Chandana Rath

Dependence on cation distribution of particle size, lattice parameter, and magnetic properties in nanosize Mn–Zn ferrite

In Mn1−xZnxFe2O4 (x=0 to 1) nanosize particles prepared through hydrothermal precipitation we observe a decrease in particle size from 13 to 4 nm with increasing Zn concentration from 0 to 1. The lattice constant, a, for all Mn/Zn concentrations is found to be less than that for the corresponding bulk values. At specific compositions within x=0.35 and 0.5, the temperature dependence of the magnetization exhibits a cusp-like behavior below the temperature at which the nanoparticles undergo a ferri- to para-magnetic transition (Tc). The Curie temperatures, Tc, of the nanoparticles are in the range of 175–500 °C, which are much higher than their corresponding bulk values. To explain these unusual features, the strong preferential occupancy of cations in chemically inequivalent A and B sites and the metastable cation distribution in nanoparticles are invoked.

OXYGEN VACANCY INDUCED STRUCTURAL PHASE TRANSFORMATION IN TIO2 NANOPARTICLES

Nanosize TiO2 powders prepared by the sol–gel technique at pH of precipitation 4.5 and 6.5 show the anatase phase after calcining at 500 °C. Anatase to rutile phase transformation (ART), however, occurs at 650 °C in the case of pH 6.5 while 850 °C is found to be the ART temperature for the lower pH sample. pH of precipitation dependent ART temperature has not been reported so far to the best of our knowledge. It is known that the smaller the particle size, the lower the ART temperature, and vice versa. The observation of higher crystallite size and lower ART temperature in the case of the higher pH sample contradicts the reported result. We realized from x-ray photoelectron spectroscopic studies that oxygen vacancy concentration drives the ART temperature to lower values in the higher pH sample compared with the sample synthesized at lower pH; even the particle size is found to be higher in the former one.

Appearance of superparamagnetism on heating nanosize Mn0.65Zn0.35Fe2O4

Nanosize particles (average size ∼12 nm) of mixed ferrite Mn0.65Zn0.35Fe2O4 were prepared by the hydrothermal precipitation route and studied using x-ray diffraction, transmission electron microscopy, differential scanning calorimetry, magnetization measurements, and Mossbauer spectroscopy. The as-prepared sample was largely ferrimagnetic and, as the sample was annealed at temperatures above 250 °C, it gradually became superparamagnetic. This unexpected behavior is explained by assuming that the cation distribution in the nanosize as-prepared sample is in a metastable state and, as the sample is heated, this distribution changes to a more stable state while the grain size remains nearly the same.

Microstructure-dependent coercivity in monodispersed hematite particles

Microstructure and magnetic properties of monodispersed pseudocubic and trapezoidal particles with varying sizes prepared through the hydrothermal precipitation route are reported. The coercivity for trapezoidal particles was similar to that of reported values. For pseudocubic particles, however, the coercivity is unusually high (∼6 kOe) as compared to the maximum value (3 kOe) reported in the literature. Detailed microstructural analysis revealed that particles with a well-defined shape are, in fact, polycrystalline. The high coercivity and its variation with particle shape and size are correlated to the internal nanostructure of the particles.

Oxygen vacancy induced phase formation and room temperature ferromagnetism in undoped and Co-doped TiO2 thin films

TiO2 and Co-doped TiO2 (CTO) thin films deposited at various oxygen partial pressures by pulsed laser deposition exhibit room temperature ferromagnetism (RTFM) independent of their phase. Films deposited at 0.1?mTorr oxygen partial pressure show a complete rutile phase confirmed from glancing angle x-ray diffraction and Raman spectroscopy. At the highest oxygen partial pressure, i.e. 300?mTorr, although the TiO2 film shows a complete anatase phase, a small peak corresponding to the rutile phase along with the anatase phase is identified in the case of CTO film. An increase in O to Ti/(Ti+Co) ratio with increase in oxygen partial pressure is observed from Rutherford backscattering spectroscopy. It is revealed from x-ray photoelectron spectroscopy (XPS) that oxygen vacancies are found to be higher in the CTO film than TiO2, while the valency of cobalt remains in the +2 state. Therefore, the CTO film deposited at 300?mTorr does not show a complete anatase phase unlike the TiO2 film deposited at the same partial pressure. We conclude that RTFM in both films is not due to impurities/contaminants, as confirmed from XPS depth profiling and cross-sectional transmission electron microscopy (TEM), but due to oxygen vacancies. The magnitude of moment, however, depends not only on the phase of TiO2 but also on the crystallinity of the films.

Si3N4 single-crystal nanowires grown from silicon micro- and nanoparticles near the threshold of passive oxidation

A simple and most promising oxide-assisted catalyst-free method is used to prepare silicon nitride nanowires that give rise to high yield in a short time. After a brief analysis of the state of the art, we reveal the crucial role played by the oxygen partial pressure: when oxygen partial pressure is slightly below the threshold of passive oxidation, a high yield inhibiting the formation of any silica layer covering the nanowires occurs and thanks to the synthesis temperature one can control nanowire dimensions.

