arXiv subjects

R. E. Dunin-Borkowski

Publications and source records attributed to R. E. Dunin-Borkowski.

At least 19 recordsLinked to original sources

arXiv2024

Strain engineering of magnetic anisotropy in the kagome magnet Fe3Sn2

The ability to control magnetism with strain offers innovative pathways for the modulation of magnetic domain configurations and for the manipulation of magnetic states in materials on the nanoscale. Although the effect of strain on magnetic domains has been recognized since the early work of C. Kittel, detailed local observations have been elusive. Here, we use mechanical strain to achieve reversible control of magnetic textures in a kagome-type Fe3Sn2 ferromagnet without the use of an external electric current or magnetic field in situ in a transmission electron microscope at room temperature. We use Fresnel defocus imaging, off-axis electron holography and micromagnetic simulations to show that tensile strain modifies the structures of dipolar skyrmions and switches their magnetization between out-of-plane and in-plane configurations. We also present quantitative measurements of magnetic domain wall structures and their transformations as a function of strain. Our results demonstrate the fundamental importance of anisotropy effects and their interplay with magnetoelastic and magnetocrystalline energies, providing new opportunities for the development of strain-controlled devices for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Quantifying the large contribution from orbital Rashba effect to the effective damping-like torque on magnetization

The generation of large spin currents, and the associated spin torques, which are at the heart of modern spintronics, have long been achieved by charge-to-spin conversion mechanisms, i.e. the spin Hall effect and/or the Rashba effect, intrinsically linked to a strong spin-orbit coupling. Recently, a novel path has been predicted and observed for achieving significant current-induced torques originating from light elements, hence possessing a weak spin-orbit interaction. These findings point out to the potential involvement of the orbital counterpart of electrons, namely the orbital Hall and orbital Rashba effects. In this study, we aim at quantifying these orbital-related contributions to the effective torques acting on a thin Co layer in different systems. First, in Pt|Co|Cu|AlOx stacking, we demonstrate a comparable torque strength coming from the conversion due to the orbital Rashba effect at the Cu|AlOx interface and the one from the effective spin Hall effect in bottom Pt|Co system. Secondly, in order to amplify the orbital-to-spin conversion, we investigate the impact of an intermediate Pt layer in Co|Pt|Cu|CuOx. From the Pt thickness dependence of the effective torques determined by harmonic Hall measurements complemented by spin Hall magneto-resistance and THz spectroscopy experiments, we demonstrate that a large orbital Rashba effect is present at the Cu|CuOx interface, leading to a twofold enhancement of the net torques on Co for the optimal Pt thickness. Our findings not only demonstrate the crucial role that orbital currents can play in low-dimensional systems with weak spin-orbit coupling, but also reveal that they enable more energy efficient manipulation of magnetization in spintronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Increasing the resolution of transmission electron microscopy by computational ghost imaging

By means of numerical simulations, we demonstrate the innovative use of computational ghost imaging in transmission electron microscopy to retrieve images with a resolution that overcomes the limitations imposed by coherent aberrations. The method requires measuring the intensity on a single pixel detector with a series of structured illuminations. The success of the technique is improved if the probes are made to resemble the sample and the patterns cover the area of interest evenly. By using a simple 8 electrode device as a specific example, a 2-fold increase in resolution beyond the aberration limit is demonstrated to be possible under realistic experimental conditions.

physics.ins-det↗

arXiv2022

Automatic alignment of an orbital angular momentum sorter in a transmission electron microscope using a convolution neural network

We report on the automatic alignment of a transmission electron microscope equipped with an orbital angular momentum sorter using a convolutional neural network. The neural network is able to control all relevant parameters of both the electron-optical setup of the microscope and the external voltage source of the sorter without input from the user. It is able to compensate for mechanical and optical misalignments of the sorter, in order to optimize its spectral resolution. The alignment is completed over a few frames and can be kept stable by making use of the fast fitting time of the neural network.

physics.ins-det↗

arXiv2022

Experimental Realisation of a π/2 Vortex Mode Converter for Electrons Using a Spherical Aberration Corrector

