Cross-tie Walls Research Articles

In-situ observation of magnetic vortices and their manipulation by external fields in Tb(Co1−xFex)5 ribbons

Nontrivial spin textures including magnetic vortices, skyrmions, and merons possess intriguing properties that are suitable for the application in further spintronics devices. The stability of topological spin textures in a broad temperature span is imperative for their potential applications as information carriers. In this study, we show the in-situ observation of the magnetic domain configuration and their manipulation under temperature and in-plane magnetic field in amorphous or nanocrystalline Tb(Co1−xFex)5 (x = 0.5, 0.8, 1.0) ribbons using Lorentz transmission electron microscopy. The results demonstrate that the formation of magnetic vortices is highly dependent on the degree of amorphization of Tb(Co1−xFex)5 ribbons and intrinsic magnetic parameters, which can be controlled by the composition. The higher the Fe content, the stronger the degree of amorphous in Tb(Co1−xFex)5 ribbons. Whereby, high-density magnetic vortices develop inside domain walls in the TbFe5 ribbon. Moreover, no matter how the degree of amorphization changes, magnetic domain structures including vortices in these ribbons have good thermal stability in the temperature range of 120–300 K, while the evolution of magnetic domains under the application of magnetic field is greatly affected by the degree of amorphization. The cross-tie domain walls consisted of magnetic vortices and antivortices form as applying a proper magnetic field in TbFe5, in contrast with the unchanged magnetic domains in Tb(Co0.5Fe0.5)5. This study provides a guideline for the design of magnetic vortex materials and soft magnetic materials.

Real-Time Observation of Magnetic Domain Structure Changes with Increasing Temperature for Z-Type Hexagonal Ferrite.

Z-type hexagonal ferrites have recently received attention for their room-temperature magnetoelectric (ME), which is activated when the temperature at which the transverse-conical spin-state transitions to a ferrimagnetic state is increased. The changes in the magnetic domain structure at the transition have been well-documented; however, they are still not understood in detail. In the present study, Lorentz transmission electron microscopy (TEM) analysis combined with an in situ heating experiment was conducted to demonstrate the shift in magnetic domain structure during the transition from the transverse-conical spin arrangement to a ferrimagnetic spin order. The dynamics of the magnetic domain structure changes with the increasing temperature were acquired in real-time. At 490 K, the magnetization transition from the transverse-conical spin state to the ferromagnetic state was demonstrated. Cross-tie domain walls formed during the magnetic transition process. The increased effect of the demagnetizing field applied to the 180° magnetic domains was caused by a lower magnetocrystalline anisotropy (MCA) at the easy axis of magnetization.

In situ observation of magnetic vortex in amorphous and nanocrystalline ribbons

In this study, the factors influencing the nucleation of magnetic vortices in CeFeB ribbons were studied using Lorentz transmission electron microscopy. Magnetic vortices and cross-tie walls exist in Ce14Fe80B6 amorphous ribbons. Via proper annealing, the amorphous ribbon is crystallized into a nanocrystalline structure whereby a magnetic vortex can exist in grains with sizes of approximately 30∼150 nm, indicating the formation of magnetic vortices is closely related to shape limitation. Moreover, it has been demonstrated magnetic vortices are the intrinsic magnetic domain structures in the amorphous alloys due to their weak magnetocrystalline anisotropy. This study provides a way to promote magnetic vortex formation in the nanocrystalline structure.

Improvement of Inductor Performance Parameters With Thin 20 nm Permalloy Patterns

In earlier works, it was shown that the controlled formation of magnetic domains by tailoring shape, size and thickness of Permalloy patterns improves the performance of the inductor compared to bulk thin films. Also, it was shown that 60 nm thick micromagnetic patterns with cross tie walls give better performance compared to 92 nm thick patterns which have asymmetric Bloch walls, owing to larger stray fields. In this work, we compare 20 nm thin film patterns with 60 nm and 92 nm micromagnetic patterns. Experimental results show that inductor embedded with the thinner film i.e. 20 nm permalloy patterns having Neel domain wall provide more improvement in inductance density and quality factor as compared to thicker 60nm having cross-tie domain wall and 92 nm with asymmetric Bloch wall over a broad range of frequencies encompassing the 5G spectrum. There is a 118% peak increment in inductance density and a 204% peak improvement in quality factor in the inductor incorporated with 20 nm thin permalloy patterns.

