PhD ceremony Ms. S. Farokhipoor: Domain walls and their conduction properties in ferroelectric BiFeO3 thin films

PhD ceremony: Ms. S. Farokhipoor, 11.00 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Domain walls and their conduction properties in ferroelectric BiFeO3 thin films

Promotor(s): prof. B. Noheda, prof. T.T.M. Palstra

Faculty: Mathematics and Natural Sciences

This dissertation deals with the conduction properties of BiFeO_3, which is typically assumed to be an insulator. Research on BiFeO₃ has become very fashionable because it is one of the rare materials that can respond strongly to both electric fields and magnetic fields at room temperature. In thin film form, BiFeO₃ can be grown under strain on a crystal substrate with a slightly smaller or larger lattice. This allows a great control of the so-called crystal domains (regions with different crystal orientations). These domains minimize the elastic energy by forming particular patterns of orthogonal domains with periodic domain walls. This is significant because the domain walls of BiFeO₃ display enhanced conductivity with respect to the domains so that the thin films actually consist of arrays of self-assembled nanodevices formed by insulating domains, having sizes of about 200nm (500 times smaller than a human hair) and conducting domain walls. We have investigated the mechanisms that cause conductivity in BiFeO₃ domain walls by using the tip of an atomic probe microscope as nanometric metal electrode. We reveal that the electrons feel a decreased energy barrier exactly at the walls because there is a deficit of oxygen in the BiFeO₃ lattice. This understanding has allowed us to control the conduction level of the domain walls in great extent. In addition, the domains themselves can be made conductive by switching the electrical polarization of BiFeO₃ (so-called ferroelectric resistive switching), which makes of BiFeO₃ a material of great interest for non-volatile memory applications.