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_2O_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. 
H. Luetkens, M.Stingaciu, Yu.G. Pashkevich, K. Conder, E.   Pomjakushina, A.A. Gusev, K.V. Lamonova, P. Lemmens, and H.-H. K...