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Thin films of magnetoelectric antiferromagnetic insulators (Cr2O3, BiFeO3 etc.) have emerged as a prospective material platform for magnonics, spin superfluidity, THz spintronics, and energy efficient spin-orbitronics. Understanding the magnetomechanical coupling in antiferromagnets offers vast advantages in the control of the primary order parameters. A standard micromagnetic approach for the description of a material relies on the effective parameters being homogeneously distributed throughout the system. Such an approach is commonly sufficient, but does not provide full characterization of the system. The family of magnetomechanical effects includes piezo- and flexomagnetic responses, which determine the modification of the magnetic order parameters due to homogeneous or inhomogeneous strain, respectively. Accounting for the flexomagnetic effects promises technological advantages for multiferroic and antiferromagnetic materials, where cross-coupling between elastic, magnetic and electric subsystems open additional degrees of freedom in the control of the respective order parameters [1, 2]. In this work, we discover the effect of strain gradient onto the magnetic behaviour of epitaxial Cr2O3 thin films [3, 4]. We demonstrate that by tuning the parameters of Cr2O3 epitaxial growth a fine control of the crystallographic and defect structure can be realized. A persistent strain gradient was obtained in Cr2O3 affecting its magnetic order parameters rendering a distribution of the Néel temperature along the thickness of the thin film. The antiferromagnetic ordering in the strained films can persist up to 100°C, rendering Cr2O3 as a prospective material for industrial electronics applications. The inhomogeneous enhancement of the antiferromagnetic order parameter induced by the strain gradient renders a flexomagnetic response of about 15 µB nm-2. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic

Dzyaloshinskii-Moriya interaction, also known as an antisymmetric exchange interaction, is the main source of chiral symmetry breaking effects in micromagnetic systems [1]. The later manifests itself in magnetic materials and layer stacks with structural space inversion symmetry breaking, where it leads to the formation of non-trivial chiral and topological spin textures (e.g. skyrmions, bubbles, homochiral spirals and domain walls). Such textures potentially could be utilized for prospective spintronic devices as a bit carrier. Still, tailoring of magnetochirality is only done by the selection of materials and adjustment of their composition in layer stacks. Alternatively, we demonstrate that space inversion symmetry breaking of the magnetic order parameter appears in geometrically curved systems [2]. In curvilinear ferromagnets, curvature governs the appearance of geometry-induced chiral and anisotropic responses, which introduce a new toolbox to create artificial chiral nanostructures from achiral magnetic materials suitable for the stabilization of non-trivial chiral textures [2,3]. Moreover, curvilinear geometry also leads to the appearance of non-local chiral effects, that arise from the asymmetry of the top and bottom surfaces and existence of both in- and out-of-plane magnetization components of different parity with respect to the reflection procedure [4]. Recently, we demonstrate the existence of non-local chiral effects in geometrically curved asymmetric permalloy cap with the vortex texture [5]. We find that the equilibrium vortex core obtain both bend and curling deformation, that are dependent on the geometric symmetries and magnetic parameters. References [1] A. Fert, N. Reyren and V. Cros, Nature Reviews Materials 2, 17031 (2017). [2] D. Makarov, O. M. Volkov, A. Kákay, O. V. Pylypovskyi, B. Budinská and O. V. Dobrovolskiy, Adv. Mater. 34, 2101758 (2021). [3] O. M. Volkov, A. Kákay, F. Kronast, I. Mönch, M.-A. Mawass, J. Fassbender and D. Makarov, Ph

Thin films of antiferromagnetic insulators (Cr2O3, Fe2O3, NiO etc.) are a prospective material platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A standard micromagnetic approach for the description of thin film system commonly relies on the effective parameters, assumed to be homogeneously distributed within a material. The family of magnetomechanical effects includes piezo- and flexomagnetic responses, which determine the modification of the magnetic order parameters due to homogeneous or inhomogeneous strain, respectively. Accounting for the strain-gradient-driven magnetomechanical coupling promises technological advantages: the cross-coupling between elastic, magnetic and electric subsystems opens additional degrees of freedom in the control of the respective order parameters [1]-[3]. In this work, we discover the presence of flexomagnetic effects in epitaxial antiferromagnetic Cr2O3 thin films [4]. We demonstrate that a gradient of mechanical strain affect the order-disorder magnetic phase transition resulting in the distribution of the Néel temperature along the thickness of Cr2O3 thin film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15 µB nm-2. The antiferromagnetic ordering in the strained films can persist up to 100 °C, rendering Cr2O3 as a prospective material for industrial spintronic applications. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters.

Thin films of antiferromagnetic insulators are a prospective material platform for magnonics, spin superfluidity, THz spintronics, and nonvolatile data storage. Here, we explore the presence of flexomagnetic effects in epitaxial Cr2O3 [1]. We demonstrate that a gradient of mechanical strain effect the order-disorder magnetic phase transition, resulting in the distribution of the Néel temperature along the thickness of a Cr2O3 film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15µB nm−2. The antiferromagnetic ordering in the strained films can persist up to 100∘C, rendering Cr2O3 as a prospective material for industrial electronics applications.

Thin films of antiferromagnetic insulators (Cr2O3, NiO etc.) are a prospective material platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A standard micromagnetic approach for the description of such thin films relies on the effective parameters being homogeneously distributed along the film thickness. The family of magnetomechanical effects includes piezo- and flexomagnetic responses, which determine the modification of the magnetic order parameters due to homogeneous or inhomogeneous strain, respectively. Accounting for the magnetomechanical coupling promises technological advantages: the cross-coupling between elastic, magnetic and electric subsystems opens additional degrees of freedom in the control of the respective order parameters [1, 2, 3]. In this work, we discover the presence of flexomagnetic effects in epitaxial Cr2O3[4]. We demonstrate that a gradient of mechanical strain affect the order-disorder magnetic phase transition resulting in the distribution of the Neel temperature along the thickness of a Cr2O3 film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15 µB nm-2. The antiferromagnetic ordering in the strained films can persist up to 100°C, rendering Cr2O3 as a prospective material for industrial electronics applications. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters.

Symmetry effects are fundamental in condensed matter physics as they define not only interactions but also resulting responses for the intrinsic order parameter depending on its transformation properties with respect to the operations of space and time reversal. Magnetic materials or layer stacks with structural space inversion symmetry breaking obtained much research attention due to the appearance of chiral Dzyaloshinskii-Moriya interaction (DMI) [1,2]. The latter manifests itself in the formation of non-trivial chiral and topological spin textures (e.g. skyrmions, bubbles, homochiral spirals and domain walls), that are envisioned to be utilized for prospective spintronic devices. At present, tailoring magnetochirality is done by the selection of materials and adjustment of their composition. Alternatively, space inversion symmetry breaking of the magnetic order parameter appears in geometrically curved systems [3]. In curvilinear ferromagnets, curvature governs the appearance of geometry-induced chiral and anisotropic responses, which introduce a new toolbox to create artificial chiral nanostructures from achiral magnetic materials suitable for the stabilization of non-trivial chiral textures [4,5]. Recently, much attention was dedicated to the exchange interaction, which enables curvature-induced extrinsic DMI as was proposed theoretically and validated experimentally for the case of conventional achiral magnetic materials [6]. Here, we demonstrate the existence of non-local chiral effects in geometrically curved asymmetric permalloy cap with the vortex texture. Using the full-scale simulation of the asymmetric nanodots we study how the vortex texture is changing with respect to the introduced sample asymmetry. We find that the equilibrium vortex core obtain both bend and curling deformation, that are dependent on the geometric symmetries and magnetic parameters. We relate the observed changes in the vortex string to the non-local chiral effects, that arise from

