Enhancement of surface spin disorder in hollow NiFe$_{2}$O$_{4}$ nanoparticles

We present synthesis and magnetic properties of hollow NiFe$_{2}$O$_{4}$ nanoparticles from core (Ni$_{33}$Fe$_{67})$/shell (NiFe$_{2}$O$_{4})$ structure based on Kirkendall effect. Morphology of the particles can be varied from hollow to solid NiFe$_{2}$O$_{4}$ nanoparticles by varying the reaction temperature and time of initial core/shell structure. Particles with hollow morphology are expected to show larger surface spin disordered or spin glass phase due to the presence of additional inner surface in addition to the usual outer surface in nanoparticles. Field cooled hysteresis loops exhibit significantly large shift due to unidirectional anisotropy resulting from the additional inner along with outer surface spin glass interface in particles with hollow morphology compared to particles with non-hollow (or solid) morphology that have only outer spin glass shell coupled with ferromagnetic core. The enhancement in the surface anisotropy is also noticeable which leads to an increase in the blocking temperature of the particles with hollow morphology.