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Magnetic properties of SmFeAsO1-xFx
superconductors for 0.15 ≤ x ≤ 0.2
G. Prando1,2, P. Carretta1, A. Lascialfari1, A. Rigamonti1, S. Sanna1, L.
3
4
4
4
Romanò , A. Palenzona , M. Putti , M. Tropeano
1 Department
of Physics “A.Volta”, University of Pavia, I-27100, Pavia (Italy) – CNISM, u.d.r.
Pavia
2 Department of Physics “E.Amaldi”, University of Roma Tre, I-00146, Roma (Italy)
3 Department of Physics, University of Parma, I-43100, Parma (Italy) – CNISM, u.d.r. Parma
4 Department of Physics, University of Genova, I-16146, Genova (Italy) – CNR/INFM-LAMIA
e-mail: [email protected]
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.
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.
SmO(F)
tri-layer
Sm
FeAs
tri-layer
• x < 0.07 : magnetic behaviour characterized by a transition to a spin density wave (SDW) phase.
• x > 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 < x < 0.1 : properties still under debate [1].
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 fieldcooled (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.
19F-NMR
linewidth, shift and relaxation rate
NMR linewidth
• Increase at T < 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 μ+SR linewidth σ 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.
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.
• Moriya’s 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-xAux[4].
Shift in resonance frequency
• Negative paramagnetic shift ΔK 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.
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)