Emergence of complex magnetism in three dimensional, yet quasi-layered, iron pnictides: CaFe$_{4}$As$_{3}$*

The class of iron pnictides has been the focus of much attention for the discovery of superconductivity in the layered compounds LaOFeAs, CaFe$_{2}$As$_{2}$, and related ones; the phase diagrams of these pnictides remain still largely unexplored. Here, we report on the electronic and magnetic structure of the recently synthesized CaFe$_{4}$As$_{3}$ compound. This material, as opposed to the layered CaFe$_{2}$As$_{2}$, shows FeAs slabs parallel to the $b$-direction and approximately perpendicular to each other, defining tunnels filled by the Ca atoms. No sign of superconductivity was found in this compound. Instead, the system shows a complex ferromagnetic state at low temperature. DFT calculations performed on the refined crystal structure using the highly precise FLAPW method\footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB, {\bf 24}, 864 (1981)} show a pronounced stabilization for the ferromagnetic state which is characterized by four distinct Fe sites with magnetic moments of between 1 $\mu_{B}$ and 2 $\mu_{B}$. The influence of the local topology of the crystal structure on the the electronic and magnetic state is analyzed.