The spontaneous magnetization of cobalt

The paper describes how measurements of the spontaneous magnetization of pure specimens of single-crystal close-packed hexagonal and polycrystalline face-centred cubic cobalt have been made over the temperature range -183 to 1121 degrees C. A single crystal of cobalt possessing a close-packed hexagonal structure was prepared by the process of slow cooling of the melt. Owing to the presence of a phase change in cobalt occurring c. 400 degrees C when the metal changes from a close-packed hexagonal structure below 400 degrees C to a face-centred cubic structure above 400 degrees C, it was necessary to determine two critical rates of cooling to prepare a crystal. Magnetic test specimens of single-crystal cobalt, cut so that they were capable of being magnetized along the easy axis of magnetization (the hexagonal axis), and of polycrystalline cobalt, of approximate dimensions 4 $\times $ 2 $\times $ 2 mm., were studied using the magnetic balance devised by Sucksmith (1939). The spontaneous magnetization of the hexagonal form was accurately measured from -183 to 431 degrees C, this latter temperature being determined as the upper transformation temperature of the cobalt. Complete tables of these measurements are given. By extrapolation the value 162$\cdot $5$_{5}$ was determined for the spontaneous magnetization of hexagonal cobalt at absolute zero of temperature. Measurements on the polycrystalline face-centred cubic material were made from 394 degrees C (this temperature being determined as the lower transformation temperature) to the Curie temperature, determined by extrapolation to be 1121 degrees C. The face-centred cubic material possessed greater magnetization than the hexagonal, and in the temperature range 431 to 950 degrees C it was found to be dependent upon the heat treatment the specimen had received; the significant factor in this heat treatment being the speed with which the specimen was allowed to cool through the lower transformation temperature. The Curie temperature was not affected by this heat treatment. To determine the spontaneous magnetization at very high temperatures where no saturation was observed the extrapolation technique devised by Weiss & Forrer (1926) was used. This procedure is described. When reduced values of magnetization and temperature were calculated it was found that the hexagonal and face-centred cubic materials obeyed different reduced equations of state although excellent agreement was obtained when the reduced values for face-centred cubic cobalt were compared with similar values for nickel. These results given in detailed tabulated form show that the reduced magnetic equation of state is structure dependent.