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.
Categories: cond-mat.supr-con