Tunning the magnetism and magnetocaloric effects of EuAl4 single crystals by Si and Cu doping

The effects of Si and Cu doping on crystal structures, magnetism, and the magnetocaloric effect of EuAl4 have been investigated. EuAl4, EuAl3.35Si0.65, and EuAl3.82Cu0.18 single crystals grown by the Al flux method are isotypic and crystallize in BaAl4-type structure with space group I4/mmm. Compared to the pristine compound EuAl4, the lattice parameter a keeps almost unchanged and c decreases obviously by Si doping, while a decreases and c increases upon Cu-substitution. These irregular changes in the lattice parameters may contribute to the electronegativity difference between Si and Cu. Cu and Si doping enhances the ferromagnetic interaction, and the direction of easy magnetization changes from the ab plane to the c axis upon the Cu-substitution. Multiple-step metamagnetic transitions are still observed in EuAl3.35Si0.65 as applying the magnetic field along the c axis, while EuAl3.82Cu0.18 shows ferromagnetic behavior along the ab plane and c axis. As a consequence, the low-field magnetocaloric effect is significantly enhanced. The maximum magnetic entropy change under a field change of 20 kOe increases from − 8.4 J/kg K for EuAl4 to − 12.9 J/kg K for EuAl3.35Si0.65 and − 15.9 J/kg K for EuAl3.82Cu0.18. The corresponding values of refrigerating capacity are 67, 140, and 147 J/kg, respectively. The remarkable magnetocaloric performances suggest that the Cu-doped EuAl4 compound is an excellent candidate material for low-temperature magnetic refrigeration.