Dynamic Properties Of Domain Walls Research Articles

Domain wall velocity asymmetries driven by saturation magnetization gradients without a Dzyaloshinskii-Moriya interaction

The discovery of asymmetric domain wall velocities in magnetic alloys and multilayers without a heavy metal underlayer has given rise to claims of a large Dzyaloshinskii-Moriya interaction (DMI) driven by compositional non-uniformities. However, these non-uniformities also give rise to vertically non-uniform magnetic properties which can influence the dynamic properties of domain walls. In this work, we use micromagnetic simulations to show that vertical modulation of the saturation magnetization can give rise to asymmetric domain wall velocities for field and current driven motion, closely resembling the presence of a DMI. For field driven motion, by comparing the velocity asymmetries for varying saturation magnetization gradients, film thicknesses, and average saturation magnetization, we show that the magnitude of effective symmetry breaking field resulting from the non-uniformity can be comparable to those resulting from DMI. These effects scale with thickness and degree of modulation, mirroring recent results in single layer materials. However, the scaling of domain velocity asymmetries is largely suppressed for SOT driven domain dynamics. Therefore, great care needs to be taken when evaluating DMI in non-uniform systems.

Magnetic Domain Walls Moving in Curved Permalloy Nanowires under Continuous and Pulsed Fields

Magnetic domain walls created and propagated in curved permally nanowires under continuous and pulsed fields in a Lorentz microscope. Using such nanowires aims to create a single or multiple magnetic domain walls in typical areas of those structures, an external magnetic field then applies along the long axis of these nanowires. Following that the created domain walls are propagated from one end to the other end of each wire by increasing the continuous/pulsed field strength. At each increased field value, a Fresnel image is recorded. The obtained results show that the characteristics of those created and propagated domain walls are dependent on various parameters, i.e. connecting structures, wall types and chiralities. Corners between the straight and linking sections of those curved nanowires also play a crucial role along witth the local defects created in these wire-edges and surfaces where a point-defect is considered as a potential well that could pin/distort those created/propagated domain walls. By the aid of this observations, the dynamic properties of domain walls with the creating and propagating processes in those curved nanowires are exposed. These outcomes are vital to design novel domain wall trap structures supporting reproducible domain wall motions. That are of interest in providing a better understanding of multiple bits moving in the furure 3D racetrack memory, logic gates, shift register and other spintronic/computing devices.

Electrical detection of internal dynamical properties of domain walls

We report here on the electrical detection of the ferromagnetic resonance in a notched Py stripe using the spin rectification effect. A lock-in detection technique is used to measure the rectified voltage generated when applying a radio-frequency field. The multiple peaks associated with the presence of a domain wall, observed during field scans, are properly identified with the help of dynamical micromagnetic simulations. It is found that at any frequency below 6 GHz some specific areas of the inhomogeneous magnetization within the domain wall can find a resonant condition and generate a voltage by rectification of the induction currents in the circuit.

Domain wall roughness and creep in nanoscale crystalline ferroelectric polymers

We report piezo-response force microscopy studies of the static and dynamic properties of domain walls (DWs) in 11 to 36 nm thick films of crystalline ferroelectric poly(vinylidene-fluoride-trifluorethylene). The DW roughness exponent {\zeta} ranges from 0.39 to 0.48 and the DW creep exponent {\mu} varies from 0.20 to 0.28, revealing an unexpected effective dimensionality of ~1.5 that is independent of film thickness. Our results suggest predominantly 2D ferroelectricity in the layered polymer and we attribute the fractal dimensionality to DW deroughening due to the correlations between the in-plane and out-of-plane polarization, an effect that can be exploited to achieve high lateral domain density for developing nanoscale ferroelectrics-based applications.

Magnetic Symmetry of the Plain Domain Walls in the Plates of Cubic Ferro- and Ferrimagnets

For sequential examination of static and dynamic properties of domain walls (DWs) in magnetically ordered media it is necessary to take into account their magnetic symmetry [1, 2]. The complete symmetry classification of plane 180◦-DWs in magnetically ordered crystals [1], similar classification of these DWs with the Bloch lines in ferromagnets and ferrites [2] and magnetic symmetry classification of plane non-180◦-DWs (all possible DW types including 0◦-DWs [3]) in ferroand ferrimagnets [4] were carried out earlier. These DW symmetry classifications allows arbitrary crystallographic point symmetry group of the crystal. The influence of the spatially restricted sample surfaces on the DW magnetic symmetry was not considered in works [1–4]. The real magnetic sample restricts a spatial (3D) magnetization distribution. Therefore, it modifies the DW symmetry in general case. This paper presents the investigation of the influence of the restricted sample surfaces on the symmetry of the all possible (0◦-, 60◦-, 70.5◦-, 90◦-, 109.5◦-, 120◦and 180◦-DW [4, 5]) plane (i.e., DW with r0 A δ, where r0 is the curvature radius of the DW [1]) DWs in an arbitrary oriented plate of the cubic (crystallographic point symmetry group m3m) ferroand ferrimagnets.

