The Magnetic Anisotropy of Cobalt

A modified form of the magnetic balance previously designed by one of us, is described. In this apparatus the gradient is made independent of the magnetizing field. With such an apparatus measurements have been made on the variation of the intensity of magnetization with magnetic field for single-crystal specimens cut along appropriate crystal axes, both for the hexagonal close-packed and face-centred cubic cobalt. Such measurements enable the magnetic anisotropy constants to be determined. The nature of the phase change from close-packed hexagonal to face-centred cubic occurring at about 400° C is such that single crystals can be cut at room temperature, which will transform to give single crystals of known orientation in the face-centred cubic phase. The values of the magnetic anistropy constants for the hexagonal phase for various temperatures are compared with those of earlier determinations and fair agreement is found. Measurements on the face-centred cubic phase were made every 50° C in the temperature range 500 to 1000° C; at the latter temperature the material became magnetically isotropic. For face-centred cubic cobalt it was found that the crystal directions in order of increasing difficulty of magnetization are [111], [110] and [100], as is the case for nickel. Moreover, the value of the anisotropy constant k1 obeys the same empirical law, giving the variation with temperature, as nickel. The similarity in the magnetic behaviour of these two ferromagnetics having the same crystal structure is thus evident.