Antiferromagnetic ordering, being prevalent over ferromagnetic one in nature, is very sensitive to the lattice structure. For example, in the absence of magnetostatics, the stress fields can be responsible for the domain formation in easy-plane antiferromagnets, while the hydrostatic pressure provides a possibility to manipulate the phase transition temperature between magnetically ordered and disordered phases [1]. Phenomena related to the strain gradient being allowed in the majority of magnetic symmetry classes are much less explored [2].Here, we provide a theoretical and experimental evidence of flexomagnetism in the uniaxial room-temperature antiferromagnet Cr2O3 [3]. In the experiment, high-quality Cr2O3 thin films grown on sapphire substrate are considered. Their magnetic state is accessed by a combination of magnetotransport measurements and Nitrogen vacancy (NV) magnetometry, which allows one to address both the uncompensated magnetization at the film surface and the interior of the film. We found a gradual transition from antiferro- to paramagnetic state by thickness with heating, which is substantially enhanced in comparison with bulk Cr2O3. To explain this observation, we provide a systematic analysis of sources of magnetization and symmetry analysis regarding the presence of a sizeable strain gradient along the film thickness. The latter enables (i) the net uniform bulk magnetization along the film thickness, which cannot be directly detected by NV magnetometry, and (ii) distribution of the Neel temperature along the film thickness. The gradual change of the magnetic phase transition temperature along the sample breaks the compensation of antiferromagnetic sublattices. The respective magnetization is proportional to the Neel vector and changes its direction betwen antiferromagnetic domains contributing to the stray fields and being detectable by NV magnetometry. We provide a theoretical description of this strain-gradient-driven effects in thin Cr2O3 fi

Here, by making use of medium and high resolution autocorrected off-axis electron holography, we directly probe the electrostatic potential as well as in-plane structural reconstruction at edges and steps in multilayer hexagonal boron nitride. In combination with ab initio calculations, the data allows revealing the formation of folded zigzag edges at steps comprising two monolayers and their absence at monolayer steps.

Symmetry effects are key building blocks of condensed matter physics as they define not only interactions but also resulting re- sponses for the intrinsic order parameter. Namely, in magnetism geometric curvature governs the appearance of chiral and anisotropic responses [1], that introduce a new toolbox to create artificial chi- ral nanostructures from achiral magnetic materials [2,3]. Here, we demonstrate both theoretically and experimentally the existence of non-local chiral effects in geometrically curved asymmetric permalloy caps with the vortex texture. We find that the equilibrium vortex core obtain bend and curling deformation, that are dependent on the geometric symmetries and magnetic texture parameters. [1] D. D. Sheka et al., Comm. Phys. 3, 128 (2020). [2] O. M. Volkov et al., Phys. Rev. Lett, 123, 077201 (2019). [3] D. Makarov et al., Adv. Mater. 34, 2101758 (2022).

Thin films of antiferromagnetic insulators (Cr2O3, Fe2O3, NiO etc.) are a prospective material platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A standard micromagnetic approach for the description of thin film system commonly relies on the effective parameters, assumed to be homogeneously distributed within a material. The family of magnetomechanical effects includes piezo- and flexomagnetic responses, which determine the modification of the magnetic order parameters due to homogeneous or inhomogeneous strain, respectively. Accounting for the strain-gradient-driven magnetomechanical coupling promises technological advantages: the cross-coupling between elastic, magnetic and electric subsystems opens additional degrees of freedom in the control of the respective order parameters [1]-[3]. In this work, we discover the presence of flexomagnetic effects in epitaxial antiferromagnetic Cr2O3 thin films [4]. We demonstrate that a gradient of mechanical strain affect the order-disorder magnetic phase transition resulting in the distribution of the Néel temperature along the thickness of Cr2O3 thin film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15 µB nm-2. The antiferromagnetic ordering in the strained films can persist up to 100 °C, rendering Cr2O3 as a prospective material for industrial spintronic applications. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters.

Thin films of antiferromagnetic insulators (Cr2O3, NiO etc.) are a prospective material platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A standard micromagnetic approach for the description of such thin films relies on the effective parameters being homogeneously distributed along the film thickness. The family of magnetomechanical effects includes piezo- and flexomagnetic responses, which determine the modification of the magnetic order parameters due to homogeneous or inhomogeneous strain, respectively. Accounting for the magnetomechanical coupling promises technological advantages: the cross-coupling between elastic, magnetic and electric subsystems opens additional degrees of freedom in the control of the respective order parameters [1, 2, 3]. In this work, we discover the presence of flexomagnetic effects in epitaxial Cr2O3[4]. We demonstrate that a gradient of mechanical strain affect the order-disorder magnetic phase transition resulting in the distribution of the Neel temperature along the thickness of a Cr2O3 film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15 µB nm-2. The antiferromagnetic ordering in the strained films can persist up to 100°C, rendering Cr2O3 as a prospective material for industrial electronics applications. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters.

Dzyaloshinskii-Moriya interaction, also known as an antisymmetric exchange interaction, is the main source of chiral symmetry breaking effects in micromagnetic systems [1]. The later manifests itself in magnetic materials and layer stacks with structural space inversion symmetry breaking, where it leads to the formation of non-trivial chiral and topological spin textures (e.g. skyrmions, bubbles, homochiral spirals and domain walls). Such textures potentially could be utilized for prospective spintronic devices as a bit carrier. Still, tailoring of magnetochirality is only done by the selection of materials and adjustment of their composition in layer stacks. Alternatively, we demonstrate that space inversion symmetry breaking of the magnetic order parameter appears in geometrically curved systems [2]. In curvilinear ferromagnets, curvature governs the appearance of geometry-induced chiral and anisotropic responses, which introduce a new toolbox to create artificial chiral nanostructures from achiral magnetic materials suitable for the stabilization of non-trivial chiral textures [2,3]. Moreover, curvilinear geometry also leads to the appearance of non-local chiral effects, that arise from the asymmetry of the top and bottom surfaces and existence of both in- and out-of-plane magnetization components of different parity with respect to the reflection procedure [4]. Recently, we demonstrate the existence of non-local chiral effects in geometrically curved asymmetric permalloy cap with the vortex texture [5]. We find that the equilibrium vortex core obtain both bend and curling deformation, that are dependent on the geometric symmetries and magnetic parameters. References [1] A. Fert, N. Reyren and V. Cros, Nature Reviews Materials 2, 17031 (2017). [2] D. Makarov, O. M. Volkov, A. Kákay, O. V. Pylypovskyi, B. Budinská and O. V. Dobrovolskiy, Adv. Mater. 34, 2101758 (2021). [3] O. M. Volkov, A. Kákay, F. Kronast, I. Mönch, M.-A. Mawass, J. Fassbender and D. Makarov, Ph

Thin films of magnetoelectric antiferromagnetic insulators (Cr2O3, BiFeO3 etc.) have emerged as a prospective material platform for magnonics, spin superfluidity, THz spintronics, and energy efficient spin-orbitronics. Understanding the magnetomechanical coupling in antiferromagnets offers vast advantages in the control of the primary order parameters. A standard micromagnetic approach for the description of a material relies on the effective parameters being homogeneously distributed throughout the system. Such an approach is commonly sufficient, but does not provide full characterization of the system. The family of magnetomechanical effects includes piezo- and flexomagnetic responses, which determine the modification of the magnetic order parameters due to homogeneous or inhomogeneous strain, respectively. Accounting for the flexomagnetic effects promises technological advantages for multiferroic and antiferromagnetic materials, where cross-coupling between elastic, magnetic and electric subsystems open additional degrees of freedom in the control of the respective order parameters [1, 2]. In this work, we discover the effect of strain gradient onto the magnetic behaviour of epitaxial Cr2O3 thin films [3, 4]. We demonstrate that by tuning the parameters of Cr2O3 epitaxial growth a fine control of the crystallographic and defect structure can be realized. A persistent strain gradient was obtained in Cr2O3 affecting its magnetic order parameters rendering a distribution of the Néel temperature along the thickness of the thin film. The antiferromagnetic ordering in the strained films can persist up to 100°C, rendering Cr2O3 as a prospective material for industrial electronics applications. The inhomogeneous enhancement of the antiferromagnetic order parameter induced by the strain gradient renders a flexomagnetic response of about 15 µB nm-2. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic

Thin films of antiferromagnetic insulators are a prospective material platform for magnonics, spin superfluidity, THz spintronics, and nonvolatile data storage. Here, we explore the presence of flexomagnetic effects in epitaxial Cr2O3 [1]. We demonstrate that a gradient of mechanical strain effect the order-disorder magnetic phase transition, resulting in the distribution of the Néel temperature along the thickness of a Cr2O3 film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15µB nm−2. The antiferromagnetic ordering in the strained films can persist up to 100∘C, rendering Cr2O3 as a prospective material for industrial electronics applications.

