Temperature-dependent magnetization and magnon Raman spectra were measured for anti-ferromagnetic NiO-nanoflowers. The results show several new magnetic behaviors, including the appearance of a ferromagnetic phase, a reduced Néel temperature (TN) and a reduced Curie temperature (TC). The temperature dependencies of the double magnon (2M) Raman wavenumber and intensity are similar to those of magnetization. A magnetic granules model (MGM) consisted of a crystalline core enclosed by a shell is proposed. The model suggests that the large quantity of spins induced by specific surface effect in the shell plays a key role in nano-magnetism. Based on the MGM, the micro-mechanism of the observed new magnetic behavior is understood by the magnon Raman spectra. The MGM is based on the general features of magnetic nano-particles, and thus it should be generally applicable to common magnetic nano-particles. Copyright © 2014 John Wiley & Sons, Ltd.