Electronic Properties and Magnetic Order in RuGd2Sr2(Cu1-xFex)2O8

There is a transition to magnetic order in RuGd2Sr2Cu2O8 at 137 K and to superconducting order at 40 K. At 4.2 K neuron scattering shows that this magnetic order is characterized to a large extent by antiferromagnetic order between nearest neighbor Ru moments within the RuO planes and between RuO planes. However there is also canting of the moments which leads to a ferromagnetic component to the order. A detailed description of electronic properties is essential to understand the coexistence of magnetic and superconducting ordered phases which are usually considered to be mutually exclusive. I use electronic structure calculations in the Local Spin Density Approximation to investigate the magnetic ordering states which arise in the material and the effect of doping of Fe. The results of these numerical calculations show that collinear alignment of moments leads to weak coupling between magnetism in adjacent RuO planes, not consistent with bulk magnetic order at 137 K. Significant polarization occurs only at the Ru and apical oxygen sites , but not the oxygen sites in the RuO planes. Conductivity between the RuO and CuO planes also seems to be an issue. Although the RuO and CuO planes are clearly conducting, there is no weight in the density of states at E=EF from the Gd sites, which prevents conduction transverse to the RuO and CuO planes. New calculations are being developed which will incorporate non-collinear magnetism. These calculations are carried in collaboration with Dr. M. Jones at the Center for Computational Research in Buffalo.