Magnetization and magnetotransport of LBaCo2O5.5 (L = Gd,Eu) single crystals -: art. no. 024425

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 LBaCo2O5.5 (L=Gd,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 GdBaCo2O5.5 and EuBaCo2O5.5 have a metal-insulator transition accompanied by a spin-state transition at T(MI)approximate to365 and 335 K respectively. The data suggest an equal ratio of LS (S=0) and IS (S=1) Co3+ 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 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 a-b 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+.