Large magnetocaloric effect in EuGd2O4 and EuDy2O4

Magnetization, heat capacity and direct measurements of the magnetocaloric effect show that EuGd₂O₄ and EuDy₂O₄ have a remarkably large magnetocaloric effect at cryogenic temperatures, owing to their high magnetic density and low ordering temperatures. The Gd derivative orders antiferromagnetically at T_N = 4.6 K, while its magnetocaloric effect largely exceeds that of the reference refrigerant Gadolinium Gallium Garnet (GGG) above 5 K. The Dy derivative undergoes two phase transitions at T_C1 = 3.65 K and T_C2 = 4.7 K, respectively, which are the result of a peculiar magnetic arrangement: the first Dy sublattice is parallel to the crystallographic c-axis, while the Eu sublattice makes a variable angle from 0^o to 45^o with the direction of the second Dy sublattice that lies in the ab-plane. EuDy₂O₄ has a lower magnetocaloric effect than EuGd₂O₄, yet larger than GGG. Both ordering mechanisms are semi-quantitatively explained within the frame of a mean-field simulation, which takes into account the magnetic anisotropy strength of the participating magnetic ions.