D. Sudfeld

Room-temperature preparation and magnetic behavior of Co2MnSi thin films

Our study presents experimental results on Co2MnSi thin-film preparation and resulting magnetic properties of the Co2MnSi Heusler alloy. The focus of our work is on the important role of the microstructure and the magnetic properties relationships of Co2MnSi thin films prepared using dc magnetron sputtering. We examined the microstructure evolution determined with x-ray diffraction for various substrates, e.g., MgO, SrTiO3, Si and SiO2, at different substrate temperatures. Polycrystalline growth observed at high substrate temperatures is independent of the nature and orientation of the substrate. These films show soft magnetic behavior at a net magnetization of 4.12μB. In contrast, textured growth is obtained at room temperature by introducing a vanadium seed layer. These samples are magnetically harder but possess a magnetization of 0.25μB only. This behavior indicates a two phase film consisting of an amorphous and textured volume. Consequently, sputtering at low argon pressure at high temperature resul...

Microcrystalline silicon formation by silicon nanoparticles

Thin silicon films are of great importance for large-area electronic applications, for example, as the basis for switching electronics in flat-panel display devices or as the active layer of solar cells. In this paper, we show that silicon nanoparticles have the potential to be used as raw material for further processing toward a microcrystalline silicon film. This can be done by thermal treatment with a reduced thermal budget because the melting point of the nanoparticles is much lower with only 60% of the equilibrium melting temperature of silicon. Coagulation processes of liquid droplets then lead to the growth of microcrystalline silicon in agglomerated nanoparticles. We demonstrate by in situ transmission electron microscopy (TEM) and differential thermal analysis that silicon nanoparticles with a size of approximately 20nm start melting at around 1000K; furthermore, the TEM observations directly demonstrate the details of the coagulation process leading to microcrystalline silicon.

Composition control in metal-organic vapor-phase epitaxy grown InGaAs nanowhiskers

InGaAs nanowhiskers were grown by metal-organic vapor-phase epitaxy on (111)B GaAs substrates using the vapor-liquid-solid growth mode. The diameter of nanowhiskers was defined by monodisperse gold nanoparticles deposited on the GaAs substrate from the liquid phase. By adjusting the triethylgallium to trimethylindium flow ratio, InxGa1−xAs whiskers with various compositions were realized. The composition characterization of the grown whiskers was done by high-resolution x-ray diffractometry. A detailed analysis of measured spectra allowed resolving the presence of an InGaAs three-dimensional layer between whiskers. High-resolution transmission electron microscopy investigation revealed the lattice constant of the grown whisker structures, which agrees with the whisker composition defined by x-ray diffractometry. Finally, low-temperature photoluminescence measurements of the realized InGaAs whiskers were carried out.

Gaas whiskers grown by metal-organic vapor-phase epitaxy using Fe nanoparticles

GaAs nanowhiskers were grown by metal-organic vapor-phase epitaxy on (111)B GaAs substrates. The diameter of the nanowhiskers was defined by monodisperse Fe nanoparticles deposited on the GaAs substrate from the vapor phase. The growth temperature of the whiskers was investigated from 480to520°C. The whiskers are preferentially directed along the crystal orientations of ⟨001⟩, ⟨111⟩, and their equivalents. High-resolution transmission electron microscopy characterization including energy disperse x-ray spectroscopy measurements revealed not only iron oxide but also arsenic inside the seed particle at the top of the GaAs whiskers. This indicates that the particle stays at the top during the whisker growth.

Giant magnetoresistance and magnetic aspects in granular structures

Abstract The microstructural influence of binary melt-spun AuCo and pseudo-binary AlNiCo5 permanent magnets in the optimum magnetic state on the giant magnetoresistance (GMR) have been compared to identify the origin of the magnetotransport in these metal/metal granular systems. In general, their high field GMR dependence is originated from small ferromagnetic clusters distributed within the nonmagnetic well-conducting matrix phase. These “heterogeneities” are determined by the corresponding phase diagrams and hence are inherently present. AlNiCo5 as an example further shows that these “heterogeneities” also determine the GMR behavior in systems were the magnetic ordering takes place on a much larger length scale compared to the electron mean free path. The microstructure and magnetic properties of well-ordered metal (Co)/organic granular superlattices are discussed in comparison so as to show their high potential for future studies of granular transport and tunneling properties in these structurally better-defined systems. It is also demonstrated that alloying of (FeCo)/organic granular superlattices is possible on a length scale of a few nanometers only.

