Heat capacity of Heusler alloys: Ferromagnetic Ni2MnSb,Ni2MnSn,NiMnSb and antiferromagnetic CuMnSb

We report the results of the investigation of the specific heat of the ferromagnetic Heusler Ni₂MnSn, Ni₂MnSb, NiMnSb and antiferromagnetic CuMnSb alloys. The low-temperature behaviour of the specific heat may be described as C=γT+βT³ for ferromagnetic compounds and as C=γT+δ T²+βT³ for antiferromagnetic CuMnSb. The values of the density of states from the heat capacity measurements are higher than those from electronic band structure calculations. Debye temperatures are in a good agreement with those obtained from thermal expansion measurements. The Grüneisen parameter is calculated for Ni₂MnSn and CuMnSb from the magnetic contribution to the specific heat in the vicinity of T_C or T_N.     

Ferromagnetism and antiferromagnetism in Ni2+x+yMn1−xAl1−y alloys

The magnetic behavior of Ni_2+xMn_1−xAl alloys around the stoichiometric 2:1:1 composition was investigated with several experimental techniques. The results of low-temperature magnetization measurements indicate that a competition mechanism between ferromagnetism and antiferromagnetism is expected in off-stoichiometric alloys. Although the Curie temperature is strongly dependent on the composition, the saturation magnetization has an unsystematic variation for deviations from the stoichiometric Ni₂MnAl alloy. A reentrant-spin-glass behavior is observed below 50 K.     

Preparation of ferromagnetic γ - Fe2O3 nanocrystallites by oxidative co-decomposition of PEG 6000 and ferrocene

Highly crystalline and ferromagnetic γ-Fe₂O₃ nanocrystallites were prepared by controlled oxidative co-decomposition of PEG 6000 and ferrocene at a temperature of 450 ^°C under air atmosphere. The morphology, crystalline structure and preliminary magnetic properties of the as-synthesized nanocrystallites have been characterized by using transmission electron microscope (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). The highly crystalline γ-Fe₂O₃ nanocrystallites are in quasi-cubic shape with an average size of 30 nm and exhibit room-temperature ferromagnetism. The capping effect of PEG 6000 has also been investigated by thermogravimetry analysis (TGA) and Fourier transform infrared (FTIR) regarding controlling the size of the nanocrystallites and preventing the volatilization of ferrocene and thus raising the yield of the products. This simple method has a high yield of over 80% as well as low cost.     

Optical and theoretical investigations of V ions in CdZnTe crystal

High-resistivity CdZnTe:V crystals are investigated by photoluminescence (PL) and by time-resolved PL in the infrared spectral range. A double peaked emission band is detected around 0.8 eV and it is related to vanadium doping. No-phonon lines of the internal transitions were detected. This emission is interpreted as a balance between the ⁴T₁(⁴P)→⁴T₁(⁴F) internal transition and an electronic transition from the conduction band to the ⁴T₁(⁴F) ground state of V²⁺. The corresponding decay time after laser excitation gives evidence to the contribution of two different recombination processes. These two emission bands are separated by time-resolved luminescence. Crystal-field calculations of the detected transition energies based on Tanabe-Sugano scheme are presented and the Racah parameter B and crystal-field intensity D_q were determined.In addition, a model is developed in terms of one-electron orbital, to explain the characteristics of the PL excitation processes of V²⁺. Excitations with above and below band edge energy confirm the proposed schemes.     

A study of nanosized Ni substituted Co–Zn ferrite prepared by coprecipitation

Nanoparticles of Ni_0.25Co_0.25Zn_0.5Fe₂O₄ with different particle sizes were synthesized by chemical coprecipitation technique. The prepared nanoparticles were characterized and studied by X-ray diffraction, room temperature ⁵⁷Fe Mössbauer spectroscopy and DC magnetization. Increase in annealing temperature leads to inversion of cations from their normal configurations which results in an increase in the hyperfine fields at both tetrahedral A and octahedral B sites. The decrease in saturation magnetization values with an increase in annealing temperature can be attributed to the variation in proportions of superparamagnetic phase and also the redistribution of cations in these ferrite nanoparticles. Variation of coercivity with annealing temperatures can be explained in terms of the size effect.     

