Transport properties of cobaltites containing holmium

Dependences of electrical conductance of ceramic samples Ho0.55Sr0.45CoO3- paragraph and HOo.45Sr0.55CoO3- paragraph on temperature and magnetic field are investigated in the interval from room to nitrogen temperatures and magnetic fields up to 7 kOe applied perpendicular to the dc current flow direction. Electrical conductivity of the samples has a character typical of a semiconductor while magnetoresistance shows a different sign varying from - 50% to + 120% in a narrow temperature interval at the same field. At low transport current, the resistance of the samples is nonlinear in the current. The magnitude of the resistance is highly sensitive to low magnetic field. Analysis of the data obtained suggests that under the condition of percolation conductivity, a magnetic contribution into the mechanism of correlation between electrons manifests itself, i. e., the states of Co3+ ions with lowered (S = 1) spin, t52ge1g, or high (S = 2) spin, t42ge2g, exist. The corresponding activation energies are estimated.