Magnetic Caps Research Articles

Size-dependent bistability of magnetic states in soft magnetic cap arrays

We have investigated the size dependent energy barrier regarding the transition between magnetic vortex and collinear states in dense arrays of magnetic cap structures hosting magnetic vortices. The cap structures were formed by the deposition of soft magnetic thin films on top of large arrays of densely packed polystyrene spheres. The energy barrier associated with the magnetic field assisted switching from a collinear magnetic state to a non-uniform vortex state (or vice versa) was tuned by tailoring the diameter and thickness of the soft magnetic caps. At a sufficient temperature, known as the bifurcation temperature, the thermal energy overcomes this energy barrier and magnetic bistability with a hysteresis-free switching occurs between the two magnetic states. In magnetic caps with a fixed thickness, the bifurcation temperature decreases with increasing cap diameter. On the other hand, for a fixed diameter, the bifurcation temperature increases with an increase in film thickness of the cap structure. This study demonstrates that the bifurcation temperature can be easily tailored by changing the magnetostatic energy contribution which in turn affects the energy barrier and thus the magnetic bistability.

Polydopamine-mediated quantity-based magnetic relaxation sensing for the rapid and sensitive detection of chloramphenicol in fish samples

Chloramphenicol is an antibiotic that cause adverse effects in humans. In this work, a novel quantity-based magnetic relaxation switching (sMRS) sensor using polydopamine (PDA) for signal amplification was successfully developed for the rapid and sensitive detection of chloramphenicol in fish samples. We first prepared the conjugation of large magnetic nanoparticles (MNPs) and CAP antigen (MNP-antigen), which performed a competitive immune reaction with chloramphenicol and chloramphenicol antibody. Horseradish peroxidase, that can bind to antibody at the surface of large MNPs, was employed to catalyze the rapid polymerization of dopamine to PDA, which was easily deposited onto the surface of large MNPs. The concentration of chloramphenicol is inversely proportional to the content of HRP after reaction, in other words, it is inversely proportional to the content of PDA. PDA then reacted with 30 nm-diameter amine-functionalized MNPs (NH2-MNP30), the consumption of which resulted in a decrease in the concentration of free NH2-MNP30 particles in solution. After magnetic separation, the remaining free NH2-MNP30 served as the magnetic probe for signal readout. The limit of detection of the sMRS assay for detecting chloramphenicol was 16.6 pg/mL, which was 49-fold lower than MRS sensor without signal amplification. This quantity-based MRS sensor can provide a powerful platform for enabling the rapid and sensitive detection of chloramphenicol in food samples.

Radio frequency analysis of partially grafted immediate dental implant with and without use of static magnetic field: An in vivo study.

Replacement of missing teeth with dental implants represents one of the most successful treatment modalities in modern dentistry. Patients desire for a shorter treatment time has made clinicians to attempt loading implants early or immediately after placement. The primary stability is determined by density and mechanical properties of the bone, the implant design, edentulous site complications, and the surgical technique. Various researchers have tried to achieve faster osseointegration static magnetic field is one of them. So the aim of this study was to investigate whether Static magnetic field created by using safer magnets was useful to promote osseointegration. Subjects were selected according to the predetermined inclusion and exclusion criteria in two groups (20 in each group). Conventional implant placement protocol was used and implant placement was performed and grafting was done. Magnetic healing cap was used in group I and conventional healing cap in group II. Implant stability assessment using radio frequency analyser was assessed at 2, 3 and 4 months on interval. Mann-Whitney U test revealed that there was significant difference was observed between the groups I and II at 2, 3 and 4 months of interval (P < 0.001). Static magnetic field improve osseointegration in group I as compared to group II. The present double-blinded RCT showed significantly improved implant stability and osseointegration in implants which were stimulated by static magnetic field by using magnetic healing cap as compared to implants with conventional healing cap.

