Magnetic studies on zinc ferrites

Fine magnetic particle systems like ZnFe2O4 have received a growing and renewed interest in the last few years for the wide possibilities of applications in the nanostructured materials technology. Nanosized magnetic particles have properties, which are drastically different from those of the corresponding bulk materials. High energy ball milling is an excellent tool to tune the grain size and modify the magnetic properties of ferrite materials.

The origin of the magnetic properties of ferrites arise from the spin magnetic moment of the unpaired transition metal 3d electrons coupled by the superexchange interaction via the oxygen separating them.It has long been known that the A-O-B exchange interaction is much stronger than the B-O-B and the A-O-A interactions.The systems where one or both the sublattices are populated with magnetic ions, the competing exchange interactions may lead to topological frustration, yielding a magnetic structure that may include states of antiferromagnetic order, local spin canting, spin glass behaviour, disordered phases and ferrimagnetic order. Experimental studies on spinel ferrites have repeatedly shown this rich variety of magnetic ordering that ranges from classical ferrimagnetic ordering to spin glass behaviour.

The magnetic behaviour of the particle surface differs from that corresponding to the core, because of the distinct atomic coordination, compositional gradients and, concentration and nature of the defects present in both the regions. Thus, whereas the core usually displays a spin arrangement similar to that of the bulk material, a much higher magnetic disorder is present in the surface, giving rise to magnetic behaviour which cover from that of a dead magnetic layer to that of a spin-glass like.

The superparamagnetism and the spin-glass like properties, which has wide applications in the technological point of view. Superparamagnetism is a finite size effect since the particle anisotropy is generally proportional to its volume and has important applications in the thermal and time stability of the bits written in recording media.Spin glass property is also has some relevant applications in the memory devices. Spin glass is a substance in which electron spins in a random direction below TG. It contains many metastable states separated by the potential barrier of KBoltzTG. Once it falls into one of the metastable states, it cannot have a more stable state at a thermal energy below TG. However the addition of some external field, for example, light irradiation, will turn the metastable state into a more stable state. This feature is exclusively used in optical magnetic memory devices.However generally, in spin glass, the TG is very low, far below room temperature. To utilize a spin glass as an actual magnetic memory device, a spin glass with a higher TG at least approximately near room temperature is desired.