Recent Neutron Studies of the Iron-based Magnetic Superconductors

We present results of recent neutron scattering investigations at the NCNR of the crystal structures, magnetic structures, and spin dynamics of the iron-based ROFe(As,P) (R=La, Ce, Pr, Nd), (Ba,Sr,Ca)Fe$_{2}$As$_{2}$, and Fe$_{1-x}$(Se-Te) superconductors [1]. All the undoped materials exhibit universal behavior, where a tetragonal-to-orthorhombic structural transition occurs between $\sim $140-220 K, at or below which the systems order antiferromagnetically. The magnetic structure within the a-b plane consists of chains of parallel Fe spins that are coupled antiferromagnetically in the orthogonal direction, with an ordered moment typically less than 1 $\mu$$_{B}$. Hence these are itinerant electron magnets, with a spin structure that is consistent with Fermi-surface nesting. The exchange interactions are strong, with spin-wave bandwidths $\sim $200 meV. The rare-earth moments order antiferromagnetically at low T like ``conventional'' magnetic-superconductors, while the crystal field excitations can be employed to study the properties of the superconducting state. With doping in CeFeAsO$_{1-x}$F$_{x}$, LaFeAsO$_{1-x}$F$_{x}$, and SrFe$_{2-y}$Ni$_{y}$As$_{2}$ [2] the structural and magnetic transitions are suppressed in favor of superconductivity. The application of pressure in CaFe$_{2}$As$_{2}$ transforms the system from a magnetically ordered orthorhombic material to a ``collapsed'' non-magnetic tetragonal system. In the superconducting doping regime, well defined spin correlations and a clear magnetic resonance in the magnetic excitation spectrum that tracks the superconducting order parameter are observed, reminiscent of the cuprate superconductors [3]. The overall results clearly indicate that the magnetic properties are a key element in these iron-based superconductors. Further information and references can be found at http://www.ncnr.nist.gov/staff/jeff \\[4pt] [1] For a recent neutron review see J. W. Lynn and P. Dai, Physica C \textbf{469}, 469 (2009). \\[0pt] [2] N. Kumar, et al., Phys. Rev. B \textbf{80}, 144524 (2009). \\[0pt] [3] S. Li, et al., Phys. Rev. B \textbf{79}, 174527 (2009). \\[4pt] It is a pleasure to acknowledge my collaborators at the NCNR and the many collaborators with the following groups: P. Dai (U. Tennessee/ORNL), N. L. Wang (Beijing), R. J. Cava (Princeton U.), A. Goldman (Ames Lab), W. Bao (LANL), S. Dhar (TIFR), J. P. Paglione (U. Maryland). Please see [1] for a complete list of co-authors.