Manganese substitution induced magnetic transformation and magnetoelectricity in SrFe₁₂O₁₉

In this study, manganese substituted strontium hexaferrite (SrFe_12-MnO₁₉; x = 0, 3, 5, and 7) prepared by the sol-gel auto-combustion method are studied. We observed that the substituted Mn preferentially goes to the 2a and 12k sites of Fe. Raman modes related to the 12k site suggest the stiffening of the lattice. The transformation of the grain's shape from hexagonal (x = 0 and 3) to rhombohedral (x = 7) was observed, as shown in the micrographs obtained from FESEM. The thermomagnetic curves show the shift of T_C to lower temperatures with the increase in the Mn content. From x = 5 onwards, the growth of another magnetic phase (FiM2) of lower coercivity apart from the parent phase (FiM1) of higher coercivity is seen. The FiM2 phase was found to increase with the Mn content in the sample (16.4(3)% for x = 5 but 66.2(5)% for x = 7). Although the magnetization for both FiM1 and FiM2 decreases with the increase in temperature, both magnetic phases behave in contrast to each other for x = 5 and x = 7. The study suggests a transformation of the compound from high magnetic anisotropy (x = 0) to low magnetic anisotropy (x = 7). The x = 5 composition sample displays the highest value of the first-order ME coefficient (0.83(2) mV × cm^-1 × Oe^-1). The observed value for x = 5 composition is ∼2.5 times higher than that of the parent x = 0 composition sample (0.33(2) mV × cm^-1 × Oe^-1). The studies thus suggest that the x = 5 composition is one of the viable candidates for magnetoelectric applications.