Local and collective magnetism of ${\mathbf{EuFe}}_2{\mathbf{As}}_2$

We present an experimental study of the local and collective magnetism of ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ that is isostructural with the high-temperature superconductor parent compound ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. In contrast to ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, where only Fe spins order, ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ has an additional magnetic transition below 20 K due to the ordering of the ${\mathrm{Eu}}^{2+}$ spins $(J=7/2$, with $L=0$ and $S=7/2)$ in an $A$-type antiferromagnetic texture (ferromagnetic layers stacked antiferromagnetically). This may potentially affect the FeAs layer and its local and correlated magnetism. $\mathrm{Fe}K\beta$ x-ray emission experiments on ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ single crystals reveal a local magnetic moment of $1.3\pm 0.15{\mu }_B$ at 15 K that increases slightly to $1.45\pm 0.15{\mu }_B$ at 300 K. Resonant inelastic x-ray scattering experiments performed on the same crystals show dispersive broad (in energy) magnetic excitations along $(0,0)\to (1,0)$ and $(0,0)\to (1,1)$ with a bandwidth on the order of 170–180 meV. These results on local and collective magnetism are in line with other parent compounds of the ${\mathrm{AFe}}_2{\mathrm{As}}_2$ series $(A=\mathrm{Ba}$, Ca, and Sr), especially the well-characterized ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. Thus, our experiments lead us to the conclusion that the effect of the high magnetic moment of Eu on the magnitude of both the Fe local magnetic moment and spin excitations is small and confined to low-energy excitations.

Figure 1

(a) Fe-K XAS-PFY for ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ and ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. Inset: zoom of the preedge region. (b) $\mathrm{Fe}{L}_{2,3}$ XAS-TFY for ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ and ${\mathrm{BaFe}}_2{\mathrm{As}}_2$.

Figure 2

(a) $\mathrm{Fe}K\beta$ XES for ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ and FeCrAs (employed for calibration) at 15 K. The shadowed part indicates the difference spectrum for ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ from which the IAD is extracted. (b) $\mathrm{Fe}K\beta$ XES for ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ and FeCrAs (employed for calibration) at 300 K. The shadowed part indicates the difference spectrum for ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ from which the IAD is extracted.

Figure 3

(a) Experimental setup for the RIXS experiment. (b) Black solid line: RIXS spectrum for ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ at 19 K at (0.44, 0). Green solid line: fitting of the emission line.

Figure 4

(a) ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ RIXS spectra at 19 K for different $q_{\parallel }$ along the $(0,0)\to (1,0)$ direction. The $q_{\parallel }$ values are (0.44, 0), (0.37, 0), (0.33, 0), (0.29, 0), (0.24, 0), (0.20, 0), and (0.10, 0). (b) RIXS spectra at 19 K and different $q_{\parallel }$ along the $(0,0)\to (1,1)$ direction. The $q_{\parallel }$ values are (0.31, 0.31), (0.26, 0.26), (0.23, 0.23), (0.20, 0.20), (0.17, 0.17), (0.14, 0.14), and (0.07, 0.07).

Figure 5

RIXS spectra of ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ at 19 K and different momentum transfer along (0, 0)$\to$(1, 0) (left panel) and (0, 0)$\to$(1, 1) (right panel). Black dots are experimental data, solid purple is the sum of resonant emission and elastic peak, green is the spin excitation peak, and red is the sum of all the components of the fitting. At (0.10, 0) and (0.07, 0.07) we do not attempt any fitting since a peak for the magnetic excitations is not clearly visible.

Figure 6

Dispersion of magnetic excitations of ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ at 19 K along (0, 0)$\to$(1, 0) and (0, 0)$\to$(1, 1) directions as black dots with error bars. The dispersion of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ is shown as red triangles with error bars for comparison purposes as extracted from Ref. [40]. At (0.1, 0) and (0.07, 0.07) the fitting was not possible because of an overlap of the magnetic peak with the elastic line. Only an estimation of the energy range is provided for these values, as depicted by error bars. The points have been shifted slightly along the horizontal axis for better visualization.

Figure 7

(a) ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ $\mathrm{Fe}{L}_{2,3}$ XAS at 19 K (red) and 300 K (cyan). (b) RIXS spectra at (0.44, 0), 19 K (red) and 300 K (cyan) for ${\mathrm{EuFe}}_2{\mathrm{As}}_2$.

Figure 8

(a) Comparison between RIXS spectra for ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ at 19 K (red) and 300 K (cyan) along the $(0,0)\to (1,0)$ direction. (b) Comparison between RIXS spectra for ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ at 19 K (red) and 300 K (cyan) along the $(0,0)\to (1,1)$ direction.