BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing a relation between a cation defect amount δ and a magnetic field degradation rate;
FIG. 2 is a graph showing a relation between a mean grain size and the magnetic field degradation rate;
FIG. 3 is a graph showing a relation between the amount of Fe2O3 and the magnetic field degradation rate;
FIG. 4 is a graph showing a relation between the amount of ZnO and the magnetic field degradation rate;
FIG. 5 is a graph showing a relation between the amount of Co3O4 and the magnetic field degradation rate;
FIG. 6 is a graph showing a relation between the amount of SiO2 and the magnetic field degradation rate; and
FIG. 7 is a graph showing a relation between the amount of CaCO3 and the magnetic field degradation rate.