Ferroelectric ground state and polarization-switching path of orthorhombic YMnO3with coexistingE-type and cycloidal spin phases

Abstract

Orthorhombic YMO${}_{3}$ ($o$-YMO) is currently being intensively investigated since a single-crystalline thin film of $o$-YMO recently showed a pronounced degree of magnetoelectric coupling. According to first-principles calculations, the ferroelectric ground state is represented by two degenerate $E$-type collinear spin configurations with the computed polarization $P$ of \ensuremath{\sim}1.4 \ensuremath{\mu}C/cm${}^{2}$. The $bc$-cycloidal spin phase is identified as the spin configuration that corresponds to the transition state in the $+P\ensuremath{\leftrightarrow}\ensuremath{-}P$ polarization switching. The remarkable coexistence of the $E$-type and cycloidal spin phases below \ensuremath{\sim}40 K is attributed to a small value of the Kohn-Sham energy difference between these two phases, 2.2 meV per formula unit. A modulated spin structure, which is characterized by the tilting of the Mn-spin vectors to the $a$-axis direction of Pbnm setting, is proposed to account for the observed strong magnetic-field-dependent polarization in $o$-YMO.