Micromagnet Arrays Research Articles

Periodically Sinusoidal Magnetic Stray Field and Improved Film Quality of CoMnP Micro-Magnet Arrays for Magnetic Encoders by Electrodeposition with the Assistance of Ultrasound

Magnetic encoders are composed of a magnetic sensor, a hard magnetic recording medium and a signal processing circuit. Electrodeposited micro-magnet arrays produced by micro-fabrication are promising recording media for enhancing encoder performance. However, two major engineering issues have yet to be resolved. One issue is an unknown relationship between the feature sizes of micro-magnet arrays and their stray field shapes, and another issue is the formation of micro-cracks due to the built-up residual stresses of thick films. In this study, we investigated the effect of feature sizes on the emanating stray field shape at various observation heights. Feature sizes include two height (i.e., film thickness) values of 78 μm and 176 μm, and both width and spacing with three values of 360 μm, 520 μm and 680 μm. Ultrasound-assisted agitation was adopted for investigating the effects of electrodepositing current densities on the film crystalline microstructures and magnetic properties. Narrowing the width of micro-magnets helps the stray field to become a sinusoidal profile. Thinner film, i.e., thickness 78 μm in this study, supports the stray field taking on a sinusoidal profile. Moreover, the spacing between the micro-magnets plays a key factor in determining the shape of the stray field. Under 37 kHz/156 W ultrasound agitation, the optimal hard magnetic properties of electrodeposited CoMnP films are residual magnetization 2329 G and coercivity 968 Oe by a current density of 10.0 mA/cm2. Ultrasound-assisted electrodeposition, along with duly designed feature size, facilitates the micro-magnet arrays having a sinusoidal stray field shape using high quality films. Furthermore, for the first time, a systematic understanding of feature-size-dependent stray field evolution and improved polarities quality has been realized for the recording media of sinusoidal magnetic encoders.

Investigation on the focusing and separation of polystyrene microbeads in an integrated microfluidic system using magnetized functionalized flexible micro-magnet arrays

Investigation on the focusing and separation of polystyrene microbeads in an integrated microfluidic system using magnetized functionalized flexible micro-magnet arrays

Fabrication of NdFeB micromagnets based on vacuum negative pressure injection

Micromagnets integrated into microelectromechanical systems devices are widely used in the fields of micro-energy harvesters, micro-actuators, and speakers. A novel method for fabricating NdFeB micromagnets based on vacuum negative pressure injection is presented. The influence of array element shape, size, and injection channel on the surface magnetic field was studied by Maxwell finite element analysis. A circle array of 600 µm diameter and 150 µm spacing could generate a stronger magnetic field at the edge of the magnet array. Micron-sized magnetic powders with a diameter of 5 µm mixed with the SU8 T2010 photoresist were filled into the microcavity on a PDMS (polydimethylsiloxane) elastomer, which was prepared by the SU8 positive membrane. The PDMS elastomer was bonded to the glass substrate and degassed by a vacuum pump to form negative pressure for rapid injection and uniform filling. By selecting a suitable binder and optimizing the mixing ratio of the magnetic powder, a 165 µm thick micromagnet with homogeneous dispersion of magnetic powder particles was achieved, and the weight percentage of magnetic powder in the SU8 photoresist was 41.8%. The remanent magnetization of micromagnets was 42 emu/g, the coercivity was 6895 Oe, and the maximum surface magnetic field strength was 1.3 mT after magnetization in a 2 T uniform magnetic field. Finally, the micromagnet array was utilized in a cantilever beam micro-actuator, which could drive the cantilever beam and exhibit an excellent linearity.

The trajectory of bio-carriers in periodic energy landscape regulated by the multiple collision history in a magnetophoretic system

Analysis of changes in particle trajectories due to collision is essential to understanding the physical information such as energy or mass exchange during collisions between particles. Most of the research has studied particles with initial energies in a relatively uniform or gradient static energy landscape. However, compared to the particle collisions of a static energy landscape, the movement becomes more diverse in a complex magnetic energy landscape. We designed micro-magnet arrays to create complex energy landscapes with temporal and spatial periodicity. Magnetic particles draw distinctly different trajectories in this magnetophoretic system depending on the coupling number. When the particles collide with each other, an immediate shift to a classified trajectory was confirmed according to the collision type. This study analyzes the particle trajectory shift that includes various information. Therefore, we inversely calculate the coupling history, such as the coupling number, time, and order due to collision. This investigation represents a unique analysis of the dynamics change of particles within a complex magnetic energy landscape.

