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Confinement of magnetic structures geometrically as well as energetically leads to novel and unexpected behavior. With advances in fabrication and lithography tools, magnetic structures can be made in complex, confined three-dimensional (3D) geometries at the nanoscale as well as patterned into a variety of interacting lattices. In order to control their behavior, it is necessary to understand the fundamental physics of such interactions along with the influence of physical shape of the nanostructures in 3D.
Lorentz transmission electron microscopy (LTEM) has been used extensively for the characterization of magnetization and domains in magnetic structures as it offers a high spatial resolution along with a direct visualization of the magnetization [1]. LTEM combined with a phase retrieval technique such as the transport-of-intensity equation (TIE) can be used to obtain quantitative information about the magnetization and interactions between nanostructures. I will present a brief introduction to quantitative phase retrieval using LTEM in 2D as well as 3D [2] with various applications in materials science. I will also present advantages of a dedicated LTEM instrument with a spherical aberration corrector, which offers an unprecedented spatial resolution of 0.45 nm while maintaining the sample in a field free region.
I will present application of these techniques to study various patterned magnetic structures in which we can control the various energy terms via geometric confinement or external coupling to produce exciting and novel behavior. This will involve study of artificial spin ices which consist of Permalloy islands on an interacting lattice. Results of magnetic induction mapping from such lattices, as well modeling of the interction energy, and magnetization reversal which leads to the formation of the so called “string defects” will be presented [3]. Further I will also show in-situ domain behavior studies of patterned Permalloy discs in which the energy terms such as interlayer coupling and exchange bias can be varied to alter their behavior. Results from trilayer discs with antiferromagnetic coupling, which show an unexpected “meron” like spin structure due to competing energy terms will be presented [4].
References:
[1] M. De Graef and Y. Zhu, eds. , Magnetic Imaging and its Applications to Magnetic Materials, Academic Press, (2000)
[2] C. Phatak, A. Petford-Long, and M. De Graef, Physical Review Letters 104, 253901 (2010).
[3] C. Phatak, et. al. , Phys. Rev. B 83, 174431 (2011).
[4] C. Phatak, A.K. Petford-Long, and O. Heinonen, Physical Review Letters 108, 067205 (2012).
[5] This research has been carried out at Argonne National Laboratory and supported by USDepartment of Energy, Office of Science, Office of Basic Energy Sciences-Materials undercontract no. DE-AC02-06CH11357. Use of the Center for Nanoscale Materials and Electron Microscopy Center for Materials Research was supported by the U. S. Department of Energy,Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
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