Modification of the domain wall structure and generation of submicron magnetic formations by local optical irradiation

Experimental and theoretical investigations are made of the generation of vertical Bloch lines in a magnetic iron garnet film exposed to pulsed optical radiation. High-speed photography and anisotropic dark-field microscopy are used to study characteristic features of the generation of Bloch lines and domain structure relaxation processes after the local action of a laser pulse. Optimum optical irradiation parameters to ensure the controlled generation of Bloch lines are established. A theoretical model is developed which links the generation of Bloch lines to the migration of domain walls induced by local changes in the distribution of the degaussing fields caused by a reduction in magnetization with temperature at the optical radiation focusing point. The experimental results indicate that the controlled formation of magnetic structures smaller than or of the order of 0.1 μm by local optical irradiation is quite feasible.     

Domain walls and ising-BLOCH transitions in parametrically driven systems

Parametrically driven systems sustaining sech solitons are shown to support a new kind of localized state. These structures are walls connecting two regions oscillating in antiphase that form in the parameter domain where the sech soliton is unstable. Depending on the parameter set the oppositely phased domains can be either spatially uniform or patterned. Both chiral (Bloch) and nonchiral (Ising) walls are found, which bifurcate one into the other via an Ising-Bloch transition. While Ising walls are at rest Bloch walls move and may display secondary bifurcations leading to chaotic wall motion.     

Nonlinear dynamics of domain walls with cross-ties

The dynamic behavior of a domain wall with cross-ties is analyzed on the basis of micromagnetic simulation with exact allowance for all main (exchange, magnetoanisotropic, and magnetostatic) interactions in thin magnetically uniaxial ferromagnetic films with planar anisotropy. It is found that the peculiarities of motion of such domain walls are closely related to the behavior of topological defects in the magnetization distribution (generation, motion, and annihilation of vortex–antivortex pairs on the film surface and Bloch points). We observe three different regimes of motion (stationary, periodic, and turbulent regimes), each of which is realized in a certain range of fields oriented along the easy magnetization axis. It is shown that the experimentally observed dynamic bends of the walls with cross-ties are determined by the type of motion of vortices and antivortices. The velocities of domain walls in different regimes are calculated, and the dynamic configurations of the magnetization and existing dynamic transitions between them are investigated.     

The increase of the spin-transfer torque threshold current density in coupled vortex domain walls

We have studied the dependence on the domain wall structure of the spin-transfer torque current density threshold for the onset of wall motion in curved, Gd-doped Ni(80)Fe(20) nanowires with no artificial pinning potentials. For single vortex domain walls, for both 10% and 1% Gd-doping concentrations, the threshold current density is inversely proportional to the wire width and significantly lower compared to the threshold current density measured for transverse domain walls. On the other hand for high Gd concentrations and large wire widths, double vortex domain walls are formed which require an increase in the threshold current density compared to single vortex domain walls at the same wire width. We suggest that this is due to the coupling of the vortex cores, which are of opposite chirality, and hence will be acted on by opposing forces arising through the spin-transfer torque effect.     

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 Néel 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.     

Generation of mesoscopic magnetic structures by local laser action

New effects are observed wherein the internal structure of the domain walls in a thin magnetic iron garnet film are modified by the action of focused laser radiation. A single laser pulse with increasing power gives rise to the following: 1) displacement of vertical Bloch lines in a domain wall; 2) generation of a pair of vertical Bloch lines on initially line-free walls; and, 3) an irreversible change in shape of a domain wall and the domain structure as a whole. The mechanism leading to the generation and displacement of Bloch lines is connected with the motion of domain walls which is induced by a local change in the distribution of demagnetizing fields as a result of a heating-induced decrease of the magnetization in the focal spot of the laser radiation. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 6, 398–402 (25 September 1997)     

Micromagnetic analysis of unusual, V-shaped domain transitions in MnAs nanowires

V-shaped domain transitions in α‐MnAs nanowires were investigated by micromagnetic simulations. These rather unusual domain patterns are commonly observed experimentally by surface-sensitive magnetic imaging techniques. It has been speculated that the accompanying inclined domain walls in MnAs are the result of either an exchange biasing effect between ferromagnetic α‐MnAs wires and antiferromagnetic β‐MnAs wires or possibly due to competing exchange mechanisms in MnAs. Here we present evidence that these domain features are in fact transitions between three-dimensional flux-closure domains of opposite chirality and can therefore rule out the involvement of an antiferromagnetic biasing effect or anisotropic exchange. The formation of the energetically unfavorable V-shaped domain transitions is discussed in the context of the magneto-structural phase transition of the sample.     

