Spin reorientation of the Fe moments in ${\mathrm{Eu}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Fe}}_2{\mathrm{As}}_2$: Evidence for strong interplay of Eu and Fe magnetism

Using complementary polarized and unpolarized single-crystal neutron diffraction, we have investigated the temperature-dependent magnetic structures of ${\mathrm{Eu}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Fe}}_2{\mathrm{As}}_2$. Upon 50% dilution of the Eu sites with isovalent ${\mathrm{Ca}}^{2+}$, the Eu sublattice is found to be still long-range ordered below $T_{\text{Eu}}=10$ K, in the $A$-type antiferromagnetic (AFM) structure. The moment size of ${\mathrm{Eu}}^{2+}$ spins is estimated to be as large as $6.74\left(4\right){\mu }_B$ at 2.5 K. The Fe sublattice undergoes a spin-density-wave transition at $T_{\text{SDW}}=192\left(2\right)$ K and displays an in-plane AFM structure above $T_{\text{Eu}}$. However, at 2.5 K, the ${\mathrm{Fe}}^{2+}$ moments are found to be ordered in a canted AFM structure with a canting angle of $14{\left(4\right)}^{\circ }$ out of the $ab$ plane. The spin reorientation of Fe below the AFM ordering temperature of Eu provides direct evidence of a strong interplay between the two magnetic sublattices in ${\mathrm{Eu}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Fe}}_2{\mathrm{As}}_2$.

Figure 1

Reciprocal-space contour maps in the $(H,0,L)$ plane for ${\mathrm{Eu}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Fe}}_2{\mathrm{As}}_2$ obtained at $T=3.5$ K via polarized neutron diffraction at DNS, with the neutron polarization parallel to the scattering vector $Q$ ($x$ polarization). Intensities in the (a) SF and (b) NSF channels correspond to the magnetic and nuclear reflections, respectively. The appearances of $(0,0,2)$ and $(0,0,4)$ reflections in the $X_{\text{SF}}$ channel are due to the imperfection of the polarization analysis and leakage from the $X_{\text{NSF}}$ channel.

Figure 2

The temperature dependence of the integrated intensity of the $(0,0,3)$ magnetic reflection in the $X_{\text{SF}}$ channel measured at DNS. The insets show the rocking scan of the $(0,0,3)$ peak at 3.5 and 11 K, respectively.

Figure 3

The temperature dependencies of the integrated intensity of the $(4,0,0)$ nuclear reflection [black squares, (a)], $(1,0,1)$ magnetic reflection [black spheres, (b)], and $(1,0,3)$ magnetic reflection [black spheres, (c)], respectively, measured at HEiDi. The peak width (FWHM) of $(4,0,0)$ is also plotted using blue open circles in (a). Red open circles in (b) and (c) represent the peak intensities of $(1,0,1)$ and $(1,0,3)$ as a function of the temperature, respectively, measured in the $X_{\text{SF}}$ channel at DNS. The insets in (b) and (c) show the rocking scans of $(1,0,1)$ and $(1,0,3)$ in the $X_{\text{SF}}$ channel at 3. 5 and 15 K, respectively. The solid curves represent the fits using the Gaussian profiles. The dashed vertical lines mark the SDW transition ($T_{\text{SDW}}$) coincident with the structural phase transition ($T_S$) and the magnetic ordering of localized ${\mathrm{Eu}}^{2+}$ moments ($T_{\text{Eu}}$), respectively.

Figure 4

Comparison between the observed intensities of four magnetic reflections from the Fe sublattice at 11 K (solid blue circles), 2.5 K (solid red squares), the calculated intensities using the in-plane AFM structure (open blue circles), and the calculated intensities using the canted AFM structure (open red squares).

Figure 5

The magnetic structures of ${\mathrm{Eu}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Fe}}_2{\mathrm{As}}_2$ at (a) 11 K and (b) 2.5 K, respectively.