R.C. Srivastava

Swift heavy ion-induced effects in Ce-doped nickel ferrite nanoparticles

Swift heavy ions of various energies are being used for material modifications. The induced modifications depend on the kind of defects produced during interaction of ions with the target material. In the present work, irradiation of 200 MeV Ag beam-induced effects in NiFe2O4 and NiCe0.04Fe1.96O4 nanoparticles are studied at two different fluences, 2×1012 and 1×1013 ions/cm2. Nanoparticles of nickel ferrite and Ce-doped nickel ferrite were prepared by chemical route. X-ray diffraction pattern shows peaks corresponding to pure spinel structure in both the systems, NiFe2O4 and NiCe0.04Fe1.96O4. The pristine as well as irradiated nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, electron paramagnetic resonance spectroscopy (EPR) and vibrating sample magnetometer (VSM). Raman spectra show bands corresponding to spinel structure. After irradiation, the position of the bands does not change significantly for both samples. The widths corresponding to the same band in both the systems show opposite trend with fluence. VSM results show that after irradiation, the magnetization decreases from 40 to 32 A m2/kg for NiFe2O4 and from 39 to 31 A m2/kg for NiCe0.04Fe1.96O4. EPR results show that after doping with Ce as well as irradiation, the EPR line width is reduced, making samples important for applications.

57Fe Mössbauer investigation of nanostructured zinc ferrite irradiated by 100 MeV oxygen beam

Zinc ferrite nanoparticles of different size were irradiated in vacuum with 100 Mev O7+ ion beam at a fluence of 5×1013 ions/cm2. Presence of ZnO phase was observed after the irradiation in the samples. 57Fe Mossbauer spectroscopy shows the presence of well defined doublets in case of pristine and irradiated samples which are the attributes of superparamagnetism in the specimen. The variation of Mossbauer hyperfine parameters is discussed.

HRTEM and FTIR investigation of nanosized zinc ferrite irradiated with 100 MeV oxygen ions

Present work aims to investigate effect of 100 MeV oxygen ion irradiation on the vibrational modes of zinc ferrite nanoparticles. Nanosize zinc ferrite systems of different crystallite size ranging from 12-62 nm were irradiated at the fluence of 1×10(13) and 5×10(13) ions/cm(2). High resolution transmission electron micrograph study indicates the structural disorder induced by ion irradiation. Bands corresponding to various vibrational modes in Fourier transform infrared spectra exhibit changes and are affected by the crystallite size/microstructure of pristine samples. The irradiation induced changes are dominated for sample ZF1000.

Effect of thermal treatment on the magnetic properties of nanostructured zinc ferrite

Present report elucidates the effect of sintering time (2 and 3h) and temperature (300, 400, 500 and 600°C) on the magnetic properties of zinc ferrite nanoparticle. XRD shows the presence of cubic spinel phase in the samples. The average crystallite sizes of the samples increase with the sintering temperature and sintering time. The Mossbauer and VSM measurements show the presence of superparamagnetic/paramagnetic phase of the samples.

100 MeV O7+ ion irradiation in nanosized zinc ferrite

For the present investigation, nanosized zinc ferrite was synthesized using the nitrate method. The synthesized sample exhibits a pure spinel phase, whereas features of partial amorphization were observed when irradiated with a 100 MeV oxygen beam at a fluence of 5×1013 ions/cm2. High-resolution transmission electron microscopy investigations of these systems show that the particle size remains almost constant within experimental errors. The change in various modes of Fourier transform infrared spectroscopy spectra is attributed to the presence of defects. This leads to an increase in the value of the peak-to-peak line width of the electron paramagnetic resonance spectroscopy spectrum of the irradiated sample. Further, the change in g-values of pristine and irradiated specimens shows attributes of cation inversion after irradiation.

XAS and XMCD investigation of zinc ferrite nanoparticles irradiated with 100 MeV O beam

