This study investigates the effect of A-site disorder, (ionic radius (〈 r _A 〉) and the cation mismatch ( σ ² )) on the structural, magnetic, critical behavior, and magnetic entropy changes of in La _0.7 (Ba,Ca,Sr) _0.3 MnO ₃ manganites.

In this paper we report the microstructure and magnetic properties of Ti⁴⁺-Mn⁴⁺ ions substitued barium hexaferrites (BHF) with formula of BaFe_12-2xMn_xTi_xO₁₉ (x = 0.2, 0.4, 0.6 and 0.8) which prepared by mechanical alloying and successive sintering at 1100 °C. Ti⁴⁺-Mn⁴⁺ ions were obtained from TiO₂ and MnO precursors which were mechanically alloyed together with BaCO₃ and Fe₂O₃ precursors. X-ray diffraction patterns for sintered samples confirmed that the materials are consisted with single phase BHF structure. Unit cell volume and crystallite size was found increase with increasing x. The crystallite size for all samples was below 70 nm, but microstructure observation shown that the particle size is in range of 200 - 400 nm, which concluded that the grains are polycrystalline. The saturation magnetization is increases up to x = 0.4 and decrease for higher x values, while the coercivity remain decreases monotonically. These results were interpreted in terms of the site preferential occupation of Ti²⁺ and Mn⁴⁺ at low level substitution.