Magnetic Pillars Research Articles

Non-uniform magnetic fields for collective behavior of self-assembled magnetic pillars

Non-uniform magnetic fields for collective behavior of self-assembled magnetic pillars

Broadband and controllable topological interface state of longitudinal wave in pillared phononic crystal beams

We design one-dimensional broadband and controllable pillared phononic crystal (PC) beams composed of magnetic pillars deposited on an Aluminum beam to realize the tunable topological interface state of longitudinal wave. Relying on the adjustable geometrical parameters in a unit cell (i.e., the distance between magnetic pillars and the number of magnetic pillars), the topological transition point can be easily generated and tuned, the controllable topological interface state of longitudinal wave can be obtained within band gap and at the interface of two pillared PC beams with different topological phases. The localization of topological interface state decreases gradually through increasing the number of magnetic pillars. The topological interface state has also been experimentally verified in the proposed PC systems. This work provides presents a simple method to flexibly control the topological interface state, further facilitating the potential applications of elastic topological systems such as energy harvesters, wave filters and switches.

Remote actuation based on magnetically responsive pillar arrays

Smart actuators, integrated with high aspect-ratio structures and stimuli-responsive materials, can take more diverse tasks skillfully. Although great advances have been achieved over the past few years, it is still difficult in manipulating the small objects such as the transporting and grasping them under the remote untethered condition. Inspired by the ciliary structures on biological organs, a magnetically responsive actuator with hierarchical pillars on the surface of the film is designed and proposed, which allows active, fast, and dynamic manipulation over the force and speed of a small object by taking an external magnetic field. Magnetic pillars of different morphologies were fabricated by a modified soft lithographic method involving inking and imprinting processes. The magnetic-force conversion unit was constructed by magnetically responsive pillar arrays. Upon applying an external magnetic field, these pillars can rhythmically bend and recover in a controlled manner, and the generated force can be used to drive the small object to move or capture the small object. All these was further addressed from experimental and theoretical perspectives. This magnetically responsive actuator demonstrates reliable actuating performance with instant field responses. No complex mechanical and electrical control systems are required. Thus, the combined use of high aspect-ratio structures and stimuli-responsive materials will facilitate many other applications such as surface engineering, soft actuators and robotics in the future.

Soft Magnetic Pillars to Secure an Upper Airway for the Relief of Obstructive Sleep Apnea

AbstractThe inherent mechanical properties of soft structures allow fragile objects to be safely manipulated without the need for precise force control. Soft structures and driving mechanisms are employed in applications ranging from gripping and locomotion to biomedical devices. Such soft structures can be especially beneficial for manipulating excessively relaxed live soft human tissue, to relieve discomfort or medical conditions. An example is obstructive sleep apnea (OSA), where soft tissue narrows or blocks the upper airway while sleeping, causing difficulty breathing. In this paper, an approach to relieve OSA using injectable soft magnetic pillars paired with a removable magnet holder that can be worn while sleeping to secure the upper airway is reported. The approach provides an alternative to currently available treatments that may cause inconveniences such as difficulty swallowing, protrusion through soft tissue, repetitive surgeries, or sleep disruption. Experimental results and computational studies are used to determine the necessary degree of pulling force and distance, which can be adjusted by controlling the number of implanted soft magnetic pillars and external magnetic field strength. Further experiments using cadavers demonstrate the opening of the upper airway confirming the feasibility of the approach.

Parallelized Manipulation of Adherent Living Cells by Magnetic Nanoparticles-Mediated Forces

The remote actuation of cellular processes such as migration or neuronal outgrowth is a challenge for future therapeutic applications in regenerative medicine. Among the different methods that have been proposed, the use of magnetic nanoparticles appears to be promising, since magnetic fields can act at a distance without interactions with the surrounding biological system. To control biological processes at a subcellular spatial resolution, magnetic nanoparticles can be used either to induce biochemical reactions locally or to apply forces on different elements of the cell. Here, we show that cell migration and neurite outgrowth can be directed by the forces produced by a switchable parallelized array of micro-magnetic pillars, following the passive uptake of nanoparticles. Using live cell imaging, we first demonstrate that adherent cell migration can be biased toward magnetic pillars and that cells can be reversibly trapped onto these pillars. Second, using differentiated neuronal cells we were able to induce events of neurite outgrowth in the direction of the pillars without impending cell viability. Our results show that the range of forces applied needs to be adapted precisely to the cellular process under consideration. We propose that cellular actuation is the result of the force on the plasma membrane caused by magnetically filled endo-compartments, which exert a pulling force on the cell periphery.

