Ferrimagnetic to spin glass transition in the mixed spinel Mg1+tFe2-2tTitO4: a Mossbauer and DC susceptibility study

Mossbauer and DC susceptibility studies are reported on the mixed spinel Mg1+tFe2-2tTitO4 at various temperatures and in an external magnetic field. Results are obtained indicating that on increasing dilution t, the collinear ferrimagnetic phase breaks down before reaching the ferrimagnetic percolation limit as a result of the presence of competing exchange interactions. In this dilution region there is a second transition, at Tf, to a spin-glass-like (SGL) state, well below the ferrimagnetic ordering temperature TN. The lower limit of re-entrant ferrimagnetism is determined by observing the spontaneous spin canting which occurs in the presence of the external magnetic field on going into the SGL state. At higher dilution a transition to a pure spin-glass state occurs. The results are discussed in terms of the model proposed by Villain (1979) based on the competition between dilution and frustration in the spinel lattice. The re-entrant ferrimagnetism found in this insulating system with near-neighbour interactions is similar to the re-entrant ferromagnetism found in some metallic alloys, and the authors propose a canting order parameter for the low-temperature state as in the metallic case.