In light optics, beams with orbital angular momentum (OAM) can be produced by employing a properly-tuned two-cylinder-lens arrangement, also called $π$/2 mode converter. It is not possible to convey this concept directly to the beam in an electron microscope due to the non-existence of cylinder lenses in commercial transmission electron microscope (TEM). A viable work-around are readily-available electron optical elements in the form of quadrupole lenses. In a proof-of-principle experiment in 2012, it has been shown that a single quadrupole in combination with a Hilbert phase plate produces a spatially-confined, transient vortex mode. Here, an analogue to an optical $π$/2 mode converter is realized by repurposing a CEOS DCOR probe corrector in an aberration corrected TEM in a way that it resembles a dual cylinder lens using two quadrupoles. In order to verify the presence of OAM in the output beam, a fork dislocation grating is used as an OAM analyser. The possibility to use magnetic quadrupole fields instead of, e.g., prefabricated fork dislocation gratings to produce electron beams carrying OAM enhances the beam brightness by almost an order of magnitude and delivers switchable high-mode purity vortex beams without unwanted side-bands.

physics.app-ph↗

arXiv2021

Symmetry and planar chirality of a protein measured on an angular basis in a transmission electron microscope

In quantum mechanics, each conserved quantity (e.g., energy, position, linear momentum and angular momentum) is associated with a Hermitian operator. Its expected value can then be determined by performing a measurement on the wavefunction. In modern electron microscopy, one can select the initial and final states of the electron and the measurement basis by performing measurements of scattering processes. For example, the orbital angular momentum (OAM) of an electron can be used to reveal the n-fold symmetry of a wavefunction scattered by a sample. Here, we introduce a new composite planar chirality operator that can be used to measure a spiral-like feature in a sample. This concept develops the concept of chirality to highlight a specific roto-scale symmetry. We show that planar chirality can be characterized using an electron OAM sorter to uncover the atomic structures of biomolecules in cryo electron microscopy, either in a stand-alone analysis for fast identification of protein structures or in the context of conventional cryo electron microscopy to produce faster and more detailed 3D reconstructions by solving upside-down orientation ambiguities.

physics.ins-det↗

arXiv2021

Influence of surface band bending on a narrow band gap semiconductor: Tunneling atomic force studies of graphite with Bernal and rhombohedral stacking orders

Tunneling atomic force microscopy (TUNA) was used at ambient conditions to measure the current-voltage ($I$-$V$) characteristics at clean surfaces of highly oriented graphite samples with Bernal and rhombohedral stacking orders. The characteristic curves measured on Bernal-stacked graphite surfaces can be understood with an ordinary self-consistent semiconductor modeling and quantum mechanical tunneling current derivations. We show that the absence of a voltage region without measurable current in the $I$-$V$ spectra is not a proof of the lack of an energy band gap. It can be induced by a surface band bending due to a finite contact potential between tip and sample surface. Taking this into account in the model, we succeed to obtain a quantitative agreement between simulated and measured tunnel spectra for band gaps $(12 \ldots 37)$\,meV, in agreement to those extracted from the exponential temperature decrease of the longitudinal resistance measured in graphite samples with Bernal stacking order. In contrast, the surface of relatively thick graphite samples with rhombohedral stacking reveals the existence of a maximum in the first derivative $dI/dV$, a behavior compatible with the existence of a flat band. The characteristics of this maximum are comparable to those obtained at low temperatures with similar techniques.

cond-mat.mtrl-sci↗

arXiv2021

Alignment of electron optical beam shaping elements using a convolutional neural network

A convolutional neural network is used to align an orbital angular momentum sorter in a transmission electron microscope. The method is demonstrated using simulations and experimentally. As a result of its accuracy and speed, it offers the possibility of real-time tuning of other electron optical devices and electron beam shaping configurations.