Slippery and magnetically responsive micropillared surfaces for manipulation of droplets and beads

Stimuli-responsive surfaces are of practical importance for applications ranging from enhanced mixing of reagents in lab-on-a-chip systems until probing cellular traction forces. Non-destructive reversible bending of cilia-inspired magnetic pillars can be used for controlled transportation of non-magnetic objects and bio-inspired sensing. Magnetic actuation of micropillars suspended in liquids allows controlled mixing, propelling, and stirring of fluids as well as droplet manipulation, which are important for various applications including generation of cell spheroids and droplet coalescence in microfluidic systems. In order to expand their practical applications, fabrication processes capable of rapid prototyping have to be developed. Inspired by biological cilia and their functionalities, actuating hairy surfaces are herein fabricated and implemented to manipulate both microbeads and droplets. The artificial cilia are based on microscale magnetic pillar arrays made of flexible polydimethylsiloxane functionalized with magnetic microparticles. The arrays are fabricated by a new method using patterned molds that relies on cryogenic separation to produce transparent cilia-inspired arrays without requiring manual interference to clean the templates during the process. Magnetic actuation of the pillar arrays is demonstrated in isopropanol and silicone oil. Filling with oil yields magnetically responsive slippery lubricated surfaces allowing directional motion of droplets by repetitive bending and recovery of the flexible magnetic pillars. The achieved structures allow manipulation of microbeads and droplets which is uncommon even at the sub-mm scale; directional motion is demonstrated for 250 μm–550 μm sized droplets. Droplet transportation is facilitated by extremely low hysteresis and a high degree of omnidirectional bending of the pillar array.

Tunable and Magnetic Thiol-ene Micropillar Arrays.

Tunable and responsive surfaces offer routes to multiple functionalities ranging from superhydrophobic surfaces to controlled adhesion. Inspired by cilia structure in the respiratory pathway, magnetically responsive periodic arrays of flexible and magnetic thiol-ene micropillars are fabricated. Omnidirectional collective bending of the pillar array in magnetic field is shown. Local non-contact actuation of a single pillar is achieved using an electromagnetic needle to probe the responsiveness and the elastic properties of the pillars by comparing the effect of thiol-ene crosslinking density to pillar bending. The suitable thiol-ene components for flexible and stiff magnetic micropillars and the workable range of thiol-to-allyl ratio are identified. The wettability of the magnetic pillars can be tailored by chemical and topography modification of the pillar surface. Low-surface-energy self-assembled monolayers are grafted by UV-assisted surface activation, which is also used for surface topography modification by covalent bonding of micro- and nanoparticles to the pillar surface. The modified thiol-ene micopillars are resistant to capillarity-driven collapse and they exhibit low contact angle hysteresis, allowing water droplet motion driven by repeated bending and recovery of the magnetic pillars in an external magnetic field. Transport of polyethylene microspheres is also demonstrated.

Magnetostatic Interactions in Self-Assembled CoxNi1-xFe2O4/BiFeO3 Multiferroic Nanocomposites.

Self-assembled vertically aligned oxide nanocomposites consisting of magnetic pillars embedded in a ferroelectric matrix have been proposed for logic devices made from arrays of magnetostatically interacting pillars. To control the ratio between the nearest neighbor interaction field and the switching field of the pillars, the pillar composition CoxNi1-xFe2O4 was varied over the range 0 ≤ x ≤ 1, which alters the magnetoelastic and magnetocrystalline anisotropy and the saturation magnetization. Nanocomposites were templated into square arrays of pillars in which the formation of a "checkerboard" ground state after ac-demagnetization indicated dominant magnetostatic interactions. The effect of switching field distribution in disrupting the antiparallel nearest neighbor configuration was analyzed using an Ising model and compared with experimental results.

Metachronal wave of artificial cilia array actuated by applied magnetic field

In this paper, a biomimetic microstructure related to cilia, which are effective fluidic and conveying systems in nature, is described. Authors have already reported that a magnetic elastomer pillar actuated by a rotating magnetic field can work like a natural cilium. In the present work, we show examples of a cilia array with a metachronal wave as the next step. A metachronal wave is a sequential action of a number of cilia. It is theoretically known that a metachronal wave gives a higher fluidic efficiency; however, there has been no report on a metachronal wave by artificial cilia. We prepared magnetic elastomer pillars that contain chainlike clusters of magnetic particles. The orientation of chains was set to be different in each pillar so that each pillar will deform with a different phase.

Multiferroic behavior of templated BiFeO3-CoFe2O4 self-assembled nanocomposites.

Self-assembled BiFeO3-CoFe2O4 nanocomposites were templated into ordered structures in which the ferrimagnetic CoFe2O4 pillars form square arrays of periods 60-100 nm in a ferroelectric BiFeO3 matrix. The ferroelectricity, magnetism, conductivity, and magnetoelectric coupling of the ordered nanocomposites were characterized by scanning probe microscopy. The insulating BiFeO3 matrix exhibited ferroelectric domains, whereas the resistive CoFe2O4 pillars exhibited single-domain magnetic contrast with high anisotropy due to the magnetoelasticity of the spinel phase. Magnetoelectric coupling was observed in which an applied voltage led to reversal of the magnetic pillars.

