LSDA study of magnetic anisotropy in undoped Fe pnictides

Dr. Alexander Yaresko, Max-Planck-Institut für Festkörperforschung,  Stuttgart, Germany

LSDA study of magnetic anisotropy in undoped Fe pnictides

Department Chemie, Lehrbereich Physikalische Chemie, Haus E, Seminarraum E0.011, Butenandtstr. 5-13 F, D-81377 München.

Abstract:

If spin-orbit coupling is taken into account, LSDA band structure calculations for parent Fe pnictides successfully reproduce the correct magnetic ground state with stripe antiferromagnetic (AFM) order and Fe moments aligned along a, i.e., along AFM Fe chains. They also predict that the out-of-plane (ac) magneto-crystalline anisotropy (MCA) in LaFeAsO and BaFe2As2 is stronger than the in-plane (ab) one. However, recent polarized inelastic neutron scattering experiment showed the opposite: the gap at zone center is larger for in-plane spin excitations than for out-of-plane ones, which means that it is easier to deviate Fe spins in the ac plane. We show that MCA in LaFeAsO, BaFe2As2, and NaFeAs is a non-monotonous function of the exchange splitting of Fe d states. For large values of the splitting the out-of-plane MCA is stronger but for sufficiently small exchange splittings the order of the MCA energies is reverted: in agreement with the experiment the in-plane ab anisotropy becomes stronger.

Another unusual feature of Fe pnictides is that two collinear stripe-like magnetic structures, characterized by ordering vectors (,0) or (0,), are separated by an energy barrier comparable to the energy difference between Neel and stripe AFM orders. The shape of the barrier can be reproduced by adding a biquadratic term to the Heisenberg model. The barrier seems to appear as a result of the peculiar band structure of Fe pnictides and nesting properties of their Fermi surfaces.

Tuesday, 11.02.14

14:15 h - 15:15 h