Microscopic evidence of spin state order and spin state phase separation in layered cobaltites RBaCo2O5.5 with R = Y, Tb, Dy, and Ho

We report muon-spin relaxation measurements on the magnetic structures of RBaCo(2)O(5.5) with R = Y, Tb, Dy, and Ho. Three different phases, one ferrimagnetic and two antiferromagnetic, are identified below 300 K. They consist of different ordered spin state arrangements of high-, intermediate-, and low-spin Co(3+) of CoO(6) octahedra. Phase separation into well separated regions with different spin state order is observed in the antiferromagnetic phases. The unusual strongly anisotropic magnetoresistance and its onset at the FM-AFM phase boundary is explained.