Electronic and magnetic phase diagram of superconductors, SmFeAsO1−xFx

A crystallographic and magnetic phase diagram of SmFeAsO1−xFx is determined as a function of x in terms of temperature based on electrical transport and magnetization, synchrotron powder x-ray diffraction, 57Fe Mössbauer spectra (MS), and 149Sm nuclear resonant forward scattering (NRFS) measurements. MS revealed that the magnetic moments of Fe were aligned antiferromagnetically at ∼144 K (TN(Fe)). The magnetic moment of Fe (MFe) is estimated to be 0.34 μB/Fe at 4.2 K for undoped SmFeAsO; MFe is quenched in superconducting F-doped SmFeAsO. 149Sm NRFS spectra revealed that the magnetic moments of Sm start to order antiferromagnetically at 5.6 K (undoped) and 4.4 K (TN(Sm)) (x=0.069). Results clearly indicate that the antiferromagnetic (AF) Sm sublattice coexists with the superconducting phase in SmFeAsO1−xFx below TN(Sm), while the AF Fe sublattice does not coexist with the superconducting phase.

Mother compounds of these superconductors crystallize in a tetragonal layered structure, which is composed of an alternate stack of carrier-conducting FePn and carrier-blocking LnO layers (Fig. 1(a)).[26,27] The former layer consists of edge-shared FePn 4 tetrahedra with an anti-PbO-type structure, which mainly contributes to the appearance of superconductivity.The Fe elements form another magnetic sublattice in the mother compounds, and are subjected to antiferromagnetic (AF) ordering.[28] Fe elements having a magnetic moment of 0.35 µ B /Fe in undoped LaFeAsO, for instance, exhibit AF ordering at temperatures below ~140 K, indicating a Néel temperature of the Fe magnetic sublattice (T N (Fe)) of ~140 K. [29,30] In addition to the magnetic transition, a crystallographic phase transition from tetragonal to orthorhombic phase takes place at slightly higher temperature than T N (Fe).[31,32] Doping with electrons in the mother compound (undoped LaFeAsO) leads to suppression of both transitions with a reduction of the Fe magnetic moment, and consequently superconductivity appears.[32] This can be performed in SmFeAsO either by doping F − ions at the O site or formation of O 2− vacancies.[4,11] F-doped SmFeAsO exhibits superconductivity with T c up to ~55 K, which is one of the highest T c in the Fe-based superconductors so far discovered.For the present system, temperature dependent heat capacity [24,25] and µ-SR [33] measurements strongly suggest that the Sm magnetic sublattice undergoes AF ordering at low temperatures, although both techniques are not element specific magnetic measurements; Heat capacity and µ-SR measurements are not capable to distinguish main phase's magnetic ordering from impurity's magnetic ordering.In contrast, 57 Fe Mössbauer spectra (MS) and 149 Sm nuclear resonant forward scattering (NRFS) spectroscopy are capable to distinguish using isomer shift values like Knight shift values in nuclear magnetic resonance (NMR).MS and NRFS are also effective in defining element-specific magnetic properties of these compounds making it possible to quantify the magnetic moments of Sm and Fe independently.MS and NRFS spectroscopy provides us information on the magnetic hyperfine field at nuclei position as a function of temperature.[34] Using these techniques, we demonstrated both Néel temperature of the Sm magnetic sublattice (T N (Sm)) and magnetic moment of the Sm ion for superconducting SmFePO.[10] For the present system, two different phase diagrams were reported to date.[33,35] Drew et al argued that T N (Fe) survives in superconducting SmFeAsO 1-x F x with a range x = 0.1 to 0.2.[33] On the other hand, Hess et al reported that an apparent coexistence of T N (Fe) and T c is observed at limited x values within a width of 0.01.[35] The latter report indicates that an apparent coexistence of the superconductivity and AF ordering of Fe (static magnetism) is due mainly to strong inhomogeneous crystallographic phases occurring at the limited x values.Such a discrepancy confuses discussion about the similarity and difference between Fe-based high-T c superconductors [36] and copper-based high-T c superconductors.[37] The discrepancy can be dissolved using well characterized samples.
Based on these transition temperatures, we make a phase diagram of SmFeAsO 1−x F x in terms of F concentration (x) and temperature.The phase diagram we proposed here is closer to that by Hess et al [35]; i.e., long range antiferromagnetic ordering of Fe (a static magnetism) does not persist in the superconducting regime.Such a relation between spin dynamics and superconductivity is common feature among LnFeAsO 1-x F x (Ln = La [41], Ce [19], Pr [42], Nd [22] and Sm [35]).Our results indicate that the relation between the static magnetism and T c of LnFeAsO 1-x F x shows similar topology to that of copper-based high-T c superconductors.[43]

