Lattice effect in perovskite and pyrochlore CMR materials

摘要

Colossal magnetoresistance (CMR) in doped La manganite thin films (La(sub 1-x)M(sub x)MnO(sub 3), where M = divalent ion, either Ca or Pb) has been shown to result in a factor of 10(sup 6) suppression of the resistance. The driving force for the CMR transition is thought to be the double-exchange interaction. Many studies of both the crystal structure and the local structure of the La(sub 1-x)M(sub x)MnO(sub 3) (M = Ca, Sr, Ba, Pb) system have now been carried out. As expected, these systems all show a strong coupling of the lattice to the CMR transition. On the other hand, neutron diffraction and x-ray absorption for the Tl(sub 2)Mn(sub 2)O(sub 7) pyrochlore, which also exhibits CMR, shows no deviations from ideal stoichiometry, mixed valency, or Jahn-Teller distortions of the MnO(sub 6) octahedron. We present results of crystallographic and local structural studies of these two important classes of CMR materials, compare the differences in structural response, and discuss the implications of these findings to our understanding of these materials.