Magnetoelectric devices for beyond CMOS applications

The Moore’s law has led the pace of innovation predicting an exponential growth in computing unit performances. However, as the dimensional and functional scaling of CMOS technologies approaches fundamental limits, the search for alternatives solutions has intensified. Spintronics emerges as a promising field among the so called beyond-CMOS technologies, manipulating the spins of electrons for different applications such as logic. Anyway, to best integrate this technology, spintronics needs the development of scalable and energy-efficient devices and an in-depth study of the magnetic materials used. In this context, this thesis studies some magnetoelectric devices made by composite form by thin piezoelectric and magnetostrictive layers aiming to quantify the magnetoelectric effect, which could be relevant for future spintronics applications. These devices are investigated using anisotropic magnetoresistance measurements, with a primary focus in the understanding of the magnetoelectric coupling in these novel structures with various configurations and to examine the magnetic properties of the magnetic layer. They are characterized by the ability to modulate the electrical resistance in response to changes in the angle between the direction of the electric current and the orientation of the magnetization within the material. The magnetoelectric effect, which involves the coupling of electric fields to intrinsic magnetization, presents a viable pathway for this low-power and controlled magnetization switching. This coupling is facilitated by the bilayer structure of piezoelectric and magnetostrictive thin films of these structures, where strain induced in the piezoelectric layer by an applied electric field is transferred to the magnetostrictive layer, thereby switching its magnetization. The study emphasizes the use of nickel as the main material, while also briefly incorporating Cobalt-Iron-Boron research as a comparison. Once the parameters are known, it is important to study how the device reacts under the effect of a magnetic and electric field. Hence the experimental data obtained in cleanroom has been compared with the results gotten by performing some simulations. Having several physical phenomena involved, COMSOL has been used for the study of the distribution of the strain in the nanostrip. On the other hand Mumax3, a GPU-accelerated micromagnetic simulation program, has been used to perform micromagnetic simulations and OOMMF to visualize the magnetic moments and domains. Simulations show how the device design is appropriate to achieve uniform strain at the centre of the strip while the coercivity, thus the ease of manipulating the magnetic domains, depends strongly on the size. Furthermore the response of the magnetization versus voltage has been measured showing that the magnetoelectric effect exists in such devices. The devices respond very well by applying small voltages, but the smaller the size of the nanostrip, the lower the maximum voltage that can be withstood. Indeed experimental data are coherent with the simulations showing that magnetization can be rotated using magnetoelectric effect although further measurements need to be made in order to get a more accurate value of coupling. In conclusion further research and enhancement are briefly discussed for improving device efficiency and magnetoelectric coupling coefficient.