H.M. Agrawal

EPR STUDY OF NANOSTRUCTURED ZINC FERRITE

Bulk zinc ferrite has been studied in the past. In the bulk form zinc ferrite crystallizes in the normal spinel structure with all the zinc ions occupying the tetrahedral and all the iron occupying the octahedral sites, respectively. But, nanosize zinc ferrite is believed to crystallize in a different way. It is now well established that in a nanosize zinc ferrite there is partial inversion in the cation occupancy, leading to some kind of magnetic ordering in this system much above its Neel temperature of 10 K. Hence, there is ample scope of maneuvering the electrical and magnetic properties of this ferrite. In the present study nanosize zinc ferrite was prepared by a chemical reaction involving the nitrates of Zn and Fe and using the citric acid as the host. The sintering of the precursor was done for 1 h at various temperatures ranging from 300°C to 1000°C. These samples were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), and electron paramagnetic resonance (EPR). The average particle sizes in these sintered samples measured by TEM are found to vary from ≈ 10 to 62 nm. The XRD measurements show the formation of single-phase spinel structure in all the samples. The X-band EPR spectrum of the precursor specimen at room temperature consists of two EPR signals: an intense signal at geff = 2.02 having peak-to-peak line width △HPP = 425 Gauss and other a weak signal at geff = 4.51 having an asymmetric line shape. As the samples are heat-treated at different temperatures, the EPR signal corresponding to the intense peak gets stabilized at geff ≈ 2.03, whereas the weak signal gets suppressed progressively. The spin–spin relaxation time in these samples have values ≈ 3 × 10-10s. The effect of sintering temperature on the properties of the nanostructured specimen of zinc ferrite and the underlying mechanism would be discussed.

Looking for the possibility of multiferroism in NiGd0.04Fe1.96O4 nanoparticle system

This paper reports the study of structural, electrical, magnetic and optical properties of Gd-doped nickel ferrite nanoparticles in order to investigate the possibility of multiferroism. The NiGd0.04Fe1.96O4 nanoparticles were prepared by a chemical route and characterized by various techniques. Doping with Gd ions induces poor ferroelectricity in nickel ferrite. This may be attributed to the small distortion in the centrosymmetric fcc structure because of the presence of large Gd ions.

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.

RELAXATION PHENOMENA IN NANOSTRUCTURED ZINC FERRITE

The electron paramagnetic resonance (EPR) is a well known tool to investigate the magnetic relaxation phenomena in the magnetic particles. For the present investigation, temperature variant EPR has been performed in order to study the relaxation mechanism in zinc ferrite nanoparticles. The magnetic nanoparticles were synthesized by using the nitrates of zinc and iron, and citric acid. The particle size of the samples were measured by the X-ray diffraction and Transmission Electron Microscopy. A more precise, Williamson–Hall (W–H) approach was used for the determination of the particle size as well as the strain in nanoparticles. The kinematics of magnetic moment has been studied with the help of temperature dependent EPR spectroscopy. Relaxation time calculations and temperature dependence of linewidth show the dominance of spin-lattice relaxation in these systems. Both nanoparticle systems show the presence of direct and Raman process in the relaxation mechanism and completely rule out the presence of Orbach process.

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.

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.

Magnetic anomalies in specific heat and dielectric properties of multiferroic GdMnO3

The specific heat and dielectric properties of multiferroic GdMnO3 synthesized by modified sol-gel technique has been investigated. GdMnO3 crystallized in orthorhombic perovskite structure. X-ray diffraction was used to investigate the structural information. Morphology has been investigated by field emission scanning electron microscope and the average grain size is found to be ∼ 552 nm. Magnetic anomalies have been observed in specific heat and dielectric properties of GdMnO3.