A single-step synthesis of magnesium ferrite (MgFe2O4) nanoparticles with an average crystallite size of about 8.5 nm synthesized via mechanochemical processing of binary oxide precursors at room temperature is reported. The study highlights the nature of the cation disorder and of the spin arrangement in mechanosynthesized MgFe2O4 as well as its response to changes in temperature. An unusual property of the magnetization enhancement in nanoscale mechanosynthesized MgFe2O4 is reported. Whereas the inner core of a MgFe2O4 nanoparticle exhibits a partly inverse spinel structure with a Neel type collinear spin alignment, the major features of the ionic and spin configurations in the grain boundary (surface) region are a nonequilibrium cation distribution and a canted spin arrangement. Although the spin-canting effect tends to reduce the magnetic moment, the magnetization enhancement exhibited by mechanosynthesized MgFe2O4 is attributed to the nearly random distribution of magnetic cations in the surface regi...

Nonequilibrium Cation Distribution, Canted Spin Arrangement, and Enhanced Magnetization in Nanosized MgFe2O4 Prepared by a One-Step Mechanochemical Route

The magnetic properties of copper ferrite (CuFe2O4) nanoparticles prepared via sol–gel auto combustion and facile solvothermal method are studied focusing on the effect of nanoparticle arrangement. Randomly oriented CuFe2O4 nanoparticles (NP) are obtained from the sol–gel auto combustion method, while the solvothermal method allows us to prepare iso-oriented uniform spherical ensembles of CuFe2O4 nanoparticles (NS). X-ray diffractometry (XRD), atomic absorption spectroscopy (AAS), infra-red (IR) spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), 57Fe Mossbauer spectroscopy and vibrating sample magnetometer (VSM) are used to investigate the composition, microstructure and magnetic properties of as-prepared ferrite nanoparticles. The field-dependent magnetization measurement for the NS sample at low temperature exhibits a step-like rectangular hysteresis loop (MR/MS ∼ 1), suggesting cubic anisotropy in the system, whereas for the NP sample, typical features of uniaxial anisotropy (MR/MS ∼ 0.5) are observed. The coercive field (HC) for the NS sample shows anomalous temperature dependence, which is correlated with the variation of effective anisotropy (KE) of the system. A high-temperature enhancement of HC and KE for the NS sample coincides with a strong spin–orbit coupling in the sample as evidenced by significant modification of Cu/Fe–O bond distances. The spherical arrangement of nanocrystals at mesoscopic scale provokes a high degree of alignment of the magnetic easy axis along the applied field leading to a step-like rectangular hysteresis loop. A detailed study on the temperature dependence of magnetic anisotropy of the system is carried out, emphasizing the influence of the formation of spherical iso-oriented assemblies.

Influence of spherical assembly of copper ferrite nanoparticles on magnetic properties: orientation of magnetic easy axis

Manganese–nickel ferrites powder with general formula MnxNi1−xFe2O4 (x = 0.0, 0.2, 0.4) were synthesized through oxalate precursor route and sintered at 1000 °C. The X-ray diffraction (XRD) patterns were measured for the prepared samples to confirm the existence of single-phase structure. The crystallite size was estimated and found to be within the range 125–170 nm. To study the radiation effect on the structure and magnetic properties, a representative group of the investigated samples were irradiated by γ-rays of 60Co source with a dose of 310 kGy. The XRD spectra were performed for the irradiated samples and compared with that of the pristine samples to estimate changes in the structure. The obtained results showed that the crystallite size increased by a factor of 10–16% after gamma irradiation. The lattice parameter also was increased due to the conversion of Fe3+ (0.64 A) to Fe2+ (0.76 A). The formula of the cation distribution of the ferrites samples was suggested at x = 0, 0.2, 0.4 before and after irradiation. The theoretical lattice parameter, sample density and porosity were calculated and compared with that obtained from the experimental data. Good agreement was found between theoretical and experimental structural data which confirms the proposed formula of cations distribution. The hysteresis curves were measured using vibrating sample magnetometer (VSM) for the unirradiated and irradiated samples and the saturation magnetization was estimated. The obtained results showed increase in saturation magnetizations (Ms) for all the samples by irradiation due to redistribution of the cations between A and B sites and changing the net magnetic moments. Theoretical calculation of magnetic moments and saturation magnetization using the proposed cations distribution of A and B sites confirmed the experimental results. The positron annihilation lifetime spectroscopy (PALS) was used to investigate the defects and changes in electron density after irradiation. The PAL parameters (τ1, I1, τ2, I2 and mean lifetime) show that the irradiation affects the size and concentration of the vacant type defects. The results reveal that there are some large voids (with radius ranged from 0.28 to 0.38 nm and mean value of 0.34 ± 0.04 nm in the studied samples). The obtained results indicate the high sensitivity of PALS technique to the enhanced structure changes due to gamma rays irradiation.

