layer is one storage element of magnetic stripe. This model is programmed with a very flexible structure to achieve the best simulation precision and efficiency, and provide easy parameter configuration interface. It is compatible with classical computer-aided design environment and can be cosimulated directly with CMOS design kits. By using the compact model, we have successfully simulated a domain wall propagation shift register.

Antiferromagnetic layer thickness dependence of the magnetization reversal in the epitaxial MnPd/Fe exchange bias system

We report an investigation on the antiferromagnetic layer thickness dependence of magnetization reversal in c-axis oriented MnPd/Fe epitaxial exchange biased bilayers. Several kinds of multistep loops were observed for different samples measured at various field orientation. The evolution of the angular dependent magnetic behavior evolving from a representative Fe film to the exchange biased bilayers was revealed. With increase of the thickness of the antiferromagnetic layers, asymmetrically shaped loops and biased two-step loops are induced by exchange bias. Including the unidirectional anisotropy, a model based on the domain nucleation and propagation was developed, which can nicely describe the evolution of the magnetic behaviors for MnPd/Fe bilayers and correctly predicts the critical angles separating the occurrence of different magnetic switching processes. For fields applied along the bias direction, the 180◦ magnetic reversal changes from two successive 90◦ domain wall nucleations to a single 180◦ domain wall nucleation at the critical thickness of the MnPd layer.

Nanoscale physical microstructure and micromagnetic behaviour of CoIr film with negative anisotropy

The physical and magnetic structure of hcp-CoIr(10 nm)/Ru(5 nm) has been systematically characterized using transmission electron microscopy. The film was observed to be polycrystalline with a mean grain size of 15.7 ± 1.1 nm. Additionally, diffraction analysis in the TEM confirmed the presence of a [0 0 0 1] texture normal to the film plane resulting from a Ru seed layer. Lorentz microscopy observation with in situ magnetizing experiments showed that the film possessed a weak-anisotropy easy axis with considerable dispersion of magnetic ripple and domain wall nucleation over a negative field of 25 Oe. Magnetization reversal on the hard axis shows a non-coherent rotation of magnetic moments from easy axis to hard axis denoted by a non-uniform nucleation of low-angle walls. Dispersion of magnetization ripple is in agreement with classical ripple theory. Generally the magnetization reversal was complete by 30 Oe; however, some small regions remained which were not fully reversed with associated 360° domain walls. These appeared to be strongly pinned locally and required much larger fields to eliminate them.

Magnetic properties of nanocrystallized nickel films on gold substrate deposited by cathodic voltammetry: Scan rate induced effects

Ni electrodeposits of thickness values ranged from 70 nm till about 1.20 μm and grown on gold substrate by cathodic voltammetry (C-V) technique are investigated varying the scan rate (r) of the related (C-V) curves in the interval 0.167&amp;lt;r&amp;lt;1.67 mV/s. The system engenders thinner films having rougher surfaces for higher r values while lower ones leads to thicker and smoother samples. Their magnetic reversal is ruled by the domain wall (DW) nucleation and motion. Their ferromagnetic-topography dependence reveals the existence of a critical thickness dc∼375 nm for both their microstructure and magnetic nanostructure. Their magnetic domain sizes (w) evolution with the sample roughness is typical of the Bloch domain type (MD)B below dc while the Néel type (MD)N appears beyond dc according to the topography-based model of Zhao et al. [J. Appl. Phys. 89, 1325 (2001)]. The magnetic anisotropy of the Ni samples exhibits a predominant parallel component for the thinnest sample while the perpendicular one grows with the thickness increase.

Domain wall propagation in single crystalline iron wires

The characterization of magnetization reversal in a magnetic domain wall (DW) in a single crystalline Fe wire with crystalline anisotropy and shape anisotropy has been investigated by the giant magnetoresistance (GMR) measurement. The DW propagation velocity reached about 1 km/s in 110 Oe field at 77 K. The broad distribution of the propagation velocity versus the switching field in the Fe wire with longitudinal axis parallel to the ⟨110⟩ direction was found. The magnetic anisotropy energy including the crystalline and shape anisotropies play an important role in the nucleation and propagation of DW.