Skyrmion Control and Phase Transitions in Cu2OSeO3

Thomas Schönenberger
EPFL, 2022
Thèse EPFL

Résumé

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.

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https://infoscience.epfl.ch/record/294769?ln=fr

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Thomas Schönenberger
EPFL, 2022
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/294769?ln=fr

À propos de ce résultat

Concepts associés (22)

Concepts associés

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Publications associées (25)

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Melting of a skyrmion lattice to a skyrmion liquid via a hexatic phase

Marco Cantoni, Fabrizio Carbone, Ping Huang, Arnaud Magrez, Henrik Moodysson Rønnow, Thomas Schönenberger

While in 3D materials melting is a single, first-order phase transition, in 2D systems, it can also proceed via an intermediate phase. For a skyrmion lattice in Cu2OSeO3, magnetic field variations can tune this quasiparticle 2D solid into a skyrmion liquid via an intermediate hexatic phase with short-range translational and quasi-long-range orientational order. The phase transition most commonly observed is probably melting, a transition from ordered crystalline solids to disordered isotropic liquids. In three dimensions, melting is a single, first-order phase transition. In two-dimensional systems, however, theory predicts a general scenario of two continuous phase transitions separated by an intermediate, oriented liquid state, the so-called hexatic phase with short-range translational and quasi-long-range orientational orders. Such hexatic phases occur in colloidal systems, Wigner solids and liquid crystals, all composed of real-matter particles. In contrast, skyrmions are countable soliton configurations with non-trivial topology and these quasi-particles can form two-dimensional lattices. Here we show, by direct imaging with cryo-Lorentz transmission electron microscopy, that magnetic field variations can tune the phase of the skyrmion ensembles in Cu(2)OSeO(3)from a two-dimensional solid through the long-speculated skyrmion hexatic phase to a liquid. The local spin order persists throughout the process. Remarkably, our quantitative analysis demonstrates that the aforementioned topological-defect-induced crystal melting scenario well describes the observed phase transitions.

NATURE PUBLISHING GROUP2020

Chargement

Thermopower as a Unique Tool to Probe Quantum Phase Transitions

Stevan Arsenijevic

In my thesis, transport measurements such as resistivity and, more importantly, thermopower S, were used to explore the phase diagram of bad metals. Bad metals are electronically correlated systems whose ground state lies close to a quantum phase transition. By tuning the control parameters, such as temperature (T ), magnetic field (B), hydrostatic pressure (p) or chemical substitution (x), we can induce phase transitions between the various electronic, magnetic and structural phases. Here, the thermopower is presented as a unique tool for probing quantum phase transition because it is a measure of the entropy of conducting electrons. The main part of the thesis is dedicated to the study of Fe-based superconductors (FeSC) discovered in 2008. Their parent compound has an antiferromagnetic (AF) ground state, where the itinerant electrons form a spin-density wave (SDW), a periodic modulation of spin density. This coincides or is preceded by a structural, tetragonal-to-orthorhombic transition. The nesting between the electron and hole Fermi surface is believed to be the driving mechanism for the SDW state. By changing the structural or chemical properties the AF ground state of FeSC is suppressed, giving way to superconductivity (SC). The remaining antiferromagnetic fluctuations above the transition can provide a glue for SC pairing. Here, the analysis of the thermopower S/T of BaFe1−xCoxAs2 (BFCA) in the x-T phase diagram shows the signatures of the spin fluctuation which have a dome-like dependence and follow the trend of superconducting Tc . The logarithmic increase of S/T upon decreasing T is ascribed to the proximity of the spin-density-wave quantum critical point. It can be understood as an increase of entropy due to the incommensurate AF spin fluctuations. We can ascribe the high values of thermopower in BFCA at intermediate- and room-temperatures to the influence of low-T quantum criticality. To probe the response of the electronic system in FeSC to structural changes, we performed measurements under pressure of the parent compound BaFe2As2 (BFA), the SC electron-doped BFCA, and hole-doped Ba1−xKxFe2As2 (BKFA). In the parent compound pressure suppresses the structural/SDW transition, similar to the effect of doping. For doped systems, in order to describe the behavior of thermopower in the high-T range (above 100K) we used a semi-metallic two-band model which was fitted to the data in order to extract the pressure dependence of the band parameters. In both doping cases the effect of pressure was similar, an increase of the band overlap and of the effective number of charge carriers. With this model we can explain the high-T , x and p dependence of thermopower in both electron- and hole-doped BFA. In a structurally simpler Fe-chalcogenide Fe1+yTe1−xSex compound, the excess of Fe has a Kondo-like influence on the charge carriers which dramatically changes the physics of the normal state. To probe the normal state, pressure, doping, magnetic Fe-excess concentration (y) and temperature were used as control parameters. At low-T a characteristic upturn of resistivity (ρmag ) is observed, followed by an increase of thermopower (Smag ), which we identify as the magnetic contribution caused by the spin-flip scattering events. Increasing the y resulted in an increase of ρmag , and a decrease of Smag , which is in agreement with the behavior of canonical Kondo-systems. Pressure suppresses the magnetic contribution to transport, thus increasing the itinerancy of the system. MnSi is another system in which the sensitivity of thermopower to entropy brings new information related to the complex magnetic structure. Pressure was used to drive the system from a helically ordered, canonical Fermi-liquid (FL) phase with T 2-resistivity to the intrinsically disordered, non-Fermi-liquid (NFL) phase above pc with T3/2-dependence. Our contour plot of S/T demonstrated how powerful the thermopower technique is, by reproducing the whole previously established T -p phase diagram. At the phase transition from the magnetically-ordered FL phase to the disordered NFL, the thermopower is dramatically enhanced. We bring useful information about the mysterious partial order (PO) phase inside the NFL phase, previously detected only by neutron scattering. The fluctuating helices scenario can describe the observed increase of entropy/thermopower in the PO phase. At ambient pressure, close to the helical transition of MnSi, a moderate magnetic field can stabilize the skyrmion lattice - the lattice of topological magnetic whirls, vortices. We observe a signature of the skyrmion lattice as a minute drop in thermopower. It is located exactly in the same region of the T − B phase diagram where an increase in magnetoresistance and Hall effect was reported previously. This feature originates from the additional scattering of conducting electrons on magnetic vortices, while the change in S is dominated by the decrease of entropy as the stable skyrmion lattice is formed. Overall, resistivity was used to confirm the established phase diagram, while thermopower, as an interesting and not sufficiently understood technique, was used to probe the sensitive changes of the charge carriers at the Fermi surface. We explored various phases showing how useful thermopower is to probe the entropy of electronic system on the verge of quantum phase transition.

