Spin-State Transition in the Fe Pnictides

We report an Fe $K\beta$ x-ray emission spectroscopy study of local magnetic moments in the rare-earth doped iron pnictide ${\mathrm{Ca}}_{1-x}{\mathrm{RE}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($\mathrm{RE}=\mathrm{La}$, Pr, and Nd). In all samples studied the size of the Fe local moment is found to decrease significantly with temperature and goes from $\sim 0.9{\mu }_B$ at $T=300\mathrm{K}$ to $\sim 0.45{\mu }_B$ at $T=70\mathrm{K}$. In the collapsed tetragonal phase of Nd- and Pr-doped samples ($T<70\mathrm{K}$) 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 ${\mathrm{Ca}}_{1-x}{\mathrm{RE}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($\mathrm{RE}=\mathrm{Pr}$ 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.

Figure 1

(a) Comparison of the $K\beta$ emission line for the Nd-doped sample and FeCrAs taken at room temperature. The difference spectrum (grey) was magnified by a factor of 4 here and the rest of the figure. The inset shows the $K\beta$ emission process in the atomic limit (see text). In (b)–(e) the temperature dependence of this difference, obtained in the same way as in (a), is shown for Pr-doped and La-doped samples.

Figure 2

The temperature dependence of (a) $c$-axis lattice parameters, and (b) the IAD values derived from the XES spectra. Same symbols are used in both panels. On the right-hand side of panel (b), the local magnetic moment scale ($\mu$) as described in Ref. 28 is shown. Thick dashed lines are guides to the eyes. The solid red line and thin dashed black lines are fits to three-state and two-state models, respectively. The spin and orbital configuration of the three-state model used for the fitting is shown in the inset. The grey area represents the detection limit of the IAD method.

Figure 3

Fe $K$-edge x-ray absorption near edge spectra taken in the partial fluorescence yield mode by monitoring the Fe $K\beta$ emission line. Spectra for Nd- and Pr-doped samples were taken at $T=75\mathrm{K}$ and 45 K, and at $T=100\mathrm{K}$ and 45 K for La-doped samples. The simulated spectrum was calculated for a Nd-doped crystal in the T and cT phase. It has been shifted in energy to match the pre-edge peak $A$ for the experimental scans. Spectra were offset for clarity.