Giant magnetocaloric effect of divalent europium-based oxide composites of Eu2TiO4 and Eu3Ti2O7 for cryogenic temperature magnetic refrigeration

Abstract

For cryogenic temperature magnetic refrigeration, we focused on materials containing Eu2+ ions and synthesized sintered composite materials comprising Eu2TiO4 and Eu3Ti2O7. Eu3Ti2O7 and Eu2TiO4 exhibited second-order phase transitions between the paramagnetic and ferromagnetic states at 7 and 8 K, respectively. The magnetocaloric effect was evaluated from magnetization and specific heat. It was shown that Eu2+ ions behave similarly to free ions with J = 7/2. The maximum magnetic entropy change per unit volume exceeded 0.3 J/cm3K at a magnetic field of 5 T. The Carnot cycle at the hydrogen liquefaction temperature was evaluated using the obtained entropy temperature diagram. It was found that the cooling capacity is several times higher than that of known materials such as Gd3Ga5O12, (Dy0.8Gd0.2)3Al5O12, and GdTiO3. It was also shown that the present composite materials are useful for extending the operating temperature range of the adiabatic demagnetization refrigerator. These results indicate that Eu2TiO4 and Eu3Ti2O7 composites are promising magnetic refrigerants for cryogenic magnetic refrigeration.