Orbital order and uniaxial spin anisotropy in EuBaCo2O5.5 single crystals -: art. no. 174401

Magnetization, resistivity, and magnetoresistance (MR) are measured on detwinned EuBaCo2O5.5 single crystals over a wide range of dc magnetic fields (up to 33 T) and temperature (from 4 K to 300 K). EuBaCo2O5.5 has a layered structure (along the c axis) with all the Co ions in the trivalent state. The magnetization is highly anisotropic. For H parallel to a axis the isothermal magnetization increases gradually until a critical field is reached, where M(H) changes its slope towards saturation. No similar transition is observed for H along b or c axes. The critical field increases linearly upon cooling, reaching an extrapolated 25 T at 0 K. From the M(H) data, a saturation moment of approximately 0.8 mu(B)/Co is determined. Coinciding with the field induced transition in the magnetization, the isothermal resistivity shows a steep decrease for H parallel to a axis, demonstrating large negative MR. The large MR is most likely associated with the orbital ordering of t(2g) states in Co3+ via spin-orbit coupling.