Structural origin of the anomalous temperature dependence of the local magnetic moments in the CaFe${}_2$As${}_2$ family of materials

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.

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