Low-temperature magnetization dynamics of oxygen-stabilized tetragonal Ni nanoparticles

A comprehensive study of the magnetization dynamics of oxygen-stabilized, tetragonal Ni (OS-Ni) nanoparticles (20 nm) prepared by the borohydride reduction method is reported. Both as-prepared and air annealed (at 573 K and 773 K) samples have been studied using ac susceptibility, aging experiments, and field cooled (FC) and zero field cooled (ZFC) magnetization measurements as a function of temperature. The OS structure, which is a deviation from the usual face-centered-cubic (fcc) structure of Ni, arises due to the presence of interstitial oxygen atoms in the unit cell of Ni and causes it to exhibit anomalous magnetic behavior such as a very large magnetization enhancement at low temperatures. Two low-temperature magnetic transitions in close succession, manifested in the form of a sharp peak at 20 K and a small hump at similar to 12 K, are observed in the ZFC curve and ac susceptibility plots of the as-prepared sample. The probable nature of these transitions has been explained on the basis of a model which associates the peak at 20 K with the occurrence of a PM (paramagnetic)-> FM (ferromagnetic) transition of the oxygen-stabilized phase. Most of these newly formed FM particles block as soon as they gain internal order, yielding a strong irreversibility between the FC and ZFC branches. Some of the macromoments formed at 20 K, however, remain unstable down to 12 K, the temperature at which they block cooperatively as shown by a critical dynamics analysis, yielding critical exponent z nu=9.84 +/- 0.48 and a relaxation prefactor of tau(0)=10(-7) s. Aging experiments at 10 K for three different wait times t(w) show wait time dependency, substantiating unequivocally the cooperative, chaotic nature of the sample magnetic dynamics. The low-temperature magnetic features displayed by the 573 K annealed sample closely resemble those of the as-prepared one, though distinctly different features are observed in the sample annealed at 773 K. These have been explained coherently taking into account the structural changes produced upon annealing