Vrijdag 25 mei 2018 , 11.30 u., Lokaal U.408 (Campus Groenenborger)
Voordracht georganiseerd door EMAT
Onderwerp: Visualizing dynamic magnetism in nanostructures using electron microscopy
Spreker: Trevor P. Almeida (SUPA, School of Physics and Astronomy, University of Glasgow, UK)
In order to better understand chemical phase transformations or magnetic behavior in naturally occurring or synthetic samples, it is often necessary to investigate the underlying processes on the nano-scale. Transmission electron microscopy (TEM) allows atomic spatial resolution imaging and the development of in situ TEM experiments over recent years has provided fundamental insight into a range of dynamic processes. Further, combining in situ TEM experiments with techniques like electron holography or differential phase contrast imaging allows for visualizing of magnetization in nanostructures whilst under the influence of external stimuli; e.g. controlled atmospheres, temperature, etc. In this context, some examples of the use of in situ TEM and magnetic imaging will be presented.
Fe3O4 is the most magnetic naturally occurring mineral on Earth, carrying the dominant magnetic signature in rocks and providing a critical tool in paleomagnetism. The oxidation of Fe3O4 to maghemite (γ-Fe2O3) is of particular interest as it influences the preservation of remanence of the Earth's magnetic field by Fe3O4. Further, the thermomagnetic behavior of Fe3O4 grains directly affects the reliability of magnetic signal recorded by rocks. Through combining electron holography with environmental TEM and in situ heating, the effects of oxidation and temperature on the magnetic behavior of vortex-state Fe3O4 NPs are visualized successfully, for the first time.
Equiatomic iron-rhodium (FeRh) has attracted much interest due to its magnetostructural transition from its antiferromagnetic to ferromagnetic phase. The co-existing phases are separated by a phase-boundary domain wall (DW) and effective control over the creation and motion of these phase boundary DWs are considered desirable for potential application in a new generation of novel nanomagnetic or spintronic devices. In this context, several scanning TEM techniques are performed to visualize the localized chemical, structural and magnetic properties of a series of FeRh films.
Dinsdag 22 mei 2018, 16.00 u., Lokaal U.244 (Campus Groenenborger)
Voordracht georganiseerd door TGM
Onderwerp: Strain in 2D materials
Spreker: Slavisa Milovanovic, CMT, University of Antwerp
In this talk, I will review the most important effects of strain in different 2D materials. The basic terminology is given in the introduction. Here I will explain what is strain/stress tensor, the difference between strain and stress, displacement field, Young’s modulus, Poisson’s ratio, etc… Furthermore, I will give a short overview about the importance of strain in the semiconductor industry.
The main part of the talk is dedicated to strain in 2D materials. Due to their strong bonds, most of the 2D materials can be considerably stretched. This stretch is accompanied with a significant change of their electronic and vibrational properties. I will show on the example of graphene that straining a 2D material is very similar to applying a gauge field. As a consequence, a pseudo-magnetic field arises in the structure. The strength and the profile of the field are determined by the strain profile. Hence, “strain engineering” developed as a promising path to alter properties of 2D materials in the desired manner.
Apart from the graphene, other 2D materials show interesting features when strained. Here, I would mention semiconducting TMDs where the size of the band gap can be modified by strain. Furthermore, these materials proved to be good candidates as quantum emitters. Density functional theory showed that monolayer BN and many of the TMDs are piezoelectric, unlike their bulk counterparts. Recently, it was shown that stacking different monolayer materials with similar lattice constants on top of each other results with an out-of-plane rippling of the structure caused by the interaction between two layers. All these effects will be discussed in the lecture.