Abstract
Detailed studies of the temperature (5–350 K) and magnetic field variations (up to of the magnetization of CuO nanoparticles of nominal size range 37–6.6 nm are reported. These particles were synthesized by the sol-gel method in combination with high-temperature annealing, followed by structural characterization by x-ray diffraction and high-resolution transmission electron microscopy. With a decrease in particle size d from 37 to 10 nm, the unit-cell volume and b axis increased and the bulk Néel temperature decreased according to For particles with there is a more rapid lattice expansion and the magnetic susceptibility χ varied as accompanied by a weak ferromagnetic component and hysteresis loops. For the 6.6-nm particles for which detailed studies are reported, there is a rapid increase in the coercivity and the remanence below 40 K accompanied by an exchange bias for the field-cooled samples in From 10 to 40 K, decreases monotonically to zero. However, above 40 K, a symmetric hysteresis loop is observed, with decreasing weakly towards zero as temperature increases towards 330 K. The hysteresis loop and the variation of χ are interpreted in terms of uncompensated surface spins, whereas the transition at 40 K is suggested to be of the spins in the core of 6.6-nm particles. Similarities to the hysteresis loops observed in the Permalloy/CoO system are noted.
- Received 28 June 2001
DOI:https://doi.org/10.1103/PhysRevB.64.174420
©2001 American Physical Society