physics.ins-det↗

arXiv2020

Room temperature skyrmions at zero field in exchange-biased ultrathin films

We demonstrate that magnetic skyrmions with a mean diameter around 60 nm can be stabilized at room temperature and zero external magnetic field in an exchange-biased Pt/Co/NiFe/IrMn multilayer stack. This is achieved through an advanced optimization of the multilayer stack composition in order to balance the different magnetic energies controlling the skyrmion size and stability. Magnetic imaging is performed both with magnetic force microscopy and scanning Nitrogen-Vacancy magnetometry, the latter providing unambiguous measurements at zero external magnetic field. In such samples, we show that exchange bias provides an immunity of the skyrmion spin texture to moderate external magnetic field, in the tens of mT range, which is an important feature for applications as memory devices. These results establish exchange-biased multilayer stacks as a promising platform towards the effective realization of memory and logic devices based on magnetic skyrmions.

cond-mat.mtrl-sci↗

arXiv2019

High-resistance YBa2Cu3O7-x grain-boundary Josephson junctions fabricated by electromigration

[100]-tilt grain-boundary YBa2Cu3O7-x (YBCO) junctions are promising for investigation of macroscopic quantum phenomena in high-Tc Josephson junctions. However, fabrication of the [100]-tilt grain-boundary YBCO junctions with a high resistance, which are required to study quantum effects, is difficult because of a high transparency of a tunnel barrier in this type of junctions. Here, we demonstrate a modification of grain-boundary barrier properties with a new approach to an oxygen electromigration in the YBCO grain-boundary junctions when the oxygen diffuses under an applied electric field from the grain-boundary to a BaTbO3 layer deposited atop of an YBCO film. Using this approach, we changed the normal-state resistance of the junctions from tens to several hundred Ohms without a degradation of their characteristic voltage IcRn and determined a barrier height and thickness by measuring the quasiparticle tunnelling current.

cond-mat.supr-con↗

arXiv2019

Energy-level quantization in YBa2Cu3O7-x phase-slip nanowires

Significant progress has been made in the development of superconducting quantum circuits, however new quantum devices that have longer decoherence times at higher temperatures are urgently required for quantum technologies. Superconducting nanowires with quantum phase slips are promising candidates for use in novel devices that operate on quantum principles. Here, we demonstrate ultra-thin YBa2Cu3O7-x nanowires with phase-slip dynamics and study their switching-current statistics at temperatures below 20 K. We apply theoretical models that were developed for Josephson junctions and show that our results provide strong evidence for energy-level quantization in the nanowires. The crossover temperature to the quantum regime is 12-13 K, while the lifetime in the excited state exceeds 20 ms at 5.4 K. Both values are at least one order of magnitude higher than those in conventional Josephson junctions based on low-temperature superconductors. We also show how the absorption of a single photon changes the phase-slip and quantum state of a nanowire, which is important for the development of single-photon detectors with high operating temperature and superior temporal resolution. Our findings pave the way for a new class of superconducting nanowire devices for quantum sensing and computing.

cond-mat.supr-con↗

arXiv2018

Experimental evidence for hotspot and phase-slip mechanisms of voltage switching in ultra-thin YBa2Cu3O7-x nanowires

We have fabricated ultra-thin YBa2Cu3O7-x nanowires with a high critical current density and studied their voltage switching behavior in the 4.2 - 90 K temperature range. A comparison of our experimental data with theoretical models indicates that, depending on the temperature and nanowire cross section, voltage switching originates from two different mechanisms: hotspot-assisted suppression of the edge barrier by the transport current and the appearance of phase-slip lines in the nanowire. Our observation of hotspot-assisted voltage switching is in good quantitative agreement with predictions based on the Aslamazov-Larkin model for an edge barrier in a wide superconducting bridge.

cond-mat.supr-con↗

arXiv2017

Low temperature pressureless immediate sintering of novel nanostructured WC/Co/NiCrSiB-alloy cemented carbide

A novel nanostructured cemented carbide formed from WC-5%Co-20%BNi2 brazing alloy is described. During sintering, the BNi2 alloy is infiltrated into a green compact of WC-5%Co at 1050-1100 °C for 2-60 minutes. Perfect wetting behavior and a zero contact angle are achieved after only 40 s. Relative densities of 98.5% and 100% and microhardness values of above 1500HV1 and 1800HV1 are obtained after 2 and 30 minutes, respectively. A change in mean particle size of about 600 nm in the precursor to a bimodal distribution of 350-400 nm and 10-20 nm is explained by a solution/reprecipitation mechanism.

