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We report on a comparative study of the exchange bias effect and magnetic properties of Ni-NiO nanoparticulate systems synthesized by the chemical reduction of NiCl2 solution of two different molar concentrations--1M (high) and 0.05M (low)--followed by annealing of the dried precipitate in the temperature range 400-600 °C in air. Interestingly, the samples derived from the low molarity solution have higher Ni content and larger crystallite size than those prepared from their high molarity counterparts. These molarity dependent features subsequently modulate the magnitude of the exchange bias field in the samples, which is found to be absent or small in the 0.05M series, but of moderate value in the 1M samples. The different physical attributes of the particles derived from different concentrations of Ni-precursor solution are explained by invoking different nucleation kinetics and supersaturation degrees surrounding the viable growing nucleus. Furthermore, an observed increase of exchange bias with increasing annealing temperature, in contrast to the reported agglomeration of particles on annealing and subsequent reduction in bias magnitude, has been explained in correlation to the Ni-NiO interface density. [ABSTRACT FROM AUTHOR]

A comparative study of the structure and magnetic properties of Ni nanoparticles (20–80 nm) prepared by the chemical reduction of NiCl2 solution of four different concentrations is reported. The concentration of the NiCl2 solution has a profound influence on the room temperature (300 K) magnetic state of the resulting Ni nanoparticles, even though all four samples show the same x-ray diffraction (XRD) pattern, i.e., have the same crystal structure (tetragonal, as proposed by us). It is found that samples obtained from lower concentration solutions (0.1 and 0.5M) show a linear response with magnetic field while those obtained from higher concentration ones (1 and 2M) have a ferromagnetic component at 300 K. This difference in magnetic behavior has been attributed to the possible presence of fcc (face centered cubic) Ni cores in the particles of higher molarity samples, which therefore leads to strong interparticle dipolar interactions in them. The strong interactions, together with the magnetocrystalline anisotropy of the cores, present a significant barrier to the relaxation of core moments in these samples, giving rise to their blocked state even above 300 K, as evident from the irreversibility in the field cooled (FC) and zero field cooled (ZFC) curves, which starts right from the measuring temperature of 390 K. Intriguing features in the form of a sharp peak at 20 K and a hump at 12 K are observed in the ZFC curve of all samples, signaling magnetic transitions at these temperatures. Appreciably high magnetization values are also seen in the M-H plots at 5 K. The presence of these low temperature features irrespective of sample molarity indicates that the low temperature magnetic states of the samples, in contrast to their room temperature states, are independent of the concentration of the NiCl2 solution. [ABSTRACT FROM AUTHOR]