Session D9: Magnetic Devices and Applications

Dual microbead-labeled DNA manipulation with magnetic traps in a microfluidic device

Biomolecular responses to mechanical force underlie many critical functions in the context of cellular physiology. In order to develop the technology to apply forces on individual biomolecules, we utilize an array of ferromagnetic disks on a silicon surface to trap and manipulate tethered DNA molecules. The force activation is achieved through remotely controlled programmable weak external magnetic fields that do not damage the biological entity. Moreover, to exploit both hydrodynamic and magnetic forces, the magnetic disks are imprinted within microfluidic channels, while a tethered microbead attached on each end of the DNA strand provides convenient force transmitting handles. Two separate approaches that are utilized involve use of two superparamagnetic beads or a superparamagnetic and nonmagnetic bead pair. The independently controlled hydrodynamic and magnetic forces allow for manipulation of the DNA in all directions within a horizontal plane. Hundreds of magnetic traps are readily patterned onto a single channel, providing the potential to multiplex an ensemble of individual molecules within the device.   

Testing of a Helmholtz Microcoil in a Diamond Anvil Cell NMR

A new designed, multi-turn tungsten Helmholtz micro-coil has been constructed and tested on the solid-state bulk NMR experiment. A Helmholtz micro-coil with diameter 950~$\mu $m is~embedded on diamond culet and produces a nearly uniform AC magnetic field inside a sample space. A Fluorine polycrystal will be used to test our Helmholtz micro-coil, and the measured NMR data will be compared with the ones produced by other type of diamond anvil cell coils. The Helmholtz micro-coil will be used for high pressure NMR and future investigation of magnetic properties of heavy fermion superconductors.   

Magneto-mechanical resonant detection of superparamagnetic microbeads trapped by magnetic domain walls

Manipulation of superparamagnetic (SPM) beads with magnetic domain walls (DWs) is of interest for lab-on-chip applications. DWs can trap SPM beads and tagged entities, enabling remote manipulation with nanoscale precision [1, 2]. Previously, we have demonstrated DW driven capture and transport of single microbeads at speeds approaching 1000 $\mu$m/s [3]. Here, we demonstrate that the strong magnetostatic bead-DW binding leads to a unique magneto-mechanical resonance [4]. We show experimentally that this resonance can be used to distinguish bead populations based on their size, presenting a new mechanism for bead metrology. Moreover, the bead-DW interaction can be used to sense and characterize magnetic beads without the need for sensor surface functionalization. Exploiting the dual functionality of DWs as both bead carriers and sensors, we present an integrated device capable of high-speed transport and electrical sensing of the magneto-mechanical resonance of individual trapped beads.\\[4pt] [1] G. Vieira et al., Phys. Rev. Lett. 103, 128101 (2009).\\[0pt][2] M. Donolato et al., Adv. Mater. 22, 2706 (2010).\\[0pt] [3] E. Rapoport, G.S.D. Beach, Appl. Phys. Lett. in press.\\[0pt] [4] E. Rapoport, G.S.D. Beach, J. Appl. Phys. in press.   

Cascaded Magneto-Optical Ring Resonator Structures for Tunable Faraday Rotation and Reduced Isolator Footprint

On-chip optical isolators are indispensible components of integrated optics, and can be modified to enable four-port and multi-port circulators and modulators. We have implemented an on-chip optical isolator by placing a racetrack resonator next to a single mode waveguide and coating half of the resonator with a uniformly magnetized magneto-optical film, which breaks the time-reversal symmetry of light propagation and provides different refractive indices and phase shifts for forward and backward propagating waves. At every pass, the optical mode inside the resonator accumulates Faraday rotation in addition to phase shift due to propagation. The transmission from the output port of the waveguide has a Lorentzian dip due to the resonance peak of the resonator. Light can only propagate in the clockwise direction inside the resonator. Here we model how cascading multiple ring resonators can increase the overall quality factor of the isolator and narrow the resonance linewidth, due to the longer photon lifetime inside the cavity. As a result of better control of Faraday rotation, the isolation ratio is enhanced and the device footprint is reduced with respect to Mach-Zehnder waveguide isolators.   

On the Energy Transfer Performance of Mechanical Nanoresonators Coupled with Electromagnetic Fields: Applications with magnetic nanoparticles

The energy transfer performance in electrically and magnetically coupled mechanical nanoresonators is studied [1]. Using the resonant scattering theory, we show that magnetically coupled resonators can achieve the same energy transfer performance as for their electrically coupled counterparts, or even outperform them within the scale of interest. Magnetic and electric coupling are compared in the \emph{Nanotube Radio}, a realistic example of a nano-scale mechanical resonator. The energy transfer performance is discussed for magnetic coupling in magnetite (Fe$_3$O$_4$) nanoparticles. \\[4pt] [1] H. Javaheri, B. Barbiellini, G. Noubir, arXiv:1108.0633.   

