a deactivatable anti - theft security strip has a soft - magnetic constituent and a hard - magnetic constituent . the hard - magnetic constituent is used for two purposes . it is used first to stress the soft - magnetic constituent and thereby to promote a fast magnetic reversal , and it is also used to deactivate the strip by magnetization . the pre - stressed , soft - magnetic constituent produces defined , steep pulses even in the presence of a slow magnetic reversal , so that the anti - theft security strip is especially suited for alternating fields having low frequencies , for example 50 or 60 hz .