Radovan Urban - Academia.edu (original) (raw)

Papers by Radovan Urban

Magnetostatic modes of yttrium iron garnet (YIG) films are investigated by ferromagnetic resonanc... more Magnetostatic modes of yttrium iron garnet (YIG) films are investigated by ferromagnetic resonance force microscopy. A thin-film "probe" magnet at the tip of a compliant cantilever introduces a local inhomogeneity in the internal field of the YIG sample. This influences the shape of the sample's magnetostatic modes, thereby measurably perturbing the strength of the force coupled to the cantilever. We present a theoretical model that explains these observations; it shows that the tip-induced variation of the internal field creates either a local "potential barrier" or "potential well" for the magnetostatic waves. The data and model together indicate that local magnetic imaging of ferromagnets is possible, even in the presence of long-range spin coupling, through the introduction of localized magnetostatic modes predicted to arise from sufficiently strong tip fields.

This thesis reports on electron transport and the magnetization dynamics of crystalline multilaye... more This thesis reports on electron transport and the magnetization dynamics of crystalline multilayers grown on Fe-whiskers(001) and clean GaAs(001) wafers by means of molecular beam epitaxy (MBE). The high quality magnetic multilayers with well defined interfaces are required to allow one to compare quantitatively the experimental results with the theoretical predictions. The electrical properties of crystalline Fe/MgO/Fe-whisker structures were characterized by in-situ scanning tunneling spectroscopy. Far mast of the scanned area, the tunneling I-V characteristics have revealed a tunneling barrier of 3.6 V which corresponds to the perfect MgO layer. At negative bias voltages, the localized spikes in the tunneling current have been observed indicating ballistic transport in crystalline tunnel junctions. Kerr microscopy has shown, that the magnetization of Fe-whisker and Fe film are coupled via stray field of the Fe-whisker domain wall. Atom force microscope (AFM) operating in an exter...

Ultramicroscopy, 2012

Field ion microscopy (FIM) using neon imaging gas was used to evaluate a W(111) nanotip shape dur... more Field ion microscopy (FIM) using neon imaging gas was used to evaluate a W(111) nanotip shape during a nitrogen assisted etching and evaporation process. Using appropriate etching parameters a narrow ring of atoms centered about the tip axis appears in a helium generated image. Etching of tungsten atoms continues exclusively on the outside of this well-defined ring. By replacing helium imaging gas with neon, normally inaccessible crystal structure of a tip apex is revealed. Comparison of the original W(111) tip (before etching) and partly etched tip shows no atomic changes at the tip apex revealing extraordinarily spatially selective etching properties of the etching process. This observation is an important step towards a detailed understanding of the nitrogen assisted etching and evaporation process and will lead to better control over atomically defined tip shapes.

Microscopy and Microanalysis, 2014

In this paper we investigate methods to characterize angular current density from atomically defi... more In this paper we investigate methods to characterize angular current density from atomically defined gas field ion sources. We show that the ion beam emitted from a single apex atom is described by a two-dimensional Gaussian profile. Owing to the Gaussian shape of the beam and the requirement to collect the majority of the ion current, fixed apertures have inhomogeneous illumination. Therefore, angular current density measurements through a fixed aperture record averaged angular current density. This makes comparison of data difficult as averaged angular current density depends on aperture size. For the same reasons, voltage normalization cannot be performed for fixed aperture measurements except for aperture sizes that are infinitely small. Consistent determination of angular current density and voltage normalization, however, can be achieved if the beam diameter as well as total ion current are known. In cases where beam profile cannot be directly imaged with a field ion microscope, the beam profile could be extracted from measurements taken at multiple acceleration voltages and/or with multiple aperture sizes.

Physical Review Letters, 2003

A long-ranged dynamic interaction between ferromagnetic films separated by normal-metal spacers i... more A long-ranged dynamic interaction between ferromagnetic films separated by normal-metal spacers is reported, which is communicated by nonequilibrium spin currents. It is measured by ferromagnetic resonance (FMR) and explained by an adiabatic spin-pump theory. In FMR the spin-pump mechanism of spatially separated magnetic moments leads to an appreciable increase in the FMR line width when the resonance fields are well apart, and results in a dramatic line-width narrowing when the FMR fields approach each other.

New Journal of Physics, 2013

This report demonstrates the most spatially-coherent electron source ever reported. A coherence a... more This report demonstrates the most spatially-coherent electron source ever reported. A coherence angle of 14.3 ±0.5 • was measured, indicating a virtual source size of 1.7 ±0.6 Å using an extraction voltage of 89.5 V. The nanotips under study were crafted using a spatially-confined, field-assisted nitrogen etch which removes material from the periphery of the tip apex resulting in a sharp, tungsten-nitride stabilized, high-aspect ratio source. The coherence properties are deduced from holographic measurements in a low-energy electron point source microscope with a carbon nanotube bundle as sample. Using the virtual source size and emission current the brightness normalized to 100 kV is found to be 7.9×10 8 A/sr cm 2 .