Tailoring magnetic nanocrystal superlattices by chemical engineering

Recognizing the technological potential of magnetic nanocrystals for the next generation of high density data storage [S. Sun and C. B. Murray, J. Appl. Phys. 85, 4325 (1999)] and as markers in biological systems for detection and manipulation of attached molecules, [L. Lagae, R. Wirix-Speetjens, J. Das, D. Graham, H. Ferreira, P. P. F. Freitas, G. Borghs, and J. De Boeck, J. Appl. Phys. 91, 7445 (2002)] ligand stabilized nanoparticles prepared via high–temperature solution–phase synthesis [C. B. Murray, S. Sun, W. Gaschler, H. Doyle, T. A. Betley, and C. R. Kagan, IBM J. Res. Dev. 45, 47 (2001); V. F. Puntes, K. M. Krishnan, and A. P. Alivisatos, Science 291, 2115 (2001)] rapidly gain interest in research and development. Following a chemical preparation route recently proposed [V. F. Puntes, K. M. Krishnan, and A. P. Alivisatos, Science 291, 2115 (2001)] Co and FeCo alloyed nanocrystals have been synthesized with different nanocrystal size distributions employing different growth rates during preparatio...

Magnetic cobalt nanocrystals organized in patches and chains

~e~~~~~~~,,j~r~~ l~~ vs. field curve 2 i is given in fig. 2. Under E diffemt preparation conditions up to 3hm long f chains consisting of lonm E

Intra- and interparticle interaction in a dense frozen ferrofluid

The zero field cooled (ZFC) and field cooled (FC) low-field magnetic moment m of a dense frozen ferrofluid containing Fe55Co45 particles of size 4.6nm in hexane exhibits irreversibility at temperatures T < T b≈ 30 K. FC in μ 0 H ≤ 1 T gives rise to shifted minor hysteresis loops below T b. At T c≈ 10 K, sharp peaks of m ZFC and of the ac susceptibility χ ′, a kink of the thermoremanent magnetic moment m TRM, a sizeable reduction of the coercive field H c, and the appearance of a spontaneous moment m SFM indicate a phase transition with near mean-field critical behaviour of both m SFM and χ ′ . These features are explained within a core-shell model of nanoparticles, whose strongly disordered shells gradually become blocked below T b, while their soft ferromagnetic cores couple dipolarly and become superferromagnetic (SFM) below T c.

Single InGaAs nanowhiskers characterized by analytical transmission electron microscopy

Single crystal In x Ga1−x As nanowhisker were grown by metal-organic vapour-phase epitaxy on (111)B GaAs substrates using the vapour–liquid–solid growth mode. Present study is aimed to the direct evaluation of the lattice parameter, the phase, and the composition of nanowhiskers with high spatial resolution by high-resolution transmission electron microscopy and analytical transmission electron microscopy including energy-dispersive X-ray spectroscopy in nano probe mode. The goal is the direct identification of the quantified concentrations and the local element distributions at characteristic whisker areas. In particular, chemical measurements provide insights into the segregation of the gallium and arsenic out of the gold seed particle and the presence of indium remaining in the Au particle of the as-prepared whiskers. For comparison, InGaAs nanowhiskers are characterized by high-resolution X-ray diffractometry providing the mean whisker composition information indirectly by substituting the X-ray data into the formula of the law of Vegard.

New magnetic FeCo nanoparticles for biotechnology

Calculated magnetophoretic mobility of a variety of magnetic compounds has identified FeCo to be an alternative for magnetite and maghemite in vitro biological cell separations. The synthesis of FeCo nanoparticles and the resulting microstructure is discussed as a function of the particle size. Their synthesis kinetics is modeled using a consecutive decomposition and growth model and is compared to experimental data.