Magnetization and Magnetotransport of LnBaCo2O5.5 (Ln=Gd, Eu) Single   Crystals
  The magnetization, resistivity and magnetoresistance (MR) of single crystals of GdBaCo2O5.5 and EuBaCo2O5.5 are measured over a wide range of dc magnetic fields (up to 30 T) and temperature. In LnBaCo2O5.5 (Ln=Gd, Eu), the Co-ions are trivalent and can exist in three spin states, namely, the S=0 low spin state (LS), the S= 1 intermediate spin state (IS) and the S=2 high spin state (HS). We confirm that GdBaCo2O5.5 and EuBaCo2O5.5 have a metal-insulator transition accompanied by a spin-state transition at TMI >> 365 and 335 K, respectively. The data suggest an equal ratio of LS (S=0) and IS (S=1) Co3+ ions below TMI, with no indication of additional spin state transitions. The low field magnetization shows a transition to a highly anisotropic ferromagnetic phase at 270 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 K. Significant anisotropy between the a-b plane and c axis was observed in magnetic and magnetotransport properties for both compounds. For GdBaCo2O5.5, the resistivity and MR data imply a strong correlation between the spin-order and charge carriers. For EuBaCo2O5.5, the magnetic phase diagram is very similar to its Gd counterpart, but the low-T MR with current flow in the ab plane is positive rather than negative as for Gd. The magnitude and the hysteresis of the MR for EuBaCo2O5.5 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+. 
Z.X. Zhou, S. McCall, C.S. Alexander, J.E. Crow, and P. Schlottmann,   S.N. Barilo, S.V. Shiryaev and G.L Bychkov, R.P. Guertin