Structural and magnetic study of $\mathrm{La}\mathrm{Ba}\mathrm{Co}\mathrm{Cu}{\mathrm{O}}_{5+\delta }$

The structure and magnetic properties of the compound $\mathrm{La}\mathrm{Ba}\mathrm{Cu}\mathrm{Co}{\mathrm{O}}_{5+\delta }$ have been studied for the non-stoichiometric oxygen concentration $\delta \approx 0.6$. The structure is pseudo-cubic with a tripled perovskite unit cell. The crystal structure was determined by a combined Rietveld fit to neutron and synchrotron x-ray powder diffraction data in the orthorhombic $Pmmm$ space group, with cell parameters $a=3.9223\left(3\right)\mathrm{\AA }$, $b=3.9360\left(3\right)\mathrm{\AA }$, $c=11.7073\left(8\right)\mathrm{\AA }$, and $V=180.74\left(2\right){\mathrm{\AA }}^3$ (room temperature). Antiferromagnetic ordering of Cu and Co magnetic moments is observed below $205\left(4\right)\mathrm{K}$. The magnetic structure with cell $a_M=2a$, $b_M=2b$, and $c_M=2c$, could be described with the Shubnikov space group $Fmm{m}^{\prime }$. The magnetic moments of both equivalent $\mathrm{Cu}∕\mathrm{Co}$ sites were determined at 50 and $170\mathrm{K}$ to be $0.83\left(3\right){\mu }_B$ and $0.58\left(3\right){\mu }_B$, respectively, consistent with one unpaired electron per atom. The fit of the intensities to a simple mean field magnetic model appeared to be insufficient to account for the variation of moments at temperatures close to $T_N$ while a three dimensional Heisenberg model could improve the fit. Susceptibility measurements between 4 and $350\mathrm{K}$ also show irreversibility below $150\mathrm{K}$. The local environments of Cu and Co were studied by extended x-ray absorption fine structure spectroscopy at both absorption edges. Cu atoms adopt an elongated octahedral or square-based pyramidal oxygen environment which suggests mainly the presence of Cu(II) in the structure. Co adopts different local environments, depending on the electronic and spin states.

Figure 1

(Color online) ORTEP view of one unit cell of $\mathrm{La}\mathrm{Ba}\mathrm{Cu}\mathrm{Co}{\mathrm{O}}_{5.6}$ at room temperature in the $Pmmm$ space group with labeling scheme. Ellipsoids are shown with 60% probability. Note that O1 and O2 atoms display occupancies lower than unit so the octahedral coordination of $\mathrm{Cu}∕\mathrm{Co}$ atoms could be pyramidal in many cases.

Figure 2

(Color online) NPD (a) and SXPD (b) patterns of LBCCO fitted by the Rietveld method with the $Pmmm$ structural model described in Table using GSAS/EXPGUI suite of programs.

Figure 3

(Color online) NPD patterns collected at 50 and $170\mathrm{K}$, and RT, the evolution of the intensity of the magnetic peak at $2\theta =19°$ is shown in the inset.

Figure 4

(Color online) Fitted NPD patterns for $\mathrm{La}\mathrm{Ba}\mathrm{Cu}\mathrm{Co}{\mathrm{O}}_5$ at (a) 50 and (b) $170\mathrm{K}$ with a magnetic model $Fmm{m}^{*}$. Upper vertical lines show magnetic peak positions while lower vertical lines represent peak positions for the nuclear model.

Figure 5

(Color online) The FTs of raw EXAFS and of the fit of $k^2\chi \left(\mathrm{k}\right)$ for $\mathrm{Co}K$ according to (a) the $a\times a\times 3a$ and (b) the $a\times a\times 2a$ models. The inverse FTs $k^2\chi \left(\mathrm{k}\right)$ for $\mathrm{Co}K$ according to (c) the $a\times a\times 3a$ and (d) the $a\times a\times 2a$ models (blue and red line, experimental data and model fit, respectively).

Figure 6

(Color online) The FTs of raw EXAFS and of the fit of $k^2\chi \left(\mathrm{k}\right)$ for $\mathrm{Cu}K$ according to (a) the $a\times a\times 3a$ and (b) the $a\times a\times 2a$ models. The inverse FTs $k^2\chi \left(\mathrm{k}\right)$ for $\mathrm{Cu}K$ according to (c) the $a\times a\times 3a$ and (d) the $a\times a\times 2a$ models (blue and red line, experimental data and model fit, respectively).

Figure 7

(Color online) (a) Integrated intensity of the ${\left(101\right)}_M$ peak as a function of temperature from $7.5\text{to}205\mathrm{K}$ with a mean-field model fit. (b) LS-fit of the exponential function $\mu =\mu _0{}^{*}{\left(T\text{−}{T}_N\right)}^{\beta }$ to the integrated intensities close to the transition temperature with $\beta =0.376$ fixed for a Heisenberg 3D critical model $[{\mu }_0=1.10\left(4\right){\mu }_B,T_N-205.04\left(4\right)\mathrm{K}]$.

Figure 8

(Color online) FC (high) and ZFC (low) branches of the magnetic susceptibility of $\mathrm{La}\mathrm{Ba}\mathrm{Cu}\mathrm{Co}{\mathrm{O}}_{5.6}$.

Figure 9

(Color online) $M$ vs $H$ measurements at different temperatures for LaBCCO. The inset shows the details of the hysteresis at $3\mathrm{K}$.