Strain relaxation induced coexistence of ferromagnetism and antiferromagnetism in (110)-oriented LuMnO3 thin films on YAlO3 : A first-principles study

Abstract

Substrate strain offers an extra dimension in enriching the functionality of perovskite thin films. The recent observation of the coexistence of ferromagnetism (FM) and antiferromagnetism (AFM) in the ${\mathrm{LuMnO}}_{3}$ thin films on (110)-oriented ${\mathrm{YAlO}}_{3}$ substrate illustrated another interesting example [Phys. Rev. Lett. 111, 037201 (2013); Phys. Rev. B 94, 054423 (2016)]. The FM signal arises from a thin atomic layer near the ${\mathrm{YAlO}}_{3}$ substrate. We propose in this paper that vertical strain relaxation may stabilize the FM structure in the most compressively strained region near the substrate, while the AFM structure is stabilized in less compressively strained outer surfaces of thin films. This physical picture was supported by the magnetic phase diagram vs compressive strain derived from comprehensive first-principles calculations. The critical $[1\overline{1}0]$ strain separating E-AFM and FM structures is $\ensuremath{\epsilon}\ensuremath{\approx}\ensuremath{-}0.0425$, which is close to the value $\ensuremath{\epsilon}\ensuremath{\approx}\ensuremath{-}0.047$ dictated by ${\mathrm{YAlO}}_{3}$ substrate. From the half-metal feature of the FM structure and ferroelectric property of E-AFM structure, the greatly reduced ferroelectric polarization and averaged magnetic moment of $0.5\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{Mn}$ can be understood.