In the strontium iron-cobalt arsenides SrFe2-xCoxAs2 (0.2 < or = x < or = 0.4) superconductivity with T_{c} up to 20 K is observed in magnetic susceptibility, electrical resistivity, and specific heat data. This first observation of bulk superconductivity induced by electron doping in this family of compounds-despite strong disorder in the Fe-As layer-favors an itinerant electronic theory in contrast to the strongly correlated cuprates and renders a p- or d-wave pairing unlikely. The magnetic ordering present in SrFe2As2 is rapidly suppressed by substitution of Fe by Co. Density functional theory calculations show that this is due to a rigid downshift of the Fe-3d_{x;{2}-y;{2}}-related band edge in the density of states.
Neutron powder-diffraction experiments were carried out on polycrystalline SrFe 2 As 2 in order to determine the magnetic structure and its relationship with the crystallographic one. Below T 0 = 205 K, magnetic reflections appear simultaneously with the onset of the orthorhombic distortion. The magnetic propagation vector of SrFe 2 As 2 is determined to be q = ͑1 0 1͒; the coupling of Fe moments is antiferromagnetic along the longer a direction within the Fe-As layer, and the interlayer coupling is antiferromagnetic as well. The size of the Fe magnetic moment is deduced to be 1.01͑3͒ B with an orientation parallel to the a axis. The temperature dependence of the magnetic moment shows excellent agreement with not only that of the muon precession frequency but also with that of the structural distortion, revealing the strong coupling of the magnetic order and the structural distortion in SrFe 2 As 2 . The discrepancy in the deduced size of the ordered moment between neutron and Mössbauer and SR may imply an unusual magnetism in SrFe 2 As 2 .The discovery of superconductivity in doped LaFeAsO triggered intensive research on layered FeAs systems. 1 So far, the superconducting ͑SC͒ transition temperature T SC of RFeAsO has rapidly been raised up to 54 K. 2-6 Furthermore, a similar high T SC exceeding 35 K was also discovered upon doping the related compounds AFe 2 As 2 ͑A =Ca,Sr,Ba͒. 7,8 AFe 2 As 2 has the well-known ThCr 2 Si 2 -type structure, which can be regarded as replacing the ͑R 2 O 2 ͒ 2+ layer in RFeAsO by a single divalent ion ͑A 2+ ͒ layer keeping the same electron count. 9 RFeAsO and AFe 2 As 2 were found to present very similar physical properties. Undoped RFeAsO compounds ͑R = La-Gd͒ present a structural transition at T 0 ϳ 150 K followed by the formation of a spin-density wave ͑SDW͒ at a slightly lower temperature T N ϳ 140 K. AFe 2 As 2 exhibits a similar structural distortion, whereas the SDW forms at the same or a slightly lower temperature. 9-13 Electron or hole doping leads to the suppression of the SDW and to the onset of superconductivity. This connection between a vanishing magnetic transition and the simultaneous formation of a SC state is reminiscent of the behavior in the cuprates and in the heavy-fermion systems, suggesting the SC state in these doped FeAs systems to be of unconventional nature. In addition, the SDW state is strongly coupled to the lattice distortion in this system and therefore it is of high interest to reveal the relationship between the lattice, the magnetism, and superconductivity.The magnetic structures of the FeAs system have been investigated by neutron-diffraction study on RFeAs͑O,F͒ for R = La, Ce, and Nd, and BaFe 2 As 2 . 14-16 The antiferromagnetic ͑AFM͒ reflection was observed around the ͑1,0͒ or ͑0,1͒ position with respect to the Fe-As layer. This corresponds to Fe moments with AFM coupling along the a or b direction called the columnar structure. In contrast, interlayer coupling is not unique among these compounds, suggesting a relatively weak coupling betwe...
We have studied the temperature-pressure phase diagram of CeCoSi by electrical-resistivity experiments under pressure. Our measurements revealed a very unusual phase diagram. While at low pressures no dramatic changes and only a slight shift of the Neel temperature T-N (approximate to 10 K) are observed, at about 1.45 GPa a sharp and large anomaly, indicative of the opening of a spin-density wave gap, appears at a comparatively high temperature T-S approximate to 38 K. With further increasing pressure, T-S shifts rapidly to low temperatures and disappears at about 2.15 GPa, likely continuously in a quantum critical point, but without evidence for superconductivity. Even more surprisingly, we observed a clear shift of T-S to higher temperatures upon applying a magnetic field. We discuss two possible origins for T-S : magnetic ordering of Co and a metaorbital type of transition of Ce
We have investigated the ground-state properties of polycrystalline CeCoAsO by means of magnetization, specific heat, and solid-state NMR. Susceptibility and specific-heat measurements suggest a ferromagnetic order at about, T C = 75 K. No further transitions are found down to 0.5 K. At 6.5 K a complex Schottky type of anomaly shows up in the specific-heat results. The interplay between Ce 4f and Co 3d magnetism being responsible for that anomaly is discussed. Furthermore 75 As-NMR investigations have been performed to probe the magnetism on a microscopic scale. As-NMR spectra are analyzed in terms of first and second-order quadrupolar interaction. The anisotropic shift component K ab and K c could be derived from the 75 As powder spectra. Towards lower temperature a strong shift anisotropy was found. Nonetheless K iso tracks the bulk susceptibility down to T = 50 K very well. Furthermore the presence of weak correlations among the Ce ions in the ferromagnetic state is discussed. The observed increase in C / T towards lower temperatures supports this interpretation.
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