Magnetostrictive Thin Films Research Articles

Magnetic anisotropy controlled FeCoSiB thin films for surface acoustic wave magnetic field sensors

Surface acoustic wave magnetic field sensors based on guided Love waves using the ΔE effect of a magnetostrictive thin film have been shown to be promising candidates for the measurement of weak fields at low frequencies as required for biomagnetic applications or as current sensors benefitting from the large dynamic range and bandwidth. The deposition of soft magnetic films with high magnetostriction is, however, more challenging on piezoelectric substrates such as quartz than on silicon. Thermally induced anisotropic expansion during the deposition process or during post-deposition magnetic field annealing leads to uniaxial stresses acting on the films, which makes the precise control of magnetic anisotropy difficult. Accordingly, this work analyzes the influence of the deposition process and heat treatment on the performance of Love wave devices. ST-cut quartz based delay line surface acoustic wave sensors with a SiO2 guiding layer are employed, and a 200 nm layer of amorphous magnetostrictive (Fe90Co10)78Si12B10 is used as the sensitive element. Magneto-optical imaging is performed for magnetic domain characterization, and the sensor performance is characterized in terms of bias field dependent phase sensitivity and frequency dependent phase noise. By performing a low temperature deposition in an external magnetic field, considerable improvement in limits of detection at biomagnetic relevant frequencies down to 70 pT/Hz at 10 Hz and 25 pT/Hz at 100 Hz is achieved.

Magnetic Field Tunable Ferromagnetic Shape Memory Alloy-Based Piezo-Resonator

In this letter, we have performed a comprehensive analysis of the influence of the magnetic field on BAW resonator consisting of a highly magnetostrictive layer and AlN thin film. The fundamental resonant frequency of the fabricated BAW resonator is about ~4.22 GHz. In the presence of a magnetic field, we studied the effect on the resonator parameters such as resonant frequency, acoustic velocity, and coupling coefficient. For the magnetic field of strength 1200 Oe, the resonant frequency significantly shifts by ~360 MHz. Resonant frequency increases and electromechanical coupling coefficient ( $\text{k}_{t}$ ) decreases with the increase in the DC magnetic field. The maximum acoustic velocity of ~7350 m/sec was observed at the magnetic field of 1500 Oe when applied parallel to the surface. Agilent Advanced Design Software (ADS) was used to extract the equivalent Modified Butterworth-Van Dyke circuit parameters (R m , C m , and L m ) of the resonator. Further, in the presence of the magnetic field, we obtained the variation in values of R m , C m , and L m of the resonator structure. Such tunable resonators can be useful and vital in dealing with varying frequency bands for sustainable growth in wireless communication and magnetic field sensor applications.

Intrinsic versus shape anisotropy in micro-structured magnetostrictive thin films for magnetic surface acoustic wave sensors

This work aims at studying the interaction between surface acoustic waves (SAW) and micro-structured magnetostrictive layers under a magnetic field with a perspective to develop magnetic field sensors. The impact of the competition between the strong intrinsic magnetic anisotropy of the magnetic material and the shape anisotropy of the interdigitated transducer (IDT) fingers introduced by the micro-structuration is investigated. Therefore, the macroscopic and microscopic magnetic properties of the IDT and their influence on the magneto-acoustic response are studied. A SAW resonator with the IDTs made of the magnetostrictive thin film was elaborated and the magnetic surface acoustic wave (MSAW) response under a magnetic field was performed and discussed. Depending on the energy balance, the anisotropy gets modified and a correlation with the MSAW sensitivity to an externally applied magnetic field is made.

Mechanical-Resonance-Enhanced Thin-Film Magnetoelectric Heterostructures for Magnetometers, Mechanical Antennas, Tunable RF Inductors, and Filters.

