Thermal and magnetic properties of (ErxY1−x)3Al5O12 for application to ADRs

An adiabatic demagnetization refrigerator (ADR) is almost the only way to keep detectors operating at low temperatures below 100 mK in the microgravity of space. We have proposed a garnet with a rare-earth element, (ErxY1−x)3Al5O12, as a suitable magnetocaloric material for ADRs, and examined the x dependence of the cooling performance using single crystals. A broad hump appearing in the temperature dependence of the specific heat at zero field shifts toward a higher temperature with increasing x, 339±36 mK for x=0.5, suggesting that the hump structure is caused by magnetic interactions among Er3+ ions. Combining magnetization and specific heat data, we obtained the field and temperature dependences of the magnetic entropy. The estimated mass needed for an ADR salt pill for x=0.5 is 20–25% smaller than that for x=0.3, at an operating temperature of 60 mK. This is due to its high density of the Er3+ magnetic ions. This mass corresponds to about 5.7 kg, when assuming the same conditions as the ADR used for the Astro-E/XRS microcalorimeters.