Synchrotron radiation based techniques have been considered superior to characterize the materials at atomic scale and to solve puzzles of physics, chemistry and biology in present decades [1, 2]. Techniques based on this radiation not only provide a description of atomic empty states but also give information about their bulk behavior. Thus simultaneous information of material characteristics at both scales by these techniques open a pathway to develop a theory regarding the behavior of materials. For more than two decades, swift heavy ion interaction with matter has been a most interesting field for researcher to understand the mechanism of the interaction which is still in debate [3, 4]. Thus investigation of this interaction with suitable techniques is demand for present era. With this motivation, synchrotron based techniques X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) have been carried out to investigate the interaction of 100 MeV O7+ ions with zinc ferrite nanoparticles of size ~ 12 nm. The nanoparticles were synthesized using nitrates of zinc and iron in presence of citric acid followed by moderate thermal treatment of 450°C, and subsequently irradiated by 100 MeV oxygen beam in vacuum (~ 10-6 Torr) with Pelletron accelerator installed at Inter University Accelerator Centre, New Delhi, India. Fluence of irradiation was kept at 5×1013 ions/cm2. Although as-deposited sample exhibits well-ordered cubic spinel phase, a slight degradation in crystalline phase is observed after irradiation. This behavior is analogues to that are reported for zinc ferrite with similar size and can be explained on the basis of distortion in the atomic position in effect of deposited energy inside the material [5]. To get glimpse of irradiation induced effects on electronic structure, XAS at O K-edge, Fe L-edge, and Zn L-edge of these samples have been measured. An O K-edge spectrum reflects approximately unoccupied oxygen p projected density of states in materials. Spectral features appear at 531, 532, 538, 541, 548, and 561 eV in O K-edge spectra for pristine and irradiated samples. Major changes occur in the pre-peak region for irradiated sample. Zn L-edge spectra shows spectral feature at 1026, 1030, 1035, 1041, and 1053 eV for pristine and irradiated samples [6]. Fe L-edge XAS spectra exhibit spectral features positioned around 707, 709, 720, and 722 eV. The spectral features at Fe L-edge occur due to Fe 2p core-level which gives rise to degenerate state of 2p3/2 and 2p1/2 showing multi-plets centered on ~708 and 721 eV in effect of spin-orbit coupling. The octahedral crystal field lifts the degeneracy of 2p3/2 and 2p1/2 levels so that two levels with t2 and e symmetry are created as indicated by two structures at about 707, 709 eV (L3) and 720, 722 eV (L2). Presence of these spectral features indicates that Fe remains in the ferromagnetic high spin 3d configuration. Intensities of various spectral features for XAS spectra at different edges were reduced after irradiation. This has been associated with the increase of broken bonds in the irradiated samples. Bond breaking in irradiated materials is consequences of energy deposition in the form of thermal spikes [7]. Further, to investigate magnetic properties, we have recorded the Fe L-edge transitions under magnetic field induced by right and left circularly polarized light. Recorded Fe L-edge XAS spectra (denoted by μ and μ- for right and left circurly polarized light in Figure 1) at 300 K for pristine and irradiated samples are very much similar to that are reported for zinc ferrite thin films and nanoparticles [8]. XMCD to XAS signal intensity ratio is estimated as 1.96 to 2.98 employing increase of 3d magnetic moment after irradiation. To explore the nature of magnetic interaction, XMCD spectral features A, B1 and B2 of L3-edge were utilized. XMCD signals for pristine sample exhibit positive peak corresponding to feature A and negative peaks corresponding to B1, B2 features (Figure 1). These features are associated with presence of Fe3+ ions at tetrahedral site and octahedral sites, respectively. Combined area of B1 and B2 peaks represent total concentration of Fe3+ ions at octahedral site. From the area ratio of A and B1, B2 features, Fe3+ ratio for tetrahedral to octahedral site is estimated as ~0.20. This ratio is similar to that are reported for zinc ferrite samples with similar thermal treatment [9]. Apart from this, irradiated sample exhibits negative peak for feature A indicating change in magnetic interaction with irradiation. The observed results have been discussed on the basis of existing theories.

Magnetic resonance in superparamagnetic zinc ferrite

In the present work, we have synthesized zinc ferrite nanoparticles by nitrate method. Presence of almost zero value of coercivity and remanence in the hysteresis of these samples shows the superparamagnetic nature at room temperature. Electron paramagnetic resonance spectroscopy performed on these samples in the temperature range 120–300xa0K indicates the systematic variation of the line-shapes of the spectra with temperature. Both g-value and peak-to-peak linewidth decrease with increase in temperature. The variation of g-values and peak-to-peak linewidth with temperature has been fitted with existing models and we observed different values of activation energies of the spins for both the samples.

Effect of sintering temperature on the structural properties of cobalt ferrite nanoparticles

The nanocrystalline CoFe2O4 was prepared by nitrate route at the sintering temperature of 300, 500, 700 and 900°C. The X-ray diffraction (XRD) patterns of all the samples show the single phase spinel structure of nanoparticles. The crystallite sizes vary from 9 to 61 nm as the sintering temperature increases from 300 to 900°C. The FTIR spectra of cobalt ferrite were recorded in the range of 400 to 5,000 cm–1 at room temperature. FTIR spectra of the cobalt ferrite nanoparticles show no residual organic compounds and confirm the formation of the organic free cobalt ferrite nanoparticles above the sintering temperature 500°C. The assigned Raman modes are the characteristics of the spinel structure of the synthesised samples which change slightly with different sintering temperature. The Raman modes of cobalt ferrite nanoparticles at 300°C temperature follow the phonon confinement as the particle size is ~9 nm.

Structural, specific heat and magnetoresistive properties of Gd0.7Ca0.3MnO3

Here, we report the structural, specific heat and magnetoresistive properties of orthorhombic perovskite structured Gd0.7Ca0.3MnO3, synthesised by modified sol-gel technique. X-ray diffraction along with the Rietveld refinement confirms the pure phase of the sample and the crystallite size comes out to be ~42 nm. Scanning electron micrograph shows the grain distribution having average particle size ~71 nm. Energy dispersive spectroscopy confirms the stoichiometry of the composition. The sample is also characterised by Raman spectroscopy for structural and phase confirmation. The functional group of Gd0.7Ca0.3MnO3 was also analysed from its Fourier transform infrared (FTIR) spectrum. The specific heat of the sample is measured in the temperature range of 2 to 300 K. Resistivity measurements with and without magnetic field was performed. The activation energy in absence of magnetic field is 0.162 eV which reduces to 0.154 eV in presence of 8 T magnetic field. The observed electrical behaviour is explained in terms of polaron hopping mechanism.