Coral-like microstructure of NiO has been synthesized via a facile template free hydrothermal technique followed by calcination. Structure and morphology of the synthesized samples have been examined by x-ray diffractometer, field emission scanning electron microscope and transmission electron microscope. Positron annihilation spectroscopy reveals high porosity (37%) and cationic vacancies within the microstructures. Magnetic study illustrates spin-glass like behaviour and asymmetric hysteresis loop at room temperature. Later one is attributed to non-inversion symmetry of the anisotropic energy barrier, introduces by defect induced ferromagnetic domain within antiferromagnetic matrix of NiO. Numerical values of the anisotropy parameters have also been evaluated using law of approach method. The present study evidences that the microstructure could be helpful for developing magnetization based memory devices due to its high barrier height similar to 2.50 x 10(6) erg/cm(3). (c) 2018 Published by Elsevier B.V.