Doping- and pressure-induced change of electrical and magnetic properties in the Fe-based spin-ladder compound ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$

We have investigated the evolution of electronic structure in the Fe-based ladder compound Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)${}_2{\mathrm{Se}}_3$ ($0\le x\le 0.15$) by means of resistivity, magnetic susceptibility, and reflectivity measurements. Although the Co substitution into ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ greatly decreases the resistivity, the compound still shows an insulating behavior at $x=0.15$. Moreover, the insulating state at $x=0.15$ is robust even under the high pressure of 8 GPa. The antiferromagnetic transition temperatures are gradually suppressed by substituting Co atoms at $x\le 0.10$, while the spin glass state is observed at $x\ge 0.125$. Our finding revealed that the substituted Co atoms play a dual effect: electron-carrier dopant and localized moment, providing a supplemental insight into the doping effect in the Fe-based superconductors.

Figure 1

The doping dependence of the lattice parameters and volume (a), the obtained activation energy ($E_a$) deduced from the resistivity in the temperature region between 230 and 300 K (b), and the magnetic transition temperature ($T_{\mathrm{N}}$ and $T$*) (c) for Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)${}_2{\mathrm{Se}}_3$ ($0\le x\le 0.15$). The inset shows the block-type magnetic structure for ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$. AFM and SG in (c) stand for the antiferromagnetic and spin glass states, respectively.

Figure 2

Temperature ($T$) dependence of resistivity (ρ) along the ladder direction for Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)${}_2{\mathrm{Se}}_3$ with $x=0$, 0.025, 0.05, 0.075, 0.10, 0.125, and 0.15 under ambient pressure (a) and the same data for Ba(${\mathrm{Fe}}_{0.85}{\mathrm{Co}}_{0.15}$)${}_2{\mathrm{Se}}_3$ under the pressure ($P$) of 0, 2, 4, 6, and 8 GPa (b). The inset of (a) shows the fitting results to the variable-range-hopping model. The inset of (b) shows the pressure dependence of the calculated activation energy ($E_a$).

Figure 3

Temperature ($T$) dependence of the magnetic susceptibility ($\chi$) for Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)${}_2{\mathrm{Se}}_3$ with $x=0$, 0.025, 0.05, 0.075, 0.10, 0.125, and 0.15. The magnetic field of 5 T is applied to the three principal axes: $a$, $b$, and $c$ axes correspond to the layer, leg, and rung directions, respectively. The data taken at the zero-field-cooled condition are shown; in addition, for $x=0.15$, the data taken in both the zero-field-cooled and field-cooled conditions are shown. The arrows indicate the magnetic transition at $T_{\mathrm{N}}$ and $T$*. Note that each curve is shifted by 1×10${}^{-3}$ emu/molOe for clarity. The inset shows the Curie-Weiss fitting between 50 and 175 K for χ along the $a$ axis at $x=0.15$.

Figure 4

Reflectivity (a) and optical conductivity (σ) spectra (b) as a function of photon energy for ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ (broken lines) and Ba(${\mathrm{Fe}}_{0.85}{\mathrm{Co}}_{0.15}$)${}_2{\mathrm{Se}}_3$ (solid lines). The modes $A$ and $B$ correspond to an excitation from in-gap states, and a Mott excitation, respectively. The inset of (a) depicts the band structure involved with the low-energy optical excitations. LH and UH indicate the lower and upper Hubbard bands of Fe $3d$ electrons.