Evidence of reduced antiferromagnetic transition in mesocrystals of CuO synthesized by a surfactant-free solution phase method

Abstract

We explore the finite size effect on the magnetic properties of CuO mesocrystals synthesized by a surfactant-free solution phase method. Significant evidence for the evolution of 3D oval, elliptical and 2D sheet-like CuO mesocrystals driven by oriented aggregation of CuO nano-subunits is presented. The dimensions of the CuO nano-subunits constituting the mesocrystals, as well as their final morphology, are tailored by the NaOH concentration in the precursor solution. Changes in the mesocrystal morphology reflect on the position and intensity of the Raman vibrational modes. DC magnetization curves reveal distinct features of antiferromagnetic transitions where the magnetization behaviour of the mesocrystal is influenced by the dimensions of the CuO nano-subunits. The magnetic ordering temperature (T_N) is reduced from the bulk value of 230 K to 58 K as the size of the CuO nano-subunit decreases to 7 nm. Moreover, all nanostructures exhibit linear field-dependent magnetization below the transition temperature similar to an antiferromagnetic system. However, below T_N field cooled isothermal magnetization displays an anomalous behaviour as a shift along the vertical axis whose origin is related to surface uncompensated spins. The vertical shift decreases as the CuO nano-subunit dimensions increase.