Magnetic properties, exchange coupling and novel stripe domains in bulk SrFe12O19/(Ni,Zn)Fe2O4 composites

Bulk SrFe12O19/(Ni0.4Zn0.6)Fe2O4 composite ferrites with mass ratios R-m = 2 : 1, 1 : 1 and 1 : 2 were prepared using nanopowders obtained via a hydrothermal method, and their phase composition, magnetic properties, exchange coupling (EC) and magnetic microstructures were systematically investigated. It is found that all the bulk specimens sintered at either 700 or 900 degrees C are composed of two phases but exhibit typical single-phase magnetic behaviours, indicating the existence of EC between the magnetically hard and soft phases. However, too much (R-m = 2 : 1) or too little (R-m = 1 : 2) soft (Ni0.4Zn0.6)Fe2O4 phase weakens the EC in the composites. It is also proved that except for the EC, the strengthening of chemical polarization of the internal chemical bonds affects saturation magnetization, and the size of nanoscale grains significantly influences the EC and magnetic properties. In addition, novel stripe domains are found in all of the bulk composite specimens, which could be ascribed to the magnetization of the soft (Ni,Zn)Fe2O4 phase induced by the hard SrFe12O19 phase.