Passivation of native defects of ZnO by doping Mg detected through various spectroscopic techniques

Native defects, responsible for the n-type behavior of ZnO are found to be reduced by Mg doping. Zn1−xMgxO nanoparticles synthesized by the conventional coprecipitation route are characterized by XRD, TEM and various spectroscopic techniques like FTIR, XPS, UV-visible, PL, Raman and PAS. Unintentional hydrogen and complex defects in addition to cationic and anionic vacancies (VZn and VO) in ZnO nanoparticles are observed from FTIR. After incorporating Mg, an additional IR mode at 856 cm−1 is observed and attributed to Mg–H. The additional mode contributing excess hydrogen in Mg doped ZnO is further confirmed from XPS. PL and PAS measurements indicate less native defects in Mg doped ZnO because of passivation of defects by excess hydrogen. We conclude that Mg doping not only diminishes the native defects in ZnO, but also could be used as a suitable material for hydrogen storage.

Anomalous x-ray diffraction peak broadening and lattice strains in Zn1?xCoxO dilute magnetic semiconductors

Nanocrystalline powders of Zn(1-x)Co(x)O synthesized by the coprecipitation technique show anomalous anisotropic broadening for x>0.05. This peak broadening is shown to be not only due to a reduction in the particle size but also due to a significant strain contribution, as confirmed by Williamson-Hall analysis. The presence of grouped Co(2+) ions, revealing the presence of clusters of high spin Co(2+) with antiferromagnetically coupled spins, as indicated by magnetization studies, seems to be responsible for the strain.

Effect of size reduction on cation distribution and magnetic transitions in CoCr2O4 multiferroic: EXAFS, magnetic and diffused neutron scattering measurements

A rich sequence of magnetic transitions such as para to long-range ferrimagnetic transition at Curie temperature, TC, to a short range non-collinear spiral ordering at spiral ordering temperature, TS, and finally to a lock in transition, TL, are examined in ∼10 and ∼50 nm samples of CoCr2O4 multiferroic through dc, ac magnetic measurements and diffused neutron scattering using polarized neutrons. Analysis of extended X-ray absorption fine structure (EXAFS) spectra and Fourier transforms of Co and Cr edges in real (r) and momentum (k) space show no change in cation distribution among A and B sites even after reducing the size to the nanometer range due to the high crystal field stabilisation energy of Cr3+ towards the B site. While TS remains independent, TC increases from 97.2 to 100.1 K and TL increases from 5 to 8.5 K with varying size from 10 to 50 nm. Temperature dependent ac susceptibility (χ) measurements demonstrate that χ′ and χ′′ do not show any dispersion behaviour in the 10 nm sample. However, we observe splitting of χ′′ into two peaks and one of them shows dispersion behaviour in the 50 nm sample, indicating a core–shell structure. Magnetization vs. magnetic field measurement show hysteresis behaviour with unsaturated magnetization at high magnetic field indicating a ferrimagnetic core surrounded by disordered surface spins. Fitting of χ′′ with an empirical relation Φ = ΔTB/TBΔlog10ω, and memory effect measurement, further confirm the spin-glass behaviour of the shell in the 50 nm sample. Magnetic ordering temperatures are examined through neutron scattering using a polarized neutron beam, and reveal that while the para- to ferrimagnetic transition, TC, is continuous and long-range in both nanocrystalline particles, TS is found to be sharp, short-range, and commensurate in 50 nm sample.

Size-dependent magnetic transitions in CoFe0.1Cr1.9O4 nanoparticles studied by magnetic and neutron-polarization analysis

Multiferroic, CoCr2O4 bulk material undergoes successive magnetic transitions such as a paramagnetic to collinear and non-collinear ferrimagnetic state at the Curie temperature (TC) and spiral ordering temperature (TS) respectively and finally to a lock-in-transition temperature (Tl). In this paper, the rich sequence of magnetic transitions in CoCr2O4 after mixing the octahedral site with 10% of iron are investigated by varying the size of the particle from 10 to 50 nm. With the increasing size, while the TC increases from 110 to 119 K which is higher than the TC (95 K) of pure CoCr2O4, the TS remains unaffected. In addition, a compensation of magnetization at 34 K and a lock-in transition at 10 K are also monitored in 50 nm particles. Further, we have examined the magnetic-ordering temperatures through neutron scattering using a polarized neutron beam along three orthogonal directions after separating the magnetic scattering from nuclear-coherent and spin-incoherent contributions. While a sharp long-range ferrimagnetic ordering down to 110 K and a short-range spiral ordering down to 50 K are obtained in 50 nm particles, in 10 nm particles, the para to ferrimagnetic transition is found to be continuous and spiral ordering is diffused in nature. Frequency-dependent ac susceptibility (χ) data fitted with different phenomenological models such as the Neel-Arrhenius, Vogel-Fulcher and power law, while ruling out the canonical spin-glass, cluster-glass and interacting superparamagnetism, reveal that both particles show spin-glass behavior with a higher relaxation time in 10 nm particles than in 50 nm. The smaller spin flip time in 50 nm particles confirms that spin dynamics does not slow down on approaching the glass transition temperature (Tg).