Neel Walls Research Articles

Ultrasensitive Sub-monolayer Palladium Induced Chirality Switching and Topological Evolution of Skyrmions.

Chiral spin textures are fundamentally interesting, with promise for device applications. Stabilizing chirality is conventionally achieved by introducing Dzyaloshinskii-Moriya interaction (DMI) in asymmetric multilayers, where the thickness of each layer is at least a few monolayers. Here we report an ultrasensitive chirality switching in (Ni/Co)n multilayer induced by capping with only 0.22 monolayer of Pd. Using spin-polarized low-energy electron microscopy, we monitor the gradual evolution of domain walls from left-handed to right-handed Néel walls and quantify the DMI induced by the Pd capping layer. We also observe the chiral evolution of a skyrmion during the DMI switching, where no significant topological protection is found as the skyrmion winding number varies. This corresponds to a minimum energy cost of <1 attojoule during the skyrmion chirality switching. Our results demonstrate the detailed chirality evolution within skyrmions during the DMI sign switching, which is relevant to practical applications of skyrmionic devices.

All-Linear Multistate Magnetic Switching Induced by Electrical Current

We present an alternative mechanism to control the domain wall motion, whose directions are manipulated by the amplitude of electrical currents when modulating the ratio of D/A (constants of Dzyaloshinskii-Moriya interaction over exchange interaction). To confirm this mechanism, we observe this type of domain wall motion and demonstrate linear magnetic switching without hysteresis effect via adjusting the D/A of Ta/Pt/Co/Ta multilayer device with ion implantations. We further find field-free biased and chirally controllable multistate switching at the lateral interface of ion exposed and unexposed area, which is due to the current induced Neel wall motion and a strong exchange coupling at this interface.

Thickness-dependence of magnetic anisotropy and domain structure in Ni thin films grown on a PMN-PT substrate

High-quality Ni ferromagnetic thin films with the thicknesses of 180, 240, 330, and 510 nm ± 5 nm were grown on (001)Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) single-crystal substrates by magnetron sputtering followed by annealing. The thickness dependence of magnetic anisotropy was investigated using ferromagnetic resonance, indicating that the 180 nm Ni thin film exhibits a large magnetic anisotropy field. Increasing the film’s thickness resulted in a reduction of the magnetic anisotropy. Well-defined stripe domain structures in Ni thin films were observed. Calculations based on the tilted partial flux closure domain structure with Bloch wall and Neel wall indicate an increase in the primary domain size and closure domain size of Ni thin films with increasing film thickness.

Constitutive modeling of polycrystalline multiconstituent and multiphase ferroic materials based on a condensed approach

The constitutive behavior of polycrystalline ferroelectric or ferromagnetic materials is essentially determined by ferroelectric domain and ferromagnetic Bloch or Neel wall motions, respectively, on the one hand and grain interactions on the other. In physically motivated models, domain switching and the rotation of elementary magnets, respectively, are directly considered coping with the first issue. In phenomenological macroscale approaches, both domain processes and grain interactions are merged in one constitutive model. The implementation of constitutive equations into a finite element code provides intrinsic interactions due to the coupling of physical quantities on the node- and element-level. The condensed approach is based on microphysical considerations of domains and accounts for grain interaction on the level of generalized residual stresses due to mismatches of individual grains and the associated effective medium. The approach is first applied to the prediction of magnetoelectric coupling in a multiferroic ferroelectric–ferromagnetic compound, where its basic idea is translated to model interactions of the constituents without going back to any discretization scheme. Second, a tetragonal–rhombohedral ferroelectric composition is considered with associated interactions in multiphase grains.

