Magnetocaloric effect for pyramid-shaped and aggregated CoO nanoparticles

We study a magnetocaloric effect for pyramid-shaped and aggregated CoO nanoparticles with variable particle size by magnetization measurements. Although CoO is antiferromagnetic in nature, all the samples exhibit ferromagnetism, which develops at low temperatures. Magnetic entropy changes calculated from magnetization curves exhibit a maximum at low temperatures below 10 K, which attains 0.49 J/K/kg at T ∼ 3 K. The surface spin freezing transition may dominate the magnetocaloric effects for CoO nanoparticles, which enhance for aggregated CoO nanoparticles with a larger surface-area-to-volume ratio as compared with pyramid-shaped ones.