Magnetic properties of the covalent chain antiferromagnet ${\mathrm{RbFeSe}}_2$

Single crystals of the ternary iron selenide ${\mathrm{RbFeSe}}_2$ have been investigated by means of x-ray diffraction, magnetic susceptibility, magnetization, and specific-heat measurements as well as by Mössbauer spectroscopy. Built up from linear chains of edge-sharing ${\mathrm{FeSe}}_4$ tetrahedra, ${\mathrm{RbFeSe}}_2$ represents a quasi-one-dimensional antiferromagnet. Below $T_{\mathrm{N}}=248$ K three-dimensional antiferromagnetic collinear magnetic order sets in, with the magnetic moments oriented perpendicularly to the chain direction. The hyperfine fields determined from our Mössbauer studies reveal strongly reduced magnetic moments. The high-temperature susceptibility data of ${\mathrm{RbFeSe}}_2$ suggest a one-dimensional metallic character along the chains.

Figure 1

Crystal structure of ${\mathrm{RbFeSe}}_2$. The ${\mathrm{FeSe}}_4$ tetrahedra, with Fe drawn as orange sphere in the center and Se as green sphere at the corners, are highlighted in transparent yellow color. White large spheres denote Rb.

Figure 2

Powder diffraction pattern of ${\mathrm{RbFeSe}}_2$. The empty circles represent the measured intensities. The black solid line shows the refined pattern. The Bragg-peak positions are indicated by vertical (green) bars. The difference pattern $I_{\mathrm{obs}}-I_{\mathrm{cal}}$ is indicated by the solid (blue) line.

Figure 3

Temperature dependence of the magnetic susceptibility in ${\mathrm{RbFeSe}}_2$ single crystals parallel and perpendicular to the chain direction ($H=10$ kOe). Inset: Field dependence of the magnetization along the chain direction in ${\mathrm{RbFeSe}}_2$ at different temperatures.

Figure 4

Temperature dependence of the specific heat $C\left(T\right)$ for ${\mathrm{RbFeSe}}_2$. The red solid line indicates the fit to the experimental results by the sum of Debye and Einstein contributions (see details in the text). Inset: Specific heat of ${\mathrm{RbFeSe}}_2$ in the temperature range $175\le T\le 300$ K measured both on field cooling (FC) and in zero field.

Figure 5

Specific heat in representation $C/T$ as a function of $T^2$ for ${\mathrm{RbFeSe}}_2$ at low temperatures (the black solid line is the fitting curve using the formula $C/T={\beta }_{\mathrm{D}}{T}^2$). Insets: (a) High-temperature magnetic specific heat $C_{\mathrm{M}}/T$ versus $T$ in zero external magnetic field for ${\mathrm{RbFeSe}}_2$ after subtracting the calculated lattice contribution; (b) low temperature $C/T$ versus $T$, the red solid line represents the fit calculated with ${\mathrm{\Theta }}_{\mathrm{D}}=110$ K.

Figure 6

Selected Mössbauer spectra of ${\mathrm{RbFeSe}}_2$ recorded at room temperature (RT, $T=300$ K, upper frame), $T=80$ K (middle frame), and $T=10$ K (lower frame). Solid red lines represent best fitting of the Mössbauer spectra obtained by least-squares fit with the assumption that the line shapes are Lorentzian. The black dashed line shows the pattern of the paramagnetic doublet coexisting with the magnetic sextet.

Figure 7

Temperature dependence of the hyperfine field on the ${}^{57}\mathrm{Fe}$ nucleus in ${\mathrm{RbFeSe}}_2$. The solid line is obtained by the best least-squares fitting of the data by power law equation (1). Inset: Temperature dependence of the total area below the ${\mathrm{RbFeSe}}_2$ Mössbauer spectrum. The solid lines indicate fits by Eq. (2).