In keeping with this invention, a ferromagnetic fiber is defined as a generally elongated article composed either of amorphous or crystalline ferromagnetic material, having a diameter from 3 to 80 microns, an aspect ratio, i.e. length to diameter ratio, of at least 150 and a magnetic switching time at half amplitude points (t1/2) of less than 10 microseconds at a sine wave driving frequency of 6 k