三角晶格反铁磁CuFeO_2的磁性和电子结构

基于广义梯度近似(GGA)的密度泛函理论(DPT),,通过构造铁磁(FM),阻挫的三角非共线反铁磁(FAFM)、上上下下型共线反铁磁(↑↑↓↓AFM)三种不同磁性构型,从非共线磁性结构计算出发.优化了低温铜铁矿CuFeO_2晶体材料的几何结构,研究了磁性结构对电子结构、能隙和磁矩等的作用.计算发现上上下下型反铁磁自旋排列能促进能隙形成,总能降低,磁矩增大.由于上上下下型反铁磁与阻挫三角非共线反铁磁相能量接近,外场的作用容易导致磁性结构相变到阻挫的三角反铁磁态,其电子态密度分布与X光发射光谱测得的结果一致,即具有高自旋的Fe离子3d电子自旋向上的子带中心位于Cu3d能态之下,O2p能态以上,而且配位场理论分析表明Fe离子3d态自旋向下的空轨道为铁电极化提供了有利的化学环境.

Magnetism and Electronic Structures of Triangular Lattice Antiferromagnetic CuFeO_2

Based on a non-collinear magnetic structure calculation, the magnetism, energy gap, and electronic structures of the triangular lattice antiferromagnetic delafossite CuFeO_2 were investigated by density functional theory (DFT) within the generalized gradient approximation (GGA) approach. By producing three types of magnetic configurations including ferromagnetic (FM), frustrated triangular non-collinear antiferromagnetic (FAFM), and up-up-down-down collinear antiferromagnetic (↑↑↓↓ AFM) ordering, a full optimization of the lattice parameters and internal coordinates was performed for the low temperature hexagonal structure. The calculations show that the up-up-down-down spin arrangement plays an important role in the formation of the band gap, the decrease in total energy and the increase in magnetic moment. Since a small difference exists between the total energy of the FAFM and ↑↑↓↓ AFM phase, the ↑↑↓↓ AFM easily undergoes a phase transition to the FAFM state when an external magnetic field is applied. Additionally, the electronic densities of states (DOS) in the ↑↑↓↓ AFM phase qualitatively agrees with the results of X-ray emission spectra, that is, the Fe ion is in a high-spin state with the spectral weight of the Fe 3d spin-up band centered slightly below the Cu 3d but above the O2p bands. Analysis with ligand field theory also indicates that the empty orbital of the Fe 3d spin-down provides a chemical environment favorable for ferroelectric polarization.