Inverse magnetoelectric effects in micro and nanostructures

The magnetoelectric (ME) effect delineates the connection between magnetism and electricity. It appears in two different forms : the direct effect, where the application of an electric field induces magnetization changes, and the inverse effect, in which a magnetic field leads to alterations in electrical polarization. The focus of this thesis will be the inverse ME effect . It will be studied in multilayer composites, specifically piezoelectric/ magnetostrictive materials, where the interaction between the two layers allows the transfer of deformation-induced magnetostriction to the piezoelectric component, resulting in dielectric polarization. The stack under investigation is consisted by PVDF-TrFe as piezoelectric layer and Ni as magnetostrictive layer. The fabrication process involves spinning a PVDF-TrFe 500 nm thick layer on either Si-SiO2(400nm)/Ta(10nm)/Pt(100nm) substrate or directly on a Ni foil substrate. Subsequently, Nickel and Gold dots are either evaporated or sputtered onto the PVDF-TrFe substrate on top of Si − SiO2/Ta/Pt, while only Gold dots are evaporated on top the PVDF-TrFe substrate on Ni foil. The dots come in varying sizes according to the pattern provided by a round shadowed mask. PVDF-TrFe layer is subjected to 40 V peak-to-peak triangular waves at 50 Hz for 200 pulses, transitioning it from the α-phase to β-phase, characterized by the highest dipolar moment. The ferroelectric and piezoelectric behavior of PVDF-TrFe has been demonstrated by measuring its Polarization-Voltage (P-V) loop, that in the case of the sputtered sample results to be very leaky, due to atom implantation in the PVDF layer. Measurements of Capacitance (C-V loop) are conducted using an LCR meter, revealing that capacitance increases with the size of dots and decreases with increasing frequency. Successive measurements involve the simultaneous application of a magnetic field In Plane (IP) and Out Of Plane (OOP). The OOP magnetic field varies from -1 Tesla to 0 and from 1 Tesla to 0, while the IP field is fixed at 500 Oe (0.05 T). The PVDF layer is poled with the condition previously mentioned and its P-V loop is measured as a function of the OOP magnetic field. However, especially for the bigger structures, a material degradation is observed. To mitigate this issue, reducing the number of loops in the poling step during the measurement to 10 is employed as a solution. What comes next is to evaluate the dependence of polarization on the magnetic field, but the measurement noise shown to be than the expected variation. One potential solution, in addition to reducing the noise, could involve proceeding with measuring the charges generated by the ME effect, rather than focusing solely on polarization.