Effects of microstructure on the magnetic properties of polycrystalline NiMn2O4 spinel oxides

•Polycrystalline NiMn2O4 samples have been prepared by a simple sol–gel method at different temperatures.•The samples present different lattice strains.•Magnetic hysteresis loops measured after FC are shifted to negative fields compared to those measured after ZFC.•EB can be attributed to the inhomogeneous magnetic regions.•The strength of the EB decreases with the decrease in microstrain.