Controlling domain wall chirality by combining hard and soft magnetic materials in planar nanostructures with wire-ring morphology

Vortex domain walls (DWs) are characterized by their chirality, an important property that needs to be controlled for the use of such walls in potential technological applications. In this work we explore a wire-ring structure in which we have alternate hard and soft magnetic materials. Our results evidence that, depending on the materials, it is possible to control the DW chirality when it goes through the ring section. Therefore, this system can be used as a device that controls domain wall chirality.     

Magnetic symmetry based definition of the chirality in the magnetically ordered media

Different definitions of the chirality have been considered for their applications to the micromagnetic structures. It was suggested to use magnetic symmetry based Barron chirality definition. The Barron chirality was obtained for all magnetic domain walls as the example. The symmetry based classification of the domain walls has been used for the chirality determination.     

Interaction between superconducting vortices and a Bloch wall in ferrite garnet films

A theoretical model for how Bloch walls occurring in in-plane magnetized ferrite garnet films can serve as efficient magnetic micromanipulators is presented. As an example, the walls' interaction with Abrikosov vortices in a superconductor in close contact with a garnet film is analyzed within the London approximation. The model explains how vortices are attracted to such walls, and excellent quantitative agreement is obtained for the resulting peaked flux profile determined experimentally in NbSe(2) using high-resolution magneto-optical imaging of vortices. In particular, this model, when generalized to include charged magnetic walls, explains the counterintuitive attraction observed between vortices and a Bloch wall of opposite polarity.     

Magnetic force microscopy studies of domain walls in nickel and cobalt films

A magnetic force microscopy is used to examine the domain walls in nickel and cobalt films deposited by argon ion sputtering. Thin nickel films deposited at high substrate temperatures exhibit coexistent Bloch and Neel walls. Films grown at room temperature display alternative Bloch lines with cap switches. These films agglomerate to form grains after annealed at high temperatures. The film composed of larger grains behaves better nucleation implying magnetic domains of closure, while the film composed of smaller grains exhibits more defects implying alternative Bloch lines. We have also observed domain displacements and cap switches, which occur due to precipitation of particles in small grain size films. Stripe domains are observed for film thicknesses larger than 100 nm. They become zigzag cells when an external field of 1.5 T is applied perpendicular to the surface of the films. This experiment indicates that the domain sizes in thin films and the strip widths for thick films both depend on the square-root of the film thickness, which varies from 5 to 45 nm and from 100 to 450 nm, respectively.     

Transition micromagnetic structures in asymmetric vortexlike domain walls (Static solutions and dynamic reconstructions)

The possible types of transition structures with a three-dimensional magnetization distribution over regions in the vortex asymmetric domain walls that exist in magnetically uniaxial soft magnetic films with in-plane anisotropy are studied by computer simulation in terms of a micromagnetic approach. It is shown that the possible structure types include both the type of vertical Bloch lines that was discussed earlier in other works and new types, namely, singular (Bloch) points and clusters consisting of vertical Bloch lines and Bloch points. The spatial configurations of the transition structures are calculated and their topological properties are found. The numerical simulation of the dynamics of closely spaced substructure regions reveals various scenarios of their interaction, including annihilation accompanied by energy release and the excitation of nonlinear waves.     

Bloch lines in domain walls of antiferromagnets

Approximants describing the substructure of 180° domain walls in antiferromagnets with a high uniaxial anisotropy are derived. The structure and spectrum of Bloch lines separating the parts of domain walls with oppositely directed antiferromagnetic vectors are determined.     