Effects of organic acids on the size-controlled synthesis of rutile TiO2 nanorods

Size-controlled synthesis of pure rutile-phase TiO₂ nanorods was carried out by a hydrothermal method using different organic acids as modifiers, and metatitanic acid and concentrated sulfuric acid as raw materials. The synthesized rutile TiO₂ nanorods were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effects of organic acid modifiers on the sizes of rutile TiO₂ nanorods were investigated. It was found that the steric effect occurred by the organic modifiers and non-polarity of organic acids were beneficial to the formation of small-sized rutile TiO₂ nanorods. The strongly coordinative interaction between the carboxyl (or hydroxyl) group of the modifier and the surface of TiO₂ nanoparticles effectively inhibited the crystal growth.     

Photofragment space distribution in the photodissociation of NaI in the spectral range 315–370 nm: the role of molecular axis rotation

The first determination of translational anisotropy parameters β in the photodissociation of NaI molecules in the spectral range 315–370 nm is reported. The anisotropy parameters were determined by the analysis of Doppler resolved absorption profiles of Na(²S_1/2) atoms produced in the photodissociation of NaI by linearly polarized light. The profiles were recorded for two orientations of the photolysis light: parallel and perpendicular to the direction of the probe beam. The value of the parameter β was obtained from a simultaneous fit of the profiles. The role of the rotation of the parent molecules on the branching ratio between parallel and perpendicular transitions in NaI during dissociation is discussed.     

Simultaneous online detection of isoprene and isoprene-d2 using infrared photoacoustic spectroscopy

A photoacoustic spectrometer for the simultaneous detection of isoprene and the deuterated species [4,4-²H]-2-methyl-1,3-butadiene (isoprene-d₂) is presented. Using a sealed-off ¹³CO₂ laser a single-component detection limit of 400 ppt (isoprene) and 600 ppt (isoprene-d₂) was achieved. Simultaneous monitoring of both compounds allowed the detection of labelling levels down to 6% (isoprene-d₂ in total isoprene) with a time resolution of 3 min. In emission studies with Eucalyptus globulus, the deuterated precursor [4,4-²H]-1-deoxy-D-xylulose was fed to a leaf through the transpiration stream. Emission of isoprene-d₂ started as early as 10 min after application of the precursor. Received: 3 May 2002 / Revised version: 31 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +49-228/733474, E-mail: frank.kuehnemann@iap.uni-bonn.de     

Detection of biogenic CO production above vascular cell cultures using a near-room- temperature QC-DFB laser

We report the first application of pulsed, near-room-temperature quantum cascade laser technology to the continuous detection of biogenic CO production rates above viable cultures of vascular smooth muscle cells. A computer-controlled sequence of measurements over a 9-h period was obtained, resulting in a minimum detectable CO production of 20 ppb in a 1-m optical path above a standard cell-culture flask. Data-processing procedures for real-time monitoring of both biogenic and ambient atmospheric CO concentrations are described. Received: 24 October 2001 / Published online: 29 November 2001     

Bose–Einstein condensates in magnetic waveguides

In this article, we describe an experimental system for generating Bose–Einstein condensates and controlling the shape and motion of a condensate by using miniaturised magnetic potentials. In particular, we describe the magnetic trap setup, the vacuum system, the use of dispenser sources for loading a high number of atoms into the magneto-optical trap, the magnetic transfer of atoms into the microtrap, and the experimental cycle for generating Bose–Einstein condensates. We present first results on outcoupling of condensates into a magnetic waveguide and discuss influences of the trap surface on the ultra-cold ensembles. Received: 21 August 2002 / Revised version: 10 December 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-7071/295-829, E-mail: fortagh@pit.uni-tuebingen.de