Composite Multiferroics and Magnetoelectric Skyrmions

Abstract

Condensed matter physics has made an enormous progress in the understanding and developing of materials, such as ferromagnets, ferroelectrics, multiferroics, chiral-magnets and composite systems. This offers possibilities to investigate new material devices, where these properties can be utilized for future technologies. In my Ph.D. thesis, we study the magnetic and electrical responses of field-driven behaviors in materials. Chapter 1: Magnetic Spins and their Dynamics. We study the conventional magnetic spins with a classical Heisenberg model to charaterize the energy of magnetization in micromagnetics. Particularly, the dynamics of spins are solved by the well-known Landau-Lifshitz-Gilbert equation. Chapter 2: Electric Pseudospins and their Dynamics. We investigate a pseudospin model to describe the electric polarization in ferroelectric materials. Pseudospins are based on a transverse Ising model to characterize the local energy, is generally believed to be a good microscopic description of the electric dipoles. The novelty here is that, a spin dynamics approach has been developed to solve the time evolution of pseudospins by a modified Landau-Lifshitz-Gilbert equation. Results show a flipping-like behavior with respect to the electric dipole moments. Chapter 3: Dynamical Responses in Composite Multiferroics. Composite multiferroics are the heterostructures of ferromagnetic and ferroelectric materials with a remarkable magnetoelectric effect at the interface. We support the ferromagnetic structure with magnetic spins and the ferroelectric structure with pseudospins who act as the electric dipoles in microscopic models. To demonstrate the performances of responses in the driving and driven parts is the aim of this chapter. In this work, four numerical models are considered, such as a 1D composite chain, 1D composite ladder, 2D composite film and 2D composite film with twisted boundary conditions, to investigate different field-driven behaviors in composite multiferroics. Chapter 4: Magnetoelectric Skyrmions in Composite Systems. Skyrmions are topologically particle-like magnetic textures realized with a range of sizes from 10nm to approximately 100nm. They occur in chiral-magnetic materials where an asymmetric exchange interaction breaks the inversion symmetry. The study of Skyrmions in composite systems has attracted much interest in recent years. This chapter reports the magnetic Skyrmions are induced by electric fields in a composite bilayer model. By using the spin dynamics method, a classical magnetic spin model and an electric pseudospin model are coupled with a strong magnetoelectric couping have been proposed in the dynamical simulations. Interestingly, we observe some Skyrmion-like objects in the electric component either by applying an electric field or a magnetic field, which is due to the connection between the electric and magnetic structures. Chapter 5: Mechanical Methods of the Skyrmion Transport. The inherent stability of Skyrmions combined with their low pinning probability to defects are intriguing characteristics that have attracted lately much attention within the spintronics community for possible applications in memory devices. In order to efficiently use Skyrmions as information carriers their motion must be controllable. Recent studies revealed several non-mechanical methods, such as by using magnetic fields, spin-polarized currents, electric currents, magnons and temperature gradients. However, inducing Skyrmions in a composite system by means of external driving has not been explored. In this chapter, we aim to develop two innovative electrical techniques to create and manipulate Skyrmions in composite systems. One is driven by a movable and polarized ferroelectric film, and the other is driven by a mobile electric field source. Effects caused by different propagation velocity, film size, and magnetoelectric coupling strength strongly impacting on stability and efficiency of the Skyrmion transport.