initial permeability exhibited a similar tendency fig5 and 6 show the direct current superposition properties at 25 u00b0 c and 100 u00b0 c respectively for the sample subjected to the annealing treatment under the conditions of 700 u00b0 c u00d7 8 hours fig5 and 6 also show the direct current superposition property of a core formed of a ni u2014 zn based ferrite as can be seen from fig5 and 6 the core of the present invention exhibits a more satisfactory direct current superposition property than the core formed of a ni u2014 zn based ferrite samples were prepared in the same manner as in example 1 except that the annealing conditions were such that 600 u00b0 c u00d7 1 hour 700 u00b0 c u00d7 1 hour 700 u00b0 c u00d7 2 hours and 700 u00b0 c u00d7 8 hours and the withstand voltage and u03c15 values thereof were measured the results thus obtained are shown in table 3 and fig7 as can be seen from table 3 and fig7 when the extent of the oxidation based on the annealing treatment is increased u03c15 is increased and the withstand voltage is improved in particular when u03c15 is made to be 1000 u03c9m or more.