Origin of the 150-K Anomaly in LaFeAsO: Competing Antiferromagnetic Interactions, Frustration, and a Structural Phase Transition

From all-electron fixed-spin-moment calculations we show that ferromagnetic and checkerboard antiferromagnetic ordering in LaFeAsO are not stable and the stripe antiferromagnetic configuration with is the only stable ground state. The main exchange interactions between Fe ions are large, antiferromagnetic, and frustrated. The magnetic stripe phase breaks the tetragonal symmetry, removes the frustration, and causes a structural distortion. These results successfully explain the magnetic and structural phase transitions in LaFeAsO recently observed by neutron scattering. The presence of competing strong antiferromagnetic exchange interactions suggests that magnetism and superconductivity in doped LaFeAsO may be strongly coupled, much like in the high- cuprates.

Figure 1

(a) The crystal structure of LaFeAsO in space group $P4/nmmm$ with origin choice 1. (b) Top view of the FeAs plane and the relations between primitive and $\sqrt{2}\times \sqrt{2}$ supercell used in our calculations. The dark and light shaded areas indicate the As atoms below and above the Fe square lattice, respectively.

Figure 2

Two antiferromagnetic configurations considered in this study. Left-hand panel shows the AFM1 configuration where nearest-neighbor spins are always aligned antiparallel. Right-hand panel shows the AFM2 configuration where the next-nearest-neighbor spins (i.e., $J_2$) are always aligned antiparallel. Note that this is the same stripe phase predicted from Fermi-surface nesting [11, 12].

Figure 3

(a) The total energy per Fe atom versus magnetic moment for FM, AFM1, and AFM2 spin configuration, indicating AFM2 is the only ground state of the system. (b) The magnetic interactions for NN and NNN Fe ions obtained from the energies of FM, AFM, and AFM2 configurations.

Figure 4

The total energy per cell versus the angle $\gamma$ for nonmagnetic (NM), ferromagnetic (FM), and two antiferromagnetic (AFM1 and AFM2) spin configurations. Note that only the AFM2 spin configuration yields structural distortion. The inset shows that as $\gamma$ increases, the ferromagnetic aligned Fe ions (i.e., ${\mathrm{Fe}}_1\mathrm{\text{−}}{\mathrm{Fe}}_2$) get closer while the antiferomagnetically aligned ions (i.e., ${\mathrm{Fe}}_1\mathrm{\text{−}}{\mathrm{Fe}}_3$) move apart, breaking the fourfold symmetry and thus the degeneracy of the $d_{xz}$ and $d_{yz}$ orbitals.

Figure 5

The total electronic DOS of LaFeAsO for high ($P4/nmm$) and low ($Cmma$ or $P2/c$) symmetry phases.