High-pressure synthesis of the layered iron oxyselenide $\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ with strong magnetic anisotropy

Using a high-pressure reaction, we successfully synthesized $\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ with a uniform stack of $[\mathrm{F}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}]$ layers. Magnetic susceptibility, heat capacity, Mössbauer spectroscopy, and neutron-diffraction measurements revealed that $\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ undergoes antiferromagnetic order at 106 K with a 2-$k$ spin structure where each Fe moment points to a neighboring oxide anion. The same spin structure has been observed in the related iron oxyselenides but with a staggered stack of $\left[\mathrm{F}{\mathrm{e}}_2\mathrm{C}{\mathrm{h}}_2\mathrm{O}\right]$ (where Ch represents chalcogen) layers $(\mathrm{Ch}=\mathrm{S},\mathrm{Se})$. We propose that the strong uniaxial anisotropy inferred from the 2-$k$ structure originates from spin-orbit coupling (SOC) induced by a $\mathit{trans}-\mathrm{Fe}{\mathrm{O}}_2\mathrm{S}{\mathrm{e}}_4$ octahedron, which provides a quasilinear coordination environment as often found in single molecule magnetic complexes. A first-principles calculation with inclusion of SOC supports the stabilization of the 2-$k$ spin structure, giving an unquenched orbital momentum of $0.1{\mu }_{\mathrm{B}}/\mathrm{Fe}$. The present paper provides an idea of how to design magnetic lattices of uniaxial anisotropy using oxychalcogenides and more generally mixed-anion compounds.

Figure 1

(a) $A{M}_2\mathrm{C}{\mathrm{h}}_2\mathrm{O}[A^{2+}={\left(\mathrm{L}{\mathrm{n}}_2{\mathrm{O}}_2\right)}^{2+},{\left(\mathrm{S}{\mathrm{r}}_2{\mathrm{F}}_2\right)}^{2+},{\left(\mathrm{B}{\mathrm{a}}_2{\mathrm{F}}_2\right)}^{2+},{\left(\mathrm{N}{\mathrm{a}}_2\right)}^{2+};M=\mathrm{Mn},\mathrm{Fe},\mathrm{Co};\mathrm{Ch}=\mathrm{S},\mathrm{Se}]$ with a staggered stacking of $[M_2\mathrm{C}{\mathrm{h}}_2\mathrm{O}]$ layers [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. (b) $hp-\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ with a uniform stacking of $\left[\mathrm{F}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}\right]$ layers.

Figure 2

The experimental (upper) and calculated (lower) XRD patterns for $hp-\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ with the $\mathrm{BaT}{\mathrm{i}}_2\mathrm{A}{\mathrm{s}}_2\mathrm{O}$-type structure.

Figure 3

Mössbauer spectra for $hp-\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ at 295 and 4 K. For each temperature, the blue line represents the subspectrum for $hp-\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$, whereas the green and yellow lines correspond to byproducts, possibly $\mathrm{F}{\mathrm{e}}_{1+x}\mathrm{Se}$ and an unknown phase with $\mathrm{F}{\mathrm{e}}^{3+}$, respectively. The red line represents the total fit.

Figure 4

(a) Temperature dependence of the magnetic susceptibility of $hp-\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ at 1 T measured in FC and ZFC processes. The inset represents the $M$($H$) curve at 2 K. (b) Specific heat for $hp-\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ in a zero field. The red curve represents a polynomial fitting for 60–120 K. The inset represents the estimated magnetic contribution to the entropy.

Figure 5

(a) Powder ND patterns of $hp-\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ at low temperatures. The ticks indicate the calculated positions of nuclear (upper) and magnetic (lower) reflections. (b) The enlarged plot around the (1/2,1,1/2) reflection. We fitted the profiles using two Gaussians. The blue and yellow lines represent the magnetic and nuclear reflections, respectively, and the red line is the total fit. The lower panels show magnetic structures compatible with ($h/2, k, l/2$) reflections: (c) a double stripe structure and (d) a 2-$k$ structure.

Figure 6

Temperature dependence of the magnetic moment obtained from powder ND (open) and hyperfine field (HF) obtained from Mössbauer (closed) for $hp-\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$. The magnetic moment shown here is obtained by normalizing the square root of the neutron-scattering intensity of (1/2,1,1/2) to the refined value of $3.28\left(5\right){\mu }_{\mathrm{B}}$ at 3 K.

Figure 7

The expected scheme of the crystal-field splitting of Fe $3d$ orbitals in the $\mathit{trans}-\mathrm{Fe}{\mathrm{O}}_2\mathrm{S}{\mathrm{e}}_4$ octahedron where only the sixth down-spin is shown for the high-spin $(S=2)$ state.

Figure 8

The partial DOS for $hp-\mathrm{BaF}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ calculated by using the 2-$k$ magnetic structure with on-site repulsion of $U=3\mathrm{eV}$.