Giant Low-Field Cryogenic Magnetocaloric Effect in a Polycrystalline EuB₄O₇ Compound

To deal with the shortage and high price of helium-3 resources, adiabatic demagnetization refrigeration technology as an alternative to helium-3-based refrigeration technology has received much attention. The magnetism and ultralow-temperature magnetocaloric effect (MCE) of the EuB₄O₇ compound have been investigated. The results of magnetic and quasi-adiabatic demagnetization measurements suggest the absence of a magnetic order above 0.4 K for EuB₄O₇. The dipolar interaction between the nearest-neighbor Eu atoms has a characteristic energy of about 800 mK, which may induce a large MCE. The maximum magnetic entropy change reaches 22.8, 36.2, and 47.6 J·kg^-1 K^-1 at μ₀H = 0-10 kOe, 0-20 kOe, and 0-50 kOe, respectively. Measurements by a quasi-adiabatic demagnetization device show that the lowest temperature achievable (289 mK) for polycrystalline EuB₄O₇ is lower than that (362 mK) for the commercial refrigerant Gd₃Ga₅O₁₂ (GGG) single crystals. The hold time is more than 70 min below 700 mK, with an environment temperature of 2 K, proving that EuB₄O₇ exhibits superior cooling performance.