Final Report Summary - MOBILE-W (Exploring Mobile Interfaces: Domain Walls as Functional Elements)
Ferroelectrics consist of regions called domains. Interfaces between adjacent domains, called domain-walls [DWs] are known to influence strongly the material’s properties. Ferroelectric DWs are interfaces that can move under external forces (electrical, mechanical, temperature variations) and be annihilated and recreated. We investigated DWs as ‘matter’ and not mere interfaces. We sought knowledge to control their internal structure, width and position, orientation, spacing between each other, to make DW arrays, and to displace them. Above all we investigated their properties.
Controlled DW patterns: Theory and practical methods of formation of arrays of ultrafine domain (periodicity <10nm) were developed. Fine and controlled patterns were obtained both in-growth and post-growth (the latter is rewritable). Concurrent formation of large arrays of ultra-dense DWs was achieved using conductive cantilever tip and through top electrodes. Controlled formation of charged DWs was shown by poling through the ferroelectric transition and by using UV light. Controlled periodicity of the charged DWs was achieved.
Internal structure of domain walls, their width, and domain-wall defect interaction: Neel-like DWs where identified in tetragonal Pb(Zr,Ti)O3 single crystals. Ferroelectric translational antiphase boundaries in non-polar material (PbZrO3) were evidenced. Theoretical investigations revealed bichiral structure of 180o DWs in tetragonal ferroelectrics, structural phase transition in 180o walls of rhombohedral BaTiO3, anomalously thick charged DWs at the morphotropic phase boundary of Pb(Zr,Ti)O3, and more. DW defect interactions were studied and asymmetric structure of 90o walls and defect accumulation next to it were found.
Controlled displacement of DWs: Displacement velocity of 180° DWs along a 1D line was modified through control of top electrode thickness or concentration of locally induced defects. Theoretically it was shown that 180° DWs can interact with each other, promising control of movement of dense arrays of 180° DWs. Displacement of ferroelectric anti-phase boundaries in non-polar material was evaluated from theory, predicting movement under realistic voltage through cantilever tip. Control of magnetic domain dynamics via persistent field effect of poled ferroelectric gates was demonstrated in ferroelectric/ferromagnetic heterostructures of PVDF/GaMnAs and PVDF/Co. Directional propagation of magnetic domains in narrow channels was obtained with the speed changing by factor 5-20 through reversed ferroelectric gate poling.
Properties and functionality of DWs: Discovery of metallic (quasi-2DG) DWs in insulator ferroelectrics is a highlight in the project. It started with a theoretical analysis showing that high density charged DWs are to enhance piezoelectric response. Same analysis showed that these walls should be highly conductive. Following this, high conductivity of charged DWs, 10 power 9 that of bulk was demonstrated in BaTiO3. Following this, tightly spaced metallic conducting channels in BiFeO3 were achieved and their reconfigurability demonstrated. Later on, metallic conductivity was shown at cryogenic temperature in nominally neutral Pb(Zr,Ti)O3 DWs, which are bent due to tailored boundary conditions. The implication of conductive DWs in reconfigurable nano-electronics was pursued in a PoC project. Currently we pursue conductive DWs compatible with silicon technology.
Indirectly obtained significant results: Other advances originated from the project though not directly related to DWs. Among them, a new light on the origin of antiferroelectricity in PbZrO3, a method to obtain free standing particles with negative pressure, and the experimental demonstration of enhanced ferroelectric and piezoelectric properties under negative pressure.
To sum up: A large volume of new results was obtained laying the foundations to understanding and exploitations of domain walls as functional entities.
Controlled DW patterns: Theory and practical methods of formation of arrays of ultrafine domain (periodicity <10nm) were developed. Fine and controlled patterns were obtained both in-growth and post-growth (the latter is rewritable). Concurrent formation of large arrays of ultra-dense DWs was achieved using conductive cantilever tip and through top electrodes. Controlled formation of charged DWs was shown by poling through the ferroelectric transition and by using UV light. Controlled periodicity of the charged DWs was achieved.
Internal structure of domain walls, their width, and domain-wall defect interaction: Neel-like DWs where identified in tetragonal Pb(Zr,Ti)O3 single crystals. Ferroelectric translational antiphase boundaries in non-polar material (PbZrO3) were evidenced. Theoretical investigations revealed bichiral structure of 180o DWs in tetragonal ferroelectrics, structural phase transition in 180o walls of rhombohedral BaTiO3, anomalously thick charged DWs at the morphotropic phase boundary of Pb(Zr,Ti)O3, and more. DW defect interactions were studied and asymmetric structure of 90o walls and defect accumulation next to it were found.
Controlled displacement of DWs: Displacement velocity of 180° DWs along a 1D line was modified through control of top electrode thickness or concentration of locally induced defects. Theoretically it was shown that 180° DWs can interact with each other, promising control of movement of dense arrays of 180° DWs. Displacement of ferroelectric anti-phase boundaries in non-polar material was evaluated from theory, predicting movement under realistic voltage through cantilever tip. Control of magnetic domain dynamics via persistent field effect of poled ferroelectric gates was demonstrated in ferroelectric/ferromagnetic heterostructures of PVDF/GaMnAs and PVDF/Co. Directional propagation of magnetic domains in narrow channels was obtained with the speed changing by factor 5-20 through reversed ferroelectric gate poling.
Properties and functionality of DWs: Discovery of metallic (quasi-2DG) DWs in insulator ferroelectrics is a highlight in the project. It started with a theoretical analysis showing that high density charged DWs are to enhance piezoelectric response. Same analysis showed that these walls should be highly conductive. Following this, high conductivity of charged DWs, 10 power 9 that of bulk was demonstrated in BaTiO3. Following this, tightly spaced metallic conducting channels in BiFeO3 were achieved and their reconfigurability demonstrated. Later on, metallic conductivity was shown at cryogenic temperature in nominally neutral Pb(Zr,Ti)O3 DWs, which are bent due to tailored boundary conditions. The implication of conductive DWs in reconfigurable nano-electronics was pursued in a PoC project. Currently we pursue conductive DWs compatible with silicon technology.
Indirectly obtained significant results: Other advances originated from the project though not directly related to DWs. Among them, a new light on the origin of antiferroelectricity in PbZrO3, a method to obtain free standing particles with negative pressure, and the experimental demonstration of enhanced ferroelectric and piezoelectric properties under negative pressure.
To sum up: A large volume of new results was obtained laying the foundations to understanding and exploitations of domain walls as functional entities.