Symmetry effects are fundamental in condensed matter physics as they define not only interactions but also resulting responses for the intrinsic order parameter depending on its transformation properties with respect to the operations of space and time reversal. Magnetic materials or layer stacks with structural space inversion symmetry breaking obtained much research attention due to the appearance of chiral Dzyaloshinskii-Moriya interaction (DMI) [1,2]. The latter manifests itself in the formation of non-trivial chiral and topological spin textures (e.g. skyrmions, bubbles, homochiral spirals and domain walls), that are envisioned to be utilized for prospective spintronic devices. At present, tailoring magnetochirality is done by the selection of materials and adjustment of their composition. Alternatively, space inversion symmetry breaking of the magnetic order parameter appears in geometrically curved systems [3]. In curvilinear ferromagnets, curvature governs the appearance of geometry-induced chiral and anisotropic responses, which introduce a new toolbox to create artificial chiral nanostructures from achiral magnetic materials suitable for the stabilization of non-trivial chiral textures [4,5]. Recently, much attention was dedicated to the exchange interaction, which enables curvature-induced extrinsic DMI as was proposed theoretically and validated experimentally for the case of conventional achiral magnetic materials [6]. Here, we demonstrate the existence of non-local chiral effects in geometrically curved asymmetric permalloy cap with the vortex texture. Using the full-scale simulation of the asymmetric nanodots we study how the vortex texture is changing with respect to the introduced sample asymmetry. We find that the equilibrium vortex core obtain both bend and curling deformation, that are dependent on the geometric symmetries and magnetic parameters. We relate the observed changes in the vortex string to the non-local chiral effects, that arise from

Antiferromagnetic ordering, being prevalent over ferromagnetic one in nature, is very sensitive to the lattice structure. For example, in the absence of magnetostatics, the stress fields can be responsible for the domain formation in easy-plane antiferromagnets, while the hydrostatic pressure provides a possibility to manipulate the phase transition temperature between magnetically ordered and disordered phases [1]. Phenomena related to the strain gradient being allowed in the majority of magnetic symmetry classes are much less explored [2].Here, we provide a theoretical and experimental evidence of flexomagnetism in the uniaxial room-temperature antiferromagnet Cr2O3 [3]. In the experiment, high-quality Cr2O3 thin films grown on sapphire substrate are considered. Their magnetic state is accessed by a combination of magnetotransport measurements and Nitrogen vacancy (NV) magnetometry, which allows one to address both the uncompensated magnetization at the film surface and the interior of the film. We found a gradual transition from antiferro- to paramagnetic state by thickness with heating, which is substantially enhanced in comparison with bulk Cr2O3. To explain this observation, we provide a systematic analysis of sources of magnetization and symmetry analysis regarding the presence of a sizeable strain gradient along the film thickness. The latter enables (i) the net uniform bulk magnetization along the film thickness, which cannot be directly detected by NV magnetometry, and (ii) distribution of the Neel temperature along the film thickness. The gradual change of the magnetic phase transition temperature along the sample breaks the compensation of antiferromagnetic sublattices. The respective magnetization is proportional to the Neel vector and changes its direction betwen antiferromagnetic domains contributing to the stray fields and being detectable by NV magnetometry. We provide a theoretical description of this strain-gradient-driven effects in thin Cr2O3 fi

Monolayer two-dimensional (2D) materials have been regarded as a hot topic in the fields of condensed matter physics, materials science, and chemistry due to their unique physical, chemical, and electronic properties. However, the research on the preparation method and properties understanding of the 2D monolayer are inadequate. In this dissertation, taking 2D nickel-iron layered double hydroxides (NiFe LDHs) and molybdenum disulfide (MoS2) as examples, the practicability of the direct synthesis of NiFe LDHs monolayer and the thermal enhancement catalytic performance of 2D MoS2 monolayer (MoS2 ML) are discussed. First, a one-pot synthetic strategy (bottom-up method) is presented to synthesize 2D NiFe-based LDHs monolayers, including NiFe, Co-, Ru-, doped, and Au-modified NiFe LDHs. The prerequisite and universality of this strategy are investigated and confirmed. The features of LDHs are characterized by advanced technologies. The obtained LDH bulks own a large interlayer spacing up to 8.2 Å, which can be facilely exfoliated into monolayers in water by hand-shaking within 10 s. As a result, the as-prepared NiFe-based LDH monolayers display a good electrocatalytic oxygen evolution reaction (OER) performance. This facile strategy paves the way for designing easily exfoliated LDHs for highly active catalysts and energy conversion devices based on other monolayer LDHs. Second, with gold-modified tape, 2D MoS2 ML is exfoliated from the bulk crystal through a micromechanical exfoliation method (top-down strategy). The thermal effects of MoS2 ML are confirmed by Raman and photoluminescence (PL) spectra. Moreover, an on-chip MoS2 ML hydrogen evolution reaction (HER) reactor is designed and fabricated. The thermal effects generate efficient electron transfer in the MoS2 ML and at the electrolyte-catalyst (MoS2 ML) interface, leading to an enhanced HER performance. Compared to the results obtained at room temperature, the MoS2 ML shows a direct thermal enhanced HER performanc

Data-Driven Design of Two-Dimensional Non-van der Waals Materials and Cation Exchange in CdSe Nanoplatelets

The combination of plasmonic nanoparticles and optical microcavities has attracted broad interest for both fundamental and applied studies. However, the conventional scheme of plasmonic nanoparticles being located at microcavity outer surfaces suffers from serious problems such as significant radiative/scattering losses and chemical/mechanical instabilities. Here, silver nanoparticles (NPs) and dispersed ions embedded in nanomembrane-formed whispering-gallery-mode (WGM) microtube cavities are prepared by ion implantations as compact and stable optoplasmonic microcavities. Upon low ion fluence implantation, dispersed silver ions are generated in the tube cavity wall, leading to a redshift of the WGM resonant cavity modes due to the increased refractive index. The silver ions start to aggregate into plasmonic NPs in the cavity wall when increasing implantation ion fluences. The competition and transition between redshift induced by the refractive index increase and blueshift induced by the formation of plasmonic NPs are investigated. Moreover, quality factor enhancement of the WGM modes is observed owing to the improved light confinement caused by the presence of NPs. This work demonstrates a convenient approach for the fabrication of stable optoplasmonic microcavities and fine tuning of resonant modes, indicating wide applications such as wavelength selective tuning and enhanced light–matter interactions.

Data-Driven Design of Two-Dimensional Non-van der Waals Materials and Cation Exchange in CdSe Nanoplatelets

Monolayer two-dimensional (2D) materials have been regarded as a hot topic in the fields of condensed matter physics, materials science, and chemistry due to their unique physical, chemical, and electronic properties. However, the research on the preparation method and properties understanding of the 2D monolayer are inadequate. In this dissertation, taking 2D nickel-iron layered double hydroxides (NiFe LDHs) and molybdenum disulfide (MoS2) as examples, the practicability of the direct synthesis of NiFe LDHs monolayer and the thermal enhancement catalytic performance of 2D MoS2 monolayer (MoS2 ML) are discussed. First, a one-pot synthetic strategy (bottom-up method) is presented to synthesize 2D NiFe-based LDHs monolayers, including NiFe, Co-, Ru-, doped, and Au-modified NiFe LDHs. The prerequisite and universality of this strategy are investigated and confirmed. The features of LDHs are characterized by advanced technologies. The obtained LDH bulks own a large interlayer spacing up to 8.2 Å, which can be facilely exfoliated into monolayers in water by hand-shaking within 10 s. As a result, the as-prepared NiFe-based LDH monolayers display a good electrocatalytic oxygen evolution reaction (OER) performance. This facile strategy paves the way for designing easily exfoliated LDHs for highly active catalysts and energy conversion devices based on other monolayer LDHs. Second, with gold-modified tape, 2D MoS2 ML is exfoliated from the bulk crystal through a micromechanical exfoliation method (top-down strategy). The thermal effects of MoS2 ML are confirmed by Raman and photoluminescence (PL) spectra. Moreover, an on-chip MoS2 ML hydrogen evolution reaction (HER) reactor is designed and fabricated. The thermal effects generate efficient electron transfer in the MoS2 ML and at the electrolyte-catalyst (MoS2 ML) interface, leading to an enhanced HER performance. Compared to the results obtained at room temperature, the MoS2 ML shows a direct thermal enhanced HER performanc