Applied Surface Science, 2014

Experiments aimed at assessing the robustness of nitrogen-etched, single-atom tips (SATs) prepare... more Experiments aimed at assessing the robustness of nitrogen-etched, single-atom tips (SATs) prepared using W(1 1 1) single crystal wire were performed. Our experiments showed that single-atoms tips sustain minimal damage when exposed to atmospheric conditions and can be readily and quickly nitrogen-etched to single-atom tips thereafter. The SATs can be annealed at temperatures up to 1100 • C with minimal shape changes. Moreover, annealing temperatures in excess of 1200 • C resulted in an apex faceting which may prove important in further single-atom tip creation. Procedures for warming of the SATs from operating temperatures of 80 K were also evaluated to determine conditions that limit tip contamination. These results show that SATS could be fabricated in a dedicated vacuum system and subsequently transferred to other instruments where they would undergo a brief conditioning procedure to recover the single-atom apex configuration prior to being subjected to operating conditions. Crown

Applied Physics Letters, 2012

ABSTRACT Tungsten W(111) oriented trimer-terminated tips as well as single atom tips, fabricated ... more ABSTRACT Tungsten W(111) oriented trimer-terminated tips as well as single atom tips, fabricated by a gas and field assisted etching and evaporation process, were investigated with a view to scanning ion microscopy and ion beam writing applications. In particular, ion current stability was studied for helium and neon imaging gases. Large ion current fluctuations from individual atomic sites were observed when a trimer-terminated tip was used for the creation of neon ion beam. However, neon ion current was stable when a single atom tip was employed. No such current oscillations were observed for either a trimer or a single atom tip when imaged with helium.

This work demonstrates the fabrication of a single atom tip electron source with exceptionally hi... more This work demonstrates the fabrication of a single atom tip electron source with exceptionally high spatial coherence. A coherence angle of 14.3 ±0.5 • was measured, indicating a virtual source size of 1.7 ±0.6 Å. The nanotips under study were crafted using a spatially-confined, field-assisted nitrogen etch, which removes material from the periphery of the tip apex, resulting in a sharp, tungsten-nitride stabilized, high-aspect ratio source. The coherence properties are measured in a low-energy electron point source microscope with a carbon nanotube bundle acting as a bi-prism. This work represents a marked improvement in spatial coherence of electron sources. arXiv:1208.2994v2 [cond-mat.mes-hall]

Magnetostatic modes of yttrium iron garnet (YIG) films are investigated by ferromagnetic resonanc... more Magnetostatic modes of yttrium iron garnet (YIG) films are investigated by ferromagnetic resonance force microscopy. A thin-film "probe" magnet at the tip of a compliant cantilever introduces a local inhomogeneity in the internal field of the YIG sample. This influences the shape of the sample's magnetostatic modes, thereby measurably perturbing the strength of the force coupled to the cantilever. We present a theoretical model that explains these observations; it shows that the tip-induced variation of the internal field creates either a local "potential barrier" or "potential well" for the magnetostatic waves. The data and model together indicate that local magnetic imaging of ferromagnets is possible, even in the presence of long-range spin coupling, through the introduction of localized magnetostatic modes predicted to arise from sufficiently strong tip fields.

This thesis reports on electron transport and the magnetization dynamics of crystalline multilaye... more This thesis reports on electron transport and the magnetization dynamics of crystalline multilayers grown on Fe-whiskers(001) and clean GaAs(001) wafers by means of molecular beam epitaxy (MBE). The high quality magnetic multilayers with well defined interfaces are required to allow one to compare quantitatively the experimental results with the theoretical predictions. The electrical properties of crystalline Fe/MgO/Fe-whisker structures were characterized by in-situ scanning tunneling spectroscopy. Far mast of the scanned area, the tunneling I-V characteristics have revealed a tunneling barrier of 3.6 V which corresponds to the perfect MgO layer. At negative bias voltages, the localized spikes in the tunneling current have been observed indicating ballistic transport in crystalline tunnel junctions. Kerr microscopy has shown, that the magnetization of Fe-whisker and Fe film are coupled via stray field of the Fe-whisker domain wall. Atom force microscope (AFM) operating in an exter...

Ultramicroscopy, 2012

Field ion microscopy (FIM) using neon imaging gas was used to evaluate a W(111) nanotip shape dur... more Field ion microscopy (FIM) using neon imaging gas was used to evaluate a W(111) nanotip shape during a nitrogen assisted etching and evaporation process. Using appropriate etching parameters a narrow ring of atoms centered about the tip axis appears in a helium generated image. Etching of tungsten atoms continues exclusively on the outside of this well-defined ring. By replacing helium imaging gas with neon, normally inaccessible crystal structure of a tip apex is revealed. Comparison of the original W(111) tip (before etching) and partly etched tip shows no atomic changes at the tip apex revealing extraordinarily spatially selective etching properties of the etching process. This observation is an important step towards a detailed understanding of the nitrogen assisted etching and evaporation process and will lead to better control over atomically defined tip shapes.

