R. Justin Joseyphus

Unusually high coercivity and critical single-domain size of nearly monodispersed CoFe2O4 nanoparticles

Nearly monodispersed CoFe2O4 nanoparticles with average sizes between 8 and 100 nm were synthesized by using seed-mediated growth dominant coprecipitation and modified oxidation methods. X-ray diffraction and Mossbauer spectroscopy analyses confirmed the spinel phase and a stoichiometric composition of (Co0.25Fe0.75)[Co0.75Fe1.25]O4 for powders with different particle diameters. Rotational hysteresis loss (Wr) analysis showed an average switching field (Hp) of 17 kOe and a magnetic anisotropy field (Hk) of 38 kOe for the 40 nm CoFe2O4 particles. The corresponding magnetocrystalline anisotropy energy constant (K) was about 5.1×106 erg/cc. The Hc and Hp results suggest that the critical single-domain size of CoFe2O4 is about 40 nm. The room temperature coercivity (Hc) of the 40 nm CoFe2O4 particles is found to be as high as 4.65 kOe.

Grain size effect on the Néel temperature and magnetic properties of nanocrystalline NiFe2O4 spinel

Nanocrystalline NiFe2O4 spinel ferrites with various grain sizes have been synthesized by ball milling the bulk NiFe2O4. The average grain sizes were estimated from the X-ray line broadening of the (3 1 1) reflection. The Neel temperatures of NiFe2O4 for various grain sizes were determined by magneto thermogravimetric method. The magnetic behaviour has been explained by combining the effects of changes in cation distribution on milling and finite size scaling. The shift in B-H loops has been correlated to the surface spin effects. The high coercivities observed here may be due to high anisotropies of the milled samples. The Hopkinson peak observed just below the Neel temperature has been explained by the mathematical formalism given by the Stoner Wohlfarth model.

Dielectric relaxation behaviour of nanostructured Mn–Zn ferrite

Dielectric measurements and modulus analysis have been made to investigate the effects of grain size, frequency and temperature for nanostructured Mn–Zn ferrite. The anomalous frequency dependence of dielectric loss (tan δ) can be attributed to the resonance effect and also to the presence of both n- and p-type charge carriers. The tan δ for 59 and 69 nm grain size samples is found to be an order of magnitude smaller than those of bulk particles. Dielectric relaxation studies using modulus formalism have shown the presence of the non-Debye type of dielectric relaxation in these materials.

Ferrimagnetic ordering in nanostructured CdFe2O4 spinel

Nanostructured CdFe2O4 spinel powders with various grain sizes ranging from 60 to 4 nm were synthesized by ball milling the bulk material. The magnetization measurements at 1 and 9 T reveal a spin-glass-like surface structure and the material is found to have a large anisotropy. Mossbauer studies at 10 K in external magnetic fields of 6 and 8.5 T applied parallel to the direction of gamma rays could clearly show that CdFe2O4 behaves like a mixed spinel on reducing the grain size to nanometer level and exhibits ferrimagnetic ordering. Fe3+ spins at both A and B sites exhibit spin canting.

Synthesis and magnetic properties of flower-like FeCo particles through a one pot polyol process

FeCo alloys of various compositions with flower-like morphology were synthesized using a unique one pot polyol process. The morphology of Fe particles was cubic, whereas the FeCo particles showed flower-like morphology, with more petals for the Co rich FeCo. The average particle size varied from 120 to 155 nm depending on the composition of the alloy. The Curie temperature as determined by thermomagnetic analysis was 985°C for Fe67Co33 and 939°C for the Fe36Co64 samples. Their corresponding bcc to fcc phase transformation temperatures were 985 and 825°C, respectively. Coercivity up to 511Oe was observed due to the shape anisotropy arising out of the flower-like morphology compared to the usual cubic or spherical morphologies. Post-annealing studies showed that Fe67Co33 is more stable compared to other compositions.

Magnetic properties of FeCo alloy nanoparticles synthesized through instant chemical reduction

The chemical synthesis of shape and composition controlled Fe based binary alloys has been challenging due to the highly oxidizing nature of Fe. Here, we report the physical properties of flower-like Fe50Co50 nanoparticles prepared by a unique polyol process based on the addition of precursors at the elevated temperature. The magnetic properties are correlated through synchrotron radiation based X-ray diffraction and 57Fe Mossbauer spectrometry. Transmission electron microscopy analysis exposed the flower-like morphology of the FeCo particles. The FeCo nanoparticles showed a coercivity of 440 Oe, attributed to the shape anisotropy of the flower-like shape. Room temperature Mossbauer investigation revealed hyperfine fields of 34.9 and 36.7 T, suggesting two different Fe environments in the disordered state. Mossbauer analysis also showed the presence of superparamagnetic Fe-oxide with a relative fraction of 17%.

ESTIMATION OF LATTICE STRAIN, STRESS, ENERGY DENSITY AND CRYSTALLITE SIZE OF THE SPHERICAL YTTRIUM OXIDE NANOPARTICLES

Yttria nanoparticles are synthesized by co-precipitation method and as-prepared nanoparticles are annealed at various temperatures. The as-prepared and annealed particles are characterized by X-ray diffraction and transmission electron microscope (TEM). Here we estimated the lattice strain, crystallite size, deformation stress, and deformation energy density for annealed (800°C) yttrium oxide nanoparticles by Williamson-Hall-Isotropic Strain Model (W-H-ISM), W-H-Anisotropic Strain Model (W-H-ASM) and W-H-Energy Density Model (W-H-EDM) based on W-H plot from powder X-ray diffraction data. The shape and size of the nanoparticles are determined using TEM. The results of the estimated crystallite size of yttria nanoparticles by various methods agreed with the TEM results.

Superparamagnetic Particle Size Limit of Mn‐Zn Ferrite Nanoparticles Synthesised Through Aqueous Method

Mn0.67Zn0.33Fe2O4 nanoparticles with size ranging from 20 to 80 nm have been synthesized using the modified oxidation method. The Curie temperatures for all the samples are found to be within 630 ± 5 K suggesting that there is no size‐dependent cation distribution. Mossbauer studies on the synthesized nanoparticles suggest that the critical particle size limit for superparamagnetism to be about 25 nm at 293 K.

Positron Annihilation Studies on Chemically Synthesized FeCo Alloy

Equiatomic flower-like FeCo magnetic nanoparticles are synthesized through a modified one-pot polyol technique. The as-prepared samples are annealed at 700 and 800 °C under reducing atmosphere. The saturation magnetization and coercivity of the flower-like FeCo are found to be 198 (1) emu/g and 243 (10) Oe respectively. The magnetic properties of FeCo approach the bulk behavior with annealing. Positron lifetime studies on the chemically synthesized equiatomic FeCo magnetic nanoparticles with flower-like morphology are reported and compared with Fe, Co and FeCo annealed at various temperatures. The FeCo is characterized by different lifetime components corresponding to positron annihilation events in vacancies and various open volume defects due to their unique morphology. The studies suggest defects arising out of cluster vacancies and interpetal gap that reduce on annealing. The average pore size obtained from positron annihilation studies closely matches with the interpetal distance obtained from the electron microscopic analysis for the flower-like FeCo.

Magnetic Nanoparticle Flow Characteristics in a Microchannel for Drug Delivery Applications

Magnetic particle flow characteristics in a microchannel were studied using Fe particles of average sizes 25 nm and 150 nm dispersed in Dextran. The particles were subjected to different applied fields and flow rate. The results suggest the requirement of an optimum field strength and flow rate for the two different particles.