Bubble Domain Research Articles

Dynamical regimes of a paramagnetic particle circulating a magnetic bubble domain

We study the dynamics of a single paramagnetic colloidal particle dispersed in water and circulating around a cylindrical magnetic domain ("magnetic bubble") when driven by an external rotating magnetic field. We record the particle trajectories and measure its angular displacement by changing the strength, frequency, and ellipticity of the applied magnetic field and show that this simple system exhibits several interesting phenomena, from synchronous-asynchronous rotations, to localized oscillations. We complement the experimental results with numerical simulations which explore the dynamical regimes of the rotating particle.

High‐Density Periodically Ordered Magnetic Cobalt Ferrite Nanodot Arrays by Template‐Assisted Pulsed Laser Deposition

AbstractA novel nanopatterning method using pulsed laser deposition through an ultrathin anodic aluminium oxide (AAO) membrane mask is proposed to synthesize well‐ordered nanodot arrays of magnetic CoFe2O4 that feature a wide range of applications like sensors, drug delivery, and data storage. This technique allows the adjustment of the array dimension from ∼35 to ∼300 nm in diameter and ∼65 to ∼500 nm in inter‐dot distance. The dot density can be as high as 0.21 Terabit in.−2. The microstructure of the nanodots is characterized by SEM, TEM, and XRD and their magnetic properties are confirmed by well‐defined magnetic force microscopy contrasts and by hysteresis loops recorded by a superconducting quantum interference device. Moreover, the high stability of the AAO mask enables the epitaxial growth of nanodots at a temperature as high as 550 °C. The epitaxial dots demonstrate unique complex magnetic domains such as bubble and stripe domains, which are switchable by external magnetic fields. This patterning method creates opportunities for studying novel physics in oxide nanomagnets and may find applications in spintronic devices.

Photo-induced control of bubble domains in chiral–nematic liquid crystal by a violet laser beam

The stable bubble domains generated by mixing 10% of chiral molecules into an azobenzene liquid crystal (LC)-doped nematic host can be optically controlled by a violet laser beam (415 nm). The photon-induced reversible trans–cis photo-isomerisation of azobenzene changes the helical twisting power (HTP) of LC mixtures in which the HTP of cis-azobenzene LC is lower than trans-azobenzene LC. Under the irradiation of an optical field (>20 mW cm − 2), the helical pitch distance, which is inverted proportional to the HTP, increases and the bubble domains disappear. Immediate obstruction of laser light irradiation initiates cholesteric nucleation, merging of domains and the subsequent generation of stably dispersed bubble domains.

Structures of confined vortex breakdown in constant diameter pipe flow

Numerical solutions of three-dimensional, incompressible and unsteady Navier-Stokes equations for constant diameter swirling pipe flows are used to study vortex breakdown, including the detailed flow structures in the bubble domain and the “tail” behind the bubble during the vortex breakdown, and a comparison is made between the numerical solutions and the experimental results.

Morphological and microstructural study of L1 0-ordered FePt and L1 0-FePt/Fe ultrathin films grown by UHV e-beam evaporation technique

Ultrathin films of L1 0-ordered FePt alloy with different thickness were grown by UHV e-beam evaporation technique. In this ultrathin regime, the increase of the thickness induces a strong improvement of the magnetic properties followed by a decrease of the grain size. Starting from these hard layers, FePt/Fe/FePt trilayers with different thickness of the outermost layer have been grown. The samples show a single-phase magnetic behaviour and a strong perpendicular anisotropy thanks to the exchange-coupling that established at the soft/hard interfaces. By increasing the thickness of the outer layer, a strong reduction of the switching field distribution has been obtained with the appearance of a predominant exchange-type interaction among the magnetic grains. The morphology is characterized by well separated and elongated islands, while the magnetic pattern shows finely dispersed bubble domains.

Magnetic Force Microscopy on Co/Pt Multilayers

Co/Pt multilayers reside among the best candidates for perpendicular magneto-optic recording. In this work, Co/Pt multilayers were prepared by electron-beam evaporation under ultrahigh vacuum conditions on polyimide. X-ray diffraction measurements revealed the high quality of multilayer stacking. Magneto-optic polar Kerr effect experiments were used in order to obtain magnetization hysteresis loops of the films. We have studied the magnetic-domain morphology on the surface of the films via Magnetic Force Microscopy. The field applied during these measurements was 2.3 kOe oriented perpendicular to the film plane; this field seems to stabilize and enhance out-of-plain stripe domains against in plain domains that may be expected from magnetization curves. Finally, we observed that when the applied field approaches the magnetic saturation field, then the domain morphology turns to be dominated by bubble domains.

Stationary structure of the domain wall of the soft magnetic bubble domain

A simple method for calculating the barrel-like curvature of a bubble domain is developed. The results obtained are compared with the results of the Thiele theory.

