Ca-induced changes in the crystal structure and magnetic properties of barium hexaferrite

Ca-doped barium hexaferrite (Ba_1-xCa_xFe₁₂O₁₉; x = 0.1-0.6) samples were synthesized successfully using solid-state reaction route. Further, the limit of the Ca solubility in BaFe₁₂O₁₉ has been investigated in the 900-1200°C temperature range. X-ray analysis reveals maximum Ca solubility in BFO at 1200°C (≈ 50%). The emergence of a hematite secondary phase at Ca concentrations ≈ 50 and 60 at% was observed. The XRD analysis also confirmed a gradual reduction in the c-parameter from 23.195 Å (at x = 0.1) to 23.147 Å (at x = 0.5). The structural data further suggested an enhancement of sample density with Ca substitution. Field-emission transmission electron microscopy micrographs reveal (a) distinct grain morphology at lower Ca concentration (x = 0.1) and (b) enhanced secondary grain growth and grain amalgamation at higher Ca concentration (x = 0.5). The M-H studies reveal that M_r and M_s almost stay constant up to x = 0.2, beyond which they start rising rapidly. M_r ≈ 42.65 emu g^-1 and M_s ≈ 92.19 emu g^-1, exhibit a maximum at x = 0.4. The coercivity first decreases rapidly from 4168 Oe (at x = 0.0) to 2326 Oe (at x = 0.1) followed by a marginal decrease to 2045 Oe (at x = 0.4). An H_c value ≥ 1200 Oe in a majority of our samples makes them potentially suitable for perpendicular recording media and permanent magnet applications. (author)