Ferroelectric ground state and polarization-switching path of orthorhombic YMnO3 with coexisting E-type and cycloidal spin phases

Orthorhombic YMO3 (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 similar to 1.4 mu C/cm(2). The bc-cycloidal spin phase is identified as the spin configuration that corresponds to the transition state in the + P <-> -P polarization switching. The remarkable coexistence of the E-type and cycloidal spin phases below similar to 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.