High-speed domain wall racetracks in a magnetic insulator

Recent reports of current-induced switching of ferrimagnetic oxides coupled to a heavy metal layer have opened realistic prospects for implementing magnetic insulators into electrically addressable spintronic devices. However, key aspects such as the configuration and dynamics of magnetic domain walls driven by electrical currents in insulating oxides remain unexplored. Here, we investigate the internal structure of the domain walls in Tm3Fe5O12 (TmIG) and TmIG/Pt bilayers and demonstrate their efficient manipulation by spin-orbit torques with velocities of up to 400 m s$^{-1}$ and minimal current threshold for domain wall flow of 5 x 10$^{6}$ A cm$^{-2}$. Domain wall racetracks embedded in TmIG are defined by the deposition of Pt current lines, which allow us to control the domain propagation and magnetization switching in selected regions of an extended magnetic layer. Scanning nitrogen-vacancy magnetometry reveals that the domain walls of thin TmIG films are N\'eel walls with left-handed chirality, with the domain wall magnetization rotating towards an intermediate N\'eel-Bloch configuration upon deposition of Pt. These results indicate the presence of a sizable interfacial Dzyaloshinskii-Moriya interaction in TmIG, which leads to novel possibilities to control the formation of chiral spin textures in magnetic insulators. Ultimately, domain wall racetracks provide an efficient scheme to pattern the magnetic landscape of TmIG in a fast and reversible wa

Frequency dependent polarisation switching in h-ErMnO3_3

We report an electric-field poling study of the geometric-driven improper ferroelectric h-ErMnO$_3$. From a detailed dielectric analysis we deduce the temperature and frequency dependent range for which single-crystalline h-ErMnO$_3$ exhibits purely intrinsic dielectric behaviour, i.e., free from extrinsic so-called Maxwell-Wagner polarisations that arise, for example, from surface barrier layers. In this regime ferroelectric hysteresis loops as function of frequency, temperature and applied electric fields are measured revealing the theoretically predicted saturation polarisation in the order of 5 - 6 $\mu$C/cm$^2$. Special emphasis is put on frequency-dependent polarisation switching, which is explained in terms of domain-wall movement similar to proper ferroelectrics. Controlling the domain walls via electric fields brings us an important step closer to their utilization in domain-wall-based electronics.Comment: 5 pages, 3 figure

Assessment of treatment response in cardiac sarcoidosis based on myocardial 18F-FDG uptake

ObjectiveImmunosuppressive therapy for cardiac sarcoidosis (CS) still largely consists of corticosteroid monotherapy. However, high relapse rates after tapering and insufficient efficacy are significant problems. The objective of this study was to investigate the efficacy and safety of non-biological and biological disease-modifying anti-rheumatic drugs (nb/bDMARDs) considering control of myocardial inflammation assessed by 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) of the heart.MethodsWe conducted a retrospective analysis of treatment response to nb/bDMARDs of all CS patients seen in the sarcoidosis center of the University Hospital Zurich between January 2016 and December 2020.ResultsWe identified 50 patients with CS. Forty-five patients with at least one follow-up PET/CT scan were followed up for a mean of 20.5 ± 12.8 months. Most of the patients were treated with prednisone and concomitant nb/bDMARDs. At the first follow-up PET/CT scan after approximately 6.7 ± 3 months, only adalimumab showed a significant reduction in cardiac metabolic activity. Furthermore, comparing all serial follow-up PET/CT scans (143), tumor necrosis factor inhibitor (TNFi)-based therapies showed statistically significant better suppression of myocardial 18F-FDG uptake compared to other treatment regimens. On the last follow-up, most adalimumab-treated patients were inactive (n = 15, 48%) or remitting (n = 11, 35%), and only five patients (16%) were progressive. TNFi was safe even in patients with severely reduced left ventricular ejection fraction (LVEF), and a significant improvement in LVEF under TNFi treatment was observed.ConclusionTNFi shows better control of myocardial inflammation compared to nbDMARDs and corticosteroid monotherapies in patients with CS. TNFi was efficient and safe even in patients with severely reduced LVEF

Electrical half-wave rectification at ferroelectric domain walls

Ferroelectric domain walls represent multifunctional 2D-elements with great potential for novel device paradigms at the nanoscale. Improper ferroelectrics display particularly promising types of domain walls, which, due to their unique robustness, are the ideal template for imposing specific electronic behavior. Chemical doping, for instance, induces p- or n-type characteristics and electric fields reversibly switch between resistive and conductive domain-wall states. Here, we demonstrate diode-like conversion of alternating-current (AC) into direct-current (DC) output based on neutral 180$^{\circ}$ domain walls in improper ferroelectric ErMnO$_3$. By combining scanning probe and dielectric spectroscopy, we show that the rectification occurs for frequencies at which the domain walls are fixed to their equilibrium position. The practical frequency regime and magnitude of the output is controlled by the bulk conductivity. Using density functional theory we attribute the transport behavior at the neutral walls to an accumulation of oxygen defects. Our study reveals domain walls acting as 2D half-wave rectifiers, extending domain-wall-based nanoelectronic applications into the realm of AC technology