The strong strain-mediated magnetoelectric (ME) coupling found in thin-film ME heterostructures has attracted an ever-increasing interest and enables realization of a great number of integrated multiferroic devices, such as magnetometers, mechanical antennas, RF tunable inductors and filters. This paper first reviews the thin-film characterization techniques for both piezoelectric and magnetostrictive thin films, which are crucial in determining the strength of the ME coupling. After that, the most recent progress on various integrated multiferroic devices based on thin-film ME heterostructures are presented. In particular, rapid development of thin-film ME magnetometers has been seen over the past few years. These ultra-sensitive magnetometers exhibit extremely low limit of detection (sub-pT/Hz1/2) for low-frequency AC magnetic fields, making them potential candidates for applications of medical diagnostics. Other devices reviewed in this paper include acoustically actuated nanomechanical ME antennas with miniaturized size by 1–2 orders compared to the conventional antenna; integrated RF tunable inductors with a wide operation frequency range; integrated RF tunable bandpass filter with dual H- and E-field tunability. All these integrated multiferroic devices are compact, lightweight, power-efficient, and potentially integrable with current complementary metal oxide semiconductor (CMOS) technology, showing great promise for applications in future biomedical, wireless communication, and reconfigurable electronic systems.

Stress compensated MEMS magnetic actuator based on magnetostrictive Fe65Co35 thin films

We have developed a stress compensated MEMS magnetic actuators based on Fe65Co35 thin films. The magnetic actuator was designed for ˜ 25 kHz mechanical resonance frequency using a finite element method. The cantilever layer stack consists of Si/SiO2/Fe65Co35 multilayers where silicon dioxide (SiO2) layer stress is compressive and Fe65Co35 layers is tensile in nature. The bilayer combination nullifies the film stress which result to a stress compensated undeformed released micro-actuator. Further, cantilevers are mounted on a piezo-disk and the experimental resonance frequency measured using laser Doppler vibrometer was found to be around 25.275 kHz which is close to simulated resonance frequency. Using cantilever deflection method, the magnetostriction coefficient (λ) measured was found to be ˜135 ppm. This study also suggests the possibility to realize variety of stress compensated magnetic MEMS devices based on stress compensation in bilayers. Further, the results also indicate that the compensation mechanism can be applied to magnetic MEMS devices for reliable device fabrication and operation. The developed magnetostrictive micro-actuator has potential applications in magnetic MEMS for low field sensing and actuation applications.

Magnetocapacitance of magnetically strained multilayered thin films

Abstract We studied on magnetoelectric behaviors of magnetically strained magnetostrictive materials-ferroelectric materials-magnetostrictive materials sandwich-type multilayered thin films. The sandwich-type multilayered thin films were deposited on Pt substrates by PLD system with a KrF excimer laser (λ = 248 nm). In particular, thin films were studied for their crystal structure and surface/interface microstructure as well as magnetic and electrical properties. The experimentally measured results have proved that saturated magnetizations (Ms) of CoFe2O4 (20 nm, 20 nm, 200 nm)/BaTiO3 (200 nm, 100 nm, 200 nm)/CoFe2O4 (20 nm, 20 nm, 200 nm) sandwich-type multilayered thin films are measured as 23.58 emu/cm3, 38.52 emu/cm3, 103.71 emu/cm3 and the capacitance of those increased more under the magnetic field (1000 Oe) than zero field by 2.4%, 7.8%, 11.5% respectively. Our work suggests the possibility of enhanced magnetocapacitance under magnetic field by controlling the interfacial compressive/tensile stress of thin film. As a results we have compared the experimental and theoretical values depending on the parameters such as thickness and potential distribution of magnetostrictive materials through the simulation program COMSOL physics.

Study of Dynamic Behavior of Magnetostrictively Patterned Flexible Micro-Wings

For the design of magnetostrictive MEMS devices, finite-element modeling (FEM), the determination of the magnetostrictive coefficient, and application examples were examined in this paper. The magnetostrictive coefficients are usually calculated by measuring the bending deflection of the cantilever on which magnetostrictive materials are deposited. In most of the research on the magnetostrictive deposited substrate for the derivation of structural formulas, it is commonly assumed that the flexural rigidities of magnetostrictive materials are negligible because magnetostrictive films are much thinner than substrate materials. In this paper, the effect of magnetostrictive thin film's elastic energy on the magnetostrictive coefficient is investigated. A self-coded and developed finite-element program for magnetostrictive actuators is formulated, where magnetic properties are considered as nonlinear, and elastic properties are assumed as linear. As a result, it is potentially shown that magnetostrictive thin-film micropatterned actuators would be accommodated for targeted actuating behavior through an FEM capability for evaluation and selective micropatterning technique for fabrication.

