Biaxial Magnetic Field Research Articles

Two-dimensional biaxial magnetic field imaging with millisecond resolution

Multi-axial dynamic magnetic field imaging with femto-Tesla sensitivity paves a promising route for acquiring comprehensive information of the extremely-weak magnetic source. Here we propose a two-dimensional (2D) scanning imaging configuration that can realize rapid biaxial magnetic field measurement. Based on the Bragg diffraction, the configuration utilizes two orthogonally-arranged acousto-optic modulators to scan the probe laser temporally and spatially, realizing a 2D (7 × 7 pixels) magnetic field detection. To solve the multi-axial dynamic magnetic field imaging problem, a non-modulated time-sharing technique is theoretically and experimentally verified. Profiting from the non-modulated biaxial working mode that avoids long demodulation time, the magnetometer realizes millisecond temporal resolution (17.15 ms) for each sensitive axis. Operation in the spin-exchange relaxation-free regime, the single-pixel measurement sensitivity reaches around 2.6 fT/Hz1/2 (y-axis) and 7.5 fT/Hz1/2 (z-axis). Furthermore, weak pump and probe lasers power (500 μW and 350 μW) make it a low-power imaging device.

On the power and efficiency of Ni2MnGa magnetic shape memory alloy power harvesters

The martensite variant reorientation in Ni2MnGa magnetic shape memory alloys (MSMAs) causes a change in their bulk magnetization, that can be harvested into useful voltage/power by means of a pick-up coil. The coil may be placed directly surrounding an MSMA element or to the side of the MSMA element wrapped around a magnetic core. This paper reports new power harvesting data generated with a bi-axial magnetic field and a surrounding coil and full strain field data for an MSMA subject to load similar to what is seen during power harvesting, then compares the performance of MSMA-based power harvesters with different designs to determine which give the best output. For this comparison, we provide a framework for evaluating the performance of MSMA-based power harvesters reported in the literature. This framework involves normalizing the results to the design characteristics of the respective harvesters, i.e. number of turns of the pickup coil, cross-sectional area of the pickup coil, frequency of excitation, and sample size, to allow for a direct comparison of power harvesters’ output. Results show that power harvesting with the bi-axial field and a surrounding coil does not generate as much power as previously thought. The strain maps reveal the potential for perpendicular twin boundaries that block each other’s motion limiting variant reorientation and correspondingly the harvester’s power output. The paper concludes that the largest change in magnetic flux density, which is the driver for power harvesting, occurs in the side coil setup with an optimized magnetic circuit and it recommends using this configuration for future MSMA-based power harvester designs for maximum power output.

Actuation of magnetoelastic membranes in precessing magnetic fields

Superparamagnetic nanoparticles incorporated into elastic media offer the possibility of creating actuators driven by external fields in a multitude of environments. Here, magnetoelastic membranes are studied through a combination of continuum mechanics and molecular dynamics simulations. We show how induced magnetic interactions affect the buckling and the configuration of magnetoelastic membranes in rapidly precessing magnetic fields. The field, in competition with the bending and stretching of the membrane, transmits forces and torques that drives the membrane to expand, contract, or twist. We identify critical field values that induce spontaneous symmetry breaking as well as field regimes where multiple membrane configurations may be observed. Our insights into buckling mechanisms provide the bases to develop soft, autonomous robotic systems that can be used at micro- and macroscopic length scales.

Biaxial Magnetic-Field Setup for Angular-Dependent Measurements of Magnetic Thin Films and Spintronic Nanodevices

The biaxial magnetic-field setup for angular magnetic measurements of thin film and spintronic devices is designed and presented. The setup allows for application of the in-plane magnetic field using a quadrupole electromagnet, controlled by power supply units and integrated with an electromagnet biaxial magnetic field sensor. In addition, the probe station is equipped with a microwave circuitry, which enables angle-resolved spin torque oscillation measurements. The angular dependencies of magnetoresistance and spin diode effect in a giant magnetoresistance strip are shown as an operational verification of the experimental setup. We adapted an analytical macrospin model to reproduce both the resistance and spin-diode angular dependency measurements.