Electronic topological states of matter exhibit novel types of responses to electromagnetic fields. The response of strong topological insulators, for instance, is characterized by a so-called axion term in the electromagnetic Lagrangian which is ultimately due to the presence of topological surface states. Here we develop the axion Mie theory for the electromagnetic response of spherical particles including arbitrary sources of fields, i.e., charge and current distributions. We derive an axion induced mixing of transverse magnetic and transverse electric modes which are experimentally detectable through small induced rotations of the field vectors. Our results extend upon previous analyses of the problem. Our main focus is on the experimentally relevant problem of electron energy loss spectroscopy in topological insulators, a technique that has so far not yet been used to detect the axion electromagnetic response in these materials., Comment: Submission to SciPost

The reduced dimensionality in two-dimensional materials leads to a wealth of unusual properties, which are currently explored for both fundamental and applied sciences. In order to study the crystal structure, edge states, the formation of defects and grain boundaries, or the impact of adsorbates, high-resolution microscopy techniques are indispensable. Here we report on the development of an electron holography (EH) transmission electron microscopy (TEM) technique, which facilitates high spatial resolution by an automatic correction of geometric aberrations. Distinguished features of EH beyond conventional TEM imaging are gap-free spatial information signal transfer and higher dose efficiency for certain spatial frequency bands as well as direct access to the projected electrostatic potential of the two-dimensional material. We demonstrate these features with the example of h-BN, for which we measure the electrostatic potential as a function of layer number down to the monolayer limit and obtain evidence for a systematic increase of the potential at the zig-zag edges.

Utilizing the Pauli equation based multislice method, introduced in a previous paper [A. Edstrom, A. Lubk, and J. Rusz, Phys. Rev. Lett. 116, 127203 (2016)], we study the atomic-resolution differential phase contrast (DPC) imaging on an example of a hard magnet FePt with in-plane magnetization. Simulated center-of-mass pattern in a scanning transmission electron microscopy experiment carries information about both electric and magnetic fields. The momentum transfer remains curl free, which has consequences for interpretation of the integrated DPC technique. The extracted magnetic component of the pattern is compared to the expected projected microscopic magnetic field as obtained by density functional theory calculation. Qualitative agreement is obtained for low sample thicknesses and a suitable range of collection angles.

Inspired by chains of ferrimagnetic nanocrystals (NCs) in magnetotactic bacteria (MTB), the synthesis and detailed characterization of ferrimagnetic magnetite NC chain-like assemblies is reported. An easy green synthesis route in a thermoreversible gelatin hydrogel matrix is used. The structure of these magnetite chains prepared with and without gelatin is characterized by means of transmission electron microscopy, including electron tomography (ET). These structures indeed bear resemblance to the magnetite assemblies found in MTB, known for their mechanical flexibility and outstanding magnetic properties and known to crystallographically align their magnetite NCs along the strongest <111> magnetization easy axis. Using electron holography (EH) and angular dependent magnetic measurements, the magnetic interaction between the NCs and the generation of a magnetically anisotropic material can be shown. The electro- and magnetostatic modeling demonstrates that in order to precisely determine the magnetization (by means of EH) inside chain-like NCs assemblies, their exact shape, arrangement and stray-fields have to be considered (ideally obtained using ET).

The complex interplay of elastic and inelastic scattering amenable to different levels of approximation constitutes the major challenge for the computation and hence interpretation of TEM-based spectroscopical methods. The two major approaches to calculate inelastic scattering cross sections of fast electrons on crystals—Yoshioka-equations-based forward propagation and the reciprocal wave method—are founded in two conceptually differing schemes—a numerical forward integration of each inelastically scattered wave function, yielding the exit density matrix, and a computation of inelastic scattering matrix elements using elastically scattered initial and final states (double channeling). Here, we compare both approaches and show that the latter is computationally competitive to the former by exploiting analytical integration schemes over multiple excited states. Moreover, we show how to include full nonlocality of the inelastic scattering event, neglected in the forward propagation approaches, at no additional computing costs in the reciprocal wave method. Detailed simulations show in some cases significant errors due to the z-locality approximation and hence pitfalls in the interpretation of spectroscopical TEM results.

Off-axis electron holographic tomography (EHT) has been successfully applied to reveal the 3D structure of III-V semiconductor core-shell nanowires (NWs) [1,2]. The technique probes the phase shift of an electron wave transmitted through such a NW that is proportional to the NWs projected electrostatic potential. Thus, a tilt series of phase images (projected potentials) can be used as input to compute a 3D tomogram of the electrostatic potential by tomographic reconstruction algorithms. Typically, the recovered 3D potential is dominated by the mean inner potential (MIP), which is related to the materials composition. Consequently, space charge potentials determining for example the electric properties, e.g., at interfaces or pn-junctions in semiconductors [2] may be superimposed by MIP variations caused by compositional changes within the heterostructures. Here, we show on the example of a GaAs/AlGaAs core-multishell NW, how the space charge potentials can be uncovered from materials contrast (MIP) by determining the latter independently: To this end, high-angle annular dark-field (HAADF) STEM tomography was applied in addition to EHT on the same NW. STEM tomograms provide solely materials contrast that depends exponentially on the atomic number. Fig. 1 compares both methods in terms of the relation between reconstructed signal and projected property, exemplary for three different tomogram regions identified as pure Au, GaAs and AlGaAs: In case of EHT between the reconstructed potential and the MIP, and in case of STEM tomography between the reconstructed intensity and the atomic number. The latter relation enables converting the STEM tomogram in units of (mean) atomic numbers. Tilt series were acquired from -70° to +71° with 3° tilt steps in holography mode, and from -68° to +68° with 2° tilt steps in STEM mode. Since phase images of axially scattered electrons are used for EHT, it suffers much more from diffraction contrast than STEM tomography (high-angle scatte

Off-axis electron holographic tomography (EHT) has been successfully applied to reveal the 3D structure of III-V semiconductor core-shell nanowires (NWs) [1,2]. The technique probes the phase shift of an electron wave transmitted through such a NW that is proportional to the NWs projected electrostatic potential. Thus, a tilt series of phase images (projected potentials) can be used as input to compute a 3D tomogram of the electrostatic potential by tomographic reconstruction algorithms. Typically, the recovered 3D potential is dominated by the mean inner potential (MIP), which is related to the materials composition. Consequently, space charge potentials determining for example the electric properties, e.g., at interfaces or pn-junctions in semiconductors [2] may be superimposed by MIP variations caused by compositional changes within the heterostructures. Here, we show on the example of a GaAs/AlGaAs core-multishell NW, how the space charge potentials can be uncovered from materials contrast (MIP) by determining the latter independently: To this end, high-angle annular dark-field (HAADF) STEM tomography was applied in addition to EHT on the same NW. STEM tomograms provide solely materials contrast that depends exponentially on the atomic number. Fig. 1 compares both methods in terms of the relation between reconstructed signal and projected property, exemplary for three different tomogram regions identified as pure Au, GaAs and AlGaAs: In case of EHT between the reconstructed potential and the MIP, and in case of STEM tomography between the reconstructed intensity and the atomic number. The latter relation enables converting the STEM tomogram in units of (mean) atomic numbers. Tilt series were acquired from -70° to +71° with 3° tilt steps in holography mode, and from -68° to +68° with 2° tilt steps in STEM mode. Since phase images of axially scattered electrons are used for EHT, it suffers much more from diffraction contrast than STEM tomography (high-angle scatte