Translatory and rotatory motion of exchange-bias capped Janus particles controlled by dynamic magnetic field landscapes

Magnetic Janus particles (MJPs), fabricated by covering a non-magnetic spherical particle with a hemispherical magnetic in-plane exchange-bias layer system cap, display an onion magnetization state for comparably large diameters of a few microns. In this work, the motion characteristics of these MJPs will be investigated when they are steered by a magnetic field landscape over prototypical parallel-stripe domains, dynamically varied by superposed external magnetic field pulse sequences, in an aqueous medium. We demonstrate, that due to the engineered magnetization state in the hemispherical cap, a comparably fast, directed particle transport and particle rotation can be induced. Additionally, by modifying the frequency of the applied pulse sequence and the strengths of the individual field components, we observe a possible separation between a combined or an individual occurrence of these two types of motion. Our findings bear importance for lab-on-a-chip systems, where particle immobilization on a surface via analyte bridges shall be used for low concentration analyte detection and a particle rotation over a defined position of a substrate may dramatically increase the immobilization (and therefore analyte detection) probability.

Symmetry of surface states in graphene superlattices terminated with magnetic cap layers

Symmetry of surface states in graphene superlattices terminated with magnetic cap layers

Magnetically responsive polymer nanopillars with nickel cap

Embedding magnetic particles within polymer matrix is a common and facile method to fabricate magnetically responsive micro-/nanoscale pillars. However, the balance between mechanical compliance and magnetic susceptibility cannot be decoupled and the particles are limited by the pillar feature size, which can limit the actuation performance. Here we demonstrate a new type of magnetically responsive nanostructure consisting of a polydimethylsiloxane (PDMS) nanopillar array with deposited nickel caps, that has successfully achieved such decoupling with multiple cap-geometry designs for a better actuation control. The actuation result of nanopillars with 540 nm period and 1.3 μm height has been analyzed using image processing, leading to a maximum displacement of 180 nm with a ratio of 13.9% with respect to the pillar height. Magnetic and mechanical models based on magnetic force and torque have been developed and used to mitigate the weakening effect of the actuation by the residual magnetic layer. This structure demonstrates a feasible strategy for magnetic actuation at the sub-micrometer scale with freedom to design magnetic cap and polymeric pillar separately. This structure can also be utilized in multiple applications such as tunable optical elements, dynamic droplet manipulation, and responsive particle manipulation.

Magnetic textures in hemispherical thin film caps with in-plane exchange bias

Hemispherical caps of in-plane exchange biased IrMn/CoFe layer systems have been fabricated on top of regularly arranged spherical silica particles by magnetron sputtering, creating magnetic Janus particles. In this thin film layer system cap, the magnetic shape anisotropy of the topographically non-flat hemispheres competes with the unidirectional anisotropy induced by the exchange bias. The magnetic properties of this non-trivial system have been investigated by longitudinal magneto-optical Kerr effect magnetometry, where a characterization method has been developed considering both the curved layer system and the signal contributions of flat parts of the sputtered thin film system. Both remagnetization curves, from Kerr magnetometry and the magnetic force microscopy images, reveal an onion state in the magnetic caps of the ensemble. Additional micromagnetic simulations show a stabilization of the onion state due to the introduced unidirectional anisotropy also in individual hollow hemispheres as compared to the vortex state exhibited by purely ferromagnetic caps.

A Two‐Parameter Model for Colloidal Particles with an Extended Magnetic Cap

Self‐assembly of magnetic colloidal particles in solution has successfully been simulated by hard‐ or soft‐sphere models with a set of embedded magnetic point dipoles, and the position and orientation of each dipole are adapted to mimic the magnetization distribution. Herein, a conceptually simpler approach is introduced for magnetically capped colloidal particles, which replaces the set of dipoles by the extended magnetization distribution of a single conductive loop. Only two parameters are required to characterize the magnetization distribution: the diameter of the loop and its radial off‐center shift within the sphere. This approach reflects the radial symmetry and the spatial extension of the magnetic cap. At larger distance and in the limit of very small loops that model reproduces the far field and particle arrangements obtained with the single, radially shifted dipole model. For larger loop radii, additional stable assembly patterns are obtained, which occur in experiments, but cannot be simulated with a single shifted dipole model.

