Electric field | EPFL Graph Search

An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field for a system of charged particles. Electric fields originate from electric charges and time-varying electric currents. Electric fields and magnetic fields are both manifestations of the electromagnetic field, one of the four fundamental interactions (also called forces) of nature. Electric fields are important in many areas of physics, and are exploited in electrical technology. In atomic physics and chemistry, for instance, the electric field is the attractive force holding the atomic nucleus and electrons together in atoms. It is also the force responsible for chemical bonding between atoms that result in molecules. The electric field is defined as a vector field that associates to each point in space the electrostatic (Coulomb) force per uni

Long Spin Lifetime in Rare-Earth Single-Atom Magnets at Surfaces

Boris Sorokin

Further miniaturisation of magnetic storage devices requires an advent of new types of magnets, since classical ferromagnetic materials show lack of remanence at nano- and subnanoscale. A single atom can represent the smallest possible bit of information. Even though the isolated single atoms are paramagnetic, the interaction of surface adsorbed atoms with a substrate can result in high magnetic anisotropy energy and long magnetisation lifetime. In this thesis the magnetic properties of surface supported rare-earth single atoms are investigated with X-ray absorption spectroscopy, X-ray magnetic circular dichroism and multiplet simulations. The first goal of the project was to find new adatom/substrate combinations that exhibit a long adatom magnetic lifetime, possibly extending the current limits of the atomic magnetic moment stability to higher temperatures and longer spin relaxation times. The second objective was to implement the external electric field control of the adatom magnetic properties.Firstly, we present the experimental and theoretical research on Dy and Ho single atoms deposited on BaO. A comparison of our results with similar studies sheds light on the impact of the substrate crystal field, in particular Dy-O bond covalency, on the magnetic stability of the rare-earth adatoms. Next, the detailed experimental investigation of Dy single atoms on ZnO is presented. This adatom/substrate pair is used as first attempt to implement the electric field control of the adatom magnetic properties. Lastly, we study the Dy adatoms on graphene layers grown on various metals. The vibrational modes of the metallic substrates are shown to strongly affect the Dy spin lifetime, while addition of a second graphene layer does not allow to significantly improve the adatom magnetic stability.

EPFL2022

Electric Field Effects in the Skyrmion Lattice Hosting Magnet Cu2OSeO3

Saumya Shivam

Magnetic skyrmions have garnered much attention in recent years because of their non trivial topology and potential to be used in next generation memory devices. They may exist individually or as a lattice of skyrmions(SkL). A number of investigations have been done on SkL in metallic samples, but recently a few magnetoelectric insulators have also begun to attract a lot of attention. The magnetoelectric nature of these materials presents a possibility to make prototype memory devices where skyrmions form a storage bit and can be controlled with only electric fields without the losses caused by Joule heating. We focus here on the material Cu2OSeO3, for which interesting electric field effects have been seen like the control of size of the region in the phase diagram where SkL is stable, creation of metastable skyrmions and the use of magnetolectric susceptibility to map the phase diagram. We use the magnetoelectric susceptibility to explore the above mentioned effects of electric fields, attempting to confirm previous reports of such effects using different techniques like neutron scattering and AC susceptibility. We also propose a new design to create metastable SkL state at low temperatures and confirm its existence using magnetoelectric susceptibility. We also attempt to measure the change in polarization between the SkL phase and the trivial phase on application of DC electric fields, which in principle could lead to electrical readout of skyrmions in a prototype skyrmion based memory device.

2018

Skyrmion Control and Phase Transitions in Cu2OSeO3

Thomas Schönenberger

Magnetic skyrmions are nanometric and non-trivial spin textures with non-zero topological charge. Their robustness against perturbations and the possibility to control them using external stimuli make them ideal candidates for future spintronic applications. In particular the magnetoelectric skyrmion host Cu2OSeO3 holds a lot of promise for low power devices since skyrmions in this compound can be controlled by electric fields alone. Using Lorentz transmission electron microscopy to perform real space and real time biasing experiments on thin lamellas of Cu2OSeO3 in a geometry that is most suitable for technological applications, we observe reproducible creation and annihilation of skyrmions. For a more quantitative analysis, we develop new feature detection algorithms to reliably extract skyrmion positions even in noisy images. We further produce Due to its low pinning, Cu2OSeO3 allows for the formation of large and well-arranged triangular skyrmion lattices. This makes this compound a perfect testbed to study the evolution of skyrmion configurations under external stimuli. Experiments are carried out again using Lorentz transmission electron microscopy on thin lamellas of Cu2OSeO3. We investigate how defects in skyrmion lattices are arranged at grain boundaries and develop algorithms to extract them and to directly visualize their alignment. These defects are at the core of the melting of skyrmion lattices in this system. We show that a controlled magnetic field ramp can induce skyrmion ensembles in Cu2OSeO3 to transition from a two-dimensional solid through a thus far unknown ordered liquid phase called the hexatic phase, to a liquid. We find that this transition is a topological defect-induced two-step process as predicted by the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. Finally, we go beyond equilibrium phenomena to explore the effect of quenching the system from its liquid phase to its solid phase using different quench rates and find first evidence that our system belongs to the Kibble-Zurek universality class.

EPFL2022

Skyrmion Control and Phase Transitions in Cu2OSeO3

Thomas Schönenberger

EPFL2022