Off-axis electron holographic tomography (EHT) has been successfully applied to reveal the 3D structure of III-V semiconductor core-shell nanowires (NWs) [1,2]. The technique probes the phase shift of an electron wave transmitted through such a NW that is proportional to the NWs projected electrostatic potential. Thus, a tilt series of phase images (projected potentials) can be used as input to compute a 3D tomogram of the electrostatic potential by tomographic reconstruction algorithms. Typically, the recovered 3D potential is dominated by the mean inner potential (MIP), which is related to the materials composition. Consequently, space charge potentials determining for example the electric properties, e.g., at interfaces or pn-junctions in semiconductors [2] may be superimposed by MIP variations caused by compositional changes within the heterostructures. Here, we show on the example of a GaAs/AlGaAs core-multishell NW, how the space charge potentials can be uncovered from materials contrast (MIP) by determining the latter independently: To this end, high-angle annular dark-field (HAADF) STEM tomography was applied in addition to EHT on the same NW. STEM tomograms provide solely materials contrast that depends exponentially on the atomic number. Fig. 1 compares both methods in terms of the relation between reconstructed signal and projected property, exemplary for three different tomogram regions identified as pure Au, GaAs and AlGaAs: In case of EHT between the reconstructed potential and the MIP, and in case of STEM tomography between the reconstructed intensity and the atomic number. The latter relation enables converting the STEM tomogram in units of (mean) atomic numbers. Tilt series were acquired from -70° to +71° with 3° tilt steps in holography mode, and from -68° to +68° with 2° tilt steps in STEM mode. Since phase images of axially scattered electrons are used for EHT, it suffers much more from diffraction contrast than STEM tomography (high-angle scatte

Off-axis electron holographic tomography (EHT) has been successfully applied to reveal the 3D structure of III-V semiconductor core-shell nanowires (NWs) [1,2]. The technique probes the phase shift of an electron wave transmitted through such a NW that is proportional to the NWs projected electrostatic potential. Thus, a tilt series of phase images (projected potentials) can be used as input to compute a 3D tomogram of the electrostatic potential by tomographic reconstruction algorithms. Typically, the recovered 3D potential is dominated by the mean inner potential (MIP), which is related to the materials composition. Consequently, space charge potentials determining for example the electric properties, e.g., at interfaces or pn-junctions in semiconductors [2] may be superimposed by MIP variations caused by compositional changes within the heterostructures. Here, we show on the example of a GaAs/AlGaAs core-multishell NW, how the space charge potentials can be uncovered from materials contrast (MIP) by determining the latter independently: To this end, high-angle annular dark-field (HAADF) STEM tomography was applied in addition to EHT on the same NW. STEM tomograms provide solely materials contrast that depends exponentially on the atomic number. Fig. 1 compares both methods in terms of the relation between reconstructed signal and projected property, exemplary for three different tomogram regions identified as pure Au, GaAs and AlGaAs: In case of EHT between the reconstructed potential and the MIP, and in case of STEM tomography between the reconstructed intensity and the atomic number. The latter relation enables converting the STEM tomogram in units of (mean) atomic numbers. Tilt series were acquired from -70° to +71° with 3° tilt steps in holography mode, and from -68° to +68° with 2° tilt steps in STEM mode. Since phase images of axially scattered electrons are used for EHT, it suffers much more from diffraction contrast than STEM tomography (high-angle scatte

Off-Axis Electron Holography permits the direct reconstruction of amplitude and phase of electron waves elastically scattered by an object (see, e.g., [1]). The technique employs the Möllenstedt biprism to mutually incline an object modulat-ed wave and a plane reference wave to form an interference pattern at the detec-tor plane. Limited coherence of the electron beam in presence of aberrations at-tenuates high spatial frequencies of the object exit wave spectrum, which is de-scribed by the sideband envelope function. We explore an extension of the con-ventional electron holography set-up given by deliberately tilting the reference wave independent from the object wave. This allows the transfer of spatial fre-quencies beyond the conventional sideband information limit as predicted by a generalized transfer theory for Off-Axis Electron Holography [2]. This is based on the idea that a reference wave tilted by q0 compensates the wave aberration for the spatial frequency q0 of the object wave spectrum. Thus, an off-axis hologram series with varying reference wave tilt allows in principle a linear synthesis of an effective coherent aperture with a radius reaching out beyond the conventional information limit. Furthermore, an object-independent measurement of aberra-tions as well as strain measurements by dark-field electron holography can be realized using this setup. The experimental realization of an arbitrarily tilted refer-ence wave is challenging and could be realized for the first time at the Hitachi HF3300C I2TEM at CEMES Toulouse for one direction [3]. We used an additional biprism placed in the illumination system. Three condenser lenses were adjusted to provide a demagnified image of the condenser biprism at the sample plane under parallel illumination. The pre-specimen deflectors were adapted to maintain the incident wave vector of the object wave and to realize a tilt of the reference wave as a function of the condenser biprism voltage. Finally, we have exper

Elastic scattering of electron vortex beams on magnetic materials leads to a weak magnetic contrast due to Zeeman interaction of orbital angular momentum of the beam with magnetic fields in the sample. The magnetic signal manifests itself as a redistribution of intensity in diffraction patterns due to a change of sign of the orbital angular momentum of the electron vortex beam. While in the atomic resolution regime the magnetic signal is most likely under the detection limits of present transmission electron microscopes, for electron probes with high orbital angular momenta, and correspondingly larger spatial extent, its detection is predicted to be feasible.

Motivated by a recent claim [Phys.Rev.Lett. 116, 127203 ( 2016)] that electron vortex beams can be used to image magnetism at the nanoscale in elastic scattering experiments, using transmission electron microscopy, a comprehensive computational study is performed to study magnetic effects in the paraxial regime of elastic electron scattering in magnetic solids. Magnetic interactions from electron vortex beams, spin polarized electron beams, and beams with phase aberrations are considered, as they pass through ferromagnetic FePt or antiferromagnetic LaMnAsO. The magnetic signals are obtained by comparing the intensity over a disk in the diffraction plane for beams with opposite angular momentum or aberrations. The strongest magnetic signals are obtained from vortex beams with large orbital angular momentum, where relative magnetic signals above 10(-3) are indicated for 10 (h) over bar orbital angular momentum, meaning that relative signals of one percent could be expected with the even larger orbital angular momenta, which have been produced in experimental setups. All results indicate that beams with low acceleration voltage and small convergence angles yield stronger magnetic signals, which is unfortunately problematic for the possibility of high spatial resolution imaging. Nevertheless, under atomic resolution conditions, relative magnetic signals in the order of 10(-4) are demonstrated, corresponding to an increase with one order of magnitude compared to previous work.

Focal series wave reconstruction in the Transmission Electron Microscope (TEM) is a well established holographic technique employed in both the medium and atomic resolution regime to study electric, magnetic and strain fields in solids as well as atomic configurations at crystal defects or grain boundaries. Focal series reconstruction does not require an undisturbed reference wave as off-axis holography and may be conducted under relaxed partial coherence provided that the latter is well-behaved and well-known in advance [1]. Moreover, focal series holography may be considered as an instance of the more general quantum state tomography (see Fig. 1) that is successfully employed to study mixed (i.e., incoherent) quantum states of matter (e.g., atoms) and light [2]. These advantages are opposed by ambiguities in the reconstructed wave function, e.g., due to inconsistent and incomplete focal series data. In reality every focal series is inconsistent, e.g., due to the presence of partial coherence, shot and detector noise, as well as geometric and chromatic aberrations depending on the defocus. Similarly, every focal series is incomplete because of a limited number of foci, typically limited to the near field regime, and the restriction to isotropic foci, where astigmatic foci are necessary to provide a dataset allowing for an unabiguous reconstruction of an underlying wave function [3]. For instance, the problematic reconstruction of low spatial frequencies can be traced back to missing focal series data in the far field. Here, we elaborate on focal series reconstruction from the perspective of quantum state tomography and use the obtained results to increase the scope of the technique in terms of convergence and uniqueness in particular for low spatial frequencies. Moreover, we explain a number of previous results by exploiting the above analogy, and open pathways to further improvements. We particularly report on the recording, preprocessing, calibration and reconstr