The effect of magnetic field orientation on the open-circuit voltage of Ni–Mn–Ga based power harvesters

Ni–Mn–Ga is a ferromagnetic alloy that can exhibit the shape memory effect or superelasticity in the presence of a magnetic field. The behavior of the material is largely due to its microstructure, which is thought to be made of tetragonal martensite variants, each exhibiting an innate magnetization aligned approximately with the short side of the unit cell. Because the reorientation strain can be induced and recovered by either magnetic field or mechanical stress, it can be induced at frequencies larger than 1 kHz, which makes the material suitable for high-frequency actuation, sensing, or power harvesting applications. This paper investigates the power harvesting capability of Ni–Mn–Ga wrapped with a pick-up coil under a bi-axial magnetic field. In this work, both experimental tests and numerical simulations are used to identify the optimal direction of the externally applied magnetic field in order to achieve maximum open-circuit voltage output from a particular Ni–Mn–Ga based power harvester. Results suggest that significantly more power can be achieved with the bias field applied at an angle of 10°–20° off the perpendicular to the coil axis and the compressive stress. We believe that this increased power output is due to the saturation of domain walls due to a small component of the magnetic field along the direction of the coil.

Magneto-actuated cell apoptosis by biaxial pulsed magnetic field

We report on a highly efficient magneto-actuated cancer cell apoptosis method using a biaxial pulsed magnetic field configuration, which maximizes the induced magnetic torque. The light transmissivity dynamics show that the biaxial magnetic field configuration can actuate the magnetic nanoparticles with higher responsiveness over a wide range of frequencies as compared to uniaxial field configurations. Its efficacy was demonstrated in in vitro cell destruction experiments with a greater reduction in cell viability. Magnetic nanoparticles with high aspect ratios were also found to form a triple vortex magnetization at remanence which increases its low field susceptibility. This translates to a larger magneto-mechanical actuated force at low fields and 12% higher efficacy in cell death as compared to low aspect ratio nanoparticles.

Magnetostriction measurements of high strength steel under the influence of bi-axial magnetic fields

A detailed knowledge of a material's microscopic texture is required in order to produce a realistic model of the magnetization process under applied fields. Previous studies on the magnetostriction in high strength steels have ignored the internal anisotropies due to prior material handling. To this end, a measurement utilizing two perpendicular fields was designed to interrogate the magnetic texture and microstructure of high-strength steel rods. These magnetization and magnetostriction measurements were then fitted to an energy-based domain rotation model which had been altered to address vector fields and uniaxial anisotropies. Given the simplicity of the model it is surprising to see that it captures a number of the general trends in the Data, however the fit is generally poor. Improving upon this data set will allow us to determine general magnetic characteristics of microstructure in the steels. These measurements will be incorporated into a future Vector Preisach model allowing detailed predictions of the magnetic state after stress and field changes in multiple directions.

Stimulation of vigorous rotational flows and novel flow patterns using triaxial magnetic fields

We have discovered that new flow patterns can be created by applying a dc field to the ac biaxial fields that are used to induce isothermal magnetic advection (IMA). IMA is a recently discovered fluid flow phenomenon that occurs in suspensions of magnetic platelets subjected to particular time-dependent, uniform, biaxial magnetic fields. IMA is characterized by the formation of emergent flow patterns called advection lattices. We find that a dc field can disrupt the antiparallel flow symmetry of the advection lattice and give rise to qualitatively new flow patterns, including vigorous rotational flows and a highly regular diamond lattice. The rotational flows are very robust and may have applications to heat transfer. The diamond lattice is an intriguing and challenging example of emergent dynamics. Both of these effects occur when the dc field is applied orthogonal to the plane of the biaxial field.

Influence of Biaxial Magnetic Field on Martensite Reorientation in Magnetic Shape Memory Alloy

Based on thermodynamics, a constitutive model for magnetic shape memory alloy (MSMA) is developed under biaxial magnetic field. Considering the driving force provided by Gibbs free energy and resistive force during martensite reorientation, a kinetic equation is got. Of special concern is the influence of biaxial magnetic field on martensite reorientation for a Ni2MnGa single-crystal specimen. The theoretical results are in agreement with experimental data. Some useful conclusions can be obtained.

Isothermal Magnetic Advection: Creating functional fluid flows for heat and mass transfer

Natural convection has been of interest for over a century due to its rich nonlinear dynamics and applications to heat transfer. However, convection occurs only when both gravity and a destabilizing thermal gradient exist. We have discovered a unique class of vigorous, emergent fluid flows that have the full functionality of natural convection but can be stimulated regardless of gravity or thermal gradients, simply by subjecting a platelet suspension to certain time-dependent biaxial magnetic fields of modest strength. This enigmatic phenomenon may facilitate cooling in microgravity environments and in other circumstances where convection fails.

