Thomas Brueckel

Magnetic lattice dynamics of the oxygen-free FeAs pnictides: how sensitive are phonons to magnetic ordering?

To shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperature orthorhombic parent compounds, which present a spin density wave. The study covers both the 122 (AFe(2)As(2); A = Ca, Sr, Ba) and 1111 (AFeAsF; A = Ca, Sr) phases. We extend our recent work on the Ca (122 and 1111) and Ba (122) cases by treating, computationally and experimentally, the 122 and 1111 Sr compounds. The effect of magnetic ordering is investigated through detailed non-magnetic and magnetic lattice dynamical calculations. The comparison of the experimental and calculated phonon spectra shows that the magnetic interactions/ordering have to be included in order to reproduce well the measured density of states. This highlights a spin-correlated phonon behavior which is more pronounced than the apparently weak electron-phonon coupling estimated in these materials. Furthermore, there is no noticeable difference between phonon spectra of the 122 Ba and Sr, whereas there are substantial differences when comparing these to CaFe(2)As(2) originating from different aspects of structure and bonding.

Structure Property Relationships in Magnetocaloric Materials

Suitable materials used for this technology exhibit a sizeable magnetocaloric effect (MCE), i.e. they show a considerable temperature change when exposed to a magnetic field. The MCE is particularly large when a transition to a magnetically ordered phase is accompanied by a structural transition, which, however, can reduce the cyclability of the material. The search for suitable materials is still ongoing as there are other demanding requirements (e.g. good moulding behaviour, cheap and nontoxic constituent elements, small hysterisis at the magnetic transition for the materials, tunable transition temperature). From the fundamental point of view there are still many open questions regarding the underlying mechanism of the magnetocaloric effect, in particular with respect to the coupling of the spin and lattice system.