Effects of La-Co Co-Substitution on the Structural and Magnetic Properties of SrM Hexaferrites Prepared by Solid-State Reaction

The effects of La-Co co-substitution on the structural and magnetic properties of strontium M-type hexaferrites (SrM) were carefully investigated for the Sr1−xLaxFe12−xCoxO19 (0.0 ≤ x ≤ 0.3) polycrystalline samples prepared by solid-state reaction in air. Without the use of sintering additives, La-Co co-substituted SrM single phases could be obtained from polycrystalline bulk samples by employing proper processing conditions. The lattice parameter a initially increases with increasing x from 0.0 to 0.1 but gradually decreases with increasing x to 0.20, and then remains almost unaltered up to x = 0.3; the lattice parameter c monotonously decreases with increasing x from 0.0 to 0.25, but it turns to an increase when x = 0.3. The reduction in c/a ratios and Vcell values with increasing x up to x = 0.25 are obviously attributable to the decreasing size effect resulting from La3+ substitution at the Sr2+ site, which surpasses the increasing size effect due to Co2+ and Fe2+ occupancy at the Fe3+ site. Meanwhile, with increasing x from 0.0 to 0.3, while the saturation magnetization (MS) continuously decreases from 75.90 to 72.07 emu/g, the magnetic anisotropy field (Ha) increases from 20.1 to 24.7 kOe, leading to an increase in the intrinsic coercivity (Hci) from 2.68 to 3.99 kOe. The gradual increase in Ha with x, probably caused by a gradual decrease in the crystallographic symmetry, is inversely proportional to the variation in the c/a ratios up to x = 0.25 as usual except when x = 0.3, of which deviation needs further study.