ErN and HoN spherical regenerator materials for 4 K cryocoolers

HoN and ErN spheres were synthesized by nitriding metallic spheres, and their regenerator performance was investigated by measuring the cooling power, CP, using a commercial Gifford-McMahon cryocooler loaded with the samples. The lowest achieved temperature was below 4.2 K. The CP of the nitride spheres was decreased by the presence of surface roughness, porosity, and oxide impurities. The intrinsic cooling power of ErN, calculated by considering these deficiencies, was greater than that measured for HoCu2. This high performance originates from the large specific heat, which peak at approximately 5K given by magnetic transition. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772531]