Magnetoelectric domain control in a multiferroic rare-earth ferrite

Abstract

Multiferroics---materials with coexisting magnetic and electric orders---have been receiving increasing attention during the last two decades. The interest in them results from the rich underlying physics due to the interplay of multiple orders, as well as their potential to be used as low-power-consumption spintronic devices. Specifically, multiferroics of spin origin where magnetic ordering induces a ferroelectric polarization seem promising in this regard. The strong coupling between the induced polarization and the magnetic order allows for electric-field control of the magnetic order in such systems. Most relevant studies on these types of materials so far concerned only the macroscopic properties while microscopic studies at the level of domains and domain walls are quite a few. Nevertheless, information about the domains and domain walls and their evolution under external magnetic and electric fields is the key to any envisaged applications. In this thesis, optical techniques such as the magnetooptic Faraday effect and second harmonic generation are used to investigate the multiferroicity in Dy$_{0.7}$Tb$_{0.3}$FeO$_3$. The main focus is put on the study of the multiferroic domains and domain walls in this system. Domains of the individual ferroic orders present in the multiferroic phase are spatially resolved for the first time and their coupling is studied. The evolution of such multiferroic domains is investigated under magnetic and electric fields and, specifically, the electric-field-induced magnetization reversal in this system is scrutinized. Full control of the multiferroic domains and domain walls is demonstrated and, by taking advantage of such control, interesting applications are conceived. For example, a ferromagnetic domain pattern is fully inverted, i.\,e., the magnetization is reversed in each domain with the original pattern unaffected, using a homogeneous electric field. Alternatively, a domain pattern in the ferromagnetic order is erased but fully transferred to the ferroelectric order using a homogeneous magnetic field. Finally, by introducing a new concept of domain and domain-wall interconversion, a domain in the multiferroic phase is spatially confined and transformed into a domain wall in the adjacent non-multiferroic phase, while retaining its multiferroic properties. In the reciprocal direction, such a 2D domain wall is deconfined to become a 3D domain using either electric or magnetic field, confirming its multiferroic nature. The results of this thesis, thus, bring new insights into complex multiferroic domains and domain walls, and demonstrate how controlling these microscopic entities can improve the overall functionalities of multiferroic materials.