Experimental
Polycrystalline samples were prepared by a two-step solid-state reaction in a sealed silica tube using dehydrated Sm 2 O 3 and a mixture of compounds composed of SmAs, Fe 2 As, and FeAs (SmAs-Fe 2 As-FeAs powder) as starting materials.The dehydrated Sm 2 O 3 was prepared by heating commercial Sm 2 O 3 powder (Rare Metallic Co. Ltd.; 99.99 wt.%) at 1000 °C for 5 h in air.To obtain the SmAs-Fe 2 As-FeAs powder, Sm (Nilaco; Sm with purity 99.9 wt.%), Fe (Ko-jundo Chemical Laboratory; >99.9 wt.%), and As (Ko-jundo Chemical Laboratory.; 99.9999 wt.%) were mixed in a stoichiometric ratio of 1:3:3 and heated at 850 °C for 10 h in an evacuated silica tube.Then, a 1:1 mixture of the two powders (dehydrated Sm 2 O 3 and SmAs-Fe 2 As-FeAs powders) was pressed and heated in a sealed silica tube at 1300 °C for 15 h to obtain a sintered pellet.To prevent the silica tube from collapsing during the reaction, the tube was filled with high-purity Ar gas with a pressure of 0.2 atm at room temperature (RT).All procedures were carried out in an Ar-filled glove box (MIWA Mfg; O 2 , H 2 O < 1 ppm).F doping was performed by replacing part of the Sm 2 O 3 with a 1:1 mixture of SmF 3 (Rare Metallic Co. Ltd.; 99.99 wt.%) and Sm metal in the starting materials.
Crystal structures, including lattice constants of the tetragonal main phase as well as impurity phases of the sintered powders, were examined by powder XRD (Bruker D8 Advance TXS) at RT using CuKα radiation from a rotating anode with the aid of the Rietveld refinement using Code TOPAS3.12.[44] In addition, SXRD measurements were performed at several temperatures (T) from 30 to 300 K at the BL02B2 beamline of SPring-8, Japan using a Debye−Scherrer camera with a 286.5 mm camera radius.[38] Two-dimensional Debye−Scherrer images on an imaging plate were obtained by irradiation with monochromatic x-rays with a fixed wavelength of 0.05 nm.For measurements at low temperatures, ground powder samples were put in capillaries and cooled using a dry N 2 or He gas-flow cooling device.SXRD patterns ranging from 4° to 73° (N 2 gas cooling) or to 53° (He gas cooling) were obtained with a 0.01° step in 2θ, which corresponds to a 0.042 nm or 0.056 nm resolution, respectively.SXRD patterns were then subjected to Rietveld analysis using Code Rietan2000.[45] Measurements for dc electrical resistivity (ρ) were performed by a four-probe technique using an Au electrode from 1.8 to 300 K. Magnetization measurements were conducted with a vibrating sample magnetometer (Quantum Design; PPMS VSM option) at T = 2.3−300 K. 57 Fe MS were obtained using conventional equipment at T = 4.2−298 K. 149 Sm NRFS spectra were taken at the BL09XU beamline of SPring-8.The MOTIF package was used for NRFS data analysis.[46,47] Details of the MS and NRFS measurements were reported elsewhere.[10,30]  indicating that the samples are dominantly composed of the tetragonal phase.Rietveld analysis further reveals that the total amount of impurity phases is less than 7 vol.%.suppression in F-doped sample agree with results reported in Ref. [49].The F-doped sample in the entire temperature region belongs to the tetragonal P4/nmm space group, but those of the undoped sample in the lower temperature region belong to the orthorhombic Cmma space group.[32,49]   confirming it as a bulk superconductor.Conversely, χ mol of the undoped sample is positive at RT and increases gradually with decreasing T (Inset (a) of Fig. 4).However, with a further decrease in T, it suffers a sharp decrease, exhibiting a maximum at T = 5.6 K, and then it increases again very sharply at T ~ 2.7 K (Inset (b) of Fig. 4).The positive χ mol value over the entire temperature range confirms that undoped SmFeAsO is a normal conductor.The maximum at 5.6 K in the χ mol −T curve is associated with a magnetic transition from paramagnetic to AF phase caused by a Sm magnetic sublattice.[24,25,33] On the other hand, the sharp increase at lower temperature can be explained by two assumptions; One of them is a Curie paramagnetic term due to a magnetic impurity phase and the other one is due to an additional magnetic configuration transition occurring in orthorhombic SmFeAsO phase, known as spin reorientation.[31,32,50] To determine the origin more clearly, further microscopic magnetic studies are essential.Inset (c) of Fig. 4  beat" [40], which relates to the effective thickness of the samples, and a quadrupole splitting component.On the other hand, a complex structure is superposed on the dynamical beat in the spectrum at 4.5 K.The structure is attributable to a "quantum beat," which results from hyperfine splitting in the ground and excited states of Sm nuclei.[10,40] The hyperfine splitting is mainly due to the internal magnetic field (H int ) produced by Sm ions.Thus, the emergence of the quantum beat verifies the AF phase of the Sm magnetic sublattice, and H int of Sm is evaluated to be ~354 tesla at 4.5 K in both the undoped and F-doped samples.Provided that the spin orbit interaction is a dominant but the magnetic exchange and crystal field are negligible small, the magnetic moment is estimated to be ~0.74µ B /Sm for a CF = 473 tesla/µ B for Sm.[10,34]  Fe.More homogeneous crystals, which might be obtained using liquid phase reactions, is required to refine the critical F content equal to phase boundary between NC and SC.Our results are similar to that reported in LnFeAsO 1-x F x (Ln = La, [41] Ce, [5,19] Pr, [43], Nd, [22] and Sm [35]).Our results reveal that the relation between antiferromagnetic ordering of Fe and

