Magnetotransport and thermal properties characterization of 55 K superconductor SmFeAsO0.85F0.15

Correlation between structural/microstructural and magneto-thermal transport properties of FeAs-based SmFeAsO and SmFeAsO0.85F0.15 has been studied in detail. The Rietveld analysis of room temperature powder X-ray diffraction (XRD) data reveals that both the samples are single phase, with very small amount of rare earth impurity in the F doped compound. Electron microscopic investigations show that compounds have layered morphology and structure, with the individual grains being surrounded by amorphous layers. The average grain boundary thickness is ~5 nm. The F free material is found to be magnetic and shows the appearance of Fe spin density wave (SDW) like order at T = 150 K. The F doped compound (SmFeAsO0.85F0.15) shows the occurrence of superconductivity at Tc(R=0, H=0)- 55 K, which decreases to 42 K at magnetic field (H) of 13 kOe. The superconducting transition was also confirmed by DC magnetization and AC susceptibility measurements. The intra-grain critical current density (Jc) calculated using the Bean critical state model is found to be around 5.26 x 10^4 A/cm2 at 5 K in zero field (H=0). The dependence of thermally activated flux flow energy (U/KB) on the applied magnetic field has been observed. AC susceptibility measurements at different amplitude of applied AC drive field confirm the granular nature of the superconducting compound. is confirmed. Both Fe (SDW) at 150K for SmFeAsO and 55K superconductivity in case of SmFeAsO0.85F0.15 sample has confirmed by Specific heat [Cp(T)] measurement too. Further Sm orders anti-ferro-magnetically at 4.5K for non-superconducting and at 3.5K for superconducting samples, also the entropy change is reduced significantly for the later than the former. Summarily complete physical property characterization for both non-superconducting SmFeAsO and 55K superconductor SmFeAsