Structural, magnetic, and transport properties of the Nd2-xCexCuO4-δ (0 ≤ x ≤ 0.15) series are presented and contrasted with the corresponding properties of the La2-xSrxCuO4-δ series. High-intensity powder neutron diffraction shows that the long-range antiferromagnetic order of the Cu sublattice persists to higher dopant levels in the Nd2-xCexCuO4-δ system compared to the effect of hole doping in the La2-xSrxCuO4-δ phases. This is consistent with different physical mechanisms operating, namely percolation upon electron doping due to the nonmagnetic CuI states versus frustration upon hole doping due to O- or CuIII states. The magnetic structure at 1.5 K remains the same for all the compositions studied, with the moment at the copper site reaching zero in the superconducting material. A representation analysis reveals the novel feature of substantial coupling between the lanthanide and the copper sublattices. This is manifested in the direction adopted by the Nd3+ moment (parallel to the CuO2 basal plane), the high Néel temperature for Nd3+ order (ca. 2 K), and the polarization-induced moment above TNNd that is apparent in the magnetic Bragg scattering. The importance of the Nd, Cu interaction along the c axis in coupling the two sublattices at the experimentally observed propagation vector (½, ½, 0) >s further demonstrated by using a simple isotropic exchange model. Finally, the assumption that the antiferromagnetic instability must be destroyed before the carriers become delocalized leads to an estimate for the transfer integral for the motion of the electrons in the upper Hubbard subband of 0.8 eV. It also appears possible that the reported occurrence of phase separation at high Ce dopant levels may be related to both the destruction of the Néel state and the delocalization of the electrons, close to the onset of superconductivity.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry