Magnetic entropy change in core/shell and hollow nanoparticles.

The development of positive magnetic entropy change in the case of ferromagnetic (FM) nanostructures is a rare occurrence. We observe positive magnetic entropy change in core/shell (Fe/γ-Fe2O3) and hollow (γ-Fe2O3) nanoparticles and its origin is attributed to a disordered state in the nanoparticles due to the random distribution of anisotropy axes which inhibits any long range FM ordering. The effect of the energy barrier distribution on the magnetic entropy change and its impact on the universal behavior based on rescaled entropy change curves for core/shell and hollow nanostructures is discussed. Our study emphasizes that the magnetic entropy change is an excellent parameter to study temperature and field dependent magnetic freezing in such complex nanostructures.