Structural and Magnetic Properties of the Insulating T' − RE2CuO4 Parent Compounds of Electron-Doped Superconductors

Abstract

The rare earth cuprates RE2 CuO4 crystallizing in the T ′ -structure are the parent compounds for electron-doped superconductors. Various investigations illustrated that these systems turn superconducting upon electron-doping and/or fine-tuning of the oxygen reduction annealing conditions. These undoped parent compounds are characterized by a very strong antiferro- magnetic coupling between the Cu moment within the CuO2 planes and a very weak cou- pling between adjacent planes. The reason for this weak interlayer coupling lies in the body- centered tetragonal (BCT) structure of these compounds which cancels out the isotropic Heisenberg interaction between one and the next nearest layer. Therefore, the BCT cuprates can be regarded as quasi-2 dimensional (2D) magnetic systems. Even though the Mermin- Wagner theorem forbids the magnetic ordering of a 2D magnetic system, the BCT cuprates are known to order at temperatures around 280 K due to the weak interlayer coupling along the third spatial direction. The actual process how these quasi-2D systems approach the ul- timate 3D magnetic order and which kinds of spin structures are realized, are anyhow still a matter of current debate. Moreover, the rare-earth cuprates exhibit underlying interactions involving both the copper and the magnetic rare-earth subsystems that demonstrated inter- esting phenomena such as spin reorientation transitions. In this work, a systematic investigation of the 3D magnetic ordering process, the influence of a magnetic rare-earth ion and the effect of oxygen reduction on the magnetism of these undoped compounds, were focused upon. The rich magnetic behavior of the BCT cuprates T ′ -RE2 CuO4 (RE= La, La/Sm, and Pr) is investigated primarily using the muon spin rotation and relaxation (µSR) technique. Complementary experimental studies were also carried out with nuclear magnetic resonance (NMR) and neutron scattering techniques. The structural properties, primarily the oxygen content and site occupation, are determined by neutron scattering and synchrotron x-ray diffraction. All the studied T ′ -RE2 CuO4 (RE= La, La/Sm, and Pr) compounds revealed series of magnetic transitions as a function of temperature.