Controlled observation of a nonequilibrium Ising-Bloch transition in a nonlinear optical cavity

We report the controlled observation of the nonequilibrium Ising-Bloch transition in a broad area nonlinear optical cavity (a quasi-1D single longitudinal-mode photorefractive oscillator in a degenerate four-wave mixing configuration). Our experimental technique allows for the controlled injection of the domain walls. We use cavity detuning as control parameter and find that both Ising and Bloch walls can exist for the same detuning values within a certain interval of detunings; i.e., the Ising-Bloch transition is hysteretic in our case. A complex Ginzburg-Landau model is used for supporting the observations.     

The sandwich domain structure in a Fe-based amorphous ribbon with uniaxial magnetic anisotropy

The formation and motion of two domain walls parallel to the ribbon surface are discovered during its dynamic magnetic reversal. The domain walls form near by the middle plane of a ribbon and move to its opposite main surfaces with different velocities.     

Experimental investigation of the magnetic structure of domain walls in thin ferromagnetic films

The structure of domain walls in thin magnetic films has been studied by the Lorentz method using electron microscopy. The possible existence of the coinciding and opposite directions of rotation of the magnetization vector in Néel domain walls has been proved experimentally. The domain walls separating 90° domains have been found in single-crystal magnetic films. These walls consist of domains with a considerably smaller area than 90° domains.     

Solitary flexural waves on a supersonic domain wall in yttrium orthoferrite

Solitary flexural waves on a supersonic domain wall in yttrium orthoferrite are observed and investigated. These waves have a sharp leading edge and a protracted trailing edge, reminiscent of the waves accompanying moving vertical Bloch lines in iron garnet films. The total velocity of the solitary flexural waves in yttrium orthoferrites for all observed amplitudes equals the maximum velocity of the domain walls. Two solitary waves with identical amplitudes colliding head-on are annihilated. The waves possess topological charges, and they move and form dynamic profiles under the influence of gyroscopic forces. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 760–765 (25 May 1997)     

Experimental evidence of the magnetic moment of Bloch walls in garnet films

Differential susceptibility measurements in low in-plane fields showed up a new peak assumed to be caused by magnetization reversal inside the Bloch walls (BW). Highly sensitive measurements carried out subsequently on different domain structures yielded various new parameters such as the BW hysteresis curve, the integral saturated and remanent BW moments and the internal BW coercivity. The mechanism proposed for the observed phenomena is based on the reversible and irreversible motion of Bloch lines inside the Bloch walls.     

Fourier algorithm for four-wave-mixing signals from optically dense systems with memory

A triple Fourier-transform algorithm for generating and propagating femtosecond four-wave-mixing signals in optically thick samples is demonstrated. The algorithm has a dynamic range that is useful for tests of theory and simulations of experiments with an arbitrary nonlinear response. Although two-pulse echoes decay faster as optical density increases for a Bloch model, we find that systems with memory exhibit the opposite trend.     

Sudden critical current drops induced in S/F structures

In the search for new physical properties of S/F structures, we have found that the superconductor critical current can be controlled by the domain state of the neighboring ferromagnet. The superconductor is a thin wire of thickness d_s ≈2ξ_S. Nb/Co and Nb/Py (Permalloy Ni₈₀Fe₂₀) bilayer structures were grown with a significant magnetic anisotropy. Critical current measurements of Nb/Co structures with ferromagnet thickness d_F > 30 nm show sudden drops in two very defined steps when the measurements are made along the hard axes direction (i.e. current track parallel to hard anisotropy axes direction). These drops disappear when they are made along the easy axis direction or when the ferromagnet thickness is below 30 nm. The drops are accompanied by vortex flux flow. In addition magnetorestistance measurements close to T_C show a sharp increase near saturation fields of the ferromagnet. Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet thickness d_F ~ 50 nm along the easy anisotropy axes. These results are explained as being due to spontaneous vortex formation and flow induced by Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain walls produce a 2D vortex-antivortex lattice structure.