Коэффициенты продольной стабильности дентальных имплантатов

Вводится понятие продольной стабильности дентальных имплантатов. Известно, что прибор Osstell ISQ ( Implant Stability Quotient, Швеция) наряду с вынужденными колебаниями дентальных имплантатов, возникающими под действием поперечных нагрузок, может возбуждать продольные колебания и регистрировать их резонансную частоту. Цель работы обоснование возможности использования коэффициентов продольной стабильности для оценки готовности дентальных имплантатов к функциональным нагрузкам, а также верификация методики их измерения с помощью прибора Osstell ISQ путем сравнения с соответствующими коэффициентами продольной жесткости, измеряемыми методом лазерного тестирования. По сравнению с традиционной предлагаемая методика обладает рядом преимуществ и позволяет создать новую, более совершенную базу для неинвазивного мониторинга процесса остеоинтеграции дентальных имплантатов. Например, использование модифицированных «магнитных заглушек» вместо магнитных штифтов ( SmartPeg ) позволяет проводить измерения коэффициентов продольной стабильности через слизистую оболочку альвеолярной кости, т.е. не раскрывая дентальный имплантат для дальнейшего использования как опору под несъемный зубной протез, а модифицированные «магнитные колпачки» дают возможность определять коэффициенты продольной стабильности мини-имплантатов со сферической головкой, используемых для дополнительной фиксации съемных зубных протезов при полном отсутствии зубов. Необходимо подчеркнуть, что традиционная методика Osstell ISQ не позволяет проводить измерения коэффициентов стабильности указанных мини-имплантатов. Введение коэффициентов продольной стабильности позволило построить простую теоретическую модель колебаний дентальных имплантатов, закрепленных в упругой и вязкоупругой среде, установить связь между коэффициентами продольной стабильности и продольной жесткости, дать оценку несущей способности дентальных имплантатов. Более того, при оценке их готовности к функциональным нагрузкам с помощью коэффициентов продольной стабильности рекомендации остаются прежними, так как отношение коэффициентов стабильности при продольных и поперечных нагрузках должно быть близко к единице.A concept of longitudinal stability is introduced for dental implants. It is well known that the Osstell ISQ device (Implant Stability Quotient, Sweden), along with forced oscillations of dental implants arising under lateral loads, may also excite longitudinal oscillations and register their resonance frequency. The objective of the study is to substantiate possibility to apply quotients of longitudinal stability in order to assess readiness of dental implants for functional loads, as well as to verify methods of their measurement using the Osstell ISQ device by comparison with appropriate quotients of longitudinal rigidity measured by laser testing. In comparison with the traditional technique, the proposed one has a number of advantages and enables creation of a new and more perfect base for noninvasive monitoring of dental implant osteointegration process. For example, the use of modified “magnetic plugs” instead of pins (SmartPeg) allows quotients of longitudinal stability to be measured through a mucous membrane of alveolar bone, i.e. a dental implant will not be open for further use as a fixed denture support, and modified magnetic caps allow determination of quotients of longitudinal stability for spherical headed mini implants used for additional fixation of detachable dentures when teeth are absent totally. Please note that the traditional Osstell ISQ technique does not enable measurements of coefficients of stability for the said mini implants. Introduction of quotients of longitudinal stability has made it possible to create a simple theoretical model of oscillations for dental implants fixed in elastic and viscous-elastic medium, to relate quotients of longitudinal stability and longitudinal rigidity to each other, and to assess bearing capacity of dental implants. Moreover, recommendations remain the same when assessing implant readiness for functional loads using quotients of longitudinal stability since the ratio of quotients of stability for longitudinal and lateral loads approximates unity.

Self-assembly of colloids with magnetic caps

In our earlier work (Steinbach et al., 2016 [1]) we investigated a homogeneous system of magnetically capped colloidal particles that self-assembled via two structural patterns of different symmetry. The particles could form a compact, equilateral triangle with a three-fold rotational symmetry and zero dipole moment and a staggered chain with mirror symmetry with a net magnetisation perpendicular to the chain. The system exhibited a bistability already in clusters of three particles. Based on observations of a real magnetic particles system, analytical calculations and molecular dynamics simulations, it has been shown that the bistability is a result of an anisotropic magnetisation distribution with rotational symmetry inside the particles. The present study is a logical extension of the above research and forms a preparatory stage for the study of a self-assembly of such magnetic particles under the influence of an external magnetic field. Since the magnetic field is only an additive contribution to the total ground state energy, we can study the interparticle interaction energies of candidate ground state structures based on the field-free terms.

