Bulk to nanoscale magnetism and exchange bias in CuO nanoparticles - art. no. 174420

Detailed studies of the temperature (5-350 K) and magnetic held variations (up to H=50kOe) of the magnetization of CuO nanoparticles of nominal size r ange 37-6.6 nm are reported. These particles were synthesized by the sol-ge l method in combination with high-temperature annealing, followed by struct ural 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 Neel temperature TN decreased according to gamma (m) = -partial derivative lnT(N)/partial deriv ative ln b=30, For particles with d<10 nm, there is a more rapid lattice ex pansion and the magnetic susceptibility <chi> varied as l/d, accompanied by a weak ferromagnetic component and hysteresis loops. For the 6.6-nm partic les for which detailed studies are reported, there is a rapid increase in t he coercivity H-c and the remanence M-r below 40 K accompanied by an exchan ge bias H-E for the field-cooled samples in H=50 kOe. From 10 to 40 K, H-E decreases monotonically to zero. However, above 40 K. a symmetric hysteresi s loop is observed, with H-c decreasing weakly towards zero as temperature increases towards 330 K. The hysteresis loop and the l/d variation of chi a re interpreted in terms of uncompensated surface Cu2+ spins, whereas the tr ansition at 40 K is suggested to be T-N of the spins in the core of 6.6-nm particles. Similarities to the hysteresis loops observed in the Permalloy/C oO system are noted.