Thermally Induced Magnetic Anisotropy in Nickel Films on Surface Acoustic Wave Devices

The combination of magnetostrictive thin films with surface acoustic wave (SAW) devices enables the magnetic sensors that exploit the AE-effect (magnetically induced change in modulus) without the need for free-standing structures that are susceptible to vibration and damage. The performance of these sensors is strongly influenced by magnetic anisotropy and, therefore, the state of stress in the magnetostrictive film. We report the fabrication and characterization of magnetic SAW devices comprised of a magnetostrictive layer and interdigital transducers on a piezoelectric substrate. The state of stress in the magnetostrictive layer is varied by annealing. Vibrating-sample magnetometer measurements indicate that the annealed devices show a strong uniaxial anisotropy with the easy axis parallel to the direction of acoustic propagation. The induced magnetic anisotropy is attributed to the anisotropic thermal expansion in the substrate. The change in frequency of one-port resonators and two-port SAW oscillators is measured as a function of dc bias field and ambient temperature. The normalized change in frequency (Δf/f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> ) is on the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> . These results demonstrate that the film stress can be used to optimize the performance of magnetic SAW devices and the SAW devices can be used to characterize the state of stress and magnetic anisotropy of magnetostrictive films.

Characterization of magnetomechanical properties in FeGaB thin films

Layered magnetic/piezoelectric heterostructures have drawn a great amount of interest for their potential use in ultra-sensitive magnetoelectric (ME) sensors, ME antennas, voltage tunable inductors, magnetic tunable resonators, etc. It is critically important to characterize the saturation magnetostriction, piezomagnetic coefficient, ΔE effect, and magnetomechanical coupling factor of magnetic thin films, which determine the performance of these ME devices. In this work, a sensitive system has been developed to measure these magnetomechanical properties, on which several different magnetostrictive thin films on the silicon substrate cantilever were characterized. A 0.015 ppm limit of detection of the magnetostriction tester and a frequency resolution of 0.01 Hz of the ΔE tester have been achieved. After magnetic anneal treatment, a record high piezomagnetic coefficient of 12 ppm/Oe, a giant magnetic field induced Young's modulus change of 153 GPa, and a high effective magnetomechanical coupling factor of 0.84 have been measured in FeGaB thin films.

Polarized neutron reflectometry study of depth dependent magnetization variation in Co thin film due to strain transfer from PMN-PT substrate

We studied the depth dependent magnetization profile of the magnetostrictive Co thin film layer in a (PbMg0.33Nb0.67)1-x:(PbTiO3)x (PMN-PT) (011)/Ta/Co/Ta structure under both zero and nonzero applied electric field using polarized neutron reflectometry. Application of an electric field across the PMN-PT substrate generates a strain, which rotates the magnetization of the Co layer consistent with the Villari effect. At low magnetic fields (near remanence and coercive field conditions), we find that the depth dependent magnetization profile is non-uniform, under both zero and nonzero applied electric fields. These variations are attributable to the depth dependent strain profile in the Co film, as determined by finite element analysis simulations.

Electric Control of the Magnetic Domains in Artificial Magnetoelectric Composite Heterostructure

Magnetic force microscopy (MFM) has been used to probe the influence of an in situ applied in-plane elastic strains and/or magnetic field on the static magnetic configuration of a 530 nm magnetostrictive FeCuNbSiB thin film. The film shows a typical magnetic stripe domain configuration with 180 nm magnetic domain width presenting its magnetic moments oriented perpendicularly to the film plane. The in-plane strain was induced via the application of a voltage in a piezoelectric (PE) actuator on which the film/substrate system was glued. The FeCuNbSiB film was deposited onto a compliant polyimide substrate (Kapton) in order to avoid clamping effects that can lead to low transmission of strains from the actuator to the film/substrate system. The instrumental setup was modified with a custom-built electromagnet for performing in situ MFM experiments under the application of an in-plane magnetic field. We could compare the strain-mediated effect upon the magnetic domain configuration with the effect of an applied magnetic field. This paper clearly shows a better-defined electric field/strain control of the local domain magnetization (size and orientation). The efficient coupling between the magnetostrictive properties of the FeCuNbSiB film and the PE ones of the actuators allows a complete switching of the out-of-plane domain magnetization toward an in-plane homogeneous magnetic one. This magnetoelectric heterostructure can thus open the way for new devices allowing the electric control of the magnetic information at the nanoscale level.

