H.C. Verma

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.

Mössbauer studies of nanosize Mn1−xZnxFe2O4

Nanosized mixed ferrite Mn1−xZnxFe2O4 (x=0.0, 0.35, 0.5, 0.65, 1) has been prepared by hydrothermal route at pH 9 and 210°C and studied by X-ray diffraction, magnetization measurement and Mossbauer spectroscopy. The size of the particles decreases gradually as the zinc concentration is increased. The cation distribution has been found to strongly depend on the process parameters. On annealing the system the cation distribution changes from a metastable state to a stable state which enhances the superparamagnetism. ZnFe2O4 with average particle size of 4 nm shows magnetic ordering even at room temperature.

A citrate process to synthesize nanocrystalline zinc ferrite from 7 to 23 nm crystallite size

Abstract Nanosized zinc ferrite of controlled crystallite size has been prepared by calcining an amorphous zinc–iron–citrate precursor at different temperatures up to 650 °C. The variation of crystallite size with calcination temperature has been investigated using X-ray diffraction (XRD), thermogravimetry–differential thermal analysis (TG–DTA), and transmission electron microscopy (TEM), and a recipe for the preparation of nano zinc ferrite of desired size has been developed. Zinc ferrite has been found to crystallize at temperatures as low as 200 °C and calcination time as little as 1 h. The crystallite size varied from 7 to 23 nm as the calcination temperature was increased from 200 to 600 °C. It has been observed that the crystallization of zinc ferrite from the amorphous powder is complete at 350 °C, forming 7-nm crystallites. Calcination at higher temperatures leads to a linear growth of the crystallites from 7 nm at 350 °C to 23 nm at 600 °C.

Fluoride adsorption studies on mixed-phase nano iron oxides prepared by surfactant mediation-precipitation technique.

Mixed nano iron oxides powder containing goethite (α-FeOOH), hematite (α-Fe(2)O(3)) and ferrihydrite (Fe(5)HO(8)·4H(2)O) was synthesized through surfactant mediation-precipitation route using cetyltrimethyl ammonium bromide (CTAB). The X-ray diffraction, FTIR, TEM, Mössbauer spectroscopy were employed to characterize the sample. These studies confirmed the nano powder contained 77% goethite, 9% hematite and 14% ferrihydrite. Fluoride adsorption onto the synthesized sample was investigated using batch adsorption method. The experimental parameters chosen for adsorption studies were: pH (3.0-10.0), temperature (35-55°C), concentrations of adsorbent (0.5-3.0 g/L), adsorbate (10-100 mg/L) and some anions. Adsorption of fluoride onto mixed iron oxide was initially very fast followed by a slow adsorption phase. By varying the initial pH in the range of 3.0-10.0, maximum adsorption was observed at a pH of 5.75. Presence of either SO(4)(2-) or Cl(-) adversely affected the adsorption of fluoride in the order of SO(4)(2-)>Cl(-). The FTIR studies of fluoride loaded adsorbent showed that partly the adsorption on the surface took place at surface hydroxyl sites. Mössbauer studies indicated that the overall absorption had gone down after fluoride adsorption that implies it has reduced the crystalline bond strength. The relative absorption area of ferrihydrite was marginally increased from 14 to 17%.

Protonation of an Oxo‐Bridged Diiron Unit Gives Two Different Iron Centers: Synthesis and Structure of a New Class of Diiron(III)‐μ‐hydroxo Bisporphyrins and the Control of Spin States by Using Counterions

Reported herein is a hitherto unknown family of diiron(III)-μ-hydroxo bisporphyrins in which two different spin states of Fe are stabilized in a single molecular framework, although both cores have identical molecular structures. Protonation of the oxo-bridged dimer (2) by using strong Brønsted acids, such as HI, HBF(4), and HClO(4), produce red μ-hydroxo complexes with I(3)(-) (3), BF(4)(-) (4), and ClO(4)(-) (5) counterions, respectively. The X-ray structure of the molecule reveals that the Fe-O bond length increases on going from the μ-oxo to the hydroxo complex, whereas the Fe-O(H)-Fe unit becomes more bent, which results in the smallest known Fe-O(H)-Fe angles of 142.5(2) and 141.2(1)° for 3 and 5, respectively. In contrast, the Fe-O(H)-Fe angle remains unaltered in 4 from the corresponding μ-oxo complex. The close approach of two rings in a molecule results in unequal core deformations in 3 and 4, whereas the cores are deformed almost equally but to a lesser extent in 5. Although 3 was found to have nearly high-spin and admixed intermediate Fe spin states in cores I and II, respectively, two admixed intermediate spin states were observed in 4. Even though the cores have identical chemical structures, crucial bond parameters, such as the Fe-N(p), Fe-O, and Fe⋅⋅⋅Ct(p) bond lengths and the ring deformations, are all different between the two Fe(III) centers in 3 and 4, which leads to an eventual stabilization of two different spin states of Fe in each molecule. In contrast, the two Fe centers in 5 are equivalent and assigned to high and intermediate spin states in the solid and solution states, respectively. The spin states are thus found to be dependent on the counterions and can also be reversibly interconverted. Upon protonation, the strong antiferromagnetic coupling in the μ-oxo dimer (J, -126.6 cm(-1)) is attenuated to almost zero in the μ-hydroxo complex with the I(3)(-) counterion, whereas the values of J are -36 and -42 cm(-1), respectively, for complexes with BF(4)(-) and ClO(4)(-) counterions.

