Nanoparticles of antiferromagnetic LaFeO3 were prepared by the sol–gel method. An exchange bias effect has been observed and is attributed to the exchange coupling between the ferromagnetic shell and antiferromagnetic core of the particles. The results provide clear evidence of the presence of spontaneous exchange bias in this system. After field cooling from room temperature, the exchange bias increases while the coercivity decreases with decreasing temperature. Taking into account the role of thermal activation, the temperature dependence of exchange bias and coercivity has been interpreted in terms of the spontaneous exchange bias mechanism proposed recently.

We have investigated the ground-state properties of polycrystalline CeCoAsO by means of magnetization, specific heat, and solid-state NMR. Susceptibility and specific-heat measurements suggest a ferromagnetic order at about, T C = 75 K. No further transitions are found down to 0.5 K. At 6.5 K a complex Schottky type of anomaly shows up in the specific-heat results. The interplay between Ce 4f and Co 3d magnetism being responsible for that anomaly is discussed. Furthermore 75 As-NMR investigations have been performed to probe the magnetism on a microscopic scale. As-NMR spectra are analyzed in terms of first and second-order quadrupolar interaction. The anisotropic shift component K ab and K c could be derived from the 75 As powder spectra. Towards lower temperature a strong shift anisotropy was found. Nonetheless K iso tracks the bulk susceptibility down to T = 50 K very well. Furthermore the presence of weak correlations among the Ce ions in the ferromagnetic state is discussed. The observed increase in C / T towards lower temperatures supports this interpretation.

We report the ferromagnetism of La0.67Ca0.33MnO3 in bulk polycrystalline, nanocrystalline, and amorphous phases. The structural change from crystalline phase to amorphous phase exhibited a systematic decrease in TC (paramagnetic to ferromagnetic transition temperature) and spontaneous magnetization (MS). The experimental results suggested few more interesting features, e.g., appearance of large magnetic irreversibility in the temperature dependence of magnetization, lack of magnetic saturation at high magnetic field, blocking of magnetization below TB, and enhancement of coercivity. In addition, the magnetic phase transition near to TC changes from first order character in bulk sample to second order character in nanocrystalline and amorphous samples. We understand the observed magnetic features as the effects of decreasing particle size and increasing magnetic (spin-lattice) disorder. The magnetic dynamics of amorphous samples is distinctly different from the nanocrystalline samples and also found to be comparable with the properties of reported amorphous ferromagnetic nanoparticles.

In this paper, we report the structural, microstructural, and magnetic properties of nanosized (particle size ranging from 20 to 30 nm) Ni0.35Zn0.65Fe2O4 (MA4) system synthesized via mechanochemical route followed by annealing. The Rietveld refinement is used for the first time to precisely resolve the crystal structure of a ferrite system at nanoscale. MA4 is a cubic spinel of Fd3¯m symmetry. According to XRD and HRTEM studies, it is a well crystalline sample which possesses large microstrain. In spite of its nanometric size, MA4 has displayed some notably distinct magnetic properties like, enhancement of magnetization (64 emu g−1 at 15 K), magnetic order, magnetic ordering temperature, coercivity (1000 Oe at 15 K), magnetic anisotropy energy, and reduction of superparamagnetic relaxation compared with its counterparts synthesized by chemical route. It exhibits clear hysteresis loop (HC = 50 Oe) at 300 K and ferrimagnetic ordering below the blocking temperature (∼250 K). These improvements in magnetic properties of the system are likely to be very helpful for its technological applications. Again, particles in the sample possess a ferrimagnetically aligned core (with small canting) surrounded by a magnetically disordered shell with canted spin structure. The magnetically disordered surface region of MA4 has an equilibrium cation distribution, whereas the ferrimagnetic core region possesses a nonequilibrium cation distribution. Moreover, the infield Mössbauer spectroscopic study reveals that the nearest neighbor ion configuration about [B] site Fe3+ ions is not identical. Thus, there is local chemical inhomogeneity in the sample. The cation redistribution, chemical inhomogeneity, lattice strain are identified as the causes for magnetic enhancement in MA4.

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Electrical resistivity, magnetic susceptibility, and the Hall voltage of Bi2Sr2Ca& "Y Cu208+~(Bi 2:2:1:2)and (Bi& Pb )2Sr2Ca&Cu30»+~(Bi 2:2:2:3)samples are measured as a function of temperature. A metal-insulator transition originating from the change of carrier concentration is found in the Bi 2:2:1:2system at x =0.55. Analysis of the electrical resistivity in the insulating region suggests that the transport is governed by a variable-range-hopping mechanism in the low-temperature region and phonon-assisted hopping of polarons in the high-temperature region. A universal dome-shaped T, versus nH variation is observed in the Bi 2:2:1:2and Bi 2:2:2:3 systems, which is similar to that reported in La& "Sr Cu04 and YBa2Cu307 "systems. Various normal-state parameters, such as the effective mass of the carrier, Fermi energy, density of states at the Fermi level, and correlation energy, are calculated and compared to those reported in the literature.

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