Finite element analysis of displacement actuator based on giant magnetostrictive thin film

With the rapid development of science and technology, mechanical and electrical equipment become more and more miniature. In order to achieve precise control in less than 1cm3, the giant magnetostrictive thin film has become a research hotspot. The micro displacement actuator with planar and arc film is designed by the dynamic coupling model based on J-A model and magneto-mechanical effect method which is proposed in this paper. The different structure and thickness of films are analyzed by COMSOL Multiphysics software when the current flows through driving coil. After comparing the simulation results with the test ones, it can be seen that the coupling model is accurate and the structure is reliable. At the same time, MATLAB is used to fit the current density-displacement curve and higher order equation is obtained, and then the feasibility of design can be verified. The actuator with arc structure had advantages of small volume, fast response, high precision, easy integration, etc., which has a broad application prospect in the field of vibration control, micro positioning, robot and so on.

Wide Band Low Noise Love Wave Magnetic Field Sensor System

We present a comprehensive study of a magnetic sensor system that benefits from a new technique to substantially increase the magnetoelastic coupling of surface acoustic waves (SAW). The device uses shear horizontal acoustic surface waves that are guided by a fused silica layer with an amorphous magnetostrictive FeCoSiB thin film on top. The velocity of these so-called Love waves follows the magnetoelastically-induced changes of the shear modulus according to the magnetic field present. The SAW sensor is operated in a delay line configuration at approximately 150 MHz and translates the magnetic field to a time delay and a related phase shift. The fundamentals of this sensor concept are motivated by magnetic and mechanical simulations. They are experimentally verified using customized low-noise readout electronics. With an extremely low magnetic noise level of ≈100 pT/sqrt{{rm{Hz}}}, a bandwidth of 50 kHz and a dynamic range of 120 dB, this magnetic field sensor system shows outstanding characteristics. A range of additional measures to further increase the sensitivity are investigated with simulations.

MEMS Magnetic Sensor with Bridge‐Type Resonator and Magnetostrictive Thin Film

SUMMARYThis paper reports on fabrication and characterization of resonant‐type mechanical MEMS magnetic sensors using magnetostrictive FePd sputtered film. The sensor has a thin Si resonator bridge with a PZT piezoelectric thin film element for excitation. The resonator is pulled by other bridges with the FePd film when magnetic flux is applied. The sensor was fabricated from an SOI wafer by sol‐gel PZT deposition, FePd sputtering, and Si etching processes. The completed sensor showed a resonant peak with a Q factor of 229 at 26.39 kHz. When magnetic flux was applied along the microbridges, resonant frequency drastically decreased due to relaxation of initial deflection of the resonant bridge. Magnetic flux can be detected by resonant frequency change with a sensing resolution of 0.2 G per 0.1 Hz. When excitation frequency was fixed at slightly below the resonant frequency, amplitude of the bridge vibration slightly decreased with increasing magnetic flux in low magnetic flux region. High sensitive sensing of magnetic flux was difficult by the amplitude change. It was found that the magnetic sensor showed large thermal drift.

Fabrication and characterization of magnetostrictive amorphous FeGaSiB thin films

In this work, amorphous FeSiB and FeGaSiB thin films have been fabricated on silicon substrates using a co-sputtering-evaporation deposition technique. The effect of adding gallium into FeSiB (Metglas) thin films on the structure, magnetic properties and magnetostriction have been studied. From X-ray diffraction (XRD), all the films were amorphous and the observed peaks were for the Si substrate. X-ray Photoelectron Spectroscopy (XPS) measurements were carried out to determine the film’s composition, which was found to be Fe83Ga11Si5.2 B0.8. Atomic force microscopy (AFM) images were taken to measure the film thickness along with studying the surface topography. It was found that the film surface had an average roughness of 0.461nm. For both FeSiB and FeGaSiB thin films, the effect of the thickness of the films on the magnetic properties and magnetostriction were investigated. The results showed that adding Ga into the FeSiB films changed the magnetic properties by reducing the saturation induction along with changing the magnetic anisotropy from uniaxial to isotropic. For the FeGaSiB films, the coercive field decreased and the saturation field (Hs) increased with film thickness. The magnetostriction constants of the FeGaSiB films were all larger than the FeSiB films for thicknesses greater than 40nm.

