Effect of annealing in reduced oxygen pressure on the structure and magnetic properties of M-type hexaferrite bulk and film

Abstract The Ca-La-Co-doped M-type Sr-hexaferrite (Sr 0.1 Ca 0.45 La 0.45 Fe 11.7 Co 0.3 O 19 ) decomposed into three different phases of orthorhombic, spinel, and hexagonal structures after annealing at 1200 °C in nitrogen atmosphere while the non-substituted SrFe 12 O 19 maintained the hexagonal structure after annealing at the same condition. It is suggested that the charge-imbalance substitution of La 3+ /Co 2+  = 1.5 induces phase instability of the M-type structure and it causes the phase transformation during the N 2 annealing. In film experiment, polycrystalline Sr-hexaferrite films (SrFe 12 O 19 ) have been prepared by pulsed laser deposition and post-annealing processes. When the film was annealed in vacuum at 500 °C, the magnetization value at the magnetic field H = 15 kOe increased by ∼80% and the coercivity decreased significantly without microstructural change.