Research On Single Phase Multiferroic Hexagonal Manganites By Electron Microscopy

Multiferroic materials have been raised as the research hotspots for decades since they have wide potential application capabilities in the field of transducers,memories,sensors and so on.Depth research on magneto-electric coupling mechanism will benefit the device application of this kind of materials.Transmission electron microscopy?TEM?,which can be treated as a bridge connecting nano-world and real world,is playing more and more crucial role in the field of multiferric materials since it has the ability to characterize samples in real space,momentum space and also energy space.Here,we have comprehensively studied the single phase multiferroic hexagonal manganites materials with the help of multiple TEM techniques,establishing the connections between properties and structures.Firstly,we have characterized single phase multiferroic materials at micro-scale with dark field images.We could uniquely justify the polarization directions of each domains with dynamic electron diffraction knowledge.Then,we use spherical aberration corrected TEM to characterize samples down to atomic scale.We realize that different sites of oxygen vacancies can be created under different external perturbations.Taking advantage of this properties,we grow YMnO₃ thin film on sapphire substrate which could provide large compressive strain and thus on-top oxygen vacancies can be created.In this case,YMnO₃ material can realize ferroelectric-ferromagnetic coupling instead of ferroelectric-antiferromagnetic coupling,which will benefit the applications of single phase multiferroic materials.Because of the symmetric regulations,hexagonal manganites always have cloverleaf shaped domain configurations.However,using aforementioned dark field technique,we have observed the existence of non-six fold domain cores.Probe corrected scanning transmission electron microscopy?STEM?technique helps us to reveal the atomic arrangement at vortex core areas.We find that partial edge dislocations are always pinning at vortex cores and thus non-six fold domains have been created.We have revealed the connections between topologically protected domains and partial edge dislocations.Further Landau based thermaldynamic calculations have been carried out to support our views.