Magnetic and magnetocaloric effects of Eu<sub>0.9</sub><i>M</i><sub>0.1</sub>TiO<sub>3</sub> (<i>M</i>=Ca, Sr, Ba, La, Ce, Sm) compounds

Abstract

EuTiO<sub>3</sub> is a direct band-gap semiconductor material and exhibits antiferromagnetism with large magnetic entropy change around the liquid helium temperature. The ferromagnetic state can be changed into antiferromagnetic state through lattice constant change and electron doping by element substitution due to strong spin-lattice coupling coexistence of ferromagnetic coupling, and antiferromagnetic coupling. The values of magnetic entropy change can be effectively improved under low magnetic field change after changing into ferromagnetism. Samples of EuTiO<sub>3</sub> and Eu<sub>0.9</sub><i>M</i><sub>0.1</sub>TiO<sub>3</sub> (<i>M</i>=Ca, Sr, Ba, La, Ce, Sm) are prepared by the sol gel method. The Eu<sub>0.9</sub>Ca<sub>0.1</sub>TiO<sub>3</sub> exhibits the antiferromagnetism due to similar ion radius. The ferromagnetic coupling between Eu<sub>0.9</sub>Sr<sub>0.1</sub>TiO<sub>3</sub> and Eu<sub>0.9</sub>Ba<sub>0.1</sub>TiO<sub>3</sub> is enhanced, for alkaline earth metal (Sr and Ba) has larger ion radius, which is beneficial to improving the magnetocaloric effect under low magnetic field. Electron doping can inhibit the antiferromagnetic coupling because the extra carrier may occupy the Ti 3d and reduce the hybridization of Eu 4f-Ti 3d-Eu 4f. When the electron doping concentration is greater than 10%, the spin polarization rate of Ti 3d state on the Fermi surface is negative, resulting in the transition from antiferromagnetic to ferromagnetic state. When the Eu ions are replaced with the Sm ions (Sm ion radius is similar to Eu ion radius), the ferromagnetic coupling is enhanced. However, when the Eu ions are replaced with the La or Ce ions, the samples show strong ferromagnetism, for the lattice constant and electron doping are increased. A giant reversible magnetocaloric effect and large refrigerant capacity for each of Eu<sub>0.9</sub><i>M</i><sub>0.1</sub>TiO<sub>3</sub> (<i>M</i>=Sr, Ba, La, Ce) compounds are observed. Under the magnetic field change of 1 T, the values of maximum magnetic entropy change and refrigeration capacity are 9.8 J/(kg·K) and 36.6 J/kg for Eu<sub>0.9</sub>Sr<sub>0.1</sub>TiO<sub>3</sub>, and 10 J/(kg·K) and 45.1 J/kg for Eu<sub>0.9</sub>Ba<sub>0.1</sub>TiO<sub>3</sub>. The values of maximum magnetic entropy change of Eu<sub>0.9</sub>La<sub>0.1</sub>TiO<sub>3</sub> and Eu<sub>0.9</sub>Ce<sub>0.1</sub>TiO<sub>3</sub> are 10.8 J/(kg·K) and 11 J/(kg·K), respectively, which are larger than that of EuTiO<sub>3</sub> (9.8 J/(kg·K)). The values of refrigeration capacity are 39.3 J/kg and 51.8 J/kg, which are also improved compared with those of EuTiO<sub>3</sub>. In a word, the results suggest that these compounds could be considered as good candidates for low-temperature and low-field magnetic refrigerant.