Control of Spin-Wave Propagation in Multiferroic Heterostructures: Magnetic Domain and Domain-Wall Driven Spin-Wave Guiding

Abstract

In today's digital age, modern information computing has become integral to our daily lives, facilitated by rapid technological advancements in transistors within central processing units (CPUs) that provide powerful computational capabilities. As the demand for more efficient and powerful processing continues to rise, the increasing density of transistors in circuits faces significant challenges, particularly in heat generation. To address these challenges, magnonics is being investigated as a promising alternative for future logic circuits. Unlike traditional electronics, which rely on electron charge and current, magnonics conveys information through the amplitude and phase of spin waves. This enables data transport via the transmission of spin angular momentum without Joule heating.

The development of efficient magnonic circuits necessitates effective manipulation of spin waves. In this thesis, I present a study on the active control of spin-wave propagation in strain-coupled multiferroic heterostructures. These structures consist of a ferromagnetic film deposited on a ferroelectric BaTiO3 substrate that exhibits alternating ferroelectric polarization domains. Strain coupling at the bilayer interface transfers the ferroelectric domain pattern onto the ferromagnetic film, resulting in regular magnetic anisotropy patterns through inverse magnetostriction. The regular alternation of magnetic anisotropy firmly pins the magnetic domain walls to the ferroelectric domain walls. In a Fe/ BaTiO3 multiferroic heterostructure, the abrupt change in the spin-wave dispersion relation and phase velocity at the magnetic anisotropy boundary results in spin-wave refraction. I experimentally demonstrate zero-field routing of spin waves across these anisotropy boundaries. This refraction effect is further confirmed by micromagnetic simulations and calculations based on a modified Snell's law for magnonics. The routing of spin waves in this system can be efficiently controlled by adjusting parameters such as the frequency, incident angle, and anisotropy strength. In a CoFeB/ BaTiO3 multiferroic heterostructure, I utilized the strongly pinned magnetic domain walls as nanoscopic channels for spin-wave propagation. The robust pinning of domain walls enables the initialization of head-to-head and head-to-tail domain wall configurations. Each type of magnetic domain wall exhibits distinct magnetization rotation and width, resulting in switchable spin-wave dispersion relations that allow for fully reversible control of propagating spin-wave modes. I showed that an external magnetic field can tune the domain wall characteristics, shifting the dispersion relation without altering its position, thereby providing continuous control over the wavelength of localized spin waves. These findings, combined with the potential for electric-field-driven domain wall motion in multiferroic heterostructures, pave the way to low-power, reconfigurable magnonic devices.