Optimization of a vibrating MEMS electromagnetic energy harvester using simulations

Nowadays, wireless sensor networks (WSN) are becoming essential in our daily life. However, a major constraint concerns the energy power supply. Indeed, batteries need to be recharged or replaced often which implies a limited lifetime for WSN nodes. One alternative consists in harvesting mechanical energy from surrounding vibrations of the environment. Using finite element simulations, we report here a complete guideline to optimize a MEMS electromagnetic energy harvester consisting of an in-plane vibrating silicon frame supporting an array of micromagnets that faces a static 2D micro-coil. The dimensioning of the magnet array and the specific design of the coil are addressed, considering patterned 50 \(\upmu \)m thick NdFeB films with out of plane magnetization. The optimization of the electromechanical coupling which allows to efficiently convert the energy results from a trade-off between the high magnetic flux gradients produced by the micromagnets and the maximum number of turns of the facing coil.

Eukaryotic Cell Capture by Amplified Magnetic in situ Hybridization Using Yeast as a Model.

A non-destructive approach based on magnetic in situ hybridization (MISH) and hybridization chain reaction (HCR) for the specific capture of eukaryotic cells has been developed. As a prerequisite, a HCR-MISH procedure initially used for tracking bacterial cells was here adapted for the first time to target eukaryotic cells using a universal eukaryotic probe, Euk-516R. Following labeling with superparamagnetic nanoparticles, cells from the model eukaryotic microorganism Saccharomyces cerevisiae were hybridized and isolated on a micro-magnet array. In addition, the eukaryotic cells were successfully targeted in an artificial mixture comprising bacterial cells, thus providing evidence that HCR-MISH is a promising technology to use for specific microeukaryote capture in complex microbial communities allowing their further morphological characterization. This new study opens great opportunities in ecological sciences, thus allowing the detection of specific cells in more complex cellular mixtures in the near future.

Performance Improvement of MEMS Electromagnetic Vibration Energy Harvester Using Optimized Patterns of Micromagnet Arrays

Scavenging mechanical energy from ubiquitous vibrations through miniaturized electromagnetic (EM) transducers is a potential solution to the problem of powering wireless sensor networks for the Internet of Things (IoT). This letter presents the design and performance analysis of fully integrated EM vibration energy harvesters on the scale of microelectromechanical systems (MEMS). Through analytical formulation and finite element analysis, we present a systematic design study to optimize the magnet-coil interaction in a precise location within a small surface area (“footprint”). The compact device topology yielded an EM coupling as high as 62.9 mWb/m with optimized stripe-shaped micromagnets and rectangular microcoils. The nonlinear spring topology demonstrated six times improvement in the half-power bandwidth compared to its linear counterpart, at a cost of reduced power density. The designs can be implemented using standard MEMS fabrication methods leveraging CMOS-compatible integration at the system level for potential applications in the IoT.

Optimized micromagnet geometries for Majorana zero modes in low g -factor materials

Solid-state experimental realizations of Majorana bound states are based on materials with strong intrinsic spin-orbit interactions. In this paper, we explore an alternative approach where spin-orbit coupling is induced artificially through a nonuniform magnetic field that originates from an array of micromagnets. Using a recently developed optimization algorithm, we find suitable magnet geometries for the emergence of topological superconductivity in wires without intrinsic spin-orbit coupling. We confirm the robustness of Majorana bound states against disorder and periodic potentials whose amplitudes do not exceed the Zeeman energy. Furthermore, we identify low g-factor materials commonly used in mesoscopic physics experiments as viable candidates for Majorana devices.

A physically unclonable function using NV diamond magnetometry and micromagnet arrays

A physically unclonable function (PUF) is an embedded hardware security measure that provides protection against counterfeiting. Here, we present our work on using an array of randomly magnetized micrometer-sized ferromagnetic bars (micromagnets) as a PUF. We employ a 4μm thick surface layer of nitrogen-vacancy (NV) centers in diamond to image the magnetic field from each micromagnet in the array, after which we extract the magnetic polarity of each micromagnet using image analysis techniques. After evaluating the randomness of the micromagnet array PUF and the sensitivity of the NV readout, we conclude by discussing the possible future enhancements for improved security and magnetic readout.

Synthesis and magnetic manipulation of hybrid nanobeads based on Fe3O4 nanoclusters and hyaluronic acid grafted with an ethylene glycol-based copolymer

Magnetic biopolymer hybrid nanoparticles have gained a lot of attention in the field of biomedical applications. Here we present the synthesis of hyaluronic acid (HA)-coated magnetic clusters of superparamagnetic nanoparticles. Firstly, the superparamagnetic magnetite nanoparticles coated with oleic acid were synthesized and characterized in terms of size, composition and magnetic properties. Secondly, magnetic nanoclusters were prepared and wrapped with HA grafted with an ethylene glycol-based copolymer. The resulting nanostructures and nanobeads were characterized using electron microscopy, dynamic light scattering, thermogravimetric analysis and magnetic measurements. The clustering of magnetite nanoparticles facilitates magnetic purification, separation, rapid attraction and controlled positioning by high field gradient micro-magnet arrays. In addition, clusters could be formed in the presence of a fluorescent drug model, which demonstrates the possibility of dual functionalization of our hybrid nanosystems: magnetic responsiveness and drug encapsulation.

