Platinum Metals Rev., 1964, 8, (2), 59
Magnetic Transformation in Iron-Rhodium Alloys
Some Thermal Switching Possibilities
The unusual magnetic properties of iron-rhodium alloys were demonstrated at the Johnson Matthey stand at the 1964 exhibition organised by the Institute of Physics and the Physical Society. Alloys containing approximately equal numbers of atoms of iron and rhodium are non-magnetic at room temperatures and become suddenly ferromagnetic when heated to about 70°C. The apparatus displayed comprised a small cylinder of 52 atomic per cent rhodium alloy suspended in a heating zone. On reaching the critical temperature, it was pulled downwards by a cobalt-platinum alloy magnet, which cooled the specimen until the antiferromagnetic state was attained; the test piece was then released upwards into the heating zone and the cycle repeated.
The curve illustrates the force exerted on an iron-rhodium armature, 0.64 cm in diameter and 2.5 cm long, hung vertically in a field tapering from 1000 to 200 oersteds along its length. The alloy had been previously ordered at 500°C for 20 hours. The force developed in the field became appreciable at 70°C and exceeded 90 g at 90°C. Over the same temperature range a geometrically identical cylinder of pure iron was attracted uniformly with a force of 118 g. Although the non-uniform field has widened the transition temperature range, the results of this simple study provide a quantitative indication of the actuating forces which an elementary iron-rhodium thermal switch could exert and might possibly help to solve some thermal protection problems.