Influence of a thermal environment on the deflection of magnetostrictive thin films

The primary application areas of magnetostrictive thin films are in the field of micro-electro-mechanical systems. Devices which use the actuation capability of thin films put to advantage the induced mechanical deformations in them upon being subjected to a magnetic field. In this study, experiments are conducted, in a thermal environment, on Tb–Dy–Fe thin film samples to determine their characteristic magnetization curves. The thin films are subjected to a periodically varying magnetic field of \(\sim \)\(\pm \)0.7 T, in a thermally controlled environment, and the deflections at the tip are measured at elevated temperatures 50, 75 and 100\(^{\circ }\)C. An analytical model constructed from the theories of elasticity, considering transversely isotropic material properties of both the film and the substrate layers, has been proposed to predict the deflections. The study has been extended to predict the tip deflections numerically (Finite Element Method) using COMSOL Multiphysics.

Reversible strain control of magnetic anisotropy in magnetoelectric heterostructures at room temperature.

The ability to tune both magnetic and electric properties in magnetoelectric (ME) composite heterostructures is crucial for multiple transduction applications including energy harvesting or magnetic field sensing, or other transduction devices. While large ME coupling achieved through interfacial strain-induced rotation of magnetic anisotropy in magnetostrictive/piezoelectric multiferroic heterostructures has been demonstrated, there are presently certain restrictions for achieving a full control of magnetism in an extensive operational dynamic range, limiting practical realization of this effect. Here, we demonstrate the possibility of generating substantial reversible anisotropy changes through induced interfacial strains driven by applied electric fields in magnetostrictive thin films deposited on (0 1 1)-oriented domain-engineered ternary relaxor ferroelectric single crystals with extended temperature and voltage ranges as compared to binary relaxors. We show, through a combination of angular magnetization and magneto-optical domain imaging measurements, that a 90° in-plane rotation of the magnetic anisotropy and propagation of magnetic domains with low applied electric fields under zero electric field bias are realized. To our knowledge, the present value attained for converse magnetoelectric coupling coefficient is the highest achieved in the linear piezoelectric regime and expected to be stable for a wide temperature range, thus representing a step towards practical ME transduction devices.

Response and Reliability of a Giant Magnetostrictive Film Actuator Subject to Axial Random Excitation

A giant magnetostrictive thin film (GMF) actuator designed as a simply supported beam under axial white noise excitation is studied. Firstly, a non-dimensional stochastic difference equation governing the motion is simplified by the modified stochastic averaging method. Subsequently, the stationary probability density function of amplitude and the joint probability density function of displacement and velocity were obtained respectively. Finally, the reliability was studied by the backward Kolmogorov (BK) equation to guarantee the safety of the actuator. Numerical simulations are carried out to verify the theoretical analysis.

Studies of the magnetostriction in thin films: Experimental, analytical and numerical analysis

Magnetostrictive thin films primarily find use for their actuation properties. Micro-electro-mechanical systems (MEMS devices) capitalize on the induced mechanical deformations when these thin films are subjected to a magnetic field. In this study, experiments are conducted on Tb–Dy–Fe thin film samples to determine their characteristic magnetization curves. The thin films are subjected to a periodically varying magnetic field of ±0.6T and the deflections at the tip are measured. A simple analytical model based on the theories of elasticity and considering transversely isotropic material properties of both the film and the substrate layers has been proposed to predict the deflections. The study has been extended to predict the tip deflections numerically using Comsol Multiphysics. The measured tip deflections are further compared with the simulated analytical and numerical results, which are found to agree with each other.

Effective 90-degree magnetization rotation in Co2FeAl thin film/piezoelectric system probed by microstripline ferromagnetic resonance

Microstripline ferromagnetic resonance technique has been used to study the indirect magnetoelectric coupling occurring in an artificial magnetoelectric heterostructure consisting of a magnetostrictive thin film cemented onto a piezoelectric actuator. Two different modes (sweep-field and sweep-frequency modes) of this technique have been employed to quantitatively probe the indirect magnetoelectric coupling and to observe a voltage induced magnetization rotation (of 90°). This latter has been validated by the experimental frequency variation of the uniform mode and by the amplitude of the sweep-frequency spectra.