Evolution of stripe and bubble domains in antiferromagnetically coupled[(Co/Pt)8/Co/Ru]18multilayers

The magnetization process of antiferromagnetically coupled ${[{(\text{Co}/\text{Pt})}_{8}/\text{Co}/\text{Ru}]}_{18}$ multilayers with perpendicular anisotropy is investigated via magnetic force microscopy at room temperature by imaging the domain configuration in magnetic fields. In the zero-field state, due to the perpendicular anisotropy, stripe domains characteristic for ferromagnetic coupling are observed. By increasing the external magnetic field, the domain configuration first modifies gradually, then transforms from continuous into isolated stripes, and changes in the end into bubbles, which collapse at higher fields. A theoretical model previously developed for bubble domains in single layer films is adapted for arbitrary complex multilayers and applied to the present layer architecture. The calculated values for strip-out and collapse fields compare well with the experimental results and demonstrate the validity of the theoretical description.

Stripe-to-bubble transition of magnetic domains at the spin reorientation of (Fe/Ni)/Cu/Ni/Cu(001)

Magnetic domain evolution at the spin reorientation transition (SRT) of (Fe/Ni)/Cu/Ni/Cu(001) is investigated using photoemission electron microscopy. While the (Fe/Ni) layer exhibits the SRT, the interlayer coupling of the perpendicularly magnetized Ni layer to the (Fe/Ni) layer serves as a virtual perpendicular magnetic field exerted on the (Fe/Ni) layer. We find that the perpendicular virtual magnetic field breaks the up-down symmetry of the (Fe/Ni) stripe domains to induce a net magnetization in the normal direction of the film. Moreover, as the virtual magnetic field increases to exceed a critical field, the stripe domain phase evolves into a bubble domain phase. Although the critical field depends on the Fe film thickness, we show that the area fraction of the minority domain exhibits a universal value that determines the stripe-to-bubble phase transition.

Surface magnetostatic oscillations in elliptical bubble domains

A theory of surface magnetostatic oscillations in magnetic bubble domains with an elliptical cross section is presented. The dependences of the eigenfrequencies of resonant modes on the applied magnetic field are analyzed for a barium hexaferrite sample with allowance made for the change in the domain size due to a variation in the bias magnetic field. The range of frequency tuning in response to a magnetic field ranging from the elliptical instability field to the collapse field is estimated. It is demonstrated that elliptical bubble domains can be used as microminiature resonators operating in the millimeter range.

Magnetic Domain-Wall Memory

Abstract : The goal of this program was to perform the necessary research and development to demostrate the feasibility of a programmable 64-bit memory utilizing Bubble Domains as the storage element. Since the target of this type of development is a low cost, high density (1,000,000 bits/sq in.), physically small replacement for drum type memories, the model must also demonstrate that it is capable of extension to meet these requirements. It was the specific objective of this program to develop and refine the techniques for the generation, propagation and detection of these small bubble domains. It was not the objective of this program to pursue the refinement of bubble domain material. Various types of replication generators employing current loop splitters are discussed and experimental data on both continuous and controlled generation is presented. Results indicate that through an optimization of design, replication generators of the type presented here will be operational at high data rates. Bubble domain switching remains a problem and for a time will be the limiting function of a bubble domain device. This function is necessary when continuous generation, passive annihilation, or memory organizations such as the major-minor loop are used. The switches presented here are limited to the conductor loop type which effects bubble switching when activated by a current pulse. Both passive and active annihilators are discussed.

Magnetic tunable confinement of the superconducting condensate in superconductor/ferromagnet hybrids

The effect of a nonuniform magnetic field induced by a ferromagnet on the magnetoresistance of thin-film superconductor/ferromagnet hybrid structures was investigated experimentally. Two different magnetic textures with out-of-plane magnetization were considered: a plain ferromagnetic film with bubble domains and a regular array of ferromagnetic dots. The stray fields of the structures are able to affect the spatial profile of the superconducting condensate, leading to a modification of the dependence of the critical temperature Tc on an external magnetic field H. We showed how the standard linear Tc(H) dependence with a single maximum at H=0 can be continuously transformed into so-called reentrant phase boundary with two Tc peaks. We demonstrated that both domain-wall superconductivity and field-induced superconductivity are different manifestations of the magnetic confinement effect in various magnetic patterns.

Complex domain structure in a magnetic film with oblique anisotropy

Analytical formulas have been derived to calculate the distortion of the stripe domain structure in a ferromagnetic film under the action of a bubble domain formed in this film. On the basis of the formulas obtained, an experimentally obtained domain configuration has been calculated. The results of the calculation coincide with the observation data within the experimental error.

Localized and Delocalized Motion of Colloidal Particles on a Magnetic Bubble Lattice

We study the motion of paramagnetic colloidal particles placed above magnetic bubble domains of a uniaxial garnet film and driven through the lattice by external magnetic field modulation. An external tunable precessing field propels the particles either in localized orbits around the bubbles or in superdiffusive or ballistic motion through the bubble array. This motion results from the interplay between the driving rotating signal, the viscous drag force and the periodic magnetic energy landscape. We explain the transition in terms of the incommensurability between the transit frequency of the particle through a unit cell and the modulation frequency. Ballistic motion dynamically breaks the symmetry of the array and the phase locked particles follow one of the six crystal directions.