Mechanism for Radio Emission of Pulsars

This is a review of work upon which a proposal for an effective mechanism for the radio emission of pulsars is based. It is shown that spin-down of a neutron star makes the system of proton vortices move toward the surface of the star’s core. The magnetic energy of these vortices is released in “magnetic caps” of limited size located on the core surface and crust of neutron stars. Magnetosonic waves propagating in the crust of the star transfer this energy to its surface and form a localized source of radio emission in pulsars. Because of the star’s rotation the emission from this source will pulsate. Some observed properties of the radio emission from pulsars can be explained by the proposed emission mechanism.

Excited eigenmodes in magnetic vortex states of soft magnetic half-spheres and spherical caps

We studied the magnetization dynamics of excitation modes in special geometrical confinements of soft magnetic half-spheres and spherical caps in magnetic vortex states using finite-element micromagnetic numerical calculations. We found additional fine features of the zeroth- and first-order gyrotropic modes and asymmetric m = +1 and m = −1 azimuthal spin-wave modes, which detailed information is unobtainable from two-dimensional mesh-cell based numerical calculations. Moreover, we examined the perpendicular bias field dependence of the excited eigenmodes, which data provide for an efficient means of control over the excited modes. Such numerical calculations offer additional details or new underlying physics on dynamic features in arbitrary-shape magnetic nano-elements such as half-spheres and spherical caps in magnetic vortex states.

Experimental investigation of magnetic arc blow in plasma arc cutting

Oxygen plasma arc cutting is widely employed in various industrial fields. In the case of cutting magnetized plates, the magnetic field is concentrated around the cutting front as cutting progresses, and the electromagnetic force induced by the leakage of magnetic field deflects the plasma jet. The deflected plasma jet leads to poor cutting quality and sometime causes damage to the electrode and nozzle because of double arc abnormal discharge. This phenomenon is called magnetic arc blow, and it is a critical issue when applying plasma cutting on magnetized plates. In this study, magnetic arc blow behavior is investigated, and a method to prevent it is devised. We examined the relationship between operating conditions and the double arc using external magnetic fields on a plasma jet. We found criteria regarding operation conditions that induce arc blow. In addition, we succeeded in suppressing the double arc caused by the leakage external magnetic field by using a magnetic shield cap composed of ferromagnetic material around the nozzle.

Temperature dependent magnetization reversal of exchange biased magnetic vortices in IrMn/Fe microcaps

We have investigated the magnetization reversal of vortex structures in Fe as well as in IrMn/Fe magnetic caps at elevated temperatures up to 450 K. The caps were formed by film deposition onto dense arrays of self-assembled silica particles of 900 nm diameter. In the investigated temperature range, the magnetization reversal in Fe caps evolves via nucleation and annihilation of magnetic vortices. However, in exchange coupled IrMn/Fe caps, the magnetic vortex at zero magnetic field vanishes as the temperature approaches the blocking temperature of IrMn accompanied by an increase in coercivity. Further increase in temperature, approaching the Néel temperature of IrMn, results in a reduction of coercivity as well as remanence indicating the re-stabilization of a vortex state.

Alternating current loss reduction for rectangular busbars by covering their edges with low permeable magnetic caps

A method to reduce ac conductive losses in a thin rectangular busbar made of copper is presented. The method is based on a technique, which makes the distribution of the ac current in the cross section of a busbar flatter. Edges of a thin busbar are covered with low permeability magnetic thin layers as caps. The magnetic cap makes the impedance experienced by the current flowing near the edge comparatively larger so that currents cannot get crowded near the edges of a busbar. This method is numerically verified.

