Amalia Garnier

Magnetic actuation of bending and torsional vibrations for 2D optical-scanner application

Abstract This paper deals with a magnetically actuated microresonator as a test device for light deflection applications (optical scanners). 2D scanning of a laser spot was achieved with this device and an optical scanning window of nearly 24° was attained. Giant magnetostrictive thin films are used. They present the novel feature to produce wireless actuation in both bending and torsional modes, which can be decoupled at the corresponding resonance frequencies. In addition, depending on working conditions (bias field, magnetization orientation), the ratio between bending and torsional responses can be adjusted over a very large scale. We show how this ratio can be fixed by two methods: (i) by an appropriate choice of the initial state of the magnetic thin film during its deposition, and (ii) by a DC magnetic field, which is applied during the device operation. The mechanical characterization of the cantilever is presented. Anharmonic behavior due to large deflections was obtained. This nonlinear behavior is lessened with a folded-type mechanical structure. Torsional deflections up to 23 μm, corresponding to mechanical rotations of ±5.9° angles have been attained at a field as low as 3.5 mT, corresponding to only 2% of the anisotropy field. Bending deflection can be twice in similar conditions.

A fast, robust and simple 2-D micro-optical scanner based on contactless magnetostrictive actuation

We have fabricated and successfully operated a new 2D micro-optical scanner based on contactless magnetostrictive (MS) actuation using only one coil. A Si cantilever coated with a sputter-deposited MS film is vibrated simultaneously in both bending and torsion modes at high frequencies (10-50 kHz). It exhibits the maximum optical deflection of /spl plusmn/12 degrees at an excitation field of 4 mT; this is only 2% of the saturation field of the MS material. Moreover, the contactless magnetic actuation enabled an easy wireless vacuum encapsulation that gave Q-factors up to 1400 and improved the vibration amplitude four times more than that in air.

Effect of direct current bias field and alternating current excitation field on vibration amplitudes and resonance frequencies of a magnetostrictively actuated bimorph microresonator

The dynamic behavior of magnetomechanical bimorph microresonators is analyzed. These resonators are composed of a sputter-deposited TbDy(CoFe)2 thin film on silicon cantilevers. This film has giant magnetostrictive properties. It is made anisotropic by postdeposition annealing under magnetic field. We prepared two samples with different orientations of the easy axis: along the cantilever width axis and with an orientation of 22°, respectively. Both bending and torsion modes of vibration can be excited by proper combination of external magnetic field and the direction of the easy axis. Vibration amplitudes and resonance frequencies are studied in those modes. Several experimental conditions are investigated. The dependence of vibration amplitudes on a magnetic dc bias field applied either along the hard axis or along the easy axis is studied. The bending/torsion vibration amplitude ratio was shown to be tunable over a very wide range, offering interesting practical applications. In addition, the dependence...

Magnetostrictive Microactuators and Application to Two-Dimensional Optical Scanners

We have fabricated and studied magnetostrictive (MS) bimorph microresonators. They consist of silicon cantilevers coated with a sputter-deposited TbDyCoFe alloy thin film. This material has giant magnetostrictive properties and it also has strong anisotropy. As a consequence, the resonators exhibit a unique behavior, that is, they are capable of vibrating in both bending and torsional modes with only one excitation field. This unique property was used to build a novel two-dimensional (2D) optical scanner. This device operates at high frequencies (10–61 kHz) and exhibits an optical deflection angle of ±12° at an excitation field of 4 mT; this is only 2% of the saturation field of the MS material. Moreover, the contactless magnetic actuation enabled an easy wireless vacuum encapsulation that resulted in Q-factors of up to 1400 and improved the vibration amplitude to four times more than that in air.

Effect of dc bias field on resonance frequency and vibration amplitude in a magnetomechanical bimorph resonator

In previous works, we have developed silicon-based, bimorph resonators, using a uniaxial giant magnetostrictive thin film. Such material has the unique feature to produce both bending and torsional vibration with a single magnetic field excitation. We have used these resonators to build a 2D-Micro-Optical scanner, which has shown actuation capabilities high enough for most applications. Even with non-optimized magnetostrictive material and mechanical design, it has shown comparable performances with those of its piezoelectric or electrostatic counterparts. In this paper, we present new characterizations, which have been made when applying a magnetic DC bias field in addition for the AC field needed for the excitation. Though this DC field is not essential for the operation of the device, it can be used for instance to tune the ratio of bending/torsional vibration amplitudes. In addition to this behavior, it was found that the bias field has also a strong effect on the resonance frequencies of the mechanical structure. This dependence was also found to be dependent on the AC excitation field amplitude. These experimental results are discussed and analyzed in both a qualitative and a quantitative way using a theoretical model. On one hand, the dependence on the AC field amplitude is ascribed to the so-called Hard- spring effect, due to the nonlinear term of the elastic restoring force in large deflection amplitude regime. On the other hand, the dependence on DC field is ascribed to the so-called Delta-E effect, which is a variation of the effective Youngs modulus due to the magneto-mechanical coupling.

Mechanical characterization of magnetostrictively actuated microresonators

A magnetostrictively actuated silicon-based micro-resonator has been fabricated in a simple process, including thin film deposition of the active material by sputtering. The aimed application is a 2D-Optical-Scanner, for which horizontal and vertical light deflections can be achieved by bending and torsional vibrations of a magneto-elastic bimorph structure. Static and dynamic magnetostriction phenomena are described according to a simple model. Mechanical characterization of three different actuators was performed by using a laser Doppler vibrometer. The vibration amplitude behavior are presented for two different orientations of the applied magnetic field, revealing two important capabilities of the actuator: there is no need of steady state biasing and the ratio of bending/torsion vibration amplitudes is tunable. Measurement of the frequency characteristics around resonance led to the evidence of different nonlinear behaviors for the two resonant modes under considerations. Moreover, some unwanted phenomena, which are induced by the fabrication process, have also been revealed.

Nonlinear behavior of a magnetic microactuator

Different nonlinear behaviors have been observed on magnetically actuated devices. These devices consist of 1 mm-long, 0.5 mm-wide, 15 /spl mu/m-thick silicon processed microstructures, on the top of which a magnetostrictive thin film was deposited. Starting from our preliminary experimental results, we are now studying the nature of these nonlinear behaviors, which can be mechanical, magnetic or a combination of both, depending on several parameters such as the structure design shape or thin film deposition conditions.

Contactless actuation of giant magnetostriction thin-film alloy bimorphs for two-dimensional scanning application

Giant magnetostriction appears to be very promising for contactless actuation. However, very few attempts have been made to actually exploit that advantage in sensors and actuators. The 2D scanning actuator that is being developed is a cantilever based structure that benefits from another unique features of the magnetostrictive materials: the ability to produce both bending and torsion at the same time with two AC magnetic fields. The ability to drive bending and torsion modes has been proven but never on a micro- structure. The actuators are processed and released from SOI wafers and all the micromachining process is done before the deposition of the active layer. A two-cantilever-based actuator has been designed i) to decrease the resonant frequency of the first rank of the torsion mode ii) to achieve a good decoupling between the bending and the torsion motion iii) to enhance the displacement. The actuation is performed using one DC 60mT and two AC 3-12 mT magnetic fields. Depending on the size of the actuator, resonant frequencies from 1 kHz to 30 kHz have been investigated for both the bending and the torsion motion.