Transition-metal-oxide/transition-metal nanocomposites, such as NiO/Ni, FeO/Fe, and CoO/Co, have been the subject of much recent investigation (i) because of their potential applications and (ii) because they are good model systems for studies of some effects on the nanoscale. They are used, for example, as catalysts, fuel-cell electrodes, magnetic memories, etc. When a nanocomposite is composed of both ferromagnetic (FM) and antiferromagnetic (AFM) nanoparticles, interesting physical properties can occur, such as the phenomenon of exchange bias (EB). A Ni/NiO nanocomposite obtained by the thermal decomposition of nickel(II) acetate tetrahydrate, Ni(CH3COO)2·4H2O, at 300 °C is composed of NiO (62%) and Ni (38%) with crystallite sizes of 11 and 278 nm, respectively. We observed an increase in the crystallite size for NiO and decrease of crystallite size for Ni, a decrease in the microstrain for both and an increase in the NiO phase content with thermal annealing in air, while high-energy ball milling leads to a decrease of the crystallite size, an increase in the size of the agglomerates, and microstrain as well as reduction, NiO → Ni. The lattice parameters of the nanosized NiO and Ni show a deviation from the value for the bulk counterparts as a consequence of crystallite size reduction and the grain-surface relaxation effect. The exchange bias found in a milled sample with particles of 10 nm (NiO) and 11 nm (Ni) disappears for larger particles as a consequence of a coupling-area decrease between the antiferromagnetic and ferromagnetic particles. Due to reduction/oxidation (NiO ↔ Ni) and size as well as surface-relaxation effects the saturation magnetization value increases/decreases with milling/annealing, respectively. Having in mind the effect of size on the exchange bias, coercivity, and magnetization values, it is possible, by annealing/milling, to tailor the composition and particle size and then control the exchange bias and improve the other magnetic properties of the Ni/NiO.

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Evolution of the structural and magnetic properties of ZnFe 1.95 Yb 0.05 O 4 nanoparticles, prepared via a high-energy ball milling route and exposed to further thermal annealing/heating, was assessed in detail and correlation of these properties explored. While as-prepared spinel nanoparticles possess a high degree of inversion, heating of the sample to ∼500 °C is found to rapidly alter the cation distribution from mixed to normal, in agreement with the known cation preferences. Under the same conditions the crystallite size only slowly grows. By further thermal treatment at higher temperatures, the crystallite size is changed more appreciably. An interrelationship among the lattice parameter, octahedral site occupancy, and crystallite size has been established. The observations are (a) both the site occupancy of Fe 3+ at octahedral 16d spinel sites (N 16d (Fe 3+ )) and the cubic lattice parameter rapidly increase with an initial increase of the crystallite size, (b) the lattice parameter increases with increasing occupancy, N 16d (Fe 3+ ), and (c) there appears to be a critical nanoparticle diameter (approximately 15 nm) above which both the site occupancy and lattice parameter values are saturated. The magnetic behavior of the annealed samples appears to be correlated to the evolution of both the cation distribution and crystallite size, as follows. As-prepared samples and those annealed at lower temperatures show superparamagnetic behavior at room temperature, presumably as a consequence of the Fe 3+ distribution and strong Fe 3+ (8a)−O−Fe 3+ (16d) superexchange interactions. Samples with a nanoparticle diameter greater than 12 nm and with almost normal distributions exhibit the paramagnetic state. The coercive field is found to decrease with an increase of the crystallite size. Partial Yb 3+ /Fe 3+ substitution is found to increase the inversion parameter and saturation magnetization. Detailed knowledge of the thermal evolution of structural/microstructural parameters allows control over the cation distribution and crystallite size and hence the magnetic properties of nanoferrites.

A study of the microstructures and magnetic properties of nanosize Zn ferrite (ZnFe 2 O 4 ), Mn ferrite (MnFe 2 O 4 ), and the cation deficit Zn-Mn ferrites Zn 0.70 Mn 0.23 Fe 1.89 O 4 (S1), Zn 0.41 Mn 0.50 Fe 1.84 O 4 (S2) and Zn 0.18 Mn 0.67 Fe 1.85 O 4 (S3) was performed. The crystallite size for all samples was determined by x-ray powder diffraction (XRPD) analysis using four different methods, and was close to the particle size found from transmission electron microphotography. Among different methods of XRPD line broadening analysis it seems that the cubic harmonic function method is more precise and reliable than the Warren-Averbach and simplified integral breadth methods. M(T ) and M(H ) magnetization curves at different fields/temperatures indicate superparamagnetic behaviour of the samples. Asymmetric hysteresis loops and differences in coercive fields, H C− (FC) − H C− (ZFC), are discussed by both the core/shell model of nanoparticles and spin canting. The magnetic measurements with a maximum in the FC magnetization branches, the difference in M/M S versus H /T curves above T max (temperature of maximum in ZFC magnetization), the nonlinearity in H C versus T 1/2 , the remanence/saturation ratio value, M R /M S and observation of the Almeida-Thouless line for low-field magnetization data (T max versus H 2/3 ) indicate that the samples consist of an interacting ferrite nanoparticle ensemble.

Spherical shaped nanoparticles of series Y(2 - x)Eu(x)O(3) (x = 0.06, 0.10, 0.20, and 2) and Gd(2 - x)Eu(x)O(3) (x = 0.06, 0.10) were prepared by thermolysis of 2,4-pentanedione complexes of Y, Gd, and Eu. The bixbyite phase of Gd(2 - x)Eu(x)O(3) samples was formed at 500 degrees C, whereas the thermal decomposition of Y and Eu complexes' mixtures occurred at higher temperatures. Linearity in the concentration dependence on lattice parameter confirmed the formation of solid solutions. The distribution of Eu(3+) in Gd(2 - x)Eu(x)O(3) was changed with thermal annealing: in the as-prepared sample (x = 0.10) the distribution was preferential at C(3i) sites while in the annealed samples, Eu(3+) were distributed at both C(2) and C(3i) sites. Rietveld refinement of site occupancies as well as emission spectra showed a random distribution of cations in Y(2 - x)Eu(x)O(3). The photoluminescence (PL) measurements of the sample showed red emission with the main peak at 614 nm ((5)D(0)-(7)F(2)). The PL intensity increased with increasing concentration of Eu(3+) in both series. Infrared excitation was required to obtain good Raman spectra. The linear dependence of the main Raman peak wavenumber offers a non-destructive method for monitoring the substitution level and its homogeneity at the micron scale.