Magnetic refrigerants for ultralow temperatures: A mini-review

Accessing the milli-Kelvin regime is increasingly important for next-generation quantum technologies and deep-space observations. Among established cryogenic techniques, adiabatic demagnetization refrigeration (ADR) is distinctive for its all-solid-state design, low vibration, and intrinsic gravity independence. Here we present a materials-centered review of ADR refrigerants, connecting classical thermodynamics to modern quantum many-body behavior. Beyond hydrated paramagnetic salts, dense rare-earth oxides and correlated-disorder ceramics, we highlight emerging quantum-engineered refrigerants, including geometrically frustrated magnets, and quantum-critical systems. In these materials, suppressing long-range order and tailoring low-energy excitations redistribute spin entropy into the sub-Kelvin window, enabling large and reversible entropy changes at the lowest accessible temperatures. We discuss the central trade-offs among volumetric entropy density, thermal transport, and magnetic ordering, and outline possible design rules for staged ADR architectures.