Site occupancy and magnetic study of Al3+ and Cr3+ co-substituted Y3Fe5O12

Single-phased polycrystalline Y3Fe5−2xAlxCrxO12 garnet samples (x=0, 0.2, 0.4 and 0.6) have been prepared by the conventional ceramic technique. Rietveld refinement of X-ray diffraction patterns of the samples shows them to crystallize in the Ia3d space group and the corresponding lattice constant to decrease with increasing Al3+ and Cr3+ contents (x). Mössbauer results indicate that Cr3+ substitutes for Fe3+ at the octahedral sites whilst Al3+ essentially replaces Fe3+ at the tetrahedral sites. This result indicates that co-doping of Y3Fe5O12 does not affect the preferential site occupancy for separate individual substitution of either Cr3+ or Al3+. The magnetization measurements reveal that the Curie temperature (Tc) monotonically decreases with increasing x while the magnetic moment per unit formula decreases up to x=0.4 and then slightly increases for x=0.6. This reflects a progressive weakening of the ferrimagnetic exchange interaction between the Fe3+ ions at octahedral and tetrahedral sites due to co-substitution. The magnetic moment was calculated using the cations distribution inferred from the Mössbauer data and the collinear ferrimagnetic model, and was found to agree reasonably with the experimentally measured value. The phenomenological amplitude crossover, characterized by the temperature T*, has also been observed in the doped YIG and briefly discussed.