Optimized process for the fabrication of PDMS membranes integrating permanent micro-magnet arrays

Here we report on the fabrication of micro-magnet arrays by powder agglomeration in a polymer matrix. The NdFeB@polydimethylsiloxane (PDMS) inner microstructure and the generated magnetic forces were studied, when prepared under two different magnetic field configurations. The initial process uses the classical crosslinking of PDMS mixed with NdFeB powder under a low magnetic field gradient provided by a permanent magnet (LG set-up for low gradient). In contrast, the optimized process uses an intermediate layer, composed of iron microstructures in a PDMS matrix that amplifies and focuses the magnetic field gradient given by the permanent magnet (HG set-up for high gradient). Both processes result in a heterogeneous material that can be described as an array of permanent micro-magnets diluted in a non-magnetic host matrix. The NdFeB@PDMS microstructure was characterized by X-ray tomography and optical microscopy. The magnetic properties were also measured by magnetometry and colloidal probe AFM. Results showed that the HG set-up leads to an array of micro-magnets localized at the surface, with higher compactness and density, resulting in stronger magnetic performances compared to the LG set-up. This technology only implies easy-to-handle and cheap fabrication processes, paving the way for the development of low-cost lab-on-chip devices integrating magnetophoretic trapping functionality.

Array of 3D permanent micromagnet for immunomagnetic separation

Immunomagnetic separation is the laboratory method used to separate targets from heterogeneous samples. It has been applied to separate or detect target entity existing rarely in the sample solution. In this study, a new type of microfluidic device with a 3D micromagnet array was designed, and it was demonstrated that this microfluidic device could be applied to immunomagnetic separation. As the 3D micromagnet is embedded in the microchannel, the height of the microchannel is extended to several hundred micrometers, and the magnetic field pulling the target can be confined to a limited region within the microchannel. Immunomagnetic separation using a microfluidic device was experimentally confirmed by fluorescence imaging. The capture efficiency was maximally 94.9% in the experimental study using the streptavidin-biotin binding of microbeads of the diameter of 1 µm. As a result, the microfluidic device with micromagnet arrays provides new solution for point-of-care device for immunomagnetic separation.

A quantitative study of magnetic interactions between a micro-magnet array and individual magnetic micro-particles by scanning particle force microscopy

Magnetic particle force microscopy has been used to measure magnetic interactions between an array of thermomagnetically patterned hard micro-magnets and both hard magnetic and superparamagnetic microspheres. Maximum interaction forces are experienced when the microspheres scan over the interface between reversed and non-reversed zones, where the stray magnetic field produced by the micro-magnet array is maximum. Comparison of experimental and simulated force maps of the hard spheres is used to validate assumptions concerning the remanent magnetisation and depth of the reversed zones. This information is in turn used to deduce the content of magnetic nanoparticles in the superparamagnetic spheres. Asymmetries in the experimental force maps of hard spheres are attributed to (i) the existence of a weak in-plane component of the force and (ii) the fact that the transition between two regions with opposite magnetization direction is not abrupt.

Optical Magnetometry of Single Biocompatible Micromagnets for Quantitative Magnetogenetic and Magnetomechanical Assays.

The mechanical manipulation of magnetic nanoparticles is a powerful approach to probing and actuating biological processes in living systems. Implementing this technique in high-throughput assays can be achieved using biocompatible micromagnet arrays. However, the magnetic properties of these arrays are usually indirectly inferred from simulations or Stokes drag measurements, leaving unresolved questions about the actual profile of the magnetic fields at the micrometer scale and the exact magnetic forces that are applied. Here, we exploit the magnetic field sensitivity of nitrogen-vacancy color centers in diamond to map the 3D stray magnetic field produced by a single soft ferromagnetic microstructure. By combining this wide-field optical magnetometry technique with magneto-optic Kerr effect microscopy, we fully analyze the properties of the micromagnets, including their magnetization saturation and their size-dependent magnetic susceptibility. We further show that the high magnetic field gradients produced by the micromagnets, greater than 104 T·m-1 under an applied magnetic field of about 100 mT, enables the manipulation of magnetic nanoparticles smaller than 10 nm inside living cells. This work paves the way for quantitative and parallelized experiments in magnetogenetics and magnetomechanics in cell biology.

