Type: Article
Publication Date: 2013-01-24
Citations: 60
DOI: https://doi.org/10.1103/physrevlett.110.047003
We report an Fe $K\ensuremath{\beta}$ x-ray emission spectroscopy study of local magnetic moments in the rare-earth doped iron pnictide ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{RE}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($\mathrm{RE}=\mathrm{La}$, Pr, and Nd). In all samples studied the size of the Fe local moment is found to decrease significantly with temperature and goes from $\ensuremath{\sim}0.9{\ensuremath{\mu}}_{B}$ at $T=300\text{ }\text{ }\mathrm{K}$ to $\ensuremath{\sim}0.45{\ensuremath{\mu}}_{B}$ at $T=70\text{ }\text{ }\mathrm{K}$. In the collapsed tetragonal phase of Nd- and Pr-doped samples ($T<70\text{ }\text{ }\mathrm{K}$) the local moment is quenched, while the moment remains unchanged for the La-doped sample, which does not show lattice collapse. Our results show that ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{RE}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($\mathrm{RE}=\mathrm{Pr}$ and Nd) exhibits a spin-state transition and provide direct evidence for a nonmagnetic ${\mathrm{Fe}}^{2+}$ ion in the collapsed tetragonal phase; spin state as predicted by Yildirim. We argue that the gradual change of the spin state over a wide temperature range reveals the importance of multiorbital physics, in particular the competition between the crystal field split Fe $3d$ orbitals and the Hund's rule coupling.