The temperature effect on the magnetoelectric response of hybrid magnetostrictive/piezoelectric laminated composites in the range from room temperature up to 85 ºC is presented. The samples analyzed consisted of alternating, stacked, layers of a magnetostrictive amorphous metal, and a piezoelectric polymer, bonded to each other with an epoxy. The maximum magnetoelectric effect was observed when the composites were driven at their electromechanical resonance. First, we present results on the fabricability of the laminated composite sensor consisting on Vitrovac 4040® (Fe39Ni39Mo4Si6B12) as the magnetostrictive amorphous component and two different piezoelectric polymers: poly(vinylidene fluoride) (PVDF) and 2,6(β-CN)APB/ODPA (poli 2,6) polyimide, a new type of high temperature piezoelectric polymer. At room temperature induced magnetoelectric voltages of 79.6 and 0.35 V/cm.Oe were measured when using PVDF and poli 2,6 polyimide respectively as the piezoelectric components. When heating, we have observed that the magnetoelectric response of the PVDF-containing device quickly decayed to about 5 V/cm.Oe, while for the poli 2,6- containing one it remained almost constat. We discuss the advantage of using this new piezoelectric polymer due to its good performance at high temperatures, making these magnetoelectric laminate composites suitable for high temperature applications.

Nanopowders of cobalt iron oxide (CoFe2O4) were successfully fabricated by the co-precipitation method followed by a technique to prevent particle agglomeration. Particle sizes were in the range of 24 to 44 nm. The size of cobalt iron oxide particles decreases with increasing the concentration of the precipitation agent. The crystal structure was confirmed by X-ray diffraction (XRD), the chemical composition by energy dispersive spectroscopy (EDS), and phase changes by thermogravimetric differential thermal analysis (TGA-TDA). The particle morphology was analyzed by scanning electron microscopy (SEM). Magnetic properties were investigated by SQUID magnetometry and Mössbauer spectroscopy. Being nearly monodisperse and non-agglomerated the prepared cobalt iron oxide powders are the base for synthesizing magnetoelectric composites embedded in a ferroelectric BaTiO3 matrix.

With the development of applications involving high sensitivity ferromagnetic-ferroelectric laminates, a systematic analysis of the noise floor for magneto-electric (ME) laminated sensor has become crucial. We report and discuss the results of such an analysis on the noise floor of magnetostrictive-piezoelectric laminates in terms of the magnetic noise spectral density at room temperature. The noise floor of highly sensitive ME laminates with a JFET charge amplifier detection method has been studied. A good correlation was found between the theoretical and experimental noise curves within the measurement bandwidth. The dominating noise sources were found to include the dielectric loss noise, mechanical loss noise of the magneto-electric laminates and the noise sources of the charge amplifier. By using an appropriate low noise JFET charge amplifier, the noise contributions from the amplifier can be made negligible, enabling the measurement of the intrinsic noise of the ME laminates sensor. Thus, we have shown that at low frequencies, below the resonant frequency, the dielectric loss noise predominates with a one-per-root-frequency dependence whereas, around the resonance, the mechanical loss noise prevails over all other noise sources as expected from our theoretical analysis.

Optimizing magnetic field sensors made by piezoelectric-magnetostrictive composites is a trade off between several parameters. Whereas large structures will cause in principle high electrical currents the mechanical coupling will lead to shear losses and therefore limit the sensitivity of the sensor and make it impossible to measure small magnetic fields. In very small structures the shear losses will decrease but the imperfections in the interfaces become more important and the typically small currents will be disturbed by, e.g., surface conductivity of the piezoelectric material and are thus difficult to measure. The best compromise is a mesoscale sensor which has relatively small losses due to shearing but still high enough electrical currents to work as a good sensor. We will present a setup which allows the use free standing ZnO micro rods as piezoelectric core material which are surrounded by a magnetostrictive layer. Since no clamping is necessary the expansion and contraction of the material is not hindered by a matrix material. The tuning of the relative layer thicknesses can be easily optimized by changing the thickness of the magnetostrictive layer so that an optimum can be achieved for different ZnO micro rods. For the growth of the ZnO a newly developed process will be presented which allows the growth of a large variety of single crystals with different aspect ratios up to needles with several millimeters in length.