The magnetic dilution of superconducting (SC) cuprates was performed to clarify a HTSC mechanism in these oxides. The magnetic properties of the series of complex cuprates based on La1.85Sr0.15O4 with the following substitutions: La - Eu; Sr - Ba, Ca; Cu - Fe, Li, Al and their solid solutions in isomorphous diamagnetic matrix were studied. The examination of temperature and concentration dependencies of magnetic susceptibility points to a very strong tendency of copper atoms to clustering. According to ESR and Mossbauer (with Fe-57 isotope as an atomic probe) study, copper atoms, being in two various oxidation states, are combined in clusters with distorted local oxygen environment. The results of microwave absorption method showed that superconducting properties are registered not only in concentrated oxides but also in extremely diluted Sr and Ba-containing solutions. The introduction of Eu, Li, Ca suppresses superconductivity in a diluted systems. All experimental results can be accounted for by suggesting the existence of nanoclusters of heterovalent Cu (II) and Cu (III) atoms, formed at spontaneous ordering of crystal structure of solid solutions. The electron structure of such clusters depends on a local distortion of nearest oxygen environment, caused by an attitude component of self-organizing process of a substance. The most important point is a temperature-dependent character of local distortions, in this case sharp changes of parameter of exchange interactions between copper atoms and, as a consequence, of superconducting properties of system are probable. By our opinion these nanoclusters are responsible for magnetic and superconducting properties of multicomponent diluted perovskite-like cuprates. The form, size and electron structure of clusters as well as the character of intraclusters interactions will be discussed in details.
The work is supported by RFBR (grant N 03-03-32355).