cond-mat.mtrl-sci↗

arXiv2016

Realization of electron vortices with large orbital angular momentum using miniature holograms fabricated by electron beam lithography

Free electron beams that carry high values of orbital angular momentum (OAM) possess large magnetic moments along the propagation direction. This makes them an ideal probe for measuring the electronic and magnetic properties of materials, and for fundamental experiments in magnetism. However, their generation requires the use of complex diffractive elements, which usually take the form of nano-fabricated holograms. Here, we show how the limitations of focused ion beam milling in the fabrication of such holograms can be overcome by using electron beam lithography. We demonstrate experimentally the realization of an electron vortex beam with the largest OAM value that has yet been reported (L = 1000h\bar), paving the way for even more demanding demonstrations and applications of electron beam shaping.

quant-ph↗

arXiv2016

Temperature and magnetic field dependence of the internal and lattice structures of skyrmions by off-axis electron holography

The internal and lattice structures of magnetic skyrmions in B20-type FeGe are investigated using off-axis electron holography. The temperature, magnetic field and angular dependence of the magnetic moments of individual skyrmions are analyzed. Whereas the internal skyrmion shape is found to vary with magnetic field, the inter-skyrmion distance remains almost unchanged in the lattice phase. The amplitude of the local magnetic moment is found to depend on temperature, while the skyrmion shape does not. Deviations from a circular to a hexagonal skyrmion structure are observed in the lattice phase.

cond-mat.mes-hall↗

arXiv2015

Suppressing twin formation in Bi2Se3 thin films

The microstructure of Bi2Se3 topological-insulator thin films grown by molecular beam epitaxy on InP(111)A and InP(111)B substrates that have different surface roughnesses has been studied in detail using X-ray diffraction, X-ray reflectivity, atomic force microscopy and probe-corrected scanning transmission electron microscopy. The use of a rough Fe-doped InP(111)B substrate results in complete suppression of twin formation in the Bi2Se3 thin films and a perfect interface between the films and their substrates. The only type of structural defects that persist in the "twin-free" films is an antiphase domain boundary, which is associated with variations in substrate height. It is also shown that the substrate surface termination determines which family of twin domains dominates.

cond-mat.mtrl-sci↗

arXiv2013

Characterization of Fe-N nano crystals and nitrogen-containing inclusions in (Ga,Fe)N thin films using transmission electron microscopy

Nanometric inclusions filled with nitrogen, located adjacent to FenN (n = 3 or 4) nanocrystals within (Ga,Fe)N layers, are identified and characterized using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). High-resolution STEM images reveal a truncation of the Fe-N nanocrystals at their boundaries with the nitrogen-containing inclusion. A controlled electron beam hole drilling experiment is used to release nitrogen gas from an inclusion in situ in the electron microscope. The density of nitrogen in an individual inclusion is measured to be 1.4 +- 0.3 g/cm3. These observations provide an explanation for the location of surplus nitrogen in the (Ga,Fe)N layers, which is liberated by the nucleation of FenN (n> 1) nanocrystals during growth.

cond-mat.mtrl-sci↗

arXiv2011

Voids and Mn-rich inclusions in a (Ga,Mn)As ferromagnetic semiconductor investigated by transmission electron microscopy

Voids adjacent to both cubic (ZnS-type) and hexagonal (NiAs-type) Mn-rich nanocrystals are characterized using aberration-corrected transmission electron microscopy in annealed Ga0.995Mn0.005As magnetic semiconductor specimen grown by molecular beam epitaxy. Nano-beam electron diffraction measurements suggest that the nanocrystals exhibit deviations in lattice parameter from that of bulk MnAs. In situ annealing inside the electron microscope is used to study the nucleation, coalescence, and grain growth of individual nanocrystals. After annealing at 903 K, the magnetic transition temperature of the specimen likely to be dominated by the presence of cubic ferromagnetic nanocrystals.

cond-mat.mtrl-sci↗