New magnetic material R₃T system with extremely large heat capacities used as heat regenerators

We developed a new magnetic material R₃T system, where R is a rare earth metal and T a transition metal, used as a regenerator matrix in a conventional two-stage Gifford-McMahon (GM) type cryocooler. These compounds, especially Er₃Ni (Erbium 3 Nickel), possess extremely large heat capacities between 4 K and room temperature because of their low Debye temperatures, about 150 K, and broadened large peaks in specific heat due to complicated ferromagnetic or antiferromagnetic spin configurations near each transition temperature, for instance 7 K for Er₃Ni. The specific heat of lead (Pb) used as a conventional regenerator matrix is too small to make it possible to produce refrigeration effectively in the temperature range between 4 and 15 K, because in most non-magnetic substance, such as lead, only the lattice vibrations and the conduction electrons make a contribution to the specific heat. In this paper, we present the thermal properties, especially specific heats, for the rare earth-3d transition metal (R-T) intermetallics rich in rare earth metal and the method of fabricating spherical particles for the newly developed regenerator matrix, which is useful for a conventional two-stage GM refrigerator.