Bulk to nanoscale magnetism and exchange bias in CuO nanoparticles

Detailed studies of the temperature (5--350 K) and magnetic field variations (up to $H=50\mathrm{kOe})$ 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 N\'eel temperature ${T}_{N}$ decreased according to ${\ensuremath{\gamma}}_{m}=\ensuremath{-}\ensuremath{\partial}\mathrm{ln}{T}_{N}/\ensuremath{\partial}\mathrm{ln}b=30.$ For particles with $d<10\mathrm{nm},$ there is a more rapid lattice expansion and the magnetic susceptibility \ensuremath{\chi} varied as $1/d,$ 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 ${H}_{c}$ and the remanence ${M}_{r}$ below 40 K accompanied by an exchange bias ${H}_{E}$ for the field-cooled samples in $H=50\mathrm{kOe}.$ From 10 to 40 K, ${H}_{E}$ decreases monotonically to zero. However, above 40 K, a symmetric hysteresis loop is observed, with ${H}_{c}$ decreasing weakly towards zero as temperature increases towards 330 K. The hysteresis loop and the $1/d$ variation of \ensuremath{\chi} are interpreted in terms of uncompensated surface ${\mathrm{Cu}}^{2+}$ spins, whereas the transition 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/CoO system are noted.