Structural origin of the anomalous temperature dependence of the local magnetic moments in the CaFe$_{2}$As$_{2}$ family of materials

TL;DR

This study combines spectroscopy and first-principles calculations to explain the unusual temperature dependence of local magnetic moments in CaFe$_{2}$As$_{2}$ family, highlighting strong magnetoelastic coupling and the dual nature of magnetism.

Contribution

It introduces a scaled magnetic interaction parameter in ab-initio calculations to accurately model temperature-dependent magnetic moments in CaFe$_{2}$As$_{2}$ compounds.

Findings

Large thermal expansion of the c-axis correlates with increased local moments.

A single scaling parameter effectively models magnetic moments across different compositions.

Strong magnetoelastic coupling explains the anomalous temperature behavior.

Abstract

We report a combination of Fe K$\beta$ x-ray emission spectroscopy and $ab$-intio calculations to investigate the correlation between structural and magnetic degrees of freedom in CaFe$_{2}$(As$_{1-x}$P$_{x} $)$_{2}$. The puzzling temperature behavior of the local moment found in rare earth-doped CaFe$_{2}$As$_{2}$ [\textit{H. Gretarsson, et al., Phys. Rev. Lett. {\bf 110}, 047003 (2013)}] is also observed in CaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. We explain this phenomenon based on first-principles calculations with scaled magnetic interaction. One scaling parameter is sufficient to describe quantitatively the magnetic moments in both CaFe$_{2}$(As$_{1-x}$P$_{x} $)$_{2}$ ($x=0.055$) and Ca$_{0.78}% $La$_{0.22}$Fe$_{2}$As$_{2}$ at all temperatures. The anomalous growth of the local moments with increasing temperature can be understood from the observed large thermal expansion of the $c$-axis lattice parameter combined with strong magnetoelastic coupling. These effects originate from the strong tendency to form As-As dimers across the Ca layer in the CaFe$_{2}$As$_{2}$ family of materials. Our results emphasize the dual local-itinerant character of magnetism in Fe pnictides.