Domain Walls In Ferromagnetic Films Research Articles

Orientation and internal structure of domain walls in ferromagnetic films with anisotropic Dzyaloshinskii-Moriya interaction

We present an analytical study of domain-wall internal structure and orientation in a ferromagnetic film with out-of-plane anisotropy and the anisotropic interfacial Dzyaloshinskii-Moriya interaction (iDMI). The term “anisotropic” means that the iDMI constant is different for different in-plane directions. The interplay between the magnetostatic interaction and the iDMI in a domain wall defines both its structure and orientation. In the case of the isotropic iDMI there is no preferable orientation of the DW in the film plane leading to formation of labyrinth domain structure with random shapes of the domains. In the case of the anisotropic iDMI an oriented domain stripe structure and zigzag domains appear. Depending on system parameters, either the Bloch domain walls, or the Néel ones or the hybrid canted walls realize in the magnetic film. The spatial orientation of the DW and the orientation of the magnetization rotation plane in the DW are intimately related by a simple linear ratio. The analytical results are corroborated by micromagnetic simulations.

SAW assisted domain wall motion in Co/Pt multilayers

The motion of domain walls in thin ferromagnetic films is of both fundamental and technological interest. In particular, the ability to use drivers other than magnetic fields to control the positions of domain walls could be exciting for memory applications. Here, we show that high frequency dynamic strain produced by surface acoustic waves is an efficient driver of magnetic domain walls in ferromagnetic films with perpendicular anisotropy. A standing surface acoustic wave of resonant frequency 96.6 MHz increases the domain wall velocities in thin films of [Co/Pt]n by an order of magnitude compared to magnetic fields alone. This effect is highly resonant, effectively ruling out thermal effects, and the velocity shows distinct variations in the domain wall velocity at the nodes and antinodes of the standing wave. The data indicate that standing strain waves can drive the domain wall motion from the creep to the flow regime as the amplitude increases. Hence, strain waves could provide an alternative route to rapid domain wall motion.

Asymmetrical pinning of vortex domain walls in ferromagnetic films in areas with increased saturation magnetization

The investigation is devoted to the pinning of vortex domain walls existing in ferromagnetic films with planar anisotropy on areas with increased saturation magnetization. The method of the investigation was the numerical micromagnetic simulation in the framework of a two-dimensional model of the magnetization distribution. The anisotropy was found of the domain wall depinning field magnitude for different directions of the external field. The depinning field is investigated as a function of sizes and location of the area with the increased saturation magnetization, the difference of magnetizations of this area and the main part of the film, and film thickness. The physical causes of derived laws have been determined.

Multifractality in domain wall dynamics of a ferromagnetic film

We investigate the multifractal properties in the dynamics of domain walls of a ferromagnetic film. We apply the Multifractal Detrended Fluctuation Analysis method in experimental Barkhausen noise time series measured in a 1000-nm-thick Permalloy film under different driving magnetic field frequencies, and calculate the fluctuation function F_{q}(s), generalized Hurst exponent h(q), multifractal scaling exponent τ(q), and the multifractal spectrum f(α). Based on this procedure, we provide experimental evidence of multifractality in the dynamics of domain walls in ferromagnetic films and identify a rich and strong multifractal behavior, revealed by the changes of the scaling properties of over the entire Barkhausen noise signal, independently of the driving magnetic field rate employed in the experiment.

Universal Nonstationary Dynamics at the Depinning Transition

We study the nonstationary dynamics of an elastic interface in a disordered medium at the depinning transition. We compute the two-time response and correlation functions, found to be universal and characterized by two independent critical exponents. We find a good agreement between two-loop functional renormalization group calculations and molecular dynamics simulations for the scaling forms, and for the response aging exponent theta_{R}. We also describe a dynamical dimensional crossover, observed at long times in the relaxation of a finite system. Our results are relevant for the nonsteady driven dynamics of domain walls in ferromagnetic films and contact lines in wetting.

Moving domain walls in ferromagnetic films with parallel anisotropy

Abstract The dependence of wall mass and energy on wall velocity in thin Permalloy films is calculated by generalizing the Ritz model of Jakubovics (1978) for two-dimensional Bloch walls. In films 1000 A thick, at low velocities, the wall mass is found to be about 52 times the value expected in bulk Permalloy. The mass increases with increasing velocity, v. Up to v≃3 × 104cm s−1 the increase is similar to that of a relativistic mass, namely proportional to (1−v 2/v ∞ 2)−½, with a critical velocity v ∞ ≈ 14.6 × 104 cm s −1 . For v 3 × 104 cm s−1 the increase of mass with velocity becomes more rapid. The model breaks down for velocities above ≃ 6 × lO4cm s−1.

Two-dimensional domain wall in (100) Ni films

The general two-dimensional model for 180° domain walls in ferromagnetic films is used here to study their structure in (100) Ni platelets. The magnetization component perpendicular to the film plane in the wall is considerably larger than in Fe, but its dependence on the anisotropy constant is disappointingly small.

Two−dimensional domain walls in ferromagnetic films. I. General theory

A model with two Ritz parameters, for which the magnetostatic self−energy can be calculated analytically, is presented for the wall separating two antiparallel domains in ferromagnetic films. The other energy terms can be only partly calculated analytically, while part of the calculations must be done by numerical computations. However, these computations are much simpler than in wall studies which relied directly on such computations, and also can be approximated at each stage by the first few terms in an analytic power series. It is shown that the model can be generalized to yield a higher accuracy in the wall structure and energy, but only at the expense of a large complication of the calculation, which

Energy of One-Dimensional Domain Walls in Ferromagnetic Films

The energy of a one-dimensional 180° domain wall in a ferromagnetic film is minimized for all possible configurations of the magnetization distribution in the wall, using an approximate expression for the magnetostatic self-energy. It is shown analytically that there is no solution other than Néel or Bloch wall. There is, however, some degeneracy, which indicates a breakdown of the one-dimensional calculation, in the region of parameter values where cross-tie walls are observed experimentally. The Bloch wall energy for this approximation is shown to be rather close to the absolute minimum computed by Brown and LaBonte. Néel walls are found to have an infinitely long tail, the behavior being that of tanh. Using the functional form of this approximate minimization in the exact energy expressions, it is shown that the Néel wall energy is at most a few percent below previous estimations.

The Determination of Domain Wall Thickness in Ferromagnetic Films by Electron Microscopy

Domain walls in ferromagnetic films may be imaged by out-of-focus electron microscopy. The wall images are light or dark depending on the relative sense of the magnetization either side of the wall, the incident electron beam being diverged or converged about the wall position. A relationship is obtained between the image widths of light and dark walls, for all distances out of focus and the true domain wall width. Values obtained for evaporated polycrystalline films are (to ±30%) permalloy, 1800 A; iron, 1000 A; nickel, 550 A.