Structural and Magnetic Properties of Ba1-xRexCo2ZnxFe16-xO27 W-Type Hexaferrites Prepared by Sol-Gel Auto-Combustion

Ba1-xRexCo2ZnxFe16-xO27 (Re = La, Nd, Pr; x = 0.0, 0.1, 0.2) hexaferrites were prepared by a sol-gel auto-combustion method and sintering at 1250 degrees C for 2 h. The effects of rare-earth (RE) substitution on the structural and magnetic properties of the hexaferrites were investigated. XRD patterns of the samples revealed crystallization of pure W-type hexaferrite phase in all samples, except for Nd-Zn (x = 0.2) sample, which contained a minor M-type phase in addition to the major W-type phase. The saturation magnetization did not change appreciably with RE-Zn substitution, although the magnetic measurements revealed slight improvement with Pr-Zn substitution. However, the magnetocrystalline anisotropy field (H-a) increased significantly with RE-Zn substitution, recording a maximum increase up to 9.23 kOe compared with 6.30 kOe for the unsubstituted sample. The results indicated the possibility of tuning the anisotropy field, and consequently, the ferromagnetic resonance frequency, to frequency ranges appropriate for desired microwave applications. The thermomagnetic curves revealed spin reorientation transitions below 300 degrees C, and magnetic phase transition in the range at 461-481 degrees C, which was associated with the Curie temperature of the W phase. In addition, the weak magnetic phase transition in the temperature range of 505-516 degrees C was associated with Co-rich impurity magnetic phase, with enhanced superexchange interactions in Co-rich regions.