Off-Axis Electron Holography permits the direct reconstruction of amplitude and phase of electron waves elastically scattered by an object (see, e.g., [1]). The technique employs the Möllenstedt biprism to mutually incline an object modulated wave and a plane reference wave to form an interference pattern at the detector plane. Limited coherence of the electron beam in presence of aberrations attenuates high spatial frequencies of the object exit wave spectrum, which is illustrated by the sideband envelope function for a non-corrected TEM in Fig. 1a. In this work, we explore an extension of the conventional setup given by deliberately tilting the reference wave independent from the object wave. This allows the transfer of spatial frequencies beyond the conventional sideband information limit in Fig. 1a as predicted by a generalized transfer theory for Off-Axis Electron Holography [2]. This is because a reference wave tilted by q0 compensates the berration impact on the spatial frequency q0 of the object wave spectrum. The resulting transfer envelope for a tilt of q0x = -10/nm perpendicular to the post-specimen biprism is shown in Fig. 1b, where the contrast maximum of the total envelope (TCC) is located at q0x. Thus, an off-axis hologram series with varying reference wave tilt allows in principle a linear synthesis of an effective coherent aperture with a radius reaching out beyond the conventional information limit. Furthermore, an object-independent measurement of aberrations as well as dark-field electron holography can be realized using this setup. The experimental realization of an arbitrarily tilted reference wave is challenging and could be realized for the first time at the Hitachi HF3300C I2TEM at CEMES Toulouse for one direction [3]. We used an additional biprism placed in the illumination system. Three condenser lenses were adjusted to provide a demagnified image of the condenser biprism at the sample plane under parallel illumination (Fig. 2). The pre-sp

Off-axis electron holography (EH) is a TEM technique that records the phase information of an electron wave transmitted through a thin specimen in an electron hologram. By reconstructing this phase information, it enables electrostatic and magnetic potentials to be mapped quantitatively with high spatial resolution and, when combined with tomography to electron holographic tomography (EHT), in three dimensions (3D) [1,2]. Tomograms obtained by EHT provide the 3D mean inner potential (MIP) distribution of nanoscale materials from which the 3D morphology and the chemical composition can be inferred [3]. Moreover, functional potentials, e.g., introduced by doping of impurities in semiconductors, have been successfully revealed in 3D [4]. Recently, we succeeded in the 3D reconstruction of the axial component of the B-field prevailing in magnetic nanowires [5,6]. EHT as applied on magnetic samples proceeds as follows (see Fig. 1): (1) an electron hologram tilt series (ideally covering a range of 360°) is acquired, (2) the phase image tilt series is reconstructed from the holograms, (3) electric and magnetic phase shifts are separated by computing half of the sum/difference between opposite (180° tilted) projections, and (4) both the electric potential and the B-field component parallel to the tilt axis are reconstructed with tomographic techniques. Here, we report EHT studies achieved by means of tomography-dedicated TEM sample holders, in combination with advanced in-house developed software packages for acquisition, alignment and tomographic reconstruction. Fig. 2 shows the 3D electric potential reconstruction of a GaAs/AlGaAs core-multishell nanowire (NW) grown by metalorganic vapour phase epitaxy (MOVPE) using an Au nanoparticle (NP) as catalyst. Such NWs may serve as novel coherent nanoscale light sources (lasers), because they provide an effective gain medium, low-loss optical waveguiding, and strong optical confinement for axially guided optical modes. The differe

Off-Axis Electron Holography permits the direct reconstruction of amplitude and phase of electron waves elastically scattered by an object (see, e.g., [1]). The technique employs the Möllenstedt biprism to mutually incline an object modulated wave and a plane reference wave to form an interference pattern at the detector plane. Limited coherence of the electron beam in presence of aberrations attenuates high spatial frequencies of the object exit wave spectrum, which is illustrated by the sideband envelope function for a non-corrected TEM in Fig. 1a. In this work, we explore an extension of the conventional setup given by deliberately tilting the reference wave independent from the object wave. This allows the transfer of spatial frequencies beyond the conventional sideband information limit in Fig. 1a as predicted by a generalized transfer theory for Off-Axis Electron Holography [2]. This is because a reference wave tilted by q0 compensates the berration impact on the spatial frequency q0 of the object wave spectrum. The resulting transfer envelope for a tilt of q0x = -10/nm perpendicular to the post-specimen biprism is shown in Fig. 1b, where the contrast maximum of the total envelope (TCC) is located at q0x. Thus, an off-axis hologram series with varying reference wave tilt allows in principle a linear synthesis of an effective coherent aperture with a radius reaching out beyond the conventional information limit. Furthermore, an object-independent measurement of aberrations as well as dark-field electron holography can be realized using this setup. The experimental realization of an arbitrarily tilted reference wave is challenging and could be realized for the first time at the Hitachi HF3300C I2TEM at CEMES Toulouse for one direction [3]. We used an additional biprism placed in the illumination system. Three condenser lenses were adjusted to provide a demagnified image of the condenser biprism at the sample plane under parallel illumination (Fig. 2). The pre-sp

Focal series wave reconstruction in the Transmission Electron Microscope (TEM) is a well established holographic technique employed in both the medium and atomic resolution regime to study electric, magnetic and strain fields in solids as well as atomic configurations at crystal defects or grain boundaries. Focal series reconstruction does not require an undisturbed reference wave as off-axis holography and may be conducted under relaxed partial coherence provided that the latter is well-behaved and well-known in advance [1]. Moreover, focal series holography may be considered as an instance of the more general quantum state tomography (see Fig. 1) that is successfully employed to study mixed (i.e., incoherent) quantum states of matter (e.g., atoms) and light [2]. These advantages are opposed by ambiguities in the reconstructed wave function, e.g., due to inconsistent and incomplete focal series data. In reality every focal series is inconsistent, e.g., due to the presence of partial coherence, shot and detector noise, as well as geometric and chromatic aberrations depending on the defocus. Similarly, every focal series is incomplete because of a limited number of foci, typically limited to the near field regime, and the restriction to isotropic foci, where astigmatic foci are necessary to provide a dataset allowing for an unabiguous reconstruction of an underlying wave function [3]. For instance, the problematic reconstruction of low spatial frequencies can be traced back to missing focal series data in the far field. Here, we elaborate on focal series reconstruction from the perspective of quantum state tomography and use the obtained results to increase the scope of the technique in terms of convergence and uniqueness in particular for low spatial frequencies. Moreover, we explain a number of previous results by exploiting the above analogy, and open pathways to further improvements. We particularly report on the recording, preprocessing, calibration and reconstr

Dynamical scattering theory describes the dominant scattering process of beam electrons at targets in the transmission electron microscope (TEM). Hence, practically every quantitative TEM study has to consider its ramifications, typically by some approximate modeling. Here, we elaborate on a hierarchy within the various approximations focusing on the two principal approaches used in practice, Bloch wave and multislice. We reveal characteristic differences in the capability of these methods to reproduce the correct local propagation of the wave function, while convergent results are obtained over larger propagation distances. We investigate the dependency of local variations of the wave function on the atomic number of the atomic scatterers and discuss their significance for, e.g., inelastic scattering.

Seminarvortrag zu: - Switching – Electrostatic field at interfaces - Strontium Titanate & Defects - Aluminium Oxide Boundaries - Quantification by DFT – EELS – HRTEM

A novel generalized linear transfer theory describing the signal and noise transfer in image detectors has been developed in Part I (Niermann, this issue, [1]) of this paper. Similar to the existing notion of a point spread function (PSF) describing the transfer of the first statistical moment (the average), a noise spread function (NSF) was introduced to characterize the spatially resolved transfer of noise (central second moment, covariance). Following the theoretic results developed in Part I (Niermann, this issue, [1]), a new experimental method based on single spot illumination has been developed and applied to measure 2D point and 4D noise spread functions of CCD cameras used in TEM. A dedicated oversampling method has been used to suppress aliasing in the measured quantities. We analyze the 4D noise spread with respect to electronic and photonic noise contributions.