Electronic Transport and Correlation Phenomena at Improper Ferroelectric Domain Walls

Domain walls are attracting significant interest in the field of (multi-)ferroic materials owing to their intriguing functional properties. The domain walls represent natural interfaces that can exhibit substantially different properties than the surrounding bulk due to their low symmetry and unusual electrostatics. The fact, that these interfaces can be moved, erased, and positioned at will is of strong technological interest and highly relevant for the design of domain-wall based nanoelectronic devices. Charged and neutral domain walls in ferroelectrics are of special interest, as they can show diverse electronic behavior ranging from highly conductive to strongly insulating states. Prior to implementation, however, further knowledge is required to generate walls with controllable output and power in order to emulate electronic nano-components such as diodes, transistors and gates. The scope of this thesis is to present novel strategies to characterize and manipulate functional domain walls in complex ferroelectric oxides. Advanced microscopy studies are realized by combining state-of-the-art imaging techniques, including scanning probe microscopy (SPM) and cathode-lens microscopy (CLM). Using hexagonal manganites (RMnO3) as model system, we demonstrate chemical impurity doping as a promising tool to engineer and improve the properties of functional domain walls. In p-type semiconducting ErMnO3, chemical doping is applied to increase the current densities at charged domain walls by two orders of magnitude and tune from p-type to n-type dominated screening and transport behavior. Moreover, we demonstrate reversible electric-field control of the electronic transport at the charged domain walls, switching between resistive and conductive domain-wall states. Aside from the charged walls, we perform a comprehensive analysis of neutral domain walls in ErMnO3 and their functionality. Under adequate boundary conditions, these walls exhibit currents, which can be influenced by thermal annealing in oxygen atmosphere. We further find that the walls facilitate AC-to-DC conversion, emulating the functionality of classical diodes. The rectifying properties, including the practical frequency regime and magnitude of the output, are controlled via the conductivity of the adjacent domains. Many of the aforementioned discoveries were enabled by the development of new experimental approaches in terms of SPM, including the application of electrostatic force microscopy (EFM) and frequency-dependent domain-wall measurements, as well as pioneering CLM experiments. In particular, x-ray photoemission electron microscopy (X-PEEM) was applied to access local domain-wall physics contact-free and with reduced data acquisition times, and the potential of low-energy electron microscopy (LEEM) was demonstrated to improve spatial resolution to a few nanometers. This work thus provides novel insight into the physical properties of functional domain walls, demonstrates new advanced characterization techniques, and highlights novel opportunities for the design of future domain-wall based devices

Charged ferroelectric domain walls for deterministic ac signal control at the nanoscale

[Image: see text] The direct current (dc) conductivity and emergent functionalities at ferroelectric domain walls are closely linked to the local polarization charges. Depending on the charge state, the walls can exhibit unusual dc conduction ranging from insulating to metallic-like, which is leveraged in domain-wall-based memory, multilevel data storage, and synaptic devices. In contrast to the functional dc behaviors at charged walls, their response to alternating currents (ac) remains to be resolved. Here, we reveal ac characteristics at positively and negatively charged walls in ErMnO(3), distinctly different from the response of the surrounding domains. By combining voltage-dependent spectroscopic measurements on macroscopic and local scales, we demonstrate a pronounced nonlinear response at the electrode-wall junction, which correlates with the domain-wall charge state. The dependence on the ac drive voltage enables reversible switching between uni- and bipolar output signals, providing conceptually new opportunities for the application of charged walls as functional nanoelements in ac circuitry

Tuning the emission colour by manipulating terbium-terbium interactions: Terbium doped aluminum nitride as an example system

Terbium-terbium interactions in terbium doped semiconductors and insulators may lead to the so-called cross-relaxation process, which increases the 5D4 (green) emission of the terbium ions at the cost of the 5D3 (blue) luminescence intensity. This effect can generally be reduced by increasing the distance between an excited ion and the nearest ion in the ground state. A straightforward measure is to use a specimen with a decreased terbium concentration. The alternative is to increase the intensity of the excitation (either by photons or electrons) and thereby to reduce the population of terbium ions in the ground state. This paper works this process out with the example of AlN:Tb on the basis of a model and respective experimental results. As will be seen, stronger excitation causes in essence more Tb ions to be excited, thus less ions in the ground state which increases the distance between an excited and the nearest ground state ions. This hinders energy transfer between the terbium ions and thus counteracts the cross-relaxation process. The advantage of changing the excitation intensity lies in the possibility to deliberately shift the apparent colour of the Tb luminescence from a single specimen between green and blue

Domain wall architecture in tetragonal ferroelectric thin films

ISSN:0935-9648ISSN:1521-409