Increased Sensitivity of Magnetoelectric Sensors at Low Frequencies Using Magnetic Field Modulation

Magnetoelectric (ME) laminate sensors are vector magnetometers that can detect pT magnetic fields at 1 kHz, although sensitivity may be reduced at lower frequencies. These passive sensors consist of alternating layers of magnetostrictive and piezoelectric materials. A magnetic field causes the magnetostrictive layer to strain the piezoelectric material and create measurable charge. We have shown\footnote{To be published in Journal of Applied Physics.} that since the strain response is a nonlinear function of the bias field, sweeping the magnetic bias on the magnetostrictive layer can modulate the ME response and increase the operating frequency of the sensor. This upward shift lowers the $1$/$f$ noise and increases the signal amplitude if the new operating frequency is near a mechanical resonance mode of the sensor. Using this modulation technique, the low frequency sensitivity has been improved by more than an order of magnitude and we have achieved a detectivity of 7 pT/$\surd $Hz at1 Hz. In addition to increasing the magnetic signal frequency, we can use magnetic modulation to increase the operating frequency of acoustic signals detected by these sensors. This occurs because the ME sensors are nonlinear devices. In these cases using magnetic field modulation, the signal appears as sidebands around the modulation frequency.   

Bipolar exchange bias modulation in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/ BiFeO$_{3}$ heterostructure based field effect devices

We have fabricated and performed electrical transport measurements on a multiferroic field effect device with a BiFeO$_{3}$ (antiferromagnetic/ferroelectric) gate dielectric and a La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (ferromagnetic) channel to investigate the effect of BiFeO$_{3}$ (BFO) polarization on interfacial magnetism by using exchange bias as a diagnostic tool. A reversible static shift in exchange bias through zero applied magnetic field is observed by electrically poling BFO. This bipolar exchange bias modulation behavior strongly suggests that interfacial magnetization is being reversed through the application of electric field. To investigate this phenomenon further we have measured temperature dependent exchange bias, temperature dependent resistivity, and Hall Effect coefficients on multiple devices. Also a comparison to an identical control device using a Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$ (ferroelectric) gate dielectric and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) channel is offered, which provides insight into the interfacial magnetic interactions uniquely occurring in the BFO/LSMO system. We analyze these results in the context of proposed models, which suggest that we are modulating both carrier density and interfacial magnetic coupling strengths; both of which have a strong effect on the bipolar modulation of exchange bias.   

Effect of the exchange bias field on the magnetoimpedance response in multilayered FeNi-IrMn films and CoFeSiB-IrMn ribbons

The magnetoimpedance effect (MI) has been widely used in sensitive magnetic field (MF) sensors, but its intrinsic nonlinear properties are disadvantageous for sensor applications near zero field. The combination of ferromagnetic (FM) and antiferromagnetic (AFM) layers produce an asymmetrical MI (AMI) peak positions which are shifted to higher MF as the probe frequency increases, so linear MI behavior can be obtained around zero external field by tuning the frequency. Here, AMI was extensively studied in multilayer strips of exchange-coupled FeNi-IrMn. The effect of the thickness of the FM layer and the angle dependence in three stripe samples with EB induced; parallel, perpendicular and forming an angle of 45 with the direction of the wire were studied. MI ratio raise with increasing thickness of the FM layer is attributed to the stronger pinning of the FM adjacent to the IrMn. Also, a combination of the EB angle affects and direction of the applied MF can tune; the number of peaks in the MI response, the asymmetry between peaks and the shift of the MI response. Besides, an antiferromagnetic layer was deposited on the top of CoFeSiB amorphous ribbons enhancing the MI effect at low frequencies and shifting the MI response at higher frequencies.   

Tailoring Giant Magneto-impedance Effect in Ultrasoft Ferromagnetic Microwires

Research on soft ferromagnetic microwires exhibiting giant magneto-impedance (GMI) effect, which is a large change of the ac impedance of a ferromagnetic conductor in a static magnetic field, for advanced magnetic sensor applications is an area of topical interest. In this study we show how the GMI effect and its field sensitivity are optimized in Co-B-Si-Mn microwires by varying the magnetic core to glass shell diameter ratio ($d)$. The microwires have been fabricated by the glass-coated melt spinning method. The largest values of GMI (245{\%}) and its field sensitivity 25{\%}/Oe are achieved at $f$ = 13MHz for the microwires with $d $= 0.86. The $d$ dependence of the magneto-impedance has been analyzed based on those of the magneto-resistance and magneto-reactance. Our studies indicate that the microwires with optimized GMI response are attractive candidate materials for structural health self-monitoring and magnetic biosensing applications.   

