The magnetization, resistivity, and magnetoresistance (MR) of single crystals of $\mathrm{Gd}\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5.5}$ and $\mathrm{Eu}\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5.5}$ are measured over a wide range of dc magnetic fields (up to $30\mathrm{T}$) and temperature. In $L\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5.5}$ $(L=\mathrm{Gd},\mathrm{Eu})$, the Co ions are trivalent and can exist in three spin states, namely, the $S=0$ low-spin (LS) state, the $S=1$ intermediate-spin (IS) state, and the $S=2$ high-spin (HS) state. We confirm that $\mathrm{Gd}\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5.5}$ and $\mathrm{Eu}\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5.5}$ have a metal-insulator transition accompanied by a spin-state transition at $T_{MI}\approx 365$ and $335\mathrm{K}$ respectively. The data suggest an equal ratio of LS $(S=0)$ and IS $(S=1)$ ${\mathrm{Co}}^{3+}$ ions below $T_{MI}$, with no indication of additional spin state transitions. The low-field magnetization shows a transition to a highly anisotropic ferromagnetic phase at $270\mathrm{K}$, followed by another magnetic transition to an antiferromagnetic phase at a slightly lower temperature. The magnetization data are suggestive of weak correlations between the Gd spins but no clear signature of ordering is seen for $T>2\mathrm{K}$. Significant anisotropy between the $a\text{‐}b$ plane and $c$ axis was observed in magnetic and magnetotransport properties for both compounds. For $\mathrm{Gd}\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5.5}$, the resistivity and MR data imply a strong correlation between the spin-order and charge carriers. For $\mathrm{Eu}\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5.5}$, the magnetic phase diagram is very similar to its Gd counterpart, but the low-$T$ MR with current flow in the $a\text{−}b$ plane is positive rather than negative as for Gd. The magnitude and the hysteresis of the MR for $\mathrm{Eu}\mathrm{Ba}{\mathrm{Co}}_2{\mathrm{O}}_{5.5}$ decrease with increasing temperature, and at higher $T$ the MR changes sign and becomes negative. The difference in the behavior of both compounds may arise from a small valence admixture in the nonmagnetic Eu ions, i.e., a valence slightly less than $3+$.

• Received 6 February 2004

DOI:https://doi.org/10.1103/PhysRevB.70.024425