Transition metal oxides exhibit a wealth of physical phenomena, among them ferroic properties such as ferroelasticity, ferroelectricity and ferromagnetism, or their combination in multiferroics. In addition, transition metal oxides are sensitive to the chemical environment via the external partial pressure of oxygen; changes induce stoichiometry deviations, which cause conductivity changes and modify the ferroic characteristics. The present study focuses on SrTiO3, YMnFeO5, and BiFeO3 and correlates local changes due to point and planar defects with changes of the elastic, polarization and magnetic properties. The microscopic interactions are determined by density functional calculations, which yield the basis for more large-scale simulations with effective Hamiltonian approaches. Under oxygen-poor conditions oxygen vacancies in SrTiO3 accumulate in an external electric field and reduce the hardness. In an Sr/O-rich environment the phases SrO(SrTiO3)n are formed, which yield a distinct change of the X-Ray reflectivity due to the regular arrangement of extrinsic SrO(001) stacking faults. YMn2O5 has a series of complex antiferromagnetic phases in coexistence with ferroelectricity. In YFeMnO5, only one commensurable ferrimagnetic phase was found and ferroelectricity is absent. Based on spin-polarized DFT calculations a Heisenberg model yields the coupling constants of the Fe-substituted and the mangenese-only compounds and relates them to crystal-field interactions. BiFeO3 is a rhombohedral multiferroic with several domain wall configurations. Among them, the 109 and 180 walls have a significant change in the component of their polarization perpendicular to the wall; the corresponding step in the electrostatic potential is consistent with a recent report of electrical conductivity at the domain walls. Changes in the Fe-O-Fe bond angles at the walls change the canting of the Fe magnetic moments which can enhance the local magnetization.

A particularly interesting class of modern materials is ferroic ceramics. Their characteristic order parameter is a result of quantum chemistry taking place on a sub-Å length scale and long-range couplings, e.g. mediated by electrostatic or stress fields. Furthermore, the particular subclass of multiferroics possesses more than one order parameter and exhibits an intriguing coupling between them, which is interesting both from the fundamental physics point of view as well as from a technological vantage point. While on a more fundamental level it is desirable to elucidate the physical details of the coupling mechanism, this knowledge could subsequently lead to new and technologically interesting multiferroic materials, which overcome their current drawback that only one of the multiple order parameters is appreciably large while the others stay small. Due to the short and long range nature of the driving forces, one challenge for thoroughly understanding ferroic ceramics is the characterization of material properties within a large interval of length scales from several tens of µm to sub-Å. To that end, it is useful to exploit that all order parameters can be described as macroscopic fields, e.g. electric polarization or strain, which, in turn, can be either directly or indirectly probed with an electron beam such as used in Transmission Electron Microscopy (TEM). Consequently, TEM is excellently suited for investigating ferroic materials, i.e., state-of-the-art instruments facilitate aberration corrected imaging within a large magnification interval covering the length scales of interest, in particular the atomic regime. A general drawback of conventional TEM techniques is the loss of phase information originally contained in the scattered electron wave introduced by recording only the electron density. Electron Holography is an advanced TEM technique that facilitates the complete evaluation of the complex electron wave, which, in combination with the manifold poss, Ferroische Keramiken bilden eine besonders interessante Klasse moderner funktionaler Werkstoffe. Ihr charakteristischer Ordnungsparameter ist das Ergebnis quantenchemischer Prozesse innerhalb einer sub- Å Längenskala und spezifischer langreichweitiger Kopplungen, welche beispielsweise durch elektromagnetische oder Spannungsfelder vermittelt werden. Des Weiteren besitzt die besondere Unterklasse der Multiferroika mehr als einen Ordnungsparameter und zeigt eine faszinierende Kopplung zwischen ihnen, was sowohl vom Standpunkt physikalischer Grundlagenforschung als auch aus technologischer Sicht von Interesse ist. Während es vom fundamentalen Standpunkt erstrebenswert ist, die physikalischen Details des Kopplungsmechanismus aufzuklären, könnte in der Folge dieses Wissen zu neuen und technologisch interessanten multiferroischen Materialien führen, welche den derzeit bestehenden Nachteil, dass nur ein Ordnungsparameter genügend groß ist, während die jeweils anderen klein bleiben, hinter sich lassen. Aufgrund der kurz- und langreichweitigen Natur der Antriebskräfte besteht eine Herausforderung für das umfassende Verständnis ferroischer Keramiken aus der Charakterisierung von Materialeigenschaften innerhalb eines breiten Intervalls von Längenskalen, welches von einigen 10 µm bis unterhalb eines Å reicht. Um dieses Ziel zu erreichen ist es zweckmäßig auszunutzen, dass alle Ordnungsparameter als makroskopische, beispielsweise elektrostatische oder Verzerrungs-, Felder beschrieben werden können, welche wiederum direkt oder indirekt mit einem Elektronenstrahl, wie er im Transmissionselektronenmikrokop (TEM) zur Anwendung kommt, gemessen werden können. Folglich ist die Transmissionselektronenmikroskopie hervorragend geeignet um ferroische Materialien zu untersuchen, das heißt, modernste Geräte ermöglichen aberrationskorrigierte Aufnahmen innerhalb eines großen Vergrößerungsbereiches, welche die interessanten Längenskalen und insbesondere den atomaren Bereich abdecken. Ein allgem

A particularly interesting class of modern materials is ferroic ceramics. Their characteristic order parameter is a result of quantum chemistry taking place on a sub-Å length scale and long-range couplings, e.g. mediated by electrostatic or stress fields. Furthermore, the particular subclass of multiferroics possesses more than one order parameter and exhibits an intriguing coupling between them, which is interesting both from the fundamental physics point of view as well as from a technological vantage point. While on a more fundamental level it is desirable to elucidate the physical details of the coupling mechanism, this knowledge could subsequently lead to new and technologically interesting multiferroic materials, which overcome their current drawback that only one of the multiple order parameters is appreciably large while the others stay small. Due to the short and long range nature of the driving forces, one challenge for thoroughly understanding ferroic ceramics is the characterization of material properties within a large interval of length scales from several tens of µm to sub-Å. To that end, it is useful to exploit that all order parameters can be described as macroscopic fields, e.g. electric polarization or strain, which, in turn, can be either directly or indirectly probed with an electron beam such as used in Transmission Electron Microscopy (TEM). Consequently, TEM is excellently suited for investigating ferroic materials, i.e., state-of-the-art instruments facilitate aberration corrected imaging within a large magnification interval covering the length scales of interest, in particular the atomic regime. A general drawback of conventional TEM techniques is the loss of phase information originally contained in the scattered electron wave introduced by recording only the electron density. Electron Holography is an advanced TEM technique that facilitates the complete evaluation of the complex electron wave, which, in combination with the manifold poss, Ferroische Keramiken bilden eine besonders interessante Klasse moderner funktionaler Werkstoffe. Ihr charakteristischer Ordnungsparameter ist das Ergebnis quantenchemischer Prozesse innerhalb einer sub- Å Längenskala und spezifischer langreichweitiger Kopplungen, welche beispielsweise durch elektromagnetische oder Spannungsfelder vermittelt werden. Des Weiteren besitzt die besondere Unterklasse der Multiferroika mehr als einen Ordnungsparameter und zeigt eine faszinierende Kopplung zwischen ihnen, was sowohl vom Standpunkt physikalischer Grundlagenforschung als auch aus technologischer Sicht von Interesse ist. Während es vom fundamentalen Standpunkt erstrebenswert ist, die physikalischen Details des Kopplungsmechanismus aufzuklären, könnte in der Folge dieses Wissen zu neuen und technologisch interessanten multiferroischen Materialien führen, welche den derzeit bestehenden Nachteil, dass nur ein Ordnungsparameter genügend groß ist, während die jeweils anderen klein bleiben, hinter sich lassen. Aufgrund der kurz- und langreichweitigen Natur der Antriebskräfte besteht eine Herausforderung für das umfassende Verständnis ferroischer Keramiken aus der Charakterisierung von Materialeigenschaften innerhalb eines breiten Intervalls von Längenskalen, welches von einigen 10 µm bis unterhalb eines Å reicht. Um dieses Ziel zu erreichen ist es zweckmäßig auszunutzen, dass alle Ordnungsparameter als makroskopische, beispielsweise elektrostatische oder Verzerrungs-, Felder beschrieben werden können, welche wiederum direkt oder indirekt mit einem Elektronenstrahl, wie er im Transmissionselektronenmikrokop (TEM) zur Anwendung kommt, gemessen werden können. Folglich ist die Transmissionselektronenmikroskopie hervorragend geeignet um ferroische Materialien zu untersuchen, das heißt, modernste Geräte ermöglichen aberrationskorrigierte Aufnahmen innerhalb eines großen Vergrößerungsbereiches, welche die interessanten Längenskalen und insbesondere den atomaren Bereich abdecken. Ein allgem