Macroscopic magnetic properties
shows similar topology to that of copper-based high-T c superconductors.[43]

Conclusions
We prepared polycrystalline SmFeAsO 1−x F x (x = 0, 0.005, 0.019, 0.037, 0.040, 0.045, 0.046, 0.060, 0.069, and 0.083).Samples were characterized by electrical resistivity, magnetization, and SXRD measurements as well as 57 Fe MS and 149 Sm NRFS spectroscopy at various temperatures to determine transition temperatures for superconductivity, magnetic ordering, and crystallographic structural phases.The results are summarized as follows; (1) Undoped SmFeAsO undergoes a crystallographic transition from tetragonal to orthorhombic at ~150 K, and an antiferromagnetic transition (T N (Fe) = 144 K) associated with the Fe magnetic sublattice, where the magnetic moment of Fe is estimated to be 0.34 µ B /Fe at 4.2 K.
(2) Both transition temperatures decrease with an increase in F − content (x) and disappear at x = 0.045.Instead, superconducting transition temperatures appear in x ≥ 0.045.The transition temperature (T c ) and onset temperature (T onset ) reach 52.5 K and 55.6 K, respectively, at x = 0.083.
(3) An apparent coexistence of T N (Fe) and T c is observed at rather limited x values (x ~ 0.04) within a width of 0.01.We tentatively attribute this apparent coexistence in a very limited x-range to crystallographic and/or compositional disorder occurring; i.e., superconductivity does not coexist with magnetic ordered Fe sublattice.
(4) 149 Sm NRFS measurements reveal that Néel temperatures of the Sm magnetic sublattice (T N (Sm)) are located at ~5.6 K for the undoped and at ~4.4 K for a superconducting one (x = 0.069).The magnetic moment of Sm ions is evaluated to be ~0.74µ B at 4.5 K.
Based on these results, a phase diagram for SmFeAsO 1−x F x in terms of temperature and x is obtained.The phase diagram provides clear evidence for the coexistence of superconductivity and the antiferromagnetic Sm sublattice in SmFeAsO 1-x F x (x ≥ 0.045) at low temperature, while the antiferromagnetic Fe sublattice does not coexist with superconductivity.T N (Fe) and T c of LnFeAsO 1-x F x show similar topology to that of copper-based high-T c superconductors.

Figure 7 Figure 8
Figure8shows a phase diagram for SmFeAsO 1−x F x in terms of x and temperature, where T anom , T c ,

Figure 1 .
Figure 1.(Color online) (a) Crystal structure of SmFeAsO.The light-grey box represents a unit cell.

Figure 7 .
Figure 7. (Color online) Temperature (T) dependence of the nuclear resonant forward scattering

Figure 8 .Figure 1 .
Figure 8. (Color online) Phase diagram of SmFeAsO 1−x F x in terms of x and temperature.T onset (open