Stabilization of E -type magnetic order caused by epitaxial strain in perovskite manganites

Abstract

The magnetism and spin-induced ferroelectricity of perovskite $R{\mathrm{MnO}}_{3}$ ($R=\text{La}$-Lu) epitaxial films grown on ${\mathrm{YAlO}}_{3}$ substrate have been investigated by means of first-principles density-functional-theory calculations. We show that the epitaxial strain leads to an $E$-type magnetic ground state for all of the rare-earth manganites, which explains the large ferroelectric polarization observed in ${R\mathrm{MnO}}_{3}$ ($R=\text{Gd}$-Lu) epitaxial films. The transition of the ground state from the $A$-type or spiral antiferromagnetic phase to the $E$-type phase can be attributed mainly to the significant weakening of ferromagnetism in the in-plane nearest-neighbor exchange interaction, which changes from ferromagnetic to antiferromagnetic with the increase of $R$ ionic radius. The enhancement of Jahn-Teller distortion caused by the epitaxial strain plays an important role in the change of exchange interactions. The predicted $E$-type phase in strained films for larger $R$ ions ($R=\text{La}$-Dy) can greatly improve the ferroelectric polarization. However, for the smaller $R$ ions ($R=\text{Ho}$-Lu) whose ground state of the bulk is the $E$-type phase, the ferroelectric polarization of strained films is slightly reduced compared to the bulks, resulting from the competition between the opposite trends of electronic and ion polarization. The ferroelectric distortion caused by the magnetostriction effect is enhanced by the epitaxial strain, while the polarization induced by the pure electronic mechanism is significantly suppressed. The introduction of Coulomb repulsion $U$ in the calculations overestimates the tendency of exchange interactions toward ferromagnetism, leading to the incorrect ground states.