Search results for "Rotating magnetic field"
showing 10 items of 28 documents
Localized domain wall nucleation dynamics in asymmetric ferromagnetic rings revealed by direct time-resolved magnetic imaging
2016
We report time-resolved observations of field-induced domain wall nucleation in asymmetric ferromagnetic rings using single direction field pulses and rotating fields. We show that the asymmetric geometry of a ring allows for controlling the position of nucleation events, when a domain wall is nucleated by a rotating magnetic field. Direct observation by scanning transmission x-ray microscopy (STXM) reveals that the nucleation of domain walls occurs through the creation of transient ripplelike structures. This magnetization state is found to exhibit a surprisingly high reproducibility even at room temperature and we determine the combinations of field strengths and field directions that all…
Dynamics of Magnetotactic Bacteria in a Rotating Magnetic Field
2007
The dynamics of the motile magnetotactic bacterium Magnetospirillum gryphiswaldense in a rotating magnetic field is investigated experimentally and analyzed by a theoretical model. These elongated bacteria are propelled by single flagella at each bacterial end and contain a magnetic filament formed by a linear assembly of approximately 40 ferromagnetic nanoparticles. The movements of the bacteria in suspension are analyzed by consideration of the orientation of their magnetic dipoles in the field, the hydrodynamic resistance of the bacteria, and the propulsive force of the flagella. Several novel features found in experiments include a velocity reversal during motion in the rotating field a…
Collective hydrodynamic transport of magnetic microrollers
2021
We investigate the collective transport properties of microscopic magnetic rollers that propel close to a surface due to a circularly polarized, rotating magnetic field. The applied field exerts a torque to the particles, which induces a net rolling motion close to a surface. The collective dynamics of the particles result from the balance between magnetic dipolar interactions and hydrodynamic ones. We show that, when hydrodynamics dominate, i.e. for high particle spinning, the collective mean velocity linearly increases with the particle density. In this regime we analyse the clustering kinetics, and find that hydrodynamic interactions between the anisotropic, elongated particles, induce p…
Stability of electrically conducting liquid flow driven by a rotating magnetic dipole in a ring channel
2020
The stability of electrically conducting liquid flow in a cylindrical ring channel is studied numerically. The flow is driven by a rotating magnetic dipole placed at the ring’s center. Depending on ring’s width, two distinct flow regimes are observed. In a narrow ring, the flow itself and its instability resemble the related rotating magnetic field driven flow in a cylinder. This changes in a wide ring when an intense radial jet develops on the midplane. Within this jet, the driving magnetic force is overwhelmed by inertial and viscous forces similar to how it occurs in the boundary layer flow. The instability develops as an azimuthally periodic wave-like deformation of this jet. Non-unifor…
Permanent magnet bottom-stirred swirling flow in coaxial shallow cylindrical containers
2021
Here, an original rotating permanent magnet (RPM) system placed coaxially with the liquid metal container is studied as an effective means of generating flow in shallow cylinders for potential application in aluminum metallurgy (e.g., for ladle stirring and metal dosing). The studied RPM system generates volume force with strong axial variation and force maximum near the radial midpoint. The numerical and experimental data show that, in the shallow cylinder case, the azimuthal velocity follows the force radial distribution. The resulting velocity maximum occurs near the radial midpoint, unlike in the traditional rotating magnetic field (RMF) stirrer systems, where the velocity maximum occur…
Deformation of flexible ferromagnetic filaments under a rotating magnetic field
2019
Research on magnetic particles dispersed in a fluid medium, actuated by a rotating magnetic field, is becoming increasingly active for both lab-on-chip and bio-sensing applications. In this study, we experimentally investigate the behaviour of ferromagnetic filaments in a rotating field. Filaments are synthesized by linking micron-sized ferromagnetic particles with DNA strands. The experiments were conducted under different magnetic field strengths, frequencies and filament sizes, and deformation of the filaments was registered via microscope and camera. The results obtained showed that the body deformation is larger for longer filaments and higher frequencies and lower for larger magnetic …
The chain length of anisotropic paramagnetic particles in a rotating field
2020
In this article, the maximal length of a chain of paramagnetic particles with magnetic anisotropy in a rotating magnetic field is studied. The theory of paramagnetic particle chains usually assumes that the particles are magnetically isotropic and do not rotate in a rotating field. In experiments it is seen that spherical paramagnetic particles rotate, which can be explained by small magnetic anisotropy. In this article, the maximal chain length is calculated for paramagnetic particles with magnetic anisotropy in a rotating magnetic field. Results show that the maximal chain length as a function of field frequency has the same trend for isotropic magnetic particles and particles with magnet…
Quantum dynamics of a nanomagnet in a rotating field
2005
Quantum dynamics of a two-state spin system in a rotating magnetic field has been studied. Analytical and numerical results for the transition probability have been obtained along the lines of the Landau-Zener-Stueckelberg theory. The effect of various kinds of noise on the evolution of the system has been analyzed.
Dynamics of a flexible magnetic chain in a rotating magnetic field.
2004
The model of an elastic magnetic rod is applied for a study of a behavior of the flexible magnetic particle chain in a rotating magnetic field. By numerical simulation it is shown that behavior of a flexible magnetic chain is characterized by the existence of a critical frequency beyond which the dynamics of the rod is periodic with subsequent stages of bending and straightening. The value of the critical frequency found is explained by a simple model. Below the critical frequency the chain is bent and rotates synchronously with a field. It is illustrated that in particular cases the considered model reproduces phenomena observed experimentally and numerically for the magnetic particle chai…
Effects of the magnetodipolar interactions in the alternating magnetic fields
1999
A macroscopic theory for the description of the structure formation in a system of magnetic dipoles under the action of a high-frequency rotating field is presented. Continuum equations for the effective magnetic field strength describing particle interactions are derived. It is shown that, contrary to the case of a constant magnetic field, where the demagnetizing field arising from a concentration fluctuation stabilizes the system with respect to the phase separation, the same concentration fluctuation will be amplified in the case of a rotating magnetic field, leading to the formation of a layered structure. The extensions of the model necessary for the description of the formation of the…