EPFL2012

Chargement

Hot tearing is a major defect of solidification processes and welding. It occurs in metallic alloys while they are in the semi-solid state and is caused by a lack of liquid feeding in the mushy zone to compensate solid network openings due to tensile and shear strains. These strains are localized in the thin liquid films that remain at grain boundaries. Therefore, it is important to know when the solid grains coalesce and percolate to form a fully coherent solid, i.e., when the material can transmit stresses and strains but also withstand thermal contraction without rupture. Coalescence is defined as the disappearance of a liquid film in between two distinct grains, forming a solid grain boundary, while percolation is defined as the gradual transition of isolated grains or clusters surrounded by a continuous liquid film to a continuous solid network across a domain, even if some liquid regions remain. If the simulation of hot tearing has to account for localization of strains and liquid feeding, it is mandatory to use a granular approach. Two previous theses done recently at EPFL have developed such granular models of hot tearing, first in 2D (PhD of S. Vernède) and then in 3D (PhD of M. Sistaninia). However, these models have used a Voronoi tessellation for the description of globular microstructures in which equiaxed grains were approximated by polygons and polyhedra, respectively. From the experimental point of view, an innovative method, based on the Bridgman furnace principle, has been developed. The standard Bridgman furnace has been substantially modified in order to obtain the desired globular-equiaxed grain structure and reduce macrosegregation in the quenched sample. This modified furnace has been characterized then in terms of thermal measurements and observed final grain size. The samples obtained with the modified Bridgman furnace were then observed post-mortem by ex situ X-ray tomography. In addition, X-ray tomography experiments under similar conditions have been performed in situ with the laser-heated furnace installed on the TOMCAT synchrotron beam line at PSI. A postprocessing analysis of the principal curvature distribution (ISD plots) was performed on both ex situ and in situ observations for various compositions of inoculated Al-Cu alloys. The ISD plots of the X-ray tomography observations indicated the formation of liquid cylinders along triple line and, at a more advanced solidification stage, the formation of liquid pocket at quadruple vertices. From the modeling point of view, a new mesoscopic model has been developed both for 2D and 3D simulations. This model was inspired by the granular model developed by Phillon et al. that considers polyhedral grains based on a Voronoi tessellation of space, but allow to obtain smoother grain shapes and more progressive coalescence. After its validation with multiphase-field simulations, the mesoscopic model has been used to predict the various percolation transitions of the solid phase. In addition, by estimating the solid moment at which the liquid starts to form isolated pockets, it was possible to predict the temperature and solid fractions for which the mushy zone is most vulnerable to hot tearing. The simulations have been performed for various conditions and copper compositions, indicating the nominal compositions that are more sensitive to hot tearing. Finally, the 3D mesoscopic model predictions were compared with the X-ray tomography observations.

EPFL2015

Chargement

Thermopower as a Unique Tool to Probe Quantum Phase Transitions

Stevan Arsenijevic

EPFL2012

EPFL2015