Search

Currently, wide-gap ZnO nanoparticles bear important potential application in electro-optical devices, transparent ultraviolet protection films, and spintronic devices. We have studied the magnetic properties of nanocrystals of ZnO(Fe, Co, Mn) prepared by two methods of synthesis. We have used the microwave assisted hydrothermal synthesis and traditional wet chemistry method followed by calcination. The detailed structural characterization was performed by means of X-ray diffraction and micro-Raman spectroscopy measurements. The morphology of the samples was studied by means of SEM and TEM microscopy. The results of systematic measurements of AC magnetic susceptibility as a function of temperature and frequency as well as SQUID magnetization are presented. The SQUID magnetization measurements revealed a clear bifurcation of the FC and ZFC plots. Such behavior suggested superparamagnetic behavior above the blocking temperature. The dynamic magnetic measurements were performed at small AC magnetic field with amplitude not exceeding 5 Oe and different frequency values (from 7 Hz to 9970 Hz). For ZnO(Fe) and ZnO(Mn), the AC susceptibility maxima has been found for in-phase susceptibility Re(χ) and for out of phase susceptibility Im(χ). We analyzed the observed frequency dependence of the peak temperature in the AC susceptibility curve using the empirical parameter ΦF that is a quantitative measure of the frequency shift and is given by the relative shift of the peak temperature per decade shift in frequency, as well as Vogel-Fulcher law. We observed two different types of magnetic behavior, spin-glasslike behavior or superparamagnetic behavior, depending on the synthesis process. For ZnO(Co) nanocrystalline samples high temperature Curie-Weiss behavior in AC magnetic susceptibility was observed. We observed that the determined negative values of the Curie-Weiss temperature θdepend strongly on the nominal content of cobalt oxide. It was shown that for calcination method the values of θ increase with the increase of magnetic ion content indicating enhancement of predominance of antiferromagnetic interactions. For hydrothermal method the opposite effect was observed indicating the breakdown of predominance of aniferromagnetic coupling with the increase of nominal magnetic ion content. This paper gives an in-depth discussion of the structural and magnetic properties of ZnO nanocrystals in addition to the technological issues such as different methods of wet chemical synthesis. [ABSTRACT FROM AUTHOR]

Nickel ferrite (NiFe2O4) and manganese ferrite (MnFe2O4) have been prepared by a soft mechanochemical route from mixture of (1) Ni(OH)2 and α-Fe2O3 and (2) Mn(OH)2 and α-Fe2O3 powders in a planetary ball mill. The mixture was activated for varying duration. Soft mechanochemical reaction leading to formation of the NiFe2O4 and MnFe2O4 spinel phases were followed by X-ray diffraction, Raman and infrared spectroscopy, scanning and transmission microscopy. The spinel phase formation was first observed after 4 h of milling (case 1) and after 3 h (case 2) and its formation was completed after 25 h in both cases. The synthesized NiFe2O4 and MnFe2O4 ferrites have a nanocrystalline structure with a crystallite size of about 30 and 40 nm for cases (1) and (2), respectively. There are five Raman and four IR active modes. In order to understand better the whole process of phase formation, the Mössbauer measurements were done. [ABSTRACT FROM AUTHOR]

We have studied the magnetic properties of ZnO nanocrystals doped with Fe2O3 in the magnetic dopant range from 5 to 70 wt%. The nanocrystals were synthesized by wet chemical method. The detailed structural characterization was performed by means of X-ray diffraction and micro-Raman spectroscopy measurements. The results of systematic measurements of magnetic AC susceptibility as a function of temperature and frequency are presented. We observed different types of magnetic behavior. For ZnO samples doped with low content of Fe2O3, the results of low-field AC susceptibility are satisfactorily explained by superparamagnetic model including inter-particle interactions. With the increase of magnetic Fe2O3 content, the spin-glass-like behavior is observed. [ABSTRACT FROM AUTHOR]

Abstract: We have synthesized CdS and Cd0.7Mn0.3S nanocrystals with diameters of about 4.5nm. These materials have been studied using photoluminescence spectroscopy with various excitation energies in the temperature range from 1.8 to 300K, in magnetic field up to 5T. We registered surface and defect states as well as two Mn2+ energy levels that correspond to Mn2+ ions in two different crystal environments. We proposed the scheme of energy levels and transitions that explain obtained experimental results. [Copyright &y& Elsevier]

We have studied the magnetic properties of nanocrystals of ZnO:MnO prepared by traditional wet chemistry method. The detailed structural and morphological characterization was performed. The results of systematic measurements of AC magnetic susceptibility as a function of temperature and frequency as well as DC magnetization are reported. We observed two different types of magnetic behavior depending on the concentration doping. For samples with low nominal content (up to 30 wt% of MnO), superparamagnetic behavior was observed. We attribute the observed superparamagnetism to the presence of nanosized ZnMnO 3 phase. For nanocrystals doped above nominal 60 wt% of MnO ferrimagnetism was detected with T C at around 42 K. This magnetic behavior we assign to the presence of nanosized Mn 3 O 4 phase. [ABSTRACT FROM AUTHOR]

