Electronic structure of the ferroelectromagnet YMnO₃

Abstract

Using the self-consistent linear muffin-tin orbitals and atomic sphere approximation (LMTO-ASA) method, electronic structure calculations of the ferroelectromagnet YMnO₃ in the magnetic and nonmagnetic phases are performed in the local density approximation (LDA) of the density-functional theory. Total energy calculations reproduce the magnetic phase observed at low temperatures. Due to the Jahn–Teller distortion, antiferromagnetic insulating solution with a very small energy gap is also obtained. After including the strong electron–electron correlation effects in Mn 3d states by the on-site Coulomb interaction correction, the energy gap is increased to 1.1 eV and the density of states is greatly redistributed. The large component of oxygen 2p states at the top of the valence states shows that the insulator YMnO₃ has a charge-transfer character.

Introduction

Recent discovery of the colossal magnetoresistance in doped LaMnO₃ near its magnetic transition temperature [1] provided renewed interest to the study of manganites. The yttrium and rare-earth manganites RMnO₃ usually have two structural phases [2], [3]: The hexagonal phase for R = Ho, Er, Tm, Yb, Lu or Y, which have a smaller ionic radius, and the orthorhombic phase for R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb or Dy, which have a larger ionic radius. While the magnetic ordering can occur in both hexagonal and orthorhombic manganites, ferroelectric ordering occurs only in the hexagonal ones, which belong to the noncentrosymmetric P6₃cm space group.

Due to coexistence of the ferroelectric and magnetic orderings at low temperatures, the hexagonal yttrium and rare-earth manganites are an interesting class of materials known as ferroelectromagnets. Indeed, the coupling phenomena between the ferroelectric and magnetic orderings have been recently observed in YMnO₃ [4], which displays a ferroelectric $\text{T}{}_{\mathrm{<mtext>E</mtext>}}\text{≈914}\text{K}$, but a rather low antiferromagnetic $\text{T}{}_{\mathrm{<mtext>M</mtext>}}\text{≈80}\text{K}$. This coupling can lead to the so-called magnetoelectric effect with an interesting potential use in devices, where the dielectric (magnetic) properties may be changed by the onset of the magnetic (electric) transition or by application of an external magnetic (electric) field. The Raman-active and infrared-active phonons spectra [5] are reported and assigned to define atomic vibrations based upon their symmetry. The atomic displacements, induced by the Γ-point phonon modes which are both Raman-active and infrared-active, modulate the macroscopic polarizability.

The electronic structures of the ferroelectric oxides have been the subject of some experimental and theoretical studies [6], [7], [8]. There has been considerable progress in achieving an understanding of the lattice dynamics and the origin of the ferroelectricity from first-principles total-energy calculations [9]. It has been shown that the hybridization between the transition metal 3d states and the oxygen 2p states is essential to ferroelectricity in the perovskite oxides [10]. However, the electronic structure of the ferroelectromagnet YMnO₃ has not been studied up to now, in which there are the more complicated interplays between magnetic ordering, ferroelectric ordering, and crystalline distortions. The first-principles density-functional calculations can enhance our microscopic understanding of this material.

In this Letter, the first-principle calculations of the hexagonal YMnO₃ have been made by the linear muffin-tin orbitals and atomic sphere approximation (LMTO-ASA) method [11], [12]. In Section 2, the computation method is described. The obtained results and discussions are given in Section 3. Finally, the conclusions are drawn in Section 4.