Structural transition, electrical and magnetic properties of the B-site Co doped Sr(14)Cu(24)O(41) compounds.

The effect of Co substitution for Cu on the structure and physical properties of Sr(14)(Cu(1-x)Co(x))(24)O(41) compounds was studied by analyzing the selected-area electron diffraction and convergent-beam electron diffraction patterns, and by measuring the magnetic susceptibility, the electrical resistivity and Raman spectra. It is found that the space group of the CuO(2) chain is changed from Amma to Ammm upon Co doping, but the structure of the Cu(2)O(3) ladder remains unchanged. This indicates that the displacement between two neighboring CuO(2) chains has disappeared due to Co doping. Once a small amount of Co ions are doped into the compound, exceptional changes in the Weiss temperature and in the number of dimers occur. The remarkable increase in the absolute value of the Weiss temperature indicates that the antiferromagnetic interaction in CuO(2) chains becomes very strong due to Co doping. The increase in the Curie coefficient and the number of dimers implies that the Co doping causes Zhang-Rice singlets in the chains to be decoupled into free spins Cu(2+) and holes. Then, the free spins Cu(2+) are coupled into dimers, and the holes transfer from chains to ladders, which causes the resistivity to decrease when the Co dopant concentration is low (x<0.10). When the Co dopant concentration is high (x>0.10), some Co ions are directly substituted for the Cu ions in the ladders, which results in an increase in resistivity with increasing Co dopant content.