Effect of Uniaxial Anisotropy and Demagnetizing Effects on the Magnetic Behavior of Polycrystalline Gadolinium

Gadolinium (Gd) is a soft ferromagnetic material with uniaxial magnetic anisotropy. The anisotropy is responsible for some magnetic features that are quite different from those observed in most common ferromagnetic materials. Polycrystalline Gd exhibits thermal hysteresis in which the memory of past magnetic history is stored in the buildup of magnetic charge density around grain boundaries on cooling in magnetic fields from above the spin-reorientation temperature $T_{\text {SR}} \sim 235$ K, in the range where the uniaxial anisotropy is positive. This has the effect of narrowing domain walls (DWs) that interact more strongly with defects, such as grain boundaries. Below $T_{\text {SR}}$ , where the negative uniaxial anisotropy is increasing in magnitude with decreasing T, the magnetic susceptibility increases by orders of magnitude. Negative uniaxial anisotropy makes it easier for DWs to move past defects, such as grain boundaries, because wider Neel walls are favored over narrower Bloch walls. Magneto-thermal measurement protocols are used to study the effect of cooling in a field $\mu _{0}H = -50$ mT to 5 K and then measuring at lower constant fields, typically $\mu _{0}H = 3.5$ mT, from 5 to 380 K. The protocols permit the extraction of the temperature dependencies of the susceptibility, which depends on anisotropy, and of the frozen fraction of the magnetization, which consists of two parts. One part freezes upon field cooling below $T = 100$ K, and a harder one freezes above 250 K. On first warming, the former unfreezes below 100 K, while the latter unfreezes above 250 K.

Tunable inertia of chiral magnetic domain walls.

The time it takes to accelerate an object from zero to a given velocity depends on the applied force and the environment. If the force ceases, it takes exactly the same time to completely decelerate. A magnetic domain wall is a topological object that has been observed to follow this behaviour. Here we show that acceleration and deceleration times of chiral Neel walls driven by current are different in a system with low damping and moderate Dzyaloshinskii–Moriya exchange constant. The time needed to accelerate a domain wall with current via the spin Hall torque is much faster than the time it needs to decelerate once the current is turned off. The deceleration time is defined by the Dzyaloshinskii–Moriya exchange constant whereas the acceleration time depends on the spin Hall torque, enabling tunable inertia of chiral domain walls. Such unique feature of chiral domain walls can be utilized to move and position domain walls with lower current, key to the development of storage class memory devices.

Spin polarized current controlled dynamics of domain walls in magnetic films with in-plane anisotropy

We study the dynamic properties of asymmetric vortex Bloch walls and classical 1D Neel walls controlled by a spin-polarized current in magnetic films with in-plane anisotropy. It is shown that fairly high velocities of domain walls (up to 100 m/s) can be obtained for the current density in the range j = 106–108 A/cm2. The nonlinear dependence of the wall velocity on the film thickness and the linear dependence of the velocity on the current density and inverse damping parameter are found.

Skyrmion Creation and Manipulation by Nano-Second Current Pulses.

Easy creation and manipulation of skyrmions is important in skyrmion based devices for data storage and information processing. We show that a nano-second current pulse alone is capable of creating/deleting and manipulating skyrmions in a spin valve with a perpendicularly magnetized free layer and broken chiral symmetry. Interestingly, for an in-plane magnetized fixed layer, the free layer changes from a single domain at zero current to a Neel wall at an intermediate current density. Reverse the current polarity, the Neel wall changes to its image inversion. A properly designed nano-second current pulse, that tends to convert one type of Neel walls to its image inversion, ends up to create a stable skyrmion without assistance of external fields. For a perpendicularly magnetized fixed layer, the skyrmion size can be effectively tuned by a current density.

Interaction Energy of Domain Walls in a Nonlocal Ginzburg–Landau Type Model from Micromagnetics

We study a variational model from micromagnetics involving a nonlocal Ginzburg-Landau type energy for S^1-valued vector fields. These vector fields form domain walls, called Neel walls, that correspond to one-dimensional transitions between two directions within the unit circle S^1. Due to the nonlocality of the energy, a Neel wall is a two length scale object, comprising a core and two logarithmically decaying tails. Our aim is to determine the energy differences leading to repulsion or attraction between Neel walls. In contrast to the usual Ginzburg-Landau vortices, we obtain a renormalised energy for Neel walls that shows both a tail-tail interaction and a core-tail interaction. This is a novel feature for Ginzburg-Landau type energies that entails attraction between Neel walls of the same sign and repulsion between Neel walls of opposite signs.