Microwave-Assisted Magnetization Reversal of Exchange-Coupled Composite Nanopillar With Large Gilbert Damping Constant

Microwave-assisted magnetization reversal (MAMR) characteristics of a single-exchange-coupled composite nanopillar, which consists of ferromagnetically coupled soft and hard magnetic sections, have been discussed in terms of Gilbert damping constant using micromagnetic simulation. The results indicate the occurrence of significant MAMR even when the damping constant of the hard magnetic section is large. The results obtained in this paper greatly contrast those obtained with single-phase hard magnetic pillars. We also found that the magnetization switching time tends to be shorter when the damping constant of the hard magnetic section is large.

Nucleation-Induced Self-Assembly of Multiferroic BiFeO3–CoFe2O4 Nanocomposites

Large areas of perfectly ordered magnetic CoFe2O4 nanopillars embedded in a ferroelectric BiFeO3 matrix were successfully fabricated via a novel nucleation-induced self-assembly process. The nucleation centers of the magnetic pillars are induced before the growth of the composite structure using anodic aluminum oxide (AAO) and lithography-defined gold membranes as hard mask. High structural quality and good functional properties were obtained. Magneto-capacitance data revealed extremely low losses and magneto-electric coupling of about 0.9 μC/cmOe. The present fabrication process might be relevant for inducing ordering in systems based on phase separation, as the nucleation and growth is a rather general feature of these systems.

Fabrication of Patterned Magnetic Nanomaterials for Data Storage Media

Patterned media (PM) for magnetic information storage have received increased attention in recent years as the primary candidate for 1 Terabit/in2 or higher recording density for computer hard disk drives. A PM consists of a periodic array of well-defined magnetic islands, each of which can store one bit of data. In the simplest scheme, the structures could be magnetic pillars and dots with a single easy axis of magnetization. The direction of detected magnetization by the read/write head is interpreted as a binary signal 1 or 0. Some of the main technical issues in the PM include the difficulty in fabricating small nanoisland arrays in a periodic fashion over large areas, reliability/reproducibility of magnetic bit characteristics, as well as wear and head flyability issues associated with the media surface roughness, and processing cost. This article deals with a recent investigation of various fabrication approaches, nanostructural features, and magnetic properties for the bit PM.

Microstructure and superconducting properties of YBa2Cu3O7−δ thin films incorporated with a self-assembled magnetic vertically aligned nanocomposite

A thin layer of a vertically aligned nanocomposite (VAN) with separated phases of ferromagnetic Fe2O3 and non-magnetic CeO2, arranged as alternating nanopillars, is introduced in YBa2Cu3O7−δ (YBCO) thin films as either a cap or a buffer layer using a pulsed laser deposition method. Detailed microstructural characterization including XRD, high resolution XTEM and STEM is conducted and correlated with the superconducting properties to investigate the flux pinning properties introduced by the magnetic VAN layers. The Tc values of both doped samples are above 89 K and the measured at 65 K increased to 150% of that of the reference YBCO prepared under the same conditions. As the measurement temperature decreases, the magnetic pinning effect increases and the field dependent Jc(H ∥ c) is further improved to more than 200% of the Jc value of the reference YBCO sample. This suggests that the Fe2O3:CeO2 VAN can provide both ordered magnetic pillars and controlled defect density. Furthermore, the magnetic pillars are very effective pinning centers especially in the high field and low temperature regime.

Micro-Fabricated Substrates to Study Mechanotransduction

Cellular processes imply an important coordination of interactions with the extracellular medium. Accumulating evidences demonstrate that cell functions can be modulated by physical factors such as the mechanical forces acting on the cells and the extracellular matrix, as well as the topography or rigidity of the matrix. These extracellular signals can be sensed by mechanosensors on the cell surface or in the cell interior to induce various cell responses. We have developed an original approach based on micro-fabricated substrates of PolyDimethylSiloxane (PDMS) to study cell migration. We used a closely spaced array of flexible micropillars of different sizes to measure the forces exerted by cells on their substrates and to modify the effective rigidity of the substrate. In particular, we performed durotaxie experiments using substrates with a well defined frontier between two regions of different rigidities. We observed that a cell arriving to the frontier is more inclined to go to the more rigid region than to the softer one. We propose also to analyze the cell response to an external applied stress by a well-controlled actuation of the substrate. To do so, we developed magnetic pillars. Using polyacrylamide hydrogels doped with ferromagnetic iron oxide particles or ferrofluids, we can make magnetic pillars with diameters of 4 to 10 µm while a magnetic field can be locally applied with a magnetic needle. With such a technique we can exert forces of the range of several nN. Those substrates can be helpful to study the mechanical response of cells to an external force or to local changes in their microenvironment.

