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

Abstract

Garnets doped with rare-earth elements can be used in adiabatic demagnetization 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 derived consistent temperature and magnetic-field dependence of the magnetic entropy. Under zero magnetic field, the magnetic entropy begins to decrease below 2 K and becomes half of R ln 2 at ∼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 range can be explained by a model in which both the crystalline-electric-field ground state and the first excited state are included. The operating temperature of the Er3+-doped YAG as a magnetic coolant is estimated to extend down to ∼100 mK, which is lower than those with nonsubstituted garnets such as gallium–gadolinium–garnet used in the range ∼4.2–15 K. With a doping level of 30%, we estimate that ∼6 kg of Er3+-doped YAG exhibits the same cooling performance at 60 mK as the 916 g of ferric–ammonium–alum salt used for the x-ray spectrometer (microcalorimeter detectors) on the Astro-E satellite.