A new magnetic cell fishing approach based on hybridization chain reaction: HCR-MISH

This paper aims at presenting a new magnetic labelling approach based on in-situ hybridization for the specific capture of bacterial cells. This approach is inspired by HCR-FISH, which was recently proposed as a new sensitive method for the detection of environmental microorganisms using a combination of FISH (Fluorescent In Situ Hybridization) and HCR (Hybridization Chain Reaction). Here we propose to adapt the technique to allow grafting of superparamagnetic nanoparticles onto target bacteria and therefore to exploit magnetism instead of fluorescence for their subsequent isolation using a micro-magnet array. The feasibility of the approach was first assessed by isolating E. coli cells using a universal bacterial probe, Eub338. Then, specific targeting of Escherichia coli DH5a, Pseudomonas putida, and Acinetobacter sp. ADP1 were demonstrated using heterologous probes. For the purpose of comparison, both HCR-FISH and HCR-MISH experimental results are presented.

Magnetically Characterized Molecular Lubrication between Biofunctionalized Surfaces.

We demonstrate an efficient approach for quantifying frictional forces (sub-piconewton) at nano-bio interfaces by controlled magnetic forces, which is based on simultaneous measurements of critical frequencies for streptavidin-coupled magnetic particles. The maximum phase angle, being corresponded with the critical frequency, is formulated in terms of magnetic, frictional, and viscous forces of the particles on DNA- and SiO2-functionalized micromagnet arrays. The streptavidin/DNA interface shows lower friction as an enhanced lubrication than the streptavidin/SiO2 interface, which is indicated by the lower transition field of quasi-static motion, the larger ratio of dynamic particles, and also the higher velocity of the particles. The friction coefficients at the streptavidin/DNA and streptavidin/SiO2 interfaces are evaluated numerically as 0.07 and 0.11, respectively, regardless of the vertical force and the velocity. The proposed method would open up new possibilities to study mechanical interactions at biological surfaces.

Micro Electromagnetic Actuator - Static Behavior

The article presents a comparative study regarding the design and experimental results for a miniature electromagnetic actuator with a modified design comparing with the classical type, including a mobile array of micro-magnets and a fixed coil in two configurations (planar spiral multi-layered and cylindrical). Previous work on design, modeling and simulation of this type of actuator indicated the optimum design and conducted to dimensions and material parameters. Different types of actuators were produced and experimentally tested, showing good results but also some drawbacks. Three of these design solutions are presented together with the static voltage-deflection and electrical impedances curves, experimentally derived. The results confirmed the feasibility of two actuating solution, showing good linearity and possibility to control the position at h accuracy and indicated improving directions.

A scanning Hall probe microscope for high resolution, large area, variable height magnetic field imaging.

We present a scanning Hall probe microscope operating in ambient conditions. One of the unique features of this microscope is the use of the same stepper motors for both sample positioning as well as scanning, which makes it possible to have a large scan range (few mm) in the x and y directions, with a scan resolution of 0.1 μm. Protocols have been implemented to enable scanning at different heights from the sample surface. The z range is 35 mm. Microstructured Hall probes of size 1-5 μm have been developed. A minimum probe-sample distance <2 μm has been obtained by the combination of new Hall probes and probe-sample distance regulation using a tuning fork based force detection technique. The system is also capable of recording local B(z) profiles. We discuss the application of the microscope for the study of micro-magnet arrays being developed for applications in micro-systems.

An on-chip micromagnet frictionometer based on magnetically driven colloids for nano-bio interfaces.

A novel method based on remotely controlled magnetic forces of bio-functionalized superparamagnetic colloids using micromagnet arrays was devised to measure frictional force at the sub-picoNewton (pN) scale for bio-nano-/micro-electromechanical system (bio-NEMS/MEMS) interfaces in liquid. The circumferential motion of the colloids with phase-locked angles around the periphery of the micromagnets under an in-plane rotating magnetic field was governed by a balance between tangential magnetic force and drag force, which consists of viscous and frictional forces. A model correlating the phase-locked angles of the steady colloid rotation was formulated and validated by measuring the angles under controlled magnetic forces. Hence, the frictional forces on the streptavidin/Teflon interface between the colloids and the micromagnet arrays were obtained using the magnetic forces at the phase-locked angles. The friction coefficient for the streptavidin/Teflon interface was estimated to be approximately 0.036 regardless of both vertical force in the range of a few hundred pN and velocity in the range of a few tenths of μm s(-1).

Optimizing the Design of a Magnetic Actuator for Scanning and Energy Harvesting MEMS Devices

Among different types of energy-harvesting devices, the electrodynamic one is widely used due to the simple structure, high stability in time (better than piezoelectric materials). In the same time, the same structure could be applied for another application – scanning device. This article presents an analysis regarding the optimum design of the magnetic structure not as a bulk magnet, with specific problems during manufacturing, but as an array of micromagnets. The target function is the force acting on a mobile plate and the magnetic inductivity at that level.