Thermal behavior of Ni,Co and Fe succinates embedded in silica matrix

This paper presents the thermal behavior of Co, Ni and Fe succinates obtained by sol-gel synthesis using Co(II), Ni(II) and Fe(III) nitrates, 1,4-butanediol and tetraethyl orthosilicate as reactants. The thermal analysis revealed the formation of succinates at 413–453 K and their decomposition to ferrites at 503–623 K. The rate constants for the decomposition of succinates to ferrites, calculated using the isotherms at 473, 523, 573 and 623 K, were used to determine the activation energy of each ferrite (NiFe₂O₄, Ni_0.3Co_0.7Fe₂O₄, Ni_0.7Co_0.3Fe₂O₄ and CoFe₂O₄) embedded in the silica matrix. By increasing the Ni content in the mixed Ni–Co ferrites, the activation energy decreases from 13.530 to 1.944 kJ mol^?1. The formation and decomposition of succinate precursors and the formation of silica matrix were confirmed by FT-IR spectroscopy, while the formation of CoFe₂O₄ and NiFe₂O₄ single-phases embedded in the silica matrix was confirmed by X-ray diffraction analysis. The nanocrystallites size decreases from 31.7 (CoFe₂O₄) to 18.5 nm (NiFe₂O₄). The optical band gap of mixed Co–Ni ferrites was significantly higher than that corresponding to CoFe₂O₄. The photocatalytic activity of the samples was evaluated against Rhodamine B under visible light. All the samples have photocatalytic activities, the best performance being obtained in the case of Ni_0.7Co_0.3Fe₂O₄.

     

Temperature and composition dependence of magnetic properties of cobalt–chromium co-substituted magnesium ferrite nanomaterials

The temperature and composition dependence of magnetic properties of Co–Cr co-substituted magnesium ferrite, Mg_1−xCo_xCr_xFe_2−xO₄ (x=0.0–0.5), prepared by novel polyethylene glycol assisted microemulsion method, are studied. The synthesized materials are characterized by the Mössbauer spectrometer and standard magnetic measurements. Major hysteresis loops are measured up to the magnetic field of 50 kOe at 300, 200 and 100 K. The high field regimes of these loops are modeled using the Law of Approach to saturation to determine the first-order cubic anisotropy coefficient and saturation magnetization. Both the saturation magnetization and the anisotropy coefficient are observed to increase with the decrease in temperature for all Co–Cr co-substitution levels. Also, both the saturation magnetization and the anisotropy coefficient achieved maximum value at x=0.3 and x=0.2, respectively. Explanation of the observed behavior is proposed in terms of the site occupancy of the co-substituent, Co²⁺ and Cr³⁺ in the cubic spinel lattice.     

Effect of boron concentration on the magnetic properties of (FeCoNi) 100−xBx (x=15 and 20 wt%) nanowire arrays

The Fe_14.5Co_16.5Ni₅₅B₁₅ and the Fe₁₃Co_15.5Ni_51.5B₂₀ ferromagnetic nanowires were deposited using the electrochemical deposition method. The structure of these nanowires was investigated using X-ray diffraction. Squid magnetometer was used to investigate the magnetic behavior. The hysteresis loops of 50 μm long nanowire arrays were studied as a function of boron concentration, nanowire diameter and field orientation. The competition between shape anisotropy and magnetostatic interactions played a vital role in determining the magnetic field necessary to saturate an array. The decrease in coercive field (H_c) and the squareness (SQ) of the hysteresis loop from 100 to 200 nm wire diameter for both types of compositions suggests the formation of multidomains in the nanowire.     

Thermal behavior of Co<Subscript>x</Subscript>Fe<Subscript>3−x</Subscript>O<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> nanocomposites obtained by a modified sol–gel method

The thermal behavior of Co_xFe_3?xO₄/SiO₂ nanocomposites obtained by direct synthesis starting from nonahydrate ferric nitrate and hexahydrate cobalt nitrate in different ratios with and without the addition of 1,4-butanediol was studied. For the synthesis of Co_xFe_3?xO₄ (x = 0.5–2.5) dispersed in the silica matrix a wide Co/Fe molar ratio was used. The decomposition processes, formation of crystalline phases, gases evolvement and mass changes during gels annealing at different temperatures were assessed by thermal analysis. The absence of succinate precursor and a low mass loss were observed in the case of the gel obtained in the absence of 1,4-butanediol. In case of gels obtained using a stoichiometric ratio of Co/Fe, no clear delimitation between Co and Fe succinates was observed, while for samples with a Fe or Co excess, the formation of the two succinates was observed. The evolution of the crystalline phase after annealing (673, 973 and 1273 K) investigated by X-ray diffraction analysis and Fourier transformed infrared spectrometry revealed that in samples with Fe excess, stoichiometric Fe/Co ratio or low Co excess, the cobalt ferrite (CoFe₂O₄) was obtained as a single phase, while in samples with higher cobalt excess, olivine (Co₂SiO₄) as a main phase, cobalt oxide and CoFe₂O₄ as secondary phases were obtained after annealing at 1273 K. The SEM images confirmed the nanoparticles embedding in the silica matrix, while the TEM and X-ray diffraction data showed that the obtained nanoparticles’ size was below 10 nm in most samples.     

In-plane anisotropy and stress detection of films deposited by RC technique

A Novel Rotating Cryostat (RC) vacuum system originally designed to fabricate organic layers has been developed in order to prepare magnetic materials for specific applications such as sensors. The RC sputtering system has a rotating drum (substrate holder) and the possibility of using multi-port deposition sources. The source material sputtered by a dc magnetron, which was positioned one of the ports around the RC, was an iron disk (25 mm diameter, 0.8 mm thick, 99.8% pure). Results show that films have exhibited isotropic and anisotropic magnetisation at various running conditions of the RC using a Magneto-Optic Loop Plotter (MOKE) and a Vibrating Sample Magnetometer (VSM). Estimation of magnetic anisotropy confirms in-plane anisotropy in the films. Moreover, when these films are subjected to a bending stress within the VSM, they show chances in their hysteresis loops. These findings indicate a possible future for this technique to produce sensing devices for stress detection. Received 29 September 2001     

Magnetic properties of dc magnetron sputtered and evaporated amorphous films

Compositions of Fe₈₁B_13.5Si_3.5C₂, Fe₆₇Co₁₈Si₁B₁₄ and Fe_5.85Co_72.15Mo₂B₁₅Si₅ were deposited on to rigid and flexible substrates for the first time using a dc sputtering source as part of a novel Rotating Cryostat (RC). The films sputtered on silicon and glass show only isotropic magnetisation, whereas those sputtered on a polyimide (Kapton^TM) substrate exhibited either isotropic or anisotropic magnetisation depending upon composition. Similar findings were obtained for equivalent evaporated films. Received 23 October 2001 and Received in final form 29 January 2002