Magnetoelectric devices for beyond CMOS applications

Abstract

The Complementary Metal Oxide Semiconductor (CMOS) can be considered as the most dominant technology and the foundation of all the electronic devices that are known today. However, for the past few years due to the excessive need of miniaturization, the CMOS scaling has been falling behind because of short-channel effects causing high leakage currents and high power densities leading to high thermal dissipation; thus the end of the Moore’s law, which states that the number of transistors in an integrated circuit doubles about every year. The development of so called beyond CMOS technologies was imperative in order to solve the problems caused by the CMOS miniaturization. One of this alternatives is the magneto- electric² devices using the magnetoelectric effect, which consist in a heterostructure coupling magnetostrictive and piezoelectric materials through strain transfer between the two. In this work, the output of the magnetoelectric device is modeled using COMSOL, a finite element method solver, in order to understand how the geometric parameter, the aspect ratio of the device, or the thickness of the layer, and the material parameters influence the strain transfer between the two layers and what impact it has on the generated voltage. In the first place, an ideal device will be analysed in order to get a general idea of its behaviour. Then, the impact of two non-idealities on the output will be considered, a modification of the geometry of piezoelectric layer, caused by etching during the fabrication process of the latter, and the influence of extended electrodes connecting to other possible pillars. For the end, a case-study of an actual device is done comparing the output voltage evolution with respect to the dimensions of the read electrode between simulations and experimental data by providing the underlying physics. ​Magnetoelectric effects naturally occur in multiferroic materials but much stronger strain-induced magnetoelectric coupling can be observed in composite materials consisting of piezoelectric and magnetostrictive materials. The application in spintronic devices requires a detailed understanding of the geometry (e.g. the relative directions of the electric field and the magnetization) as well as thermal fluctuations on the magnetization dynamics. In this thesis, the student will perform simulations to study the magnetoelectric coupling in different geometries and different material systems. The goal of the thesis is to develop efficient strategies to control the magnetization orientation and switching by the magnetoelectric effect, as well as to assess the voltage generated by the magnetization rotation via inverse effect. The work will be in close collaboration with experimentalists working on integration of magnetoelectrics into spintronic devices for exploratory logic.