Acta Physica Academiae Scientiarum Hungaricae 6. (1957)
1956 / 2. szám - L. Pál - T. Tarnóczi: Temperature Dependence of the Differential Susceptibility of Cobalt in Strong Magnetic Fields
226 L. PÁL and T. TARNÓCZI tűre dependence of the saturation magnetization of cobalt but, as has been pointed out by Myers and Sucksmith [5], the measurements of none of them can be regarded as reliable, because the magnetic properties of cobalt are very greatly dependent on the heat treatment applied near the transformation temperature. Basing themselves upon the findings of Edward and Lipson [6] as well as Troiano and Tokich [7], Myers and Sucksmith [5] found it more expedient to determine the exact value of the saturation magnetization on a monocrystal specimen. Edward and Lipson [6] showed by X-ray structure determinations that even at room temperature occlusions of face-centred cube structure are present in the hexagonal phase, and their concentration is contingent on the conditions of the heat treatment. Earlier than by Myers and Sucksmith [5], monocrystals of cobalt were measured by Kaya [8] and by Honda and Masumoto [9]. On the basis of the temperature dependence of the magnetization curve as obtaine l by these authors Bozorth [10] determined the temperature dependence of tlie first and second anisotropy constants of hexagonal cobalt. The mechanism of the a^zß allotropie transformation in cobalt was clarified in 1936 by the work of JNishiy ama [11], who showed that the plane (111) of the face-centred cubic modification forms parallel to the (0001) base of the hexagonal modification, while the direction (1Ï0) forms parallel to the direction (1120). In the course of the a ->/3 transformation the hexagonal crystal can only give rise to a single face-centred cubic crystal, while during the reverse process each face-centred crystal may yield four different hexagonal crystals, with well-defined orientations. However, the four hexagonal crystals arising from a single cubic crystal invariably re-transform into a single cubic crystal. These findings of Nishiyama [11] were later confirmed by Kehrer and Leidheiser [12]. Accordingly, from the hexagonal cobalt monocrystal a cubic crystal of known orientation can be produced, which will re-form after any number of phase transformations. Sucksmith and Thompson [13] made use of this fact and were the first to determine the temperature dependence of the anisotropy constants of cubic cobalt. Their measurements showed the anisotropy constants of cubic cobalt to be smaller by an order of magnitude than those of hexagonal cobalt and, rather strikingly, the second anisotropy constant to be of greater absolute value than the first. On these grounds, it is to be expected that in a strong magnetic field the differential susceptibility of polycrystalline cobalt markedly decreases near the phase transformation, since one of the most important factors determining its magnitude, the magnetic anisotropy, likewise decreases considerably. It has been known ever since the work of Honda and Masumoto [9] that in the hexagonal phase the first anisotropy constant changes its sign at about 220° C, while the second, changing slowly, stays positive throughout. Therefore, at a temperature slightly above 220° C the differential susceptibility of the polycrystalline cobalt is expected to show a marked minimum.