Abstract
An adiabatic demagnetization refrigerator (ADR) is almost the only way to keep detectors operating at low temperatures below 100mK 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±36mK 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 60mK. This is due to its high density of the Er3+ magnetic ions. This mass corresponds to about 5.7kg, when assuming the same conditions as the ADR used for the Astro-E/XRS microcalorimeters.
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