L.T. Baczewski

ZnTe nanowires grown on GaAs(100) substrates by molecular beam epitaxy

ZnTe nanowires with an average diameter of about 30nm and lengths above 1μm were grown on GaAs(100) substrate by molecular beam epitaxy. The growth process was based on the Au-catalyzed vapor-liquid-solid mechanism. A thin gold layer (3–20A thick) annealed in high vacuum prior to the nanowire growth was used as a source of catalytic nanoparticles. The nanowires are inclined about 55° to the (100) substrate surface normal. They have a zinc-blende crystal structure and their growth axis is ⟨111⟩.

Local light-induced magnetization using nanodots and chiral molecules.

With the increasing demand for miniaturization, nanostructures are likely to become the primary components of future integrated circuits. Different approaches are being pursued toward achieving efficient electronics, among which are spin electronics devices (spintronics). In principle, the application of spintronics should result in reducing the power consumption of electronic devices. Recently a new, promising, effective approach for spintronics has emerged, using spin selectivity in electron transport through chiral molecules. In this work, using chiral molecules and nanocrystals, we achieve local spin-based magnetization generated optically at ambient temperatures. Through the chiral layer, a spin torque can be transferred without permanent charge transfer from the nanocrystals to a thin ferromagnetic layer, creating local perpendicular magnetization. We used Hall sensor configuration and atomic force microscopy (AFM) to measure the induced local magnetization. At low temperatures, anomalous spin Hall effects were measured using a thin Ni layer. The results may lead to optically controlled spintronics logic devices that will enable low power consumption, high density, and cheap fabrication.

Giant Spin Splitting in Optically Active ZnMnTe/ZnMgTe Core/Shell Nanowires

An enhancement of the Zeeman splitting as a result of the incorporation of paramagnetic Mn ions in ZnMnTe/ZnMgTe core/shell nanowires is reported. The studied structures are grown by gold-catalyst assisted molecular beam epitaxy. The near band edge emission of these structures, conspicuously absent in the case of uncoated ZnMnTe nanowires, is activated by the presence of ZnMgTe coating. Giant Zeeman splitting of this emission is studied in ensembles of nanowires with various average Mn concentrations of the order of a few percent, as well as in individual nanowires. Thus, we show convincingly that a strong spin sp-d coupling is indeed present in these structures.

Zn1−xMnxTe Diluted Magnetic Semiconductor Nanowires Grown by Molecular Beam Epitaxy

It is shown that the growth of II-VI diluted magnetic semiconductor nanowires is possible by the catalytically enhanced molecular beam epitaxy (MBE). Zn(1-x)MnxTe NWs with manganese content up to x=0.60 were produced by this method. X-ray diffraction, Raman spectroscopy, and temperature dependent photoluminescence measurements confirm the incorporation of Mn(2+) ions in the cation substitutional sites of the ZnTe matrix of the NWs.

Drastic changes of the domain size in an ultrathin magnetic film

A general framework for the domain size in any ultrathin film with perpendicular magnetic anisotropy is here discussed. The domain structure is analyzed by using the classical theory taking into consideration the demagnetization field contribution to the domain wall energy. A sinusoidal model is considered to describe the domain structure while approaching, in two different cases, the monodomain state with in-plane magnetization. The first case is realized applying a large enough in-plane magnetic field. The second one is obtained by decreasing the perpendicular magnetic anisotropy, which is connected in many ultrathin systems with the increase of film thickness. A change in the domain size of several orders of magnitude is obtained while approaching the magnetization reorientation region. The minimal stripe domain period p=8πlex2/d is calculated from the sinusoidal model, where lex is the exchange length and d is the thickness of the film. The range of possible domain size changes in ultrathin films is p...

Irreversible modification of magnetic properties of Pt/Co/Pt ultrathin films by femtosecond laser pulses

Annealing ultrathin Pt/Co/Pt films with single femtosecond laser pulses leads to irreversible spin-reorientation transitions and an amplification of the magneto-optical Kerr rotation. The effect was studied as a function of the Co thickness and the pulse fluence, revealing two-dimensional diagrams of magnetic properties. While increasing the fluence, the creation of two branches of the out-of-plane magnetization state was found.

Magnetic ordering in ultrathin cobalt film covered by an overlayer of noble metals

The spatial distribution of magnetic properties studied by polar Kerr-effect-based magnetometry are carried out on an ultrathin cobalt wedge covered with a silver wedge whose slopes are perpendicular to each other and subsequently capped with a gold layer. The coercivity field was studied as a function of both cobalt thickness d and silver thickness h. A few monolayers of Ag were found to have a substantial influence on the coercivity field. A similar strong influence of silver coverage on the magnetic anisotropy field was observed. Domain structure evolution during magnetization reversal is investigated using an optical microscope. With an increase of the magnetic field, a magnetization reversal process occurs: first by domain nucleation and next by a domain wall movement towards the higher coercivity region of the sample. Changes of the fractal dimension of the domain wall as well as nucleation center density are investigated. A rapid increase of these parameters is observed while d increases towards the value of spin reorientation transition, which goes from easy-axis to easy-plane magnetization. Similar strong changes were observed while decreasing d to the lowest thickness available for observation. The problem discussed here, of tuning magnetic properties by silver–gold structure, is important from both a general physics and an application point of view, especially because of the possibilities for magnetic film patterning.

Ion irradiation induced enhancement of out-of-plane magnetic anisotropy in ultrathin Co films

Ga+ or He+ irradiated MBE grown ultrathin films of sapphire/Pt/Co(dCo)/Pt(dPt) were studied using polar Kerr effect in wide ranges of both cobalt dCo and platinum dPt thicknesses as well as ion fluences F. Two branches of increased magnetic anisotropy and enhanced Kerr rotation angle induced by Ga+ or He+ irradiation are clearly visible in two-dimensional (dCo, LogF) diagrams. Only Ga+ irradiation induces two branches of out-of-plane magnetization state.

Growth and optical properties of CdTe quantum dots in ZnTe nanowires

We report on the formation of optically active CdTe quantum dots in ZnTe nanowires. The CdTe/ZnTe nanostructures have been grown by a gold nanocatalyst assisted molecular beam epitaxy in a vapor-liquid solid growth process. The presence of CdTe insertions in ZnTe nanowire results in the appearance of a strong photoluminescence band in the 2.0 eV–2.25 eV energy range. Spatially resolved photoluminescence measurements reveal that this broad emission consists of several sharp lines with the spectral width of about 2 meV. The large degree of linear polarization of these individual emission lines confirms their nanowire origin, whereas the zero-dimensional confinement is proved by photon correlation spectroscopy.

Self-assembled growth of Au islands on a Mo(110) surface.

The self-assembled growth of epitaxial Au(111) islands on a Mo(110) buffer layer has been investigated as a function of growth temperature and amount of deposited material by reflection high energy electron diffraction and atomic force microscopy. At the growth temperature of 385 degrees C the dendrite-shaped islands coexist with the compact ones. The uniform islands formed at 500 degrees C adopt mostly a shape of truncated pyramids with a well developed (111) top plane and {111} and {100} side facets. As the growth temperature reaches 800 degrees C the Au islands take less regular shapes due to occurrence of coalescence. The averaged area and height of the islands increase with the deposition temperature and the amount of deposited material. The surface density of the islands decreases with increasing temperature. The epitaxial relations at the interface between the Au islands and the Mo buffer determined from the angular dependence of the electron diffraction pattern favour the Nishiyama-Wassermann growth mode. Factors responsible for the island-like growth and possible mechanisms of diffusion are discussed in details.