Laser-drilled PVC template for electrodeposition of multi-core orthogonal fluxgate sensing element

The sensing performance of the multi-core orthogonal fluxgate sensor has been found to be significantly enhanced as compared to single-core orthogonal fluxgate sensor, with the magnitude of the enhancement proportional to the number of core wires in sensing element. One of the methods of making the multi-core sensing element could be a template deposition approach, in which laser drilling of hole arrays in PVC templates and then electrodeposition of the soft magnetic pillars in the holes are involved. As such, the optimization of the Nd:YAG laser parameters on the quality of the holes on the PVC templates has been carried out in this study. The effect of laser processing parameters, including: (1) laser fluence; (2) laser irradiation time; (3) number of pulses; (4) laser focal point, was investigated in relation to the hole quality and tapering effect. In particular, the laser fluence was the major factor in determining hole diameter and depth. The laser irradiation time affects the heat affected zone the most severely, and the number of pulses used ensures a hole that is relatively clear of debris. In order to obtain holes of the best quality finish, high laser fluence and optimum values of laser irradiation time and number of pulses should be used. By setting the focal point above the surface of the PVC, the tapering effect was found to be minimum.

Evolution of magnetic pillars using 200 MeV Ag15+ ion irradiation

We have studied the evolution of micro and nano magnetic pillars, created by 200MeV Ag15+ ion irradiation on the surface of Mg0.95Mn0.05Fe2O4 thin films, using X-ray diffraction, atomic force and dc magnetization. Thin films of Mg0.95Mn0.05Fe2O4 were deposited using pulsed laser deposition technique on platinum coated silicon (Pt–Si) substrate. The X-ray diffraction study clearly indicates that the evaluated magnetic pillars have single phase spinel cubic structure. From dc magnetization hysteresis loop, it is observed that the film deposited on Pt–Si exhibits a ferrimagnetic ordering at room temperature and the saturation magnetization increases with fluence value up to 5×1011ions/cm2 which may be due to grain growth. However, with further increase in fluence value, the magnetization decreases, but this value is still larger than that of as-deposited film. The average size of magnetic pillars calculated from atomic force microscopy is found to be ∼63 and ∼121nm at a fluence of 1×1011ions/cm2 and then increases up to ∼136nm at a fluence of 5×1011ions/cm2 and finally, attains a value of ∼107nm at a fluence of 1×1012ions/cm2. The formations of these magnetic nano-pillars are explained on the basis of metallic catalyst assisted growth after SHI irradiation.

PBG structures of two-dimensional magnetic photonic crystals in square lattice

Photonic band gap(PBG) structures of the two-dimensional magnetic photonic crystals (MPCs) are calculated by use of the plane-wave expansion method. The MPCs consist of magnetic pillars in rectangular, square, hexagonal and circular shapes, respectively embedded in dielectric medium in a square lattice. The optimal MPC samples that possess the largest gap-mid gap ratio for each type of MPC are obtained by scanning the three parameters(permeability, filling factor and rotating angle). The changes of the PBG structures are investigated when varying independently each parameter and fixing the others. Calculated results show that the frequency in the middle of the PBG hardly changes with the variation in filling factor or rotating angle, but decreases monotonicly with the increase of the per meability. The PBG width increases to a top value and then decreases with the in crease of each parameter.

Guide modes in magnetic photonic crystal heterostructures in two-dimensional square lattices

The photonic band structure of two-dimensional magnetic photonic crystals(MPCs) composed of magnetic pillars in different shapes (rectangular, square, hexagonal and circular) sitting on square lattice is investigated by use of the plane wa ve expansion method. The optimal MPCs, which possess the largest photonic band g ap and the lowest frequency in the middle of the photonic band gap, are picked u p when scanning the parameters (filling factor, orientation angle, and permeabil ity). Based on the results, we construct theoretically the mixed MPC heterostruc tures and investigate the guide modes at interface. The guide modes along the in terface of the heterostructures can be generated without any lattice distortion when the scattering pillars are in shapes of rectangle, hexagon, and circle.

Projective dynamics analysis of magnetization reversal

The computational Projective Dynamics method is used to analyze simulations of magnetization reversal in nanoscale magnetic pillars. It is shown that this method can be used to determine the magnetizations corresponding to the metastable minimum and saddle point in the free energy, and the free-energy barrier associated with those points. For the nanopillars studied here, entropy is found to provide a significant contribution to the free-energy barrier which determines the reversal time scale.