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 Mg 1+t Fe 2-2t Ti t O 4 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 T f , to a spin-glass-like (SGL) state, well below the ferrimagnetic ordering temperature T N . 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.