Pure, crystalline, ∼10 nm lithium ferrite phase (Li0.5Fe2.5O4), was successfully synthesized at very low temperature using a modified combustion method. The crystal structure and microstructure evolution of this system upon annealing were monitored by a careful investigation of X-ray diffractograms collected on a synchrotron source. Comparative analysis of the results obtained from the full profile Rietveld method (in reciprocal space) and the pair distribution function method (in direct space) was carried out. Nanocrystalline samples exhibit similar crystal structure, on average, with a partial ordering of Li+ and Fe3+ ions between octahedral 4b and 12d sites on the spinel crystal lattice (space group P4332). After annealing at 973 K, cation distribution changes to a completely ordered, resembling that which is seen in the bulk lithium ferrite. The PDF analysis reveals abnormally high values of oxygen atomic displacement parameters in tetrahedral 8c sites (O1) indicating a significant disordering of the O1 network and suggests migration of lithium ions from 4b sites to the outer layers of nanoparticles. Analysis of room temperature Mössbauer spectra has shown that the hyperfine field for Fe3+ ions in tetrahedral 8c sites is the most sensitive on increasing the particle size and improving the crystallinity. From the differential thermal analysis, it was found that a lower driving force is required to induce an order−disorder phase transition in nanocrysalline samples, compared to the bulk-like sample, presumably due to the higher crystal disordering in these samples.

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Herein we present the results of specific loss power (SLP) analysis of polydisperse water based ferrofluids, Fe 3 O 4 /PEG200 and Fe 3 O 4 /PEG6000, with average Fe 3 O 4 particle size of 9 nm and 11 nm, respectively. Specific loss power was measured in alternating magnetic field of various amplitudes and at fixed frequency of 580.5 kHz. Maximum SLP values acquired were 195 W/g for Fe 3 O 4 /PEG200 and 60 W/g for Fe 3 O 4 /PEG6000 samples. The samples were labeled as superparamagnetic by magnetization measurements, but SLP field dependence showed deviation from the behavior predicted by the commonly employed linear response theory. The scope of this theory for both samples with wide particle size distribution is discussed. Deviation from the expected behavior is explained by referring to polydisperse nature of the samples and field dependent relaxation rates. V C 2015 AIP Publishing LLC. [http://dx

In this paper we report results of structural, spectroscopic, and magnetic investigations of MgFe2O4 nanoparticles prepared by soft mechanochemical synthesis. MgFe2O4 nanoparticles crystallize in Fd3¯m space group with mixed cation distribution and reduced percentage of Fe3+ at tetrahedral (8a) sites. Discrepancy in the cation distribution compared to that in the bulk Mg–ferrite is one of the highest known. X-ray line broadening analysis reveals crystallite size and strain anisotropy. The saturation magnetization, Msat=62 emu/g measured at 5 K is twice higher than that found in the bulk counterparts. Such high value of Msat is attributed to the low value of cation inversion parameter (δ=0.69), to the core/shell structure of the nanoparticles and to the surface/volume ratio. Mössbauer spectrum collected at room temperature reveals ferrimagnetic ordering between Fe3+ ions in 8a and 16d sites, while zero-field-cooled (ZFC) and field-cooled (FC) M(T) measurements were shown superparamagnetic state above 350 K.

In this paper we present the results of the synthesis, crystal structure investigations and in situ X-ray diffraction studies of the order−disorder phase transition in cobalt substituted lithium titanate oxide spinels, Li1.33xCo2−2xTi1+0.67xO4 (0 ≤ x ≤ 1). Depending on the chemical composition the samples crystallize in two space groups (S.G.): Fd3m (0 ≤ x ≤ 0.40 and x = 1) and P4332 (0.50 ≤ x ≤ 0.875). Samples crystallizing in the S.G. P4332 are ordered spinels with a cation ordering of the 1 : 3 type at octahedral 4b and 12d sites. The cation ordering in octahedral sites is full in the sample with x = 0.75 (Li and Ti occupy 4b and 12d sites, respectively) and decreases for samples with higher/smaller x. Changes of the extinction conditions and nonlinearities in the concentration dependence of the lattice parameter in the regions 0.40 < x < 0.50 and 0.875 < x < 1 indicate changes of the crystal symmetry (Fd3m ↔ P4332). The partially ordered spinel x = 0.50 has a convergent, reversible, order−disorder phase transition at TC = (1083 ± 10) K. Samples with x = 0.875 and 0.75 have an order−disorder phase transition out of our experimental ranges with TC(x = 0.875) < 973 K and TC(x = 0.75) > 1173 K. The mechanism of the phase transition is based on cation migration.

We present a study of magnetic and structural properties of CoFe2O4nanoparticles suspended in an organic liquid. Transmission electron microscopy shows that the nanoparticles have a narrow size distribution of average particle size 5.9 ± 1.0 nm. X-ray diffraction shows that the particles are of cubic spinel crystal structure. Dynamic light scattering measurements reveal the existence of an organic shell around the CoFe2O4nanoparticles with an average hydrodynamic diameter of 14.4 nm. Coercive magnetic field atT=5 K is found to be 11.8 kOe. Disappearance of the coercive field and remanent magnetization at about 170 K suggests that the CoFe2O4nanoparticles are superparamagnetic at higher temperatures which is confirmed by the room temperature Mössbauer spectrum analysis. Saturation magnetization of the nanoparticles of 80.8 emu/g(CoFe2O4) at 5 K reaches the value detected in the bulk material and remains very high also at room temperature. The cobalt ferrite nanoparticle system synthesized in this work exhibits magnetic properties which are very suitable for various biomedical applications.

DC magnetization and AC susceptibility measurements point to the formation of a spin glass state in the ternary spinel-type compounds Li 1.33x Co 2−2x Ti 1+0.67x O 4. The dynamics of spin freezing was analysed with both the critical slowing down and the thermally activated dynamics models. The parameter values obtained, as well as the behaviour of the zero-field-cooled and field-cooled magnetization as a function of temperature, indicate the existence of a cluster glass state in disordered spinel samples with x = 0.25 and 0.40, and probably also in the ordered spinel with x = 0.50. Ordered spinel samples with x = 0.75 and 0.875 were found to be paramagnetic down to the temperature of 1.7 K with a random distribution of Co 2+ ions.