A particularly interesting class of modern materials is ferroic ceramics. Their characteristic order parameter is a result of quantum chemistry taking place on a sub-Å length scale and long-range couplings, e.g. mediated by electrostatic or stress fields. Furthermore, the particular subclass of multiferroics possesses more than one order parameter and exhibits an intriguing coupling between them, which is interesting both from the fundamental physics point of view as well as from a technological vantage point. While on a more fundamental level it is desirable to elucidate the physical details of the coupling mechanism, this knowledge could subsequently lead to new and technologically interesting multiferroic materials, which overcome their current drawback that only one of the multiple order parameters is appreciably large while the others stay small. Due to the short and long range nature of the driving forces, one challenge for thoroughly understanding ferroic ceramics is the characterization of material properties within a large interval of length scales from several tens of µm to sub-Å. To that end, it is useful to exploit that all order parameters can be described as macroscopic fields, e.g. electric polarization or strain, which, in turn, can be either directly or indirectly probed with an electron beam such as used in Transmission Electron Microscopy (TEM). Consequently, TEM is excellently suited for investigating ferroic materials, i.e., state-of-the-art instruments facilitate aberration corrected imaging within a large magnification interval covering the length scales of interest, in particular the atomic regime. A general drawback of conventional TEM techniques is the loss of phase information originally contained in the scattered electron wave introduced by recording only the electron density. Electron Holography is an advanced TEM technique that facilitates the complete evaluation of the complex electron wave, which, in combination with the manifold poss, Ferroische Keramiken bilden eine besonders interessante Klasse moderner funktionaler Werkstoffe. Ihr charakteristischer Ordnungsparameter ist das Ergebnis quantenchemischer Prozesse innerhalb einer sub- Å Längenskala und spezifischer langreichweitiger Kopplungen, welche beispielsweise durch elektromagnetische oder Spannungsfelder vermittelt werden. Des Weiteren besitzt die besondere Unterklasse der Multiferroika mehr als einen Ordnungsparameter und zeigt eine faszinierende Kopplung zwischen ihnen, was sowohl vom Standpunkt physikalischer Grundlagenforschung als auch aus technologischer Sicht von Interesse ist. Während es vom fundamentalen Standpunkt erstrebenswert ist, die physikalischen Details des Kopplungsmechanismus aufzuklären, könnte in der Folge dieses Wissen zu neuen und technologisch interessanten multiferroischen Materialien führen, welche den derzeit bestehenden Nachteil, dass nur ein Ordnungsparameter genügend groß ist, während die jeweils anderen klein bleiben, hinter sich lassen. Aufgrund der kurz- und langreichweitigen Natur der Antriebskräfte besteht eine Herausforderung für das umfassende Verständnis ferroischer Keramiken aus der Charakterisierung von Materialeigenschaften innerhalb eines breiten Intervalls von Längenskalen, welches von einigen 10 µm bis unterhalb eines Å reicht. Um dieses Ziel zu erreichen ist es zweckmäßig auszunutzen, dass alle Ordnungsparameter als makroskopische, beispielsweise elektrostatische oder Verzerrungs-, Felder beschrieben werden können, welche wiederum direkt oder indirekt mit einem Elektronenstrahl, wie er im Transmissionselektronenmikrokop (TEM) zur Anwendung kommt, gemessen werden können. Folglich ist die Transmissionselektronenmikroskopie hervorragend geeignet um ferroische Materialien zu untersuchen, das heißt, modernste Geräte ermöglichen aberrationskorrigierte Aufnahmen innerhalb eines großen Vergrößerungsbereiches, welche die interessanten Längenskalen und insbesondere den atomaren Bereich abdecken. Ein allgem

Transition metal oxides exhibit a wealth of physical phenomena, among them ferroic properties such as ferroelasticity, ferroelectricity and ferromagnetism, or their combination in multiferroics. In addition, transition metal oxides are sensitive to the chemical environment via the external partial pressure of oxygen; changes induce stoichiometry deviations, which cause conductivity changes and modify the ferroic characteristics. The present study focuses on SrTiO3, YMnFeO5, and BiFeO3 and correlates local changes due to point and planar defects with changes of the elastic, polarization and magnetic properties. The microscopic interactions are determined by density functional calculations, which yield the basis for more large-scale simulations with effective Hamiltonian approaches. Under oxygen-poor conditions oxygen vacancies in SrTiO3 accumulate in an external electric field and reduce the hardness. In an Sr/O-rich environment the phases SrO(SrTiO3)n are formed, which yield a distinct change of the X-Ray reflectivity due to the regular arrangement of extrinsic SrO(001) stacking faults. YMn2O5 has a series of complex antiferromagnetic phases in coexistence with ferroelectricity. In YFeMnO5, only one commensurable ferrimagnetic phase was found and ferroelectricity is absent. Based on spin-polarized DFT calculations a Heisenberg model yields the coupling constants of the Fe-substituted and the mangenese-only compounds and relates them to crystal-field interactions. BiFeO3 is a rhombohedral multiferroic with several domain wall configurations. Among them, the 109 and 180 walls have a significant change in the component of their polarization perpendicular to the wall; the corresponding step in the electrostatic potential is consistent with a recent report of electrical conductivity at the domain walls. Changes in the Fe-O-Fe bond angles at the walls change the canting of the Fe magnetic moments which can enhance the local magnetization.

We present a first-principles density-functional study of the structural, electronic, and magnetic properties of the ferroelectric domain walls in multiferroic BiFeO3. We find that domain walls in which the rotations of the oxygen octahedra do not change their phase when the polarization reorients are the most favorable and of these, the 109° domain wall centered around the BiO plane has the lowest energy. The 109° and 180° walls have a significant change in the component of their polarization perpendicular to the wall; the corresponding step in the electrostatic potential is consistent with a recent report of electrical conductivity at the domain walls. Finally, we show that changes in the Fe-O-Fe bond angles at the domain walls cause changes in the canting of the Fe magnetic moments which can enhance the local magnetization at the domain walls.

In YMn2O5 the Mn atoms occupy two non-equivalent Wyckoff sites within the unit cell exhibiting different oxygen coordinations, i. e. the system can be characterized as a mixed-valence compound. For the formation of the orthorhombic crystal structure Jahn-Teller distortions are assumed to play an important role. In this study, we aimed at the investigation of the crystal structure on the substitution of Mn by the non-Jahn-Teller cation Fe+3. Therefore, we synthesized a series of YMn(2-x)FexO5 powder samples with x = 0; 0:5; 1 by a citrate technique. We utilized extended x-ray absorption fine structure (EXAFS), x-ray absorption near edge structure (XANES) analysis and Density Functional Theory to investigate the two non-equivalent Wyckoff sites within the orthorhombic crystal structure (confirmed for all compositions) occupied by transition-metal atoms. For quantitative determination of structural short-range order firstly all plausible options of substitution of Fe for Mn were discussed. On basis of these evaluations the EXAFS and XANES behavior was analyzed and appropriate crystallographic weights were assigned to the subset of structural models in accordance with the experimental data. From EXAFS analysis, using multiple scattering theory, we concluded only the 4h Wyckoff site to be occupied by Fe (occupancy refined was (100 +- 3)% in case of x = 1). Furthermore, taking the XANES spectra into account, we could verify the EXAFS results and additionally explain the differences in the Mn-K XANES spectra in dependence on x to be caused by changes of the dipole transitions to 4p final states. Since only one Wyckoff site is involved the experimentally observed limit to a maximum amount of x = 1 is explained. Additionally, a possible disorder, discussed in the literature, was not proven for our samples. With DFT calculations the experimental findings were verified on base of the total energy of the different possible electronic configurations. Crystal field effects were i

We present a first-principles density-functional study of the structural, electronic, and magnetic properties of the ferroelectric domain walls in multiferroic BiFeO3. We find that domain walls in which the rotations of the oxygen octahedra do not change their phase when the polarization reorients are the most favorable and of these, the 109° domain wall centered around the BiO plane has the lowest energy. The 109° and 180° walls have a significant change in the component of their polarization perpendicular to the wall; the corresponding step in the electrostatic potential is consistent with a recent report of electrical conductivity at the domain walls. Finally, we show that changes in the Fe-O-Fe bond angles at the domain walls cause changes in the canting of the Fe magnetic moments which can enhance the local magnetization at the domain walls.