Domain Wall Configurations and Interactions in Nanowires With Perpendicular Magnetic Anisotropy

Magnetic thin film wires with perpendicular magnetic anisotropy (PMA) containing domain walls (DWs) have been proposed for magnetic logic and memory devices. Using micromagnetic simulations, the magnetic configuration of a DW as a function of the magnitude of the PMA, wire width and thickness is described, showing the regimes of stability of Bloch and Neel walls. Stray field leads to repulsive interactions preventing the formation of 360 degree DWs, and promote uniform spacing between multiple DWs confined within a nanowire.

Domain configurations in Co/Pd and L10-FePt nanowire arrays with perpendicular magnetic anisotropy.

Perpendicular magnetic anisotropy [Co/Pd]15 and L10-FePt nanowire arrays of period 63 nm with linewidths 38 nm and 27 nm and film thickness 27 nm and 20 nm respectively were fabricated using a self-assembled PS-b-PDMS diblock copolymer film as a lithographic mask. The wires are predicted to support Néel walls in the Co/Pd and Bloch walls in the FePt. Magnetostatic interactions from nearest neighbor nanowires promote a ground state configuration consisting of alternating up and down magnetization in adjacent wires. This was observed over ∼75% of the Co/Pd wires after ac-demagnetization but was less prevalent in the FePt because the ratio of interaction field to switching field was much smaller. Interactions also led to correlations in the domain wall positions in adjacent Co/Pd nanowires. The reversal process was characterized by nucleation of reverse domains, followed at higher fields by propagation of the domains along the nanowires. These narrow wires provide model system for exploring domain wall structure and dynamics in perpendicular anisotropy systems.

Domain wall of a ferromagnet on a three-dimensional topological insulator.

Topological insulators (TIs) show rich phenomena and functions which can never be realized in ordinary insulators. Most of them come from the peculiar surface or edge states. Especially, the quantized anomalous Hall effect (QAHE) without an external magnetic field is realized in the two-dimensional ferromagnet on a three-dimensional TI which supports the dissipationless edge current. Here we demonstrate theoretically that the domain wall of this ferromagnet, which carries edge current, is charged and can be controlled by the external electric field. The chirality and relative stability of the Neel wall and Bloch wall depend on the position of the Fermi energy as well as the form of the coupling between the magnetic moments and orbital of the host TI. These findings will pave a path to utilize the magnets on TI for the spintronics applications.

Polar structure of disclination loops in nematic liquid crystals probed by second-harmonic-light scattering.

Angle-resolved, second-harmonic-light scattering (SHLS) measurements are reported for three different classes of thermotropic nematic liquid crystals (NLCs): polar and nonpolar rodlike compounds and a bent-core compound. Results revealing well-defined scattering peaks are interpreted in terms of the electric polarization induced by distortions of the nematic orientational field ("flexopolarity") associated with inversion wall defects, nonsingular disclinations, analogous to Neel walls in ferromagnets, that often exhibit a closed loop morphology in NLCs. Analysis of the SHLS patterns based on this model provides a "proof-of-concept" for a potentially useful method to probe the flexopolar properties of NLCs.

Nonlinear rearrangement of the structure of domain walls in a thin magnetic film with a uniaxial in-plane anisotropy

The structures of domain walls have been investigated by the method of three-dimensional computer simulation in films with uniaxial in-plane anisotropy at different film thicknesses. For the calculations, the parameters characteristic of permalloy films have been taken. It has been established that, depending on the film thickness, the walls that prove to be stable are one-dimensional Neel walls, cross-tie walls, and C- and S-shaped walls. Film thicknesses at which the transition between the different types of walls occurs have been determined. The structure of Bloch lines and Bloch points in C- and S-shaped walls has been investigated. The values of topological charges for different micromagnetic structures have been calculated.