Change in the magnetic state of a surface upon formation of hydrogen bonds during adsorption

It is established that reversible adsorption of water and methyl alcohol molecules, occurring via formation of hydrogen bonds, changes the dynamic properties of domain walls in the surface region of soft ferromagnets, as well as their initial static magnetic susceptibility. A mechanism is proposed for the effects revealed.

Magnetooptic Setup for Investigating the Dynamic Properties of Domain Walls in Thin Ferromagnetic Films

A magnetooptic Kerr setup is designed for investigating the dynamic properties of domain walls in thin ferromagnetic films subjected to an external magnetic field in the temperature range 20–150°C. With this setup, the method of interrupted magnetization is implemented and the magnetic (domain) structure is visualized based on the meridional Kerr effect. The domain structure is displayed on a PC monitor using a Nikon DXM 1200 high-resolution digital video camera. A dedicated software makes it possible to automatize measurements and data processing.

Perturbation theory for domain walls in the parametric Ginzburg-Landau equation.

We demonstrate that in the parametrically driven Ginzburg-Landau equation arbitrarily small nongradient corrections lead to qualitative differences in the dynamical properties of domain walls in the vicinity of the transition from rest to motion. These differences originate from singular rotation of the eigenvector governing the transition. We present analytical results on the stability of Ising walls, deriving explicit expressions for the critical eigenvalue responsible for the transition from rest to motion. We then develop a weakly nonlinear theory to characterize the singular character of the transition and analyze the dynamical effects of spatial inhomogeneities.

Study of magnetization reversal in submicron wires using the giant magnetoresistance effect

Magnetization reversal phenomena in submicron magnetic wire systems were investigated from magnetoresistance measurements. Wire arrays and also single wire samples consisting of two magnetic layers with different coercivities were prepared and the resistivity was measured by applying external magnetic fields. Owing to the non-coupled type of giant magnetoresistance effect, the resistivity of wires is enhanced in proportion to the relative area of antiparallel magnetization. It is demonstrated that the resistivity measurements can sensitively detect the magnetization reversal phenomena in submicron wires. The dynamic properties of domain walls in wire samples are studied and the velocity of domain wall propagation is estimated.

Domain walls in one-dimensional 3-periodic structure

The static and dynamic properties of domain walls in the incommensurate phase near the lock-in transition was studied numerically in the frame of the elastically hinged molecule model of crystal which was recently proposed by the present authors. The case of discommen-surations in the 3-periodic commensurate phase was discussed and the analytical expression for the displacements near the domain wall was derived in continum approximation. The interaction of the domain pattern with the external field, processes of collision and breakup of domain walls were investigated.

Static and dynamic properties of domain walls in KDP-type crystals

We have studied properties of domain walls within the mean field approximation of the pseudospin model appropriate for the description of KDP-type crystals. Besides the planar structure the domain wall of the step-like shape has been considered. The energetical barriers controlling the movement of the walls over the unit cells strongly depend on the temperature and causes slowing-down of the dynamics on cooling. In contrast to it the relaxation of domain walls localized inside a unit cell shifts to higher frequencies with decreasing temperature.

Antiferromagnetic domain walls.

Antiferromagnetic domain walls are shown to exhibit a nonvanishing total magnetic moment. This result is established numerically for a discrete spin system, as well as analytically within a suitable continuum approximation that leads to the nonlinear \ensuremath{\sigma} model extended to include anisotropy. The moment is due to certain parity-breaking terms that are implicit in arguments pertaining to the Haldane gap but have been missing in earlier treatments of domain walls. In this paper we present a study of both static and dynamical properties of domain walls in antiferromagnets with an easy-axis anisotropy, but some of the results should prove relevant also for weak ferromagnets.

Investigation of magnetization reversal in double layer FeNiCo/CoW films

The dependence of the dynamic properties of domain walls in FeNiCo layers on the CoW thickness is investigated. The FeNiCo thickness is about 80 nm. While the FeNiCo layer coercivity increases, domain wall mobility decreases in weak magnetic fields. The observed mechanism of layers interaction is associated with interaction between FeNiCo domain wall stray fields and the CoW layer. The possibility of the increase of coercivity without mobility loss is considered. >

Dynamic properties of domain walls in 3d ferroelectrics

Abstract MC simulations are used to investigate dynamic properties of domain walls in 3 dim. s.c. Ising systems. For zero field only diffuse wall motion takes place. We distinguish between short time diffusion (migration over less than a lattice constant - “inside a Peierls well”) and long time diffusion (over several lattice constants) and show that the diffusion constants exhibit Arrhenius - and T · Arrhenius-type temperature dependences respectively. These dependences as deduced from the simulations are corroborated by model calculations. An external field induces a drift of the average wall position zw(t). The latter function exhibits a characteristic steplike structure with statistically fluctuating step widths (lattice pinning). Introducing point defects with a stronger coupling in a plane parallel to the wall leads to a pronounced slowing down of the motion before and a speeding up directly after the defect plane. These effects are explained by a detailed investigation of the mechanism of wall motion in terms of cluster growth.

