Magnetic ordering of high-T_c superconducting systems studied by Mossbauer spectroscopy

This review article is concerned with the interrelation between the phenomena of superconductivity and antiferromagnetic order in high-T_c superconductors. Magnetometry, Mossbauer spectroscopy and X-ray diffraction, have been utilized to study the R₂CuO₄, RBa ₂Cu₃O₇, RBa₂Cu₄O ₈, Bi₂Sr₂CaCu₂O₈ and La₂(Ca,Sr)Cu₂O₈ systems in their pure form as well as in a wide variety of substituted systems. For each compound, T _c was determined by magnetometry and T_n of the Cu sublattice by Mossbauer spectroscopy of a dilute ⁵⁷Fe probe. Full superconductivity-antiferromagnetism phase diagrams were established. All high-T_c superconductor compounds have a layered structure with CuO₂ planes which are responsible for superconductivity. It is shown here that in all systems studied, whenever substitution of cations is made outside these CuO₂ planes in an amount sufficient to make superconductivity disappear, static long-range antiferromagnetism is induced in these planes. On the other hand substitution of Cu in the sites responsible for superconductivity even by non-magnetic ions, suppresses both superconductivity and magnetic order. It is thus concluded that in the investigated high-T _c systems, superconductivity and antiferromagnetism are closely related, they compete with each other and rarely coexist. Moreover, in all cases T_n obtained in the antiferromagnetic branch is directly proportional to T_c observed in the superconducting branch of the phase diagram. The close relationship between the two physical phenomena suggest a new guideline in searching for new high-T_c superconductor materials.