based on the concept of an enhanced electron- phonon interaction which leads to an attraction between electrons forming superconducting pairs. This interaction is due to the exchange spin fluctu­ ations (exchange enhancement effect). In compounds in which there is no magnetic ordering except at very low temperatures, such as in rare-earth metal oxides, the electron-phonon interaction is strengthened due to fluctuations in the spins of the conducting electrons. If there is magnetic ordering in a superconductor at a tempera­ ture higher than or of the same order as the critical superconducting temperature Tc, then the attraction in the electron pairs will be further increased because the Coulomb repulsion is overwhelmed by fluctuations in the spins forming the long-range anti fer­ romagnetic order.