Surface Roughness Influence on Néel-, Crosstie, and Bloch-Type Charged Zigzag Magnetic Domain Walls in Nanostructured Fe Films.

Charged magnetic domain walls have been visualized in soft magnetic nanostructured Fe thin films under both static and dynamic conditions. A transition in the core of these zigzagged magnetic walls from Néel-type to Bloch-type through the formation of crosstie walls has been observed. This transition in charged zigzagged walls was not previously shown experimentally in Fe thin films. For film thicknesses t < 30 nm, Néel-type cores are present, while at t ≈ 33 nm, walls with crosstie cores are observed. At t > 60 nm, Bloch-type cores are observed. Along with the visualization of these critical parameters, the dependence on the film thickness of the characteristic angle and length of the segments of the zigzagged walls has been observed and analyzed. After measuring the bistable magneto-optical behavior, the values of the wall nucleation magnetic field and the surface roughness of the films, an energetic fit to these nucleation values is presented.

Coupling between vortices and antivortices in a cross-tie wall studied by time-resolved SEMPA

Using time-resolved scanning electron microscopy with polarization analysis (TR-SEMPA) the dynamics of magnetic vortices and antivortices in a cross-tie wall is investigated. Under quasistatic external field drive both quasiparticles oscillate at opposite phase to each other (in analogy to an optical mode). An additional common motion, which stems from the finite boundary conditions in a patterned FeCoSiB rectangle, is observed (in analogy to an acoustic mode), leading in total to a stronger field-dependent displacement for the vortices compared to the antivortices. Both types of magnetic solitons are mutually coupled via the minimization of the total magnetic energy of the structure. It is shown that the TR-SEMPA results can be explained via coupling in the framework of micromagnetic simulations and that the coupling constants can be extracted. The motion is composed from the contributions of the two terminating vortices and a mutual vortex-antivortex coupling along the chain.

In situ observation of magnetic vortex manipulation by external fields in amorphous CeFeB ribbon

In this study, we show the real-space observation of the magnetic domain configuration in amorphous Ce14Fe80B6 ribbon using Lorentz transmission electron microscopy. Cross-tie domain walls composed of magnetic vortices (Vs) and antivortices (AVs) are observed. The evolution of Vs/AVs manipulated by temperature, in-plane magnetic field, and electrical current is clearly demonstrated. Magnetic V nucleation and annihilation in pair are observed because of the stimulus of external fields.

Three-dimensional magnetization structures revealed with X-ray vector nanotomography.

In soft ferromagnetic materials, the smoothly varying magnetization leads to the formation of fundamental patterns such as domains, vortices and domain walls. These have been studied extensively in thin films of thicknesses up to around 200 nanometres, in which the magnetization is accessible with current transmission imaging methods that make use of electrons or soft X-rays. In thicker samples, however, in which the magnetization structure varies throughout the thickness and is intrinsically three dimensional, determining the complex magnetic structure directly still represents a challenge. We have developed hard-X-ray vector nanotomography with which to determine the three-dimensional magnetic configuration at the nanoscale within micrometre-sized samples. We imaged the structure of the magnetization within a soft magnetic pillar of diameter 5 micrometres with a spatial resolution of 100 nanometres and, within the bulk, observed a complex magnetic configuration that consists of vortices and antivortices that form cross-tie walls and vortex walls along intersecting planes. At the intersections of these structures, magnetic singularities-Bloch points-occur. These were predicted more than fifty years ago but have so far not been directly observed. Here we image the three-dimensional magnetic structure in the vicinity of the Bloch points, which until now has been accessible only through micromagnetic simulations, and identify two possible magnetization configurations: a circulating magnetization structure and a twisted state that appears to correspond to an 'anti-Bloch point'. Our imaging method enables the nanoscale study of topological magnetic structures in systems with sizes of the order of tens of micrometres. Knowledge of internal nanomagnetic textures is critical for understanding macroscopic magnetic properties and for designing bulk magnets for technological applications.