Electron- and ion-beam-induced maneuvering of nanostructures: phenomenon and applications

Electron-and ion-induced bending (EIB/IIB) phenomena have been studied in self-supported polycrystalline metallic and metal-amorphous bilayered nanocantilevers. The experiments reveal many interesting facts regarding electron/ion-matter interaction, which builds a proper foundation for the understanding of the phenomenon. The mechanism for bending of metallic cantilevers has been proposed to be primarily due to void-induced stress generation during ion beam irradiation. On the other hand, thermal effects have been found to play the dominant role in the case of bending of bilayer (amorphous-metal) nanocantilevers. The instantaneous, reversible, highly controllable and permanent nature of the process has been exploited to fabricate several complicated nanostructures in three dimensions. IIB of the fabricated cantilevers is shown to have a high precession mass sensing aptitude, capable of detecting a change in mass of the order of femtograms.

Finite Size Effects in Magnetic and Optical Properties of Antiferromagnetic NiO Nanoparticles

We report systematic investigations on structural, magnetic and optical properties of NiO nanoparticles prepared by mechanical alloying. As-milled powders exhibit face centred cubic structure, but average particle size decreases and effective strain increases for the initial periods of milling. Lattice volume increases monotonically with a reduction in particle size. Antiferromagnetic NiO particles exhibit significant room temperature (RT) ferromagnetism with modest moment and coercivity. A maximum moment of 0.0147 μB/f.u at 12 kOe applied field and a coercivity of 160 Oe were obtained for 30 h milled NiO powder. Exchange bias decreases linearly with a decrease in NiO particle size. Thermo-magnetization data reveal the presence of mixed magnetic phases in milled powders and shifts magnetic phase transition towards high temperature with increasing milling. Annealing of milled NiO powder and photoluminescence studies show a large reduction in RT magnetic moment and blue-shifting of band edge emission peak. The observed properties are discussed on the basis of finite size effect, defect density, oxidation/reduction of Ni, increase in number of sublattices, uncompensated spins from surface to particle core, and interaction between uncompensated surfaces and particle core with lattice expansion.

Quadrupole interaction at57Fe in zirconium metal

The nuclear quadrupole interaction at57Fe nuclei in hcp α-zirconium metal is measured in the temperature range 4.2 to 560 K using Mössbauer spectroscopy of57Fe. The quadrupole splitting at room temperature is measured to be 0.660(8) mm/sec which corresponds to an electric field gradient of |eq|=3.17×1017 V/cm2 at the57Fe nucleus in a α-Zr host. As has been observed in many other systems, the results show significant electronic contributions. The temperature variation of the quadrupole interaction is much stronger than is expected from the lattice contributions and is found to follow theT3/2 dependence approximately.57FeZr does not follow the universal correlation betweeneqion andeqel observed in most of the normal metal hosts but follows the trends recently observed by Krusch and Forker for the transition metal hosts. Our results are compared with the predictions of the conduction electron charge shift model recently proposed by Bodenstedt and Perscheid.

Preparation and characterization of Cu(II), Ni(II) or Co(II) ion-doped goethite samples and their conversion to magnetite in NH3–FeSO4–H2O medium

Abstract Crystalline goethite samples doped with Cu(II), Ni(II) and Co(II) ions were prepared by a coprecipitation method using ferric nitrate, sodium hydroxide and metal ion salt solutions. These samples were characterized by XRD and were also chemically analyzed. The XRD patterns of Cu(II), Ni(II) or Co(II) ion-doped goethite matched with that of pure goethite but showed slight shift in their d-values. These samples were treated in ammoniacal medium in the presence of ferrous sulphate under the following conditions [Hydrometallurgy (2002) in press]: temperature=403 K; ammonia=25 g L −1 ; time=2 h; and ferrous iron concentration=45 g L −1 . The converted products were analyzed both for ferrous and total iron. The characterization was carried out using instrumental techniques such as TEM, TG-DTA and Mossbauer Spectroscopy. Though the XRD patterns of the converted products showed presence of only magnetite, the Mossbauer spectra and TEM showed unreacted goethite in the converted product of Cu ion-doped goethite. Pure magnetite was confirmed by chemical and instrumental analysis for the product obtained from Ni-doped goethite. A nonmagnetic ferrous iron fraction along with magnetite was confirmed from the Mossbauer spectra in the product obtained from Co ion-doped goethite. These results are compared with the products obtained in another study [Int. J. Miner. Process (2002) submitted for publication] related to the addition of Cu(II), Ni(II) or Co(II) ions during treatment of pure crystalline goethite in NH 3 –FeSO 4 –H 2 O system.

The electric field gradient at 57Fe in zinc and comparisons with the conduction-electron charge-shift model

Abstract The magnitude of the electric field gradient at 57Fe probe nuclei in Zn is measured using Mossbauer effect and the value is 2.34 × 1017 V/cm2. The electric field gradients at Fe probe nuclei in Ti, Zn, Zr, Cd and Hf hosts are compared with the predictions of the conduction-electron charge-shift model.