PhD ceremony Ms. S. Farokhipoor: Domain walls and their conduction properties in ferroelectric BiFeO3 thin films
|When:||Fr 26-04-2013 at 11:00|
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 BiFeO3, 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.