This paper describes the structural and optical properties of Cu–Se thin films. The surface morphology of thin films was investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Formation of Cu–Se thin films is concluded to proceed unevenly, in the form of islands which later grew into agglomerates. The structural characterization of Cu–Se thin film was investigated using X-ray diffraction pattern (XRD). The presence of two-phase system is observed. One is the solid solution of Cu in Se and the other is low-pressure modification of CuSe 2 . The Raman spectroscopy was used to identify and quantify the individual phases present in the Cu–Se films. Red shift and asymmetry of Raman mode characteristic for CuSe 2 enable us to estimate nanocrystal dimension. In the analysis of the far-infrared reflection spectra, numerical model for calculating the reflectivity coefficient of layered system, which includes film with nanocrystalite inclusions (modeled by Maxwell-Garnett approximation) and substrate, has been applied. [ABSTRACT FROM AUTHOR]

Influence of laser power on nanocrystalline samples of ZnO(Co) prepared by commonly used wet chemistry method followed by calcination was investigated. Previous confirmation of the existence of ZnO and Co 3 O 4 phases was based on the X-ray diffraction measurements. Here we report the experimental spectra of non-resonant Raman scattering in the range between 100 cm −1 and 1600 cm –1 , for a series of samples irradiated with four different laser power densities. The laser power density has different influence on relative intensity of peaks that belong to ZnO phase than on those corresponding to Co 3 O 4 phase. Both peak types show characteristic broadening and red shift toward lower frequencies. The laser power densities used in our study did not cause thermal destruction in any of the investigated samples. Laser-induced local heating effects in samples caused formation of cobalt dimers on the surface of Co 3 O 4 . [ABSTRACT FROM AUTHOR]

Abstract: The aim of the present work is to study the magnetic properties of nanocrystals of ZnO(Co) prepared by two methods of synthesis. We used the microwave assisted hydrothermal synthesis and traditional wet chemistry method followed by calcination. The detailed structural characterization was performed by means of X-ray diffraction and micro-Raman spectroscopy measurements. The magnetic properties were studied by means of AC susceptibility χ. All samples demonstrated Curie–Weiss behavior at higher temperatures. For calcination process, the negative values of the Curie–Weiss θ temperature were observed indicating the predominance of antiferromagnetic interactions. For samples prepared by hydrothermal synthesis, we observed the breakdown of predominance of antiferromagnetic interactions. [Copyright &y& Elsevier]

Abstract: Nickel ferrite, NiFe2O4 has been prepared by a soft mechanochemical route from mixture of (1) Ni(OH)2 and α-Fe2O3 and (2) Ni(OH)2 and Fe(OH)3 powders in a planetary ball mill for varying duration. Soft mechanochemical reaction leading to formation of the NiFe2O4 spinel phase was followed by X-ray diffraction, Raman and infrared spectroscopy, TGA, scanning and transmission microscopy. The spinel phase formation was first observed after 4h of milling and its formation was completed after 25h in the both cases. The synthesized NiFe2O4 ferrite has a nanocrystalline structure with a crystallite size of about 20 and 10nm respectively for the cases (1) and (2). The final grain size in the system (1) is about twice as large as that in the system (2), what is a consequence of different reaction paths in these two processing routes. There are five Raman and four IR active modes. Mössbauer spectroscopy studies implied on the possible cation distribution between the tetrahedral and octahedral sites in formed NiFe2O4 spinel structure. We were able to estimate the degree of inversion at most 0.82 in the case (1) and 0.66 in the case (2). [Copyright &y& Elsevier]

Abstract: The aim of the present work is to study the magnetic properties of ZnO(Fe) nanocrystalline samples prepared by two methods of synthesis. We have used the microwave assisted hydrothermal synthesis and traditional wet chemistry method followed by calcination. The detailed structural characterization was performed by means of X-ray diffraction and micro-Raman spectroscopy measurements. The dynamic magnetic properties were studied by means of AC susceptibility χ. The measurements were performed at small AC magnetic field with amplitude not exceeding 5Oe and different frequency values (from 7Hz to 9970Hz). The AC susceptibility maxima have been found for in-phase susceptibility Re(χ) and for out of phase susceptibility Im(χ). We analyzed the observed frequency dependence of the peak temperature in the AC susceptibility curve using the empirical parameter Φ that is a quantitative measure of the frequency shift and is given by the relative shift of the peak temperature per decade shift in frequency, as well as Vogel–Fulcher law. [Copyright &y& Elsevier]

Abstract: Nanocrystaline samples of ZnO(Fe) were synthesized by wet chemical method. Samples were characterized by X-ray diffraction to determine composition of the samples (ZnO, Fe2O3, ZnFe2O4) and the mean crystalline size (8–52nm). In this paper we report the experimental spectra of Raman scattering. Main characteristics of experimental Raman spectrum in 200–1600cm−1 spectral region are: sharp peak at 436cm−1 and broad two-phonon structure at ∼1150cm−1, typical for ZnO; broad structure below 700cm−1 that has different position and shape in case of ZnFe2O4 or Fe2O3 nanoparticles. Low-frequency Raman modes were measured and assigned according to confined acoustic vibrations of spherical nanoparticles. Frequencies of these vibrational modes were analyzed in elastic continuum approximation, which considers nanoparticle as homogeneous elastic sphere. [ABSTRACT FROM AUTHOR]