Soft ferromagnetic microribbons with enhanced GMI effect for advanced magnetic sensor applications

Soft ferromagnetic ribbons with giant magneto-impedance (GMI) effect are attractive candidate materials for high-performance magnetic sensor applications. GMI is a large change in the ac impedance of a ferromagnetic conductor subject to a dc magnetic field. There is a need for further improving GMI response of existing materials, as well as reducing the size of a GMI-based sensor for use in micro-sensing systems. In this work, we report the enhancement of GMI in soft ferromagnetic ribbons (Metglas{\textregistered} 2714A) at high frequencies by reducing the width of the ribbon to the micrometer scale. This finding is of practical importance, as sensors with enhanced field sensitivity and reduced size find wider ranging applications. The origin of the enhanced GMI effect in the microribbon is explained in terms of the skin and demagnetization effects. The relative contributions to the magneto-impedance from the magneto-resistance and magneto-reactance have been analyzed and discussed in detail.   

Designing New Metal-Semiconductor Hybrid Structures With Large Geometric Magnetoresistance

The extraordinary magnetoresistance (EMR) in metal-semiconductor hybrid structures was first demonstrated using a four-contact configuration for a circular semiconductor wafer with a concentric metallic inclusion in it. The EMR effect, which is observed at room temperature, is very suitable for use in read heads in magnetic data storage devices. This effect depends on the orbital motion of carriers in an external magnetic field, and the remarkably high magnetoresistance response (the change in resistance with a magnetic field) observed suggests that the geometry of the metallic inclusion can be optimized to significantly enhance the EMR. The theory and simulations to achieve this goal are considered by comparing various 2D structures in an external magnetic field to evaluate the EMR in them using finite element analysis and geometric optimization. For a 10 $\mu$m square semiconductor wafer with a square metallic inclusion we see a range in resistance from -400 to 400 $\Omega$ for -1 T $\leq B \leq$ 1 T. This response can be optimized by changes in contact orientation and the size and shape of the metallic region. Extension to 3D is being investigated at present, which would allow for modeling of magnetic field sensors that also provide the direction of the field.   

Ultra wideband, high sensitivity magneto-optic field sensor

Using the bismuth rare-earth iron garnet thick films we have demonstrated a magneto-optic (MO) field sensor. The sensor made of all dielectric materials is nearly noninvasive, and is operated at room temperature. The sensor's sensitivity is scalable; the same sensor design can be used for a low-field sensor to measure fields below nano-Tesla or for a high-field sensor to measure several hundred Tesla. The highest sensitivity that we have achieved with the sensor is about 30 pico-Tesla/(Hz)$^{1/2}$. Presently its frequency range is limited from DC to 2 GHz. We have carried out several different experiments with this sensor to explore a few interesting applications, such as electromagnetic signal interception tests over a very broad frequency range. In this presentation we will report our experimental results obtained from this MO field sensor.   

Diamagnetic repulsion, the method of magnetic images {\&} suitability of the solenoid and dipole models

The repulsion of a permanent magnet from a diamagnetic region was investigated. A magnet of moment m can be described by two models (i) solenoid - a circulating current of appropriate value; second (ii) a magnetic dipole comprising of a pair of north and south poles of separated by a distance.The magnetic field (B) of a permanent magnet was measured. The magnet was modeled as a solenoid with a circulating surface current. The Biot-Savart law field (B) was of computed in Matlab. The experimental data of was in excellent agreement with the Matlab results. However, for computing the repulsion force (F) between the magnet and its diamagnetic image by the direct integration of the current-current interaction require detailed knowledge of the two current densities. However such knowledge is not essential if image is modeled as a dipole. When the magnet is a distance z above the diamagnetic interface then the image current I2 gives rise to a image dipole m2 and the F $\sim $ m2div B, where the div of the holding field is computed at the distance 2z below the magnet. In this model F is directly proportional to both m' and the derivative of the field and a negative slope indicates repulsion, all three were confirmed.   

Magnetic Field Assisted assembly using programmable array of solenoids - A Manufacturing Approach

The use of an array of programmable solenoids to implement a magnetic field driven assembly of ~heterogeneous micro-components onto a substrate is studied. ~A lower limit of component size, the upper limit of the rate of assembly and the efficiency of the assembly from various perspectives is presented. Various statistical tests are performed on the assembly process to determine its feasibility. A comparison is made between this method of assembly and established assembly techniques in the literature.