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Sujet de stage / Master 2 Internship

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Imaging and control of polar domain walls in ferroelastic materials for high density storage media

Contact: BARRETT Nick, , nick.barrett@cea.fr, +33 1 69 08 32 72
Summary:
Ferroelastic materials can contain a very high density of polar domain walls and could serve as robust, nanoscale memory cells. The aim of the internship is to study the local chemistry in the vicinity of the domain wall and the polarity screening by electron injection and defects such as oxygen vacancies.
Possibility of continuation in PhD: Oui
Deadline for application:27/04/2018

Full description:
Context
Downscaling of memory devices for ultra-high storage densities and low power consumption is a major challenge for post-CMOS electronics in order to implement new functionalities. Domain wall (DW) engineering in ferroic materials is one possible route where the DW rather than the bulk material becomes the active element. The challenge then is to predict and control the nanoscale DW functionality [1]. DWs are transition regions where the changes of the order parameter from one domain to another result in strong gradient effects. Ferroelastic materials can contain a very high density of polar DWs [2] with dipole moments in the wall aligned parallel or antiparallel [3] to the apex and therefore serve as robust, nanoscale functional devices such as memory cells. CaTiO3 is the prototypical non-polar ferroelastic showing DW polarity.

Objectives
Control of the wall polarity would make them suitable for high density information storage. The aim of the internship is to study the local chemistry in the vicinity of the DW and the screening of DW polarity by free charge carriers (injected electrons) and defects such as oxygen vacancies.
The internship work will be carried out at SPEC (UMR CEA/CNRS) of the IRAMIS institute in the CEA-Saclay in collaboration with Raphael Haumont (ICMMO, Université Paris Saclay). The controlled elaboration of crystals, exhibiting different ferro/piezo-domains configurations, as a function of applied electric field during growth will provide a range of DW densities, orientations and polarity. The student will use low energy electron microscopy (LEEM) and photoelectron emission microscopy (PEEM) to characterize the DW arrays [4]. The subject requires a good grounding in solid state physics and a desire for experimental teamwork.

Figure (left) Orientation and hence DW density in single crystal CaTiO3 controlled by in-situ electric field during growth. (centre) Octahedral tilts in CaTiO3 on either side of a ferroelastic DW. (right) low energy electron image of DWs, R and V indicate ridges and valleys. Dark (light) contrast indicates positive (negative) polarity.

[1] G. Catalan, J. Seidel, R. Ramesh, Rev. Mod. Phys. 84, 119 (2012)
[2] H. Poettker and E.K.H. Salje, J. Phys.: Conden. Matt. 26, 342201 (2014)
[3] T. Zykova-Timan & E.K.H. Salje, Appl. Phys. Lett. 104, 082907 (2014)
[4] N. Barrett et al. J. Appl. Phys. 113, 187203 (2013)
Technics/methods used during the internship:
Photoelectron emission microscopy (PEEM) Low energy electron microscopy (LEEM)

Laboratory
Tutor of the internship

 

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