Microscopy and Microanalysis, 2014

In this paper we investigate methods to characterize angular current density from atomically defi... more In this paper we investigate methods to characterize angular current density from atomically defined gas field ion sources. We show that the ion beam emitted from a single apex atom is described by a two-dimensional Gaussian profile. Owing to the Gaussian shape of the beam and the requirement to collect the majority of the ion current, fixed apertures have inhomogeneous illumination. Therefore, angular current density measurements through a fixed aperture record averaged angular current density. This makes comparison of data difficult as averaged angular current density depends on aperture size. For the same reasons, voltage normalization cannot be performed for fixed aperture measurements except for aperture sizes that are infinitely small. Consistent determination of angular current density and voltage normalization, however, can be achieved if the beam diameter as well as total ion current are known. In cases where beam profile cannot be directly imaged with a field ion microscope, the beam profile could be extracted from measurements taken at multiple acceleration voltages and/or with multiple aperture sizes.

Physical Review Letters, 2003

A long-ranged dynamic interaction between ferromagnetic films separated by normal-metal spacers i... more A long-ranged dynamic interaction between ferromagnetic films separated by normal-metal spacers is reported, which is communicated by nonequilibrium spin currents. It is measured by ferromagnetic resonance (FMR) and explained by an adiabatic spin-pump theory. In FMR the spin-pump mechanism of spatially separated magnetic moments leads to an appreciable increase in the FMR line width when the resonance fields are well apart, and results in a dramatic line-width narrowing when the FMR fields approach each other.

New Journal of Physics, 2013

This report demonstrates the most spatially-coherent electron source ever reported. A coherence a... more This report demonstrates the most spatially-coherent electron source ever reported. A coherence angle of 14.3 ±0.5 • was measured, indicating a virtual source size of 1.7 ±0.6 Å using an extraction voltage of 89.5 V. The nanotips under study were crafted using a spatially-confined, field-assisted nitrogen etch which removes material from the periphery of the tip apex resulting in a sharp, tungsten-nitride stabilized, high-aspect ratio source. The coherence properties are deduced from holographic measurements in a low-energy electron point source microscope with a carbon nanotube bundle as sample. Using the virtual source size and emission current the brightness normalized to 100 kV is found to be 7.9×10 8 A/sr cm 2 .

Applied Surface Science, 2014

Experiments aimed at assessing the robustness of nitrogen-etched, single-atom tips (SATs) prepare... more Experiments aimed at assessing the robustness of nitrogen-etched, single-atom tips (SATs) prepared using W(1 1 1) single crystal wire were performed. Our experiments showed that single-atoms tips sustain minimal damage when exposed to atmospheric conditions and can be readily and quickly nitrogen-etched to single-atom tips thereafter. The SATs can be annealed at temperatures up to 1100 • C with minimal shape changes. Moreover, annealing temperatures in excess of 1200 • C resulted in an apex faceting which may prove important in further single-atom tip creation. Procedures for warming of the SATs from operating temperatures of 80 K were also evaluated to determine conditions that limit tip contamination. These results show that SATS could be fabricated in a dedicated vacuum system and subsequently transferred to other instruments where they would undergo a brief conditioning procedure to recover the single-atom apex configuration prior to being subjected to operating conditions. Crown

Applied Physics Letters, 2012

ABSTRACT Tungsten W(111) oriented trimer-terminated tips as well as single atom tips, fabricated ... more ABSTRACT Tungsten W(111) oriented trimer-terminated tips as well as single atom tips, fabricated by a gas and field assisted etching and evaporation process, were investigated with a view to scanning ion microscopy and ion beam writing applications. In particular, ion current stability was studied for helium and neon imaging gases. Large ion current fluctuations from individual atomic sites were observed when a trimer-terminated tip was used for the creation of neon ion beam. However, neon ion current was stable when a single atom tip was employed. No such current oscillations were observed for either a trimer or a single atom tip when imaged with helium.

This work demonstrates the fabrication of a single atom tip electron source with exceptionally hi... more This work demonstrates the fabrication of a single atom tip electron source with exceptionally high spatial coherence. A coherence angle of 14.3 ±0.5 • was measured, indicating a virtual source size of 1.7 ±0.6 Å. The nanotips under study were crafted using a spatially-confined, field-assisted nitrogen etch, which removes material from the periphery of the tip apex, resulting in a sharp, tungsten-nitride stabilized, high-aspect ratio source. The coherence properties are measured in a low-energy electron point source microscope with a carbon nanotube bundle acting as a bi-prism. This work represents a marked improvement in spatial coherence of electron sources. arXiv:1208.2994v2 [cond-mat.mes-hall]