Numerical studies of micromagnetic configurations in stripes with in-plane anisotropy and low quality factor

Abstract Magnetization distributions and energy of 180-degrees domain walls in a stripe-film were investigated over a wide film thickness range. Three-dimensional numerical simulations are performed. Two kinds of transitions between stable domain wall configurations were obtained: from Neel walls to cross-tie walls and from cross-tie walls to asymmetric Bloch (C-shaped) walls. The latter kind of transition was investigated for the first time. The transition from the two-dimensional cross-tie structure to the three-dimensional one during the rise of the film thickness was demonstrated.

Penetration of a Magnetic Wall into Thin Ferromagnetic Electrodes of a Nano-Contact Spin Valve

We theoretically analyze a magnetic wall confined in a nano-contact spin valve paying special attention to the penetration of the magnetic wall into thin ferromagnetic electrodes. We show that, compared with the Bloch wall, the penetration of the Neel wall is suppressed by increases of the demagnetization energy. We find the optimal conditions of the radius and height of the nano-contact to maximize the power of the current-induced oscillation of the magnetic wall. We also find that the thermal stability of the Bloch wall increases when the nano-contact's radius increases or height decreases.

Intrawall configurations of magnetization near Bloch-Néel transitions in magnetically triaxial films

The micromagnetic approach is used to study the structure of domain walls and find their energies depending on values of magnetically anisotropic interactions and thicknesses b of magnetically triaxial films with various orientation of the surface relative to (100) plane. This is performed based on the two-dimensional magnetization distribution with the exact consideration of the principal interactions, including dipoledipole interaction. The calculations are carried out for film thicknesses bN that correspond to the transitions of domain wall structures from Neel to Bloch walls. It is shown that the magnetic anisotropy and orientation of film surfaces become significant at great distances from the point bN. Initially, with increasing b, the transition of the one-dimensional Neel domain wall to the asymmetric vortex-free Neel domain wall occurs; the energy of the latter is lower than that of the asymmetric Bloch wall. As b continues to increase, the vortex-free asymmetric Neel wall transforms to a double-vortex wall and its energy begins to exceed that of the asymmetric Bloch wall.

Sub-nanosecond switching of vortex cores using a resonant perpendicular magnetic field

We performed micromagnetic numerical studies on ultrafast switching of magnetic vortex cores (VCs) using a perpendicular magnetic field that oscillates at the eigenfrequency of a permalloy nanodisk. Our calculations show that a resonant magnetic field with amplitude of 30 mT stimulates strong axially symmetric magnetization oscillation and forces the vortex core to stay at the center of the nanodisk. The compression of the vortex core by spin wave leads to core reversal at 602 ps. This switching process is mediated by the propagation of a Neel wall across the sample thickness.

Effect of surface anisotropy on the dynamic properties of Néel domain walls in the films with planar anisotropy

This work is devoted to the investigation into the effect of surface magnetic anisotropy on the motion of domain walls in magnetic uniaxial films with an in-plane anisotropy in the range of thicknesses corresponding to the stability of one-dimensional Neel walls. It has been established that the surface anisotropy with a constant K s > 0 (K s < 0) increases (decreases) the velocity of both stationary and nonstationary motion of the domain wall, as well as the period of oscillations of the velocity of nonstationary motion. The magnitude of the critical field as a function of K s has been determined. Effect of surface anisotropy with K s < 0 on the scenario of the dynamic transformation of the domain-wall structure has been revealed.

Magnetic properties of sputtered Permalloy/molybdenum multilayers

In this work, we report the magnetic properties of sputtered Permalloy (Py: Ni80Fe20)/molybdenum (Mo) multilayer thin films. We show that it is possible to maintain a low coercivity and a high permeability in thick sputtered Py films when reducing the out-of-plane component of the anisotropy by inserting thin film spacers of a non-magnetic material like Mo. For these kind of multilayers, we have found coercivities which are close to those for single layer films with no out-of-plane anisotropy. The coercivity is also dependent on the number of layers exhibiting a minimum value when each single Py layer has a thickness close to the transition thickness between Neel and Bloch domain walls.