Magnetic properties of nanostructured systems based on TbFe2

The aim of this work is to study the magnetic properties of annealed [Fe3Ga/TbFe2]n heterostructures grown by sputtering at room temperature. The interest of investigating multilayers comprised of TbFe2 and Fe3Ga is their complementary properties in terms of coercivity and magnetostriction. We have studied the thickness combination which optimizes the magnetic and magnetostrictive properties of the annealed multilayers. The crystallization of the Laves phase upon the thermal treatment in heterostructures with thick TbFe2 layers promotes the increase of the coercivity. This crystallization seems to be prevented by the low mechanical stiffness of the Fe3Ga. [Fe3Ga/TbFe2]n heterostructures show promising characteristics, λ of 340 ppm and a HC of 220 Oe, for the development of new magnetostrictive devices

Magnetomechanical performance of directionally solidified Fe-Ga alloys

Iron-gallium alloys can produce magnetostrictions of ~400 ppm and might serve as mechanically robust actuator/sensing materials. However, for polycrystalline Fe-Ga alloys, the magnetostrictive performance decreases with the increasing deviations from the ideal <100> texture. In this paper, three directionally solidified Fe-Ga alloys with gallium contents of 17, 18.4, and 19.5 at. % were characterized at ambient temperature. These specimens exhibit high d33 and magnetic permeability when subjected to applied magnetic fields, indicating their suitability for light weight actuator applications but not for high force applications due to their low saturation magnetostriction and hence low blocking force. All the alloys produce significant changes in magnetization, around 0.7Ms-0.8Ms when subjected to cyclic compressive stresses of 51 MPa, making them promising candidate materials for sensing and energy harvesting applications. However, eddy current effects may easily become a problem when such materials are subjected to a high frequency vibration or magnetic field due to their intrinsic high magnetic permeability

Vibration energy harvesting using Galfenol based transducer

In this paper the novel design of Galfenol based vibration energy harvester is presented. The device uses Galfenol rod diameter 6.35 mm and length 50mm, polycrystalline, production grade, manufactured by FSZM process by ETREMA Product Inc. For experimental study of the harvester, the test rig was developed. It was found by experiment that for given frequency of external excitation there exist optimal values of bias and pre-stress which maximize generated voltage and harvested power. Under optimized operational conditions and external excitations with frequency 50Hz the designed transducer generates about 10 V and harvests about 0,45 W power. Within the running conditions, the Galfenol rod power density was estimated to 340mW/cm3. The obtained results show high practical potential of Galfenol based sensors for vibration-to-electrical energy conversion, structural health monitoring, etc

ABNORMAL GRAIN GROWTH IN MAGNETOSTRICTIVE GALFENOL ROLLED SHEET

Highly textured Fe-Ga (Galfenol) rolled sheet with Cube (100) or Goss (110) preferred orientation is under investigation to provide easy magnetization, enhanced magnetostrictive performance and a cost-effective option for production of these alloys for use in applications as sensors and actuators. In this study, 1-2.5% NbC added Galfenol rolled sheet was used because NbC particles enhance the rollability of and abnormal grain growth (AGG) in polycrystalline Galfenol rolled sheet. Driving forces, due to grain boundary energy, surface energy, deformation energy and magnetic fields are generally considered to explain grain growth phenomena. In this dissertation, the effect on grain boundary energy for influencing AGG was studied for the case of high temperature annealing at 1200°C. Both Coincident Site Lattice (CSL) and High Energy Grain Boundary (HEGB) models were investigated as possible mechanisms to explain the contribution of grain boundary energy to Goss-textured AGG. Results support the HEGB model as a suitable model for the observed development of Goss-textured AGG in Galfenol rolled sheet. Next, the effect of deformation energy on AGG was studied by using tension annealing and strain annealing methods in the temperature range of 900°C to 1100°C. This study was built on results from studies of grain boundary energy on other alloys. For the tension annealing investigation, Galfenol rolled sheet was simultaneously subjected to tensile loading during high temperature annealing. No AGG was observed from the tension annealing method. For the strain-annealing investigation, homogeneously recrystallized Galfenol rolled sheet with a taper was subjected to tensile loading under different strain rates and post-strain high temperature anneal conditions to investigate the resultant grain growth phenomena. Different grain growth modes, including Cube- and Goss-textured AGG, were observed in this study. Assessment of the extent of AGG resulting from these was conducted using Electron Backscattering Diffraction (EBSD) patterns that were captured and analyzed using Orientation Imaging Microscope (OIM) software to obtain Inverse Pole Figures (IPF) and Orientation Distribution Function (ODF). Additionally, Ga loss, which lowers the magnetostrictive properties, under different conditions was investigated by Electron Probe Micro Analyzer (EPMA). No significant Ga loss was observed during the annealing process at 1000°C, however, about 2% Ga loss was observed during the annealing process at 1100°C and 1200°C in the areas with a high density of grain boundaries

The effect of dynamical compressive and shear strain on magnetic anisotropy in a low symmetry ferromagnetic film

Dynamical strain generated upon excitation of a metallic film by a femtosecond laser pulse may become a versatile tool enabling control of magnetic state of thin _lms and nanostructures via inverse magnetostriction on a picosecond time scale. Here we explore two alternative approaches to manipulate magnetocrystalline anisotropy and excite magnetization precession in a low-symmetry _lm of a magnetic metallic alloy galfenol (Fe,Ga) either by injecting picosecond strain pulse into it from a substrate or by generating dynamical strain of complex temporal profile in the film directly. In the former case we realize ultrafast excitation of magnetization dynamics solely by strain pulses. In the latter case optically-generated strain emerged abruptly in the film modifies its magnetocrystalline anisotropy, competing with heat-induced change of anisotropy parameters. We demonstrate that the optically-generated strain remains efficient for launching magnetization precession, when the heat-induced changes of anisotropy parameters do not trigger the precession anymore. We emphasize that in both approaches the ultrafast change of magnetic anisotropy mediating the precession excitation relies on mixed, compressive and shear, character of the dynamical strain, which emerges due to low-symmetry of the metallic film under study

Development of a Bio-Inspired Magnetostrictive Flow and Tactile Sensor

A magnetostrictive sensor was designed, constructed, and evaluated for use as flow or tactile sensor. Vibrissa-like beams (whiskers) were cut from sheets of the magnetostrictive iron-gallium alloy, Galfenol. These beams were cantilevered, with the fixed end of the whisker attached to a permanent magnet to provide the whisker with a magnetic bias. The free portion of the whisker was quasi-statically loaded, causing the whisker-like sensor to bend. The bending-induced strain caused the magnetization of the whisker to change, resulting in a changing magnetic field in the area surrounding the whisker. The change in magnetic field was detected by a giant magnetoresistance (GMR) sensor placed in proximity to the whisker. Therefore, the electrical resistance change of the GMR sensor was a function of the bending in the whisker due to external forces. Prototype design was aided using a bidirectionally coupled magnetoelastic model for computer simulation. The prototype was tested and evaluated under tactile loading and low speed flow conditions