Magnetic Bubble Domain Phase at the Spin Reorientation Transition of UltrathinFe/Ni/Cu(001)Film

Magnetic domain phases of ultrathin Fe/Ni/Cu(001) are studied using photoemission electron microscopy at the spin reorientation transition (SRT). We observe a new magnetic phase of bubble domains within a narrow SRT region after applying a nearly in-plane magnetic field pulse to the sample. By applying the magnetic field pulse along different directions, we find that the bubble domain phase exists only if the magnetic field direction is less than approximately 10 degrees relative to the sample surface. A temperature dependent measurement shows that the bubble domain phase becomes unstable above 370 K.

Thermal stability of bubble domains in ferromagnetic discs

The transition and thermal stability of disc-shaped ferromagnetic particles at the temperature of T = 300 K with a uniaxial anisotropy along the symmetry axis from a bi-domain to a single domain state has been studied. The nudge elastic band method was used to map the energy landscape and to calculate the energy barrier between the transition states. For single FePt disc-shaped particles with perpendicular anisotropy three transition configurations have been found: single domain, stripe- and stable bubble domains at zero applied field. The single domain configuration along the positive anisotropy axis is reached by an annihilation process of the domain wall and the all-down state by a complex domain expansion process. Magnetization configurations in two interacting discs show an increase in thermal stability compared with single disc systems, which is attributed to the interacting magnetostatic energy between the two particles.

Magnetically Driven Colloidal Microstirrer

Paramagnetic colloidal particles dispersed in water and deposited above magnetic bubble domains of a uniaxial ferrimagnetic garnet film are used as microscopic stirrer when subjected to external rotating magnetic fields. The hydrodynamic flow field above the stirrer is detected by tracking of nonmagnetic microspheres. The vorticity of the flow falls off inversely proportionally to the distance from the bubble center and is proportional to the field frequency. The device provides complete control over the mixing capability. This alternative method of active mixing might be used for microfluidics applications where mechanical stirring cannot be achieved easily with other machinery parts.

Charge states of strongly correlated 3d oxides: from typical insulator to unconventional electron–hole Bose liquid

We present a model approach to describe charge fluctuations and different charge phases in strongly correlated 3d oxides. As a generic model system we consider that of centers each with three possible valence states M0,± described in the framework of an S=1 pseudospin (isospin) formalism by an effective anisotropic non-Heisenberg Hamiltonian which includes two types of single-particle correlated hopping and also the two-particle hopping. Simple uniform mean-field phases include an insulating monovalent M0 phase, mixed-valence binary (disproportionated) M± phase, and a mixed-valence ternary (“under-disproportionated”) M0,± phase. We consider the first two phases in more detail, focusing on the problem of electron–hole states and different types of excitons in the M0 phase and the formation of electron–hole Bose liquid in the M± phase. The pseudospin formalism provides a useful framework for revealing and describing different topological charge fluctuations, such as, in particular, domain walls or bubble domains in antiferromagnets. Electron–lattice polarization effects are shown to be crucial for the stabilization of either phase. All the insulating systems such as M0 phase are subdivided to two classes: stable and unstable ones with respect to the formation of self-trapped charge transfer (CT) excitons. The latter systems appear to be unstable with respect to the formation of CT exciton clusters, or droplets of the electron–hole Bose liquid. The model approach suggested is believed to apply to the description of the physics of strongly correlated oxides such as cuprates, manganites, bismuthates, and other systems with charge transfer excitonic instability and/or mixed valence. We briefly discuss an unconventional scenario of the essential physics of cuprates which implies their instability with respect to the self-trapping of charge-transfer excitons and the formation of electron–hole Bose liquid.

Observation of antiferromagnetic coupling in epitaxial ferrite films

Defects in ferrite can change the local magnetic coupling from ferromagnetic to strongly antiferromagnetic. In eptiaxial magnetite Fe3O4 films, defects locally altering exchange interaction, i.e., “antiphase boundaries” APB , essentially determine magnetic and magnetotransport properties. We locally observed specific magnetization reversal events in epitaxial magnetite Fe3− O4/MgO films 0.03 by magnetic force microscopy in external magnetic fields. A dominating phenomenon is pinning of bubble domains at single APB. Pinning is a particular consequence of antiferromagnetic coupling across APB. Antiferromagnetic coupling across APB could be directly verified for a sample with an APB domain size above the resolution limit of magnetic force microscopy.

Magnetic domain pinning in an anisotropy-engineered GdTbFe thin film

Focused ion beam irradiation was used to reduce locally the perpendicular anisotropy of magnetic thin films in rectangular lattices of 50nm sized dots. The effect of the anisotropy patterns, differing in ion fluence and interdot spacing, on the magnetization reversal process was determined in q space with x-ray resonant magnetic scattering and in real space with magnetic force microscopy. At remanence only a slight alignment of the irregularly shaped domains is observed. In perpendicular magnetic fields, however, the high field bubble domains display a pronounced localization on the dots, showing that this form of local anisotropy reduction is a highly efficient way of domain positioning.