Superparamagnetic behavior of antiferromagnetic CuO nanoparticles

We report on the magnetic properties of CuO nanoparticles prepared by the sol-gel method. M (T) curves show a blocking temperature at T_B=135 K and 160 K for 13 nm and 17 nm size particles, respectively, in measuring field H=100 Oe. The M-H data for both the samples show a reversal magnetization at 300 K, whereas below T_B it exhibit symmetrical hysteresis loops with a coercive field of 200 Oe (155 Oe) and a remanence 0.306 emu/mole (0.171 emu/mole) for 13 nm (17 nm) at 5 K. Presence of superparamagnetism and net magnetic moments of CuO nanoparticles are attributed to the uncompensated Cu²⁺ions at the surface of the particles.