Microstructure investigation and magnetic study of permalloy thin films grown by thermal evaporation

We study the effect of thickness on the structural and magnetic properties of permalloy thin films, evaporated on glass substrate. The films thicknesses range from 16 to 90 nm. From X-ray diffraction spectra analysis, we show that the thinner films present a ‹1,1,1› preferred orientation. However, the thicker films exhibit a random orientation. The grains size increases and the lattice parameter decreases with increasing thickness. The magnetic force microscopy observations display cross-tie walls features only for the two thicker films (60 and 90 nm thick films). The magnetic microstructure, carried out by Kerr microscopy technique, shows the presence of magnetic domains changing with the direction of applied magnetic field. The coercive field, H c , was found to decrease from 6.5 for 16 to 1.75 Oe for 90 nm. All these results will be discussed and correlated.

Effects of a spin-polarized current assisted Ørsted field in magnetization patterning

A spin-polarized electrical current leads to a variety of periodical magnetic structures in nanostripes. In the presence of the Ørsted field, which always assists an electrical current, the basic types of magnetic structures, i.e., a vortex-antivortex crystal and cross-tie domain walls, survive. The Ørsted field prevents saturation of the nanostripe and a longitudinal domain wall appears instead. Possible magnetization structures in stripes with different geometrical and material properties are studied numerically and analytically.

Understanding the dense stripe domains in soft magnetic film

The FeCoAlON films in different thickness have been studied. The films show crosstie wall on 50nm and the Bloch line with weak stripe domains on 110nm. It means the stripe domains generated within a domain. The in-plane hysteresis loops and in-plane field MFM images of the stripe domains samples show a typical magnetization process mainly caused by magnetization moment rotation. For such film with stripe domain, the magnetization reversal differs from traditional domain wall displacements. The FeCoZr films in different Zr component also have been studied. The smooth stripe domains found for the amorphous. Due to the demagnetizing field generated in the stripe domains, a model of the magnetization distribution has been found. The free energy in the stripe domains film has been calculated and the relation between the domain width and the deviation angle (the angle of the magnetization and the external field) has been derived. It is found that the minimal of the static magnetic energy and the exchange energy can also result in the dense stripe domains in soft magnetic film.

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.

Magnetic Microstructure of Rolled‐Up Single‐Layer Ferromagnetic Nanomembranes

The magnetic microstructure of rolled-up magnetic nanomembranes is revealed both theoretically and experimentally. Two types of nanomembranes are considered, one with a non-magnetic spacer layer and the other without. Experimentally, by using different materials and tuning the dimensions of the rolled-up nanomembranes, domain patterns consisting of spiral-like and azimuthally magnetized domains are observed, which are in qualitative agreement with the theoretical predictions.

Study of thermally evaporated thin permalloy films by the Fresnel mode of TEM and AFM

Thin permalloy films 10nm and 60nm thick were investigated. They were thermally evaporated at an incidence angle of 0° in a vacuum of about 10−5mbar. The magnetic structure of the films was observed with the Fresnel mode of transmission electron microscopy (TEM), while their morphological structure was revealed using atomic force microscopy (AFM). The magnetic structure consisted of domains typically 10–30μm in size. The films were substantially magnetized in the plane of the film. The domain walls of Néel type as well as cross-tie walls occurred in the films 10nm thick, while in the films 60nm thick the presence of cross-tie walls was only observed. The presence of cross-tie walls in the films 10nm thick is reported for the first time. The coexistence of Néel type and cross-tie walls in the films 10nm thick means that their wall energies are comparable at this film thickness, and this statement is supported by the results of theoretical works. The morphological structure of the films was composed of nanocrystalline grains smaller than about 30nm in size; the films 60nm thick had grains somewhat larger in size than the films 10nm thick. The random distribution of the magnetocrystalline anisotropy of the individual nanocrystalline grains is found to be practically averaged out by exchange interaction, which leads consequently to the strongly reduced effective magnetic anisotropy and the wide magnetic domains on a 10μm scale.