Effect of γ-rays irradiation on Mn–Ni ferrites: Structure, magnetic properties and positron annihilation studies

Nanocrystalline nickel ferrite (NiFe 2 O 4 ) is synthesized at room temperature by high-energy ball milling the stoichiometric mixture of (1:1 mol%) of NiO and α-Fe 2 O 3 powders. The structural and microstructural evolution of NiFe 2 O 4 caused by milling is investigated by X-ray powder diffraction. The relative phase abundance, particle size, r.m.s. strain, lattice parameter changes of different phases have been estimated employing Rietveld structure refinement analysis of X-ray powder diffraction data. Particle size and content (wt%) of both NiO and α-Fe 2 O 3 phases reduce rapidly with increasing milling time and a significant amount of nanocrystalline NiFe 2 O 4 is formed within 1 h of ball milling. Particle sizes of all the phases reduce to ∼10 nm within 5 h of milling and remain almost unchanged with increasing milling time up to 20 h. Lattice parameter of cubic NiO decreases linearly with increasing milling time, following the Vegard's law of solid-solution alloy. A continuous decrease in lattice parameter of cubic NiFe 2 O 4 phase clearly suggests that smaller Ni atoms have occupied some of the vacant oxygen sites of ferrite lattice. Cation distribution both in octahedral and tetrahedral sites changes continuously with milling time and the normal spinel lattice formed at early stage of milling, transforms to inverse spinel lattice in the course of milling. High-resolution transmission electron microscope (HRTEM) micrographs of 11 h milled sample corroborates the findings of X-ray profile analysis.

Microstructure characterization of mechanosynthesized nanocrystalline NiFe2O4 by Rietveld's analysis

Effect of chromium substitution on the structural and magnetic properties of nanocrystalline zinc ferrite

Nanocrystalline non-stoichiometric cadmium ferrite (CdFe2O4) has been synthesized by high-energy ball milling the mixture (1:1 mol%) of CdO and α-Fe2O3 at room temperature. Formation of nanocrystalline CdFe2O4 as normal spinel structures has been noticed after ball milling the mixture for different durations. The structural and microstructural evolution of CdFe2O4 caused by milling is investigated by X-ray powder diffraction. The relative phase abundances, particle sizes, r.m.s strains, lattice parameter changes of different phases have been estimated employing Rietveld powder structure refinement analysis of XRD data. Particle size and amount (wt%) of CdO reduce very fast in comparison to α-Fe2O3 and a significant amount of nanocrystalline CdFe2O4 is formed within 5 h ball milling of CdO+α-Fe2O3 powder mixture. It appears that solid state diffusion of nanocrystalline CdO and α-Fe2O3 particles results in formation of nanocrystalline CdFe2O4 spinel. A significant amount of α-Fe2O3 did not take part in ferrite formation within 25 h of milling time and even after post annealing the 5 h milled samples at different temperatures (up to 1273 K) for different durations, ~13 wt% of α-Fe2O3 remained as an excess phase. It indicates that the ferrite prepared by ball milling is a non-stoichiometric Cd-riched phase. However the ferrite phase content increases considerably when the 5 h milled sample is annealed at different elevated temperatures for different durations.

http://iopscience.iop.org/article/10.1143/JJAP.43.5455/pdf

Microstructure Characterization and Phase Transformation Kinetic Study of Mechanosynthesized Non-Stoichiometric CdFe2O4 by Rietveld's Analysis

powder under open air. Formation of both normal and inverse spinel ferrite phases was noticed after 30 minutes and 2.5 hours ball milling respectively and the content of inverse spinel phase increased with increasing milling time. The phase transformation kinetics towards formation of ferrite phases and microstructure characterization of ball milled ZnFe

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Microstructural Changes and Effect of Variation of Lattice Strain on Positron Annihilation Lifetime Parameters of Zinc Ferrite Nanocomposites Prepared by High Enegy Ball-milling

Srinjoy Bid

Papers

Microstructure Characterization and Phase Transformation Kinetic Study of Mechanosynthesized Non-Stoichiometric CdFe2O4 by Rietveld's Analysis

Microstructural Changes and Effect of Variation of Lattice Strain on Positron Annihilation Lifetime Parameters of Zinc Ferrite Nanocomposites Prepared by High Enegy Ball-milling

Microstructure and positron annihilation studies of mechanosynthesized CdFe2O4

Microstructure and positron annihilation studies of mechanosynthesized