Characterization and applications of FeGa/PZT multiferroic cantilevers

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Multiferroic materials and structures, which possess two or more ferroic properties, have been widely investigated because of their ability to transfer one different form of signals. The magnetoelectric (ME) effect, which results in induced voltage under applied magnetic field, makes multiferroic materials promising in applications for new types of transducers, sensors, and information storage devices. The laminated bulk composite multiferric devices had attracted a lot of attention because of their high ME coefficients, which define the strength of ME coupling.

  We fabricated mechanically-resonant ME devices by depositing magnetostrictive FeGa and piezoelectric PZT thin films on Si cantilevers. Various sized cantilevers were found to exhibit different behaviors. With a 1 Oe AC magnetic driving field HAC, the small cantilever (0.95 mm × 0.2 mm × 5 &#956;m) shows a high ME coefficient (33 V/(cm×Oe)) with a bias DC magnetic field of 66.1 Oe at the resonant frequency fr of 3833 Hz in vacuum. We found that the fr of the small cantilever continuously shifts with the bias magnetic field. A magnetic cantilever theory was used to explain this shift. In addition, we are able to demonstrate application of magnetic cantilevers in AC magnetic energy harvesters with an efficiency of 0.7 mW/cm<super>3</super>.

  By driving the cantilever into the nonlinear regime with an AC magnetic field larger than 3 Oe or AC electric field larger than 5 mV, we are able to demonstrate its application in a robust multi-mode memory device based on bistable solutions of the Duffing oscillation. We can use the driving frequency, the driving amplitude, DC magnetic field, or DC electric field as the input, and use bistable vibration amplitudes of the device as the output. 

  We also show that parametric amplification can be used to substantially increase the ME coefficient by adding a pump voltage on the PZT layer. The parametric gain is sensitive to both the phase of pumping signal and the phase of the driving signal. The gain diverges as the pump voltage approaches the threshold. With parametric amplification, the ME coefficient can be boosted to a value as large as 2×10<super>6</super> V/(cm×Oe) from 33 V/(cm×Oe).