Shape- and field-dependent Morin transitions in structured α-Fe2O3

We report shape- and field-dependent magnetic properties of ellipsoid-, spindle-, flattened- and rhombohedra-shaped α-Fe2O3 samples prepared by solvothermal technique. We observed that a magnetic spin-flip mechanism, mostly known as Morin transition (TM), depends on the shape of α-Fe2O3 as well as on the applied magnetic field. In each of these structures the obtained value of TM was less than its bulk value of 263 K. We observed that TM shifted from highest 251.4 K for ellipsoid to lowest 220.8 K for rhombohedra structure, with intermediate values of TM for the other two structures. However, for rhombohedra structure TM shifted from 220.8 to 177.5 K under the external magnetic field of 100 Oe–30 kOe, respectively. The observed lowering of TM in the structured sample was analyzed in terms of elementary size, shape of the nanocrystallites, lattice parameters and occupancy of Fe+3 ions as well. These parameters were determined from the Rietveld refinement process using MAUD software.