Domain wall dynamics in a single CrO2 grain

Recently we have reported on the magnetization dynamics of a single CrO2 grain studied by micro Hall magnetometry (P. Das et al., Appl. Phys. Lett. 97 042507, 2010). For the external magnetic field applied along the grain's easy magnetization direction, the magnetization reversal takes place through a series of Barkhausen jumps. Supported by micromagnetic simulations, the ground state of the grain was found to correspond to a flux closure configuration with a single cross-tie domain wall. Here, we report an analysis of the Barkhausen jumps, which were observed in the hysteresis loops for the external field applied along both the easy and hard magnetization directions. We find that the magnetization reversal takes place through only a view configuration paths in the free-energy landscape, pointing to a high purity of the sample. The distinctly different statistics of the Barkhausen jumps for the two field directions is discussed.

Magnetic antivortex-core reversal by rotating magnetic fields

Magnetic vortices and antivortices are usually observed coexisting in cross-tie walls within ferromagnetic thin films. An antivortex can be isolated by utilizing the shape anisotropy. The isolation offers the possibility to investigate its fundamental dynamics as a two-dimensional oscillator in a confining potential. Hitherto, the gyration mode of vortices and its excitation has been studied intensely. Here, a detailed investigation of the coupling of an antivortex to in-plane rotating magnetic fields is presented. The resonant response is imaged by time-resolved scanning transmission x-ray microscopy to determine the amplitude as well as the phase of the gyration. The experimental results are compared with the analytical model of a two-dimensional harmonic oscillator derived from the Thiele equation. As a cause for deviations from the model, a buckled energy landscape due to local defects is discussed.

Vortex-antivortex creation and annihilation on CoFeB crosstie patterns

The creation and annihilation of vortex and antivortex pairs in crosstie walls on CoFeB islands were investigated using magnetic force microscopy (MFM) in the presence of applied field. The first vortex-antivortex pair (V-AV) is nucleated in a 4-domain Landau pattern from a section of a 180° degree Néel wall as a consequence of the formation of an end domain closure pattern. The V-AV pair creation satisfies the condition for the closure domain vortices to have the same chiralities. Pair annihilation occurs in two steps: a rapid advance of the vortices toward the more or less fixed antivortices followed by a deceleration caused by the decrease in local susceptibility. There is a spread in the annihilation fields that does not depend upon the core polarity but is determined by energy minimization.

Magnetization dynamics of a CrO2 grain studied by micro-Hall magnetometry

Micro-Hall magnetometry is employed to study the magnetization dynamics of a single, micron-size CrO2 grain. With this technique, we track the motion of a single domain wall, which allows us to probe the distribution of imperfections throughout the material. An external magnetic field along the grain’s easy magnetization direction induces magnetization reversal, giving rise to a series of sharp jumps in magnetization. Supported by micromagnetic simulations, we identify the transition to a state with a single cross-tie domain wall, where pinning/depinning of the wall results in stochastic Barkhausen jumps.

Measurement of the remnant magnetic-field in Lorentz mode using permalloy

A novel method was reported to measure the remnant magnetic field in Lorentz mode in a FEI Tecnai F20 transmission electron microscope equipped with a Lorentz lens. The movement of the circle Bloch line of the cross-tie wall in Permalloy is used to measure the remnant magnetic field by tilting the specimen and adjusting the objective lens current. The remnant magnetic field is estimated to be about 17 Oe, in a direction opposite to that of the objective lens magnetic field. The remnant magnetic field can be compensated by adjusting the value of the objective lens current.