Fuel-Free Locomotion of Janus Motors: Magnetically Induced Thermophoresis

We present fuel-free locomotion of magnetic spherical Janus motors driven by magnetically induced thermophoresis--a self-diffusive propulsion of an object in any liquid media due to a local temperature gradient. Within this approach an ac magnetic field is applied to induce thermophoretic motion of the objects via heating a magnetic cap of the particles, while an additional dc magnetic field is used to orient Janus motors and guide their motion on a long time scale. Full control over the motion is achieved due to specific properties of ultrathin 100-nm-thick Permalloy (Py, Fe₁₉Ni₈₁ alloys) magnetic films resulting in a topologically stable magnetic vortex state in the cap structure of Janus motors. Realized here magnetically induced thermophoretic locomotion does not require catalytic chemical reactions that imply toxic reagents. In this respect, we addressed and successfully solved one of the main shortcomings in the field of artificial motors, namely being fully controlled and remain biocompatible. Therefore, our approach is attractive for biotechnological in vitro assays and even in vivo operations, since the functioning of Janus motors offers low toxicity; it is not dependent on the presence of the fuel molecules in solution. Furthermore, the suggested magnetic ac excitation is superior compared to the previously proposed optically induced heating using lasers as it does not require transparent packaging.

Equilibrium magnetic states in individual hemispherical permalloy caps

The magnetization distributions in individual soft magnetic permalloy caps on non-magnetic spherical particles with sizes ranging from 50 to 800 nm are investigated. We experimentally visualize the magnetic structures at the resolution limit of the x-ray magnetic circular dichroism photoelectron emission microscopy (XMCD-PEEM). By analyzing the so-called tail contrast in XMCD-PEEM, the spatial resolution is significantly enhanced, which allowed us to explore magnetic vortices and their displacement on curved surfaces. Furthermore, cap nanostructures are modeled as extruded hemispheres to determine theoretically the phase diagram of equilibrium magnetic states. The calculated phase diagram agrees well with the experimental observations.

Energy source for the radio emission from pulsars

The vortex structure of the superfluid core of neutron stars is examined, taking the residual (RP), generated(poloidal) (GP), and toroidal (TP) magnetic fields into account. The motion of the vortex structure toward the core-crust boundary as the star spins down leads to the formation of two local “magnetic caps,” where the magnetic field increases with time and reaches the limit of the second critical field. The rate of energy release in the magnetic cap owing to the “collapse” of the magnetic field is found to be on the order of the intensity of the radio emission from pulsars.

Magnetic Films on Nanoparticle Arrays

Novel properties originating from the combination of magnetism with the reduced dimensionality of nanoobjects are of considerable interest for fundamental science but are also readily used in a wide range of technological applications. To fulfill the demand for larger capacities of storage systems, complex magnetic heterostructures of a nanometer size are envisioned as future magnetic storage media. Here we review recent advances on a novel gradient nanomaterial fabricated by deposition of magnetic thin films onto self-assembled arrays of nonmagnetic spherical particles, resulting in arrays of magnetic caps. Due to the curvature of the particle's surface, magnetic and structural properties of the magnetic thin films differ substantially from their planar counterparts with respect to the equilibrium magnetic domain pattern as well as magnetization reversal behavior. It will be shown that varying the size of the particles as well as the thickness of the deposited layers, the magnetostatic and exchange coupling between the neighboring caps can be systematically modified, providing direct access to the change of fundamental magnetic interactions at the nanoscale. Furthermore, these magnetic cap structures were employed to realize so called bit patterned magnetic media, which is one of the most promising concepts in magnetic data storage to provide areal densities beyond 1 Tbit/inch 2 . In this respect, important aspects regarding the potential application of the magnetic cap structures in magnetic data storage will be addressed.

Catalytic Janus Motors on Microfluidic Chip: Deterministic Motion for Targeted Cargo Delivery

We fabricated self-powered colloidal Janus motors combining catalytic and magnetic cap structures, and demonstrated their performance for manipulation (uploading, transportation, delivery) and sorting of microobjects on microfluidic chips. The specific magnetic properties of the Janus motors are provided by ultrathin multilayer films that are designed to align the magnetic moment along the main symmetry axis of the cap. This unique property allows a deterministic motion of the Janus particles at a large scale when guided in an external magnetic field. The observed directional control of the motion combined with extensive functionality of the colloidal Janus motors conceptually opens a straightforward route for targeted delivery of species, which are relevant in the field of chemistry, biology, and medicine.