Neutron Studies of the Iron-based Family of High Tc Magnetic Superconductors
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1148-PP02-05
Neutron Studies of the Iron-based Family of High Tc Magnetic Superconductors Jeffrey W. Lynn NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899-6102 ABSTRACT We briefly review recent neutron scattering investigations carried out at the NIST Center for Neutron Research on the crystal structures, magnetic structures, and spin dynamics of the iron-based ROFe(As,P) (R=La, Ce, Pr, Nd), and (Ba,Sr,Ca)Fe2As2 systems. All the undoped materials exhibit universal behavior, where a tetragonal-to-orthorhombic structural transition occurs between ~100-220 K, below which the systems become antiferromagnets. 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 one Bohr magneton. Hence these are itinerant electron magnets, with a spin structure that is consistent with Fermi-surface nesting and a spin wave bandwidth ~200 meV. The rare-earth moments order antiferromagnetically at low T like ‘conventional’ magnetic-superconductors. With doping, the structural and magnetic transitions are suppressed in favor of superconductivity, with superconducting transition temperatures up to 56 K, while the Ce crystal field linewidths are affected when superconductivity sets in. The application of pressure in CaFe2As2 transforms the system from a magnetically ordered orthorhombic material to a ‘collapsed’ non-magnetic tetragonal system which is superconducting at lower T when anisotropic pressure is applied. Fe1+xTe shows a transition from a monoclinic to orthorhombic low T structure with increasing x, and a crossover from commensurate to incommensurate magnetic order. Se doping suppresses the magnetic order, while incommensurate magnetic scattering is observed in the superconducting regime. INTRODUCTION The nature of the magnetic order in superconductors has had a rich and interesting history, and has been a special topic of interest ever since the parent materials of the high TC cuprates were found to be antiferromagnetic Mott insulators that exhibit huge exchange energies within the Cu-O planes. For the newly discovered iron oxypnictide class of superconductors, the observation of long range spin-density-wave-type antiferromagnetic order in the undoped materials has naturally led to strong parallels being drawn between these two classes of materials. Here we provide a very brief review, which by necessity will be restricted to the neutron scattering experiments carried out at the NCNR. We discuss the structure and magnetic transitions of the undoped materials and how these progress with doping into the superconducting regime. We also discuss the spin dynamics for SrFe2As2, and the search for a possible magnetic resonance in the superconducting state. DIFFRACTION Diffraction measurements have been carried out on the undoped La [1,2], Ce [3], Nd [4,5], and PrOFeAs [6] (1:1:1:1) materials, as well as the Ba- [7], Sr-[8], and CaFe2As2 [9,10]
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