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Abstract

• The recent discovery of high-Tc superconductivity
  in Fe-based oxypnictides has renewed the interest
  in the study of the interplay between
  superconductivity and magnetism in layered
  materials. We performed static magnetization and
  19F nuclear magnetic resonance (NMR) measurements
  on loose powder samples of superconducting
  SmFeAsO1-xFx (x 0.15 and x 0.2). Our results
  indicate that
• the low-energy excitations of the magnetic Sm3
  sublattice are not directly involved in the
  pairing mechanisms leading to the superconducting
  state.
• the 4f electrons of Sm3 ions are coupled to a
  sea of weakly itinerant and antiferromagnetically
  (AF) interacting fermions.

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Fe-based pnictides the 1111-phase

• SmFeAsO1-xFx is composed of insulating
  tri-layers of Sm-O-Sm alternating metallic
  tri-layers of As-Fe-As.
• Sm3 ions have incomplete electronic shells
  leading to net magnetic moments.
• For one F- ion replacing one O2- ion the
  injection of an electron into the Fe-As layers is
  obtained.

• x lt 0.07 magnetic behaviour characterized by a
  transition to a spin density wave (SDW) phase.
• x gt 0.1 superconductivity confined onto the
  As-Fe-As tri-layers. Tc(x) depends very
  smoothly on x. AF ordering of Sm3 ions at T
  5K.
• 0.07 lt x lt 0.1 properties still under debate
  1.

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Magnetization measurements

• Estimate of the bulk Tc
• x 0.15 broad superconducting (SC)
  transition. Low contribution of magnetic
  impurities.
• x 0.2 sharper SC transition. The effect of
  magnetic impurities causes the difference between
  zero-field cooled (ZFC) and field-cooled (FC)
  curves above Tc.

• Analysis of SC fluctuations above Tc
• Precursor diamagnetism characterized by an
  upturn field (HUP 200 300 Oe) in the
  isothermal fluctuative contributions.
• The behaviour of HUP vs. T (see inset) is
  reminescent of the anomalous diamagnetic
  contributions (superimposed to the conventional
  Ginzburg-Landau terms) already observed in
  underdoped samples of cuprate materials.
• Dashed lines (fitting curves) hypothesis that
  small stable superconducting droplets lacking of
  phase coherence appear already above Tc.

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19F-NMR linewidth, shift and relaxation rate

• NMR linewidth
• Increase at T lt Tc due to both dynamical 1/T2
  processes and spatial modulation of the magnetic
  field as induced by the fluxoids lattice.
• Fluxoids contribution possibly implies motional
  narrowing effects due to the liquid phase of the
  vortices lattice 3.
• Direct comparison with the µSR linewidth s in
  the x 0.2 sample 1 unexplained huge
  difference of values. Further examinations are
  planned in order to investigate the details of
  such a discrepancy.

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• Nuclear spin-lattice relaxation rate
• 1/T1 is unaffected by the transition to the SC
  phase, being possibly entirely driven by the
  low-energy fluctuations of the magnetic moments
  associated with the Sm3 ions.
• 1/T1 vs. T is fairly well described by a
  power-law over the entire explored range
  independently of the sample and of the magnetic
  field.
• The picture Sm3 moments decoupled from the Fermi
  sea is not satisfactory.
• Interpretation of the results
• 4f electrons of the rare-earth ions are weakly
  delocalized and develop AF correlations.
• Moriyas theory of magnetic relaxation in weakly
  itinerant metals is valid.
• The magnetic correlation length follows the
• proportionality
• Similar trends have already been observed in
  some heavy fermions systems characterized by
  antiferromagnetic ground states, like
  CeCu6-xAux4.

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• Shift in resonance frequency
• Negative paramagnetic shift ?K strictly obeing a
  Curie-Weiss-like trend.
• Direct comparison with susceptibility data
  value of the scalar hyperfine coupling Ahyp
  between the magnetic moments associated with Sm3
  ions and the NMR probes, namely the 19F nuclei.

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References 1 S. Sanna et al., Phys. Rev. B 80
052503 (2009) 2 A. Lascialfari et al., Phys.
Rev. B 65 144523 (2002) 3 P. Carretta et al.,
Phys. Rev. Lett. 68 1236 (1992) 4 P. Carretta
et al., Phys. Rev. B 68 220404 (2003)