Nanocrystalline oxide spinels are of interest both for fundamental studies in magnetism and technological applications. These materials display properties that differ from their bulk counterparts due to size dependent quantum confinement effects. Spinel Mg-Mn ferrite is a well-known soft magnetic material with very low cubic magnetocrystalline anisotropy, low coercivity and moderate saturation moment. These make it a promising candidate for various technological applications. The interest in the swift heavy ion (SHI) irradiation on nanoferrites is because of the effect of irradiation on the superexchange interactions, which are highly sensitive to any disorder in the ferrites. SHI has been proved a good source to produce a wide variety of defects to create structural strain and disorder in oxide materials that can be further responsible to modify their physical properties. In the present book, we have investigated the structural and magnetic properties of Mg-Mn spinel ferrites and how they get affected, when the particle size approaches to nanoscale range. Finally, how these properties get modified when irradiated with SHI.