2, ETH Zurich, Zurich, , Switzerland
3, KAIST, Daejeon, , Korea (the Republic of)
4, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Multiferroic materials that exhibit simultaneous -and strongly coupled- magnetic and ferroelectric order above room temperature offer exciting potential for room-temperature device integration. Thus, magnetoelectric multiferroic films are ideal candidates for applications in next-generation memory devices which utilize low consuming electric fields to control magnetic order. However, due to competing requirements for displacive ferroelectricity and magnetism, only a hand-full of single-phase materials displaying multiferroic properties above room temperature are known. Most multiferroics are not suitable for practical applications either because they exhibit antiferromagnetic or weak ferromagnetic alignments, small spontaneous polarization, week coupling between the order parameters, or because their properties only emerge at extremely low temperatures. Therefore, much effort is devoted to search new single-phase multiferroic materials that exhibit high ordering temperatures.
Additionally, the multiferroic domain structures in these materials are considered to be an important factor to improve the efficiency and performance of future multiferroic devices. Therefore, it is crucial to investigate the domain structures in multiferroic oxides. Recent advances in aberration-corrected (scanning) transmission electron microscopy (S/TEM) and in microelectromechanical (MEMS) technology for miniaturized TEM specimen holders have opened up a wide range of new opportunities for in-situ studies. Thus, probing ferroelectric domain dynamics at atomic resolution by means of in-situ heating/electrical biasing TEM is now feasible thanks to the better spatial and temporal resolution of in-situ TEM.
In this contribution, we will show recent advances on the characterization of ferroelectric domain structures in multiferroic oxides. In particular, we will address three different multiferroic oxide systems. Firstly, we will show that substitutional Ca dopants in BiFeO3 lead to the spontaneous formation of a layered structure of substitutional dopants which creates a complex ferroelectric structure constituted by the alternation of polar and non-polar domains. Secondly, the effect of doping and epitaxial strain on the structure and the ferroelectric properties of the potential multiferroic Aurivillius compound Bi5FeTi3O15 will be discussed. Finally, the improper ferroelectric phase transition in the hexagonal YMnO3 perovskite will be addressed.