Modelling of rearrangement yield surface under biaxial magnetic field in Ni-Mn-Ga shape memory alloys

Abstract. A predictive model of field-induced strain in Ni-Mn-Ga ferromagnetic shape memory alloys is proposed. Theaction of both biaxial magnetic field and mechanical stress on the martensitic rearrangement in a Ni-Mn-Ga single crystal aremodelled. Yield surfaces of starting rearrangement are simulated. They show how applied mechanical stress helps or opposesto the applied magnetic field to make the rearrangement easier or harder. The results are fully compatible with the experimentsof Mullner et al. [8] describing the action of a rotating magnetic field.¨Keywords: Magneto-mechanical coupling, shape memory alloy, model 1. Introduction In the search of new active materials, ferromagnetic shape memory alloys (FSMA) such as Ni-Mn-Gahave a prominent position thanks to their capability to developa strain more important than conventionalactive materials such as piezoelectric and magnetostrictive. Contrary to classical shape memory alloys,theyareabletodevelopthisstrainveryrapidly,underanappliedmagneticfield. Thisparticularbehaviouris in fact due to variant structure rearrangement of twinned martensite platelets: when a magnetic fieldis applied, this one tends to increase the volume fraction of the best oriented martensite between the twotwinned ones. To describe and understand this mechanism of magnetic field-induced-strain, differentmodels were proposed (Murray et al. [1], James et al. [2], Likhachev et al. [3] and Mullner et al. [4]).¨In this paper, the authors propose a magneto-mechanical model based on phenomenology, thermo-dynamics of irreversible processes and changing of scale. This model is written in order to predict theeffect of biaxial magneticfieldin asingle crystalof 5 M or 7 M twinnedmartensite, with appliedstressornot. For sake of simplicity, no demagnetizing field is considered. A Representative Elementary Volumeis classically introduced with different microstructural parameters, i.e. volume fractions of martensiticbands, magnetization rotations, width ratios of two consecutive magnetic domains. It must be noticedthat these variables are linked to physical phenomenon, for example, the movements of 180

Magnetostriction of Field-Structured Composites

ABSTRACTField-structured magnetic particle composites are an important new class of materials that have great potential as both sensors and actuators. These materials are synthesized by suspending magnetic particles in a polymeric resin and subjecting these to magnetic fields while the resin polymerizes. If a simple uniaxial magnetic field is used, the particles will form chains, yielding composites whose magnetic susceptibility is enhanced along a single direction. A biaxial magnetic field, comprised of two orthogonal ac fields, forms particle sheets, yielding composites whose magnetic susceptibility is enhanced along two principal directions. A balanced triaxial magnetic field can be used to enhance the susceptibility in all directions, and biased heterodyned triaxial magnetic fields are especially effective for producing composites with a greatly enhanced susceptibility along a single axis. Magnetostriction is quadratic in the susceptibility, so increasing the composite susceptibility is important to developing actuators that function well at modest fields. To investigate magnetostriction in these field-structured composites we have constructed a sensitive, constant-stress apparatus capable of 1 ppm strain resolution. The sample geometry is designed to minimize demagnetizing field effects. We have demonstrated field-structured composites with nearly 10,000 ppm strain, and have shown that at large magnetic fields a structural phase transition occurs within the composite. These experimental results are compared to microscopic, self-consistent field simulations of magnetostriction in these complex, disordered materials.

Ferromagnetic shape memory in the NiMnGa system

Strain versus field measurements for a ferromagnetic shape memory alloy in the NiMnGa system demonstrate the largest magnetostrictive strains to date of nearly 1.3%. These strains are achieved in the martensitic state through field-induced variant rearrangement. An experimental apparatus is described that provides biaxial magnetic fields and uniaxial compressive prestress with temperature control while recording microstructural changes with optical microscopy. The magnetostrictive response is found to be sensitive to the initial state induced by stress-biasing the martensitic variant structure, and exhibits rate effects related to twin boundary mobility. Experiments performed with constant stress demonstrate work output capacity. Experimental results are interpreted by using a theory based on minimization of a micromagnetic energy functional that includes applied field, stress, and demagnetization energies. It is found that the theory provides a good qualitative description of material behavior, but significantly overpredicts the amount of strain produced. Issues concerning the martensitic magnetic anisotropy and variant nucleation are discussed with regard to this discrepancy.