Determining the Magnetic Structure of a Superconductor

Superconducting materials are the subject of widespread research because of their potential contributions to improved magnetic energy-storage systems, motors, generators, transformers, computer parts, medical imaging devices, and other types of instrumentation. Computational simulations are necessary for understanding the properties of these materials and improving their design. For the first time, the complex nature of the coexistence of magnetism and superconductivity on a microscopic scale has been determined from first-principles calculations.

	Figure 3   Schematic magnetic ordering of Ru and Gd moments in (a) collinear AFM and (b) noncollinear AFM structures of RuSr2GdCu2O8, where the Cu, ORu, OCu, and Oapical atoms are not given. The ordering of the Ru moments in (b) is similar to a C-type AFM ordering in (a), but the moments cant perpendicular to the AFM axis, i.e., along the FM axis direction. Note that the moment directions are defined in a spin space, since spin-orbit coupling is not taken into account.

Nakamura and Freeman have determined the magnetic structures of the ruthenocuprate compound RuSr₂GdCu₂O₈—a ferromagnetic superconductor—from first principles by using the highly precise full-potential linearized augmented plane-wave (FLAPW) method. FLAPW includes a newly developed intra-atomic noncollinear magnetism capability that can describe the canting of Ru moments.

The researchers found that the magnetism of RuSr₂GdCu₂O₈ arises from the RuO₆ octahedra, where the moments on neighboring Ru sites order antiferromagnetically but cant perpendicular to the antiferromagnetic (AFM) axis, and so induce a weak ferromagnetism (Figure 3). From the canting of the Ru moments, a double exchange interaction is exerted via itinerant t_2g electrons which can travel through the neighboring O p states. The projected Ru moments along the AFM and FM axes result in magnetic moments of 1.16 and 0.99_B, respectively. The results are consistent with the possible coexistence of canted ferromagnetism and superconductivity in RuSr₂GdCu₂O₈ as inferred from experiments.

INVESTIGATORS
A. J. Freeman, Northwestern University, and K. Nakamura, Mie University, Japan.

PUBLICATION
K. Nakamura and A. J. Freeman, “Canted ferromagnetism in RuSr₂GdCu₂O₈,” Phys. Rev. B 66, 140405(R) (2002).