Erbium-doped yttrium aluminum garnet as a magnetic refrigerant for low temperature x-ray detectors

Garnets doped with rare-earth elements can be used in adiabatic demagnetiza tion refrigerators. We have measured the specific heat and magnetization of a single crystal yttrium aluminum garnet (YAG) doped with 30% Er3+ ion at temperatures between 93 mK and 8 K under magnetic fields up to 8.0 T along the < 111 > crystal axis. From the specific heat and magnetization, we deri ved consistent temperature and magnetic-field dependence of the magnetic en tropy. Under zero magnetic field, the magnetic entropy begins to decrease b elow 2 K and becomes half of R ln 2 at similar to 160 mK. This decrease is considered to be due to an antiferromagnetic short-range ordering among Er3 + ions. This behavior of the specific heat in the measured temperature rang e can be explained by a model in which both the crystalline-electric-field ground state and the first excited state are included. The operating temper ature of the Er3+-doped YAG as a magnetic coolant is estimated to extend do wn to similar to 100 mK, which is lower than those with nonsubstituted garn ets such as gallium-gadolinium-garnet used in the range similar to4.2-15 K. With a doping level of 30%, we estimate that similar to6 kg of Er3+-doped YAG exhibits the same cooling performance at 60 mK as the 916 g of ferric-a mmonium-alum salt used for the x-ray spectrometer (microcalorimeter detecto rs) on the Astro-E satellite.(C) 2001 American Institute of Physics.