Dynamics of elliptic domain walls

With the use of the generalized Landau-Lifshitz-Bloch equation of motion for a ferromagnet at finite temperatures we investigate the dynamic properties of domain walls (DWs) taking into account their ellipticity, i.e., the deficit of the magnetization value M in the DW in comparison with that of the domains. The translational motion of elliptic DWs is accompanied by longitudinal relaxation, which governs the DW dynamics even in the case of small ellipticity, if the relaxation constants are small. The linear DW mobility μ calculated in the whole temperature range 0⩽ T⩽ T c shows a singular behavior in the point T=T ∗ where the elliptic DW restructures into a linear one. At low temperatures the calculated dependence of μ on the transverse field H x is consistent with the experimental observations. The longitudinal relaxation mechanism of the DW damping switches off for the DW velocities υ≳ υ 0≈ lΓ 1 ( l is the DW width, Γ 1 is the longitudinal relaxation rate), and the dependence υ( H) may show a hysteresis. We also derive Slonczewski-like reduced equations of motion for the DW parameters with an additional equation for the magnetization deficit. With the use of these equations the stability of the stationary DW motion is investigated. It is shown that in some cases there are three separate stable sections of the curve υ( H). Finally, the dynamic susceptibility of a ferromagnetic sample due to the displacement of elliptic domain walls is calculated.

Static and Dynamic Properties of Domain Walls in Crystals with Combined Anisotropy

Static and Dynamic Properties of Domain Walls in Crystals with Combined Anisotropy

Static and dynamic properties of domain walls in finite systems

Abstract Characteristics of domain walls and related properties in 2d Ising systems were calculated from the analytic theory and by Monte Carlo simulations. Both ideal lattices and lattices with point defects were considered. Static quantities determined and discussed as functions of temperature, interactions, and specimen size are: correlation-functions, -lengths and -(effective) decay constants for small separations, wall profiles and -widths, and free and internal energies and entropies of walls. Dynamic quantities considered are: individual wall migration paths - which exhibit a pronounced stepwise motion at low temperatures -, averaged paths and -velocities, and wall mobilities. The retarding effect of defects is discussed.

Dynamical properties of domain walls in garnet films

Domain wall resonance (DWR) has been investigated in a parallel stripe lattice using a new high sensitivity DWR excitation and detection technique. Experiments were performed on garnets with pure uniaxial and orthorhombic anisotropy. A strong dependence of the DWR frequency on the intrinsic medium parameters, such as the saturation magnetization and the anisotropy constants, was observed. The influence on the domain oscillation frequency of an in-plane field applied parallel and perpendicular to the wall plane was analyzed and the corresponding changes of the wall mass and restoring constant was determined in (111)- and (110)-oriented garnet films.

Peculiarities of dynamical properties of domain walls and phase boundaries in some ferroelectric single crystals and ceramics near phase transition

Abstract Low- and infralow frequencies dielectric spectra of some ferroelectric single crystals (RS, KTL, PMN, CDP) and ceramics (PZT, PSN, PLZT) are studied in the vicinity of phase transition temperatures. Oscillation parameters of 180° domain walls in weak fields are determined on the basis of phenomenological consideration of polydomain ferroelectrics dielectric properties. Quantitative estimates are presented for elasticity and viscosity coefficients, electric compliance and domain-walls displacement amplitude in field intensity of E ≈ 1V·m−1. The connection of the above parameters with “weak” and “strong” defects in crystals is discussed. Examples are adduced of revealing the processes of domain structure rearrangement, domain-wall pinning, polarized defects interrelations, formation of clusters and other phenomena associated with phase transition o in real ferroelectric single crystals and ceramics.

Mc-Studies of static and dynamic properties of domain walls

Abstract Static and dynamic properties of ferroelectric domain walls are studied by Monte-Carlo-simulations of the two-dimensional Ising model. The width, the free energy and the distribution of the center positions relative to the lattice are determined as functions of temperature and of the interaction anisotropy. Free energies are obtained separately for walls with centers nearly midway between and nearly coinciding with lattice planes, and the former are shown to be more stable. The motion of walls in external fields is studied for various temperatures. For the finite lattices considered, the walls are found to move rather stepwise, especially at low temperatures.