transformer core particularly for a transformer core in a so called medium frequency power supply methods for making cores are well known to the prior art during processing by the teachings of this invention a tape is preferably exposed to or maintained in during the cooling step a magnetic field at least sufficient to magnetize such tape nearly to its saturation point by the term 34 saturation point 34 or equivalent reference is had to the condition in which after a magnetic field strength becomes sufficiently large further increase in the magnetic field strength produces no additional magnetization in a magnetic material in this invention the field is preferably applied either longitudinally or transversely relative to a given tape preferably longitudinally members of preferred class of starting tapes consist of an alloy characterized by the formula y z ranges from 15 to 30 atomic percent and in any given such metal alloy the sum total of w x y and z is 100 atomic percent within this class of starting tapes a particularly preferred member is an alloy of the formula where the subscript values are expressed in hundredths of atomic percents a tape of such class following the teachings of this invention is formed into a core and then heated to a temperature ranging from about 280 u00b0 to 350 u00b0 c for a time of at least about 0 5 to 2 hours also following such teachings such a so heated shaped tape is cooled preferably to a temperature below about 200 u00b0 c at a cooling rate of from about 100 u00b0 to 250 u00b0 per hour the heating and the cooling take place.