Local conductivity and the role of vacancies around twin walls of (001)-BiFeO3 thin films

BiFeO3 thin films epitaxially grown on SrRuO3-buffered (001)-oriented SrTiO3 substrates show orthogonal bundles of twin domains, each of which contains parallel and periodic 71o domain walls. A smaller amount of 109o domain walls are also present at the boundaries between two adjacent bundles. All as-grown twin walls display enhanced conductivity with respect to the domains during local probe measurements, due to the selective lowering of the Schottky barrier between the film and the AFM tip (see S. Farokhipoor and B. Noheda, Phys. Rev. Lett. 107, 127601 (2011)). In this paper we further discuss these results and show why other conduction mechanisms are discarded. In addition we show the crucial role that oxygen vacancies play in determining the amount of conduction at the walls. This prompts us to propose that the oxygen vacancies migrating to the walls locally lower the Schottky barrier. This mechanism would then be less efficient in non-ferroelastic domain walls where one expects no strain gradients around the walls and thus (assuming that walls are not charged) no driving force for accumulation of defects

Domain walls and their conduction properties in ferroelectric BiFeO3 thin films

Geleidingseigenschappen BiFeO3 onderzocht In haar proefschrift behandelt Saeedeh Farokhipoor geleidingseigenschappen van BiFeO3, een materiaal waarvan typisch isolerend gedrag verwacht mag worden. Het onderzoek naar BiFeO3 staat recent in de belangstelling. Het is namelijk één van de weinige materialen die bij kamertemperatuur een sterke materiaalrespons vertoont in zowel een aangelegd elektrisch- als een magnetisch veld. BiFeO3 kan in dunne lagen onder roosterspanning worden gegroeid op een substraat met een kristalrooster dat een fractie groter of kleiner is. Zo kunnen kristaldomeinen (gebiedjes met verschillende kristaloriëntaties) op controleerbare wijze worden gevormd. Deze domeinen verminderen de elastische energie door specifieke patronen te vormen van loodrechte domeinen met periodieke domeinwanden. Dit is van belang, aangezien de domeinwanden van BiFeO3 een hogere geleiding vertonen, vergeleken met de domeinen zelf. Dunne films kunnen dus eigenlijk worden beschouwd als zelf geassembleerde nano-devicestructuren die gevormd worden door isolerende domeinen met een grootte van ongeveer 200 nm (500 keer kleiner dan een mensenhaar) en geleidende domeinwanden. We hebben de mechanismen onderzocht die geleiding in BiFeO3 domeinwanden veroorzaken met behulp van een microscoop met metalen elektrodenaald die op atomaire schaal het oppervlak aftast. We hebben ontdekt dat elektronen ter hoogte van de domeinwanden een verlaagde energiebarrière ondervinden, omdat juist daar een sterk tekort aan zuurstofatomen in het BiFeO3 rooster heerst. Dankzij dit inzicht zijn we in staat om het geleidingsniveau van domeinwanden sterk te beïnvloeden. Bovendien kunnen de domeinen zelf geleidend gemaakt worden door de elektrische polarisatie van BiFeO3 te schakelen (het zogenaamde ferro-elektrische weerstand schakelen). Dit effect maak BiFeO3 zeer geschikt voor permanente geheugenopslag

Conduction through 71 degrees DomainWalls in BiFeO3 Thin Films

Local conduction at domains and domains walls is investigated in BiFeO3 thin films containing mostly 71o domain walls. Measurements at room temperature reveal conduction through 71o domain walls. Conduction through domains could also be observed at high enough temperatures. It is found that, despite the lower conductivity of the domains, both are governed by the same mechanisms: in the low voltage regime electrons trapped at defect states are temperature-activated but the current is limited by the ferroelectric surface charges; in the large voltage regime, Schottky emission takes place and the role of oxygen vacancies is that of selectively increasing the Fermi energy at the walls and locally reducing the Schottky barrier. This understanding provides the key to engineering conduction paths in oxides.Comment: RevTeX, four two-column pages, 5 color figure

Tuning the atomic and domain structure of epitaxial films of multiferroic BiFeO3

Recent works have shown that the domain walls of room-temperature multiferroic BiFeO3 (BFO) thin films can display distinct and promising functionalities. It is thus important to understand the mechanisms underlying domain formation in these films. High-resolution x-ray diffraction and piezo-force microscopy, combined with first-principles simulations, have allowed us to characterize both the atomic and domain structure of BFO films grown under compressive strain on (001)-SrTiO3, as a function of thickness. We derive a twining model that describes the experimental observations and explains why the 71o domain walls are the ones commonly observed in these films. This understanding provides us with a new degree of freedom to control the structure and, thus, the properties of BiFeO3 thin films.Comment: RevTeX; 4 two-column pages; 4 color figures. Figure 2b does not seem to display well. A proper version can be found in the source fil

Tunable resistivity exponents in the metallic phase of epitaxial nickelates

We report a detailed analysis of the electrical resistivity exponent of thin films of NdNiO3 as a function of epitaxial strain. Thin films under low strain conditions show a linear dependence of the resistivity versus temperature, consistent with a classical Fermi gas ruled by electron-phonon interactions. In addition, the apparent temperature exponent, n, can be tuned with the epitaxial strain between n = 1 and n = 3. We discuss the critical role played by quenched random disorder in the value of n. Our work shows that the assignment of Fermi/Non-Fermi liquid behaviour based on experimentally obtained resistivity exponents requires an in-depth analysis of the degree of disorder in the material

Conduction at domain walls in insulating Pb(Zr0.2_{0.2}Ti0.8_{0.8})O3_3 thin films

Among the recent discoveries of domain wall functionalities, the observation of electrical conduction at ferroelectric domain walls in the multiferroic insulator BiFeO3 has opened exciting new possibilities. Here, we report evidence of electrical conduction also at 180{\deg} ferroelectric domain walls in the simpler tetragonal ferroelectric PZT thin films. The observed conduction shows nonlinear, asymmetric current-voltage characteristics, thermal activation at high temperatures and high stability. We relate this behavior to the microscopic structure of the domain walls, allowing local defects segregation, and the highly asymmetric nature of the electrodes in our local probe measurements

Domain wall magnetoresistance in BiFeO₃ thin films measured by scanning probe microscopy

We measure the magnetotransport properties of individual 71° domain walls in multiferroic BiFeO₃ by means of conductive-atomic force microscopy (C-AFM) in the presence of magnetic fields up to one Tesla. The results suggest anisotropic magnetoresistance at room temperature, with the sign of the magnetoresistance depending on the relative orientation between the magnetic field and the domain wall plane. A consequence of this finding is that macroscopically averaged magnetoresistance measurements for domain wall bunches are likely to underestimate the magnetoresistance of each individual domain wall

Voltage-driven displacement of magnetic vortex cores

Abstract Magnetic vortex cores in polycrystalline Ni discs underwent non-volatile displacements due to voltage-driven ferroelectric domain switching in single-crystal BaTiO3. This behaviour was observed using photoemission electron microscopy to image both the ferromagnetism and ferroelectricity, while varying in-plane sample orientation. The resulting vector maps of disc magnetization match well with micromagnetic simulations, which show that the vortex core is translated by the transit of a ferroelectric domain wall, and thus the inhomogeneous strain with which it is associated. The non-volatility is attributed to pinning inside the discs. Voltage-driven displacement of magnetic vortex cores is novel, and opens the way for studying voltage-driven vortex dynamics.The Royal Society, Gates Cambridge, the Winton Programme for the Physics of Sustainability, Trinity College (Cambridge), Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) from the Catalan governmen