Strain-driven magnetic phase transitions from an antiferromagnetic to a ferromagnetic state in perovskite RMnO3 films

The effects of epitaxial strain on the structural, electronic, and magnetic properties in perovskite $R{\mathrm{MnO}}_{3}$ ($R=\mathrm{La}--\mathrm{Lu}$) films grown on (001) and (110) ${\mathrm{SrTiO}}_{3}$ substrates have been studied by density functional theory calculations. We demonstrate that the epitaxial strain causes the magnetic phase transitions from antiferromagnetic to ferromagnetic in most of the $R{\mathrm{MnO}}_{3}$ films, except those with smaller $R$ ions. These transitions can be attributed to the suppression of ${Q}_{3}$-type Jahn-Teller distortion caused by epitaxial strain, which significantly enhances the ferromagnetism in the nearest-neighbor exchange interactions. The strain gradient resulting from the strain relaxation provides the possibility to realize the coexistence of ferroelectricity and ferromagnetism, and a magnetoelectric coupling interface can be formed at the phase boundary between ferromagnetic and ferroelectric antiferromagnetic phases. Our results not only explain the observed ferromagnetism and the coexistence of multiple magnetic orders but also shed light on how to achieve the coexistence and coupling between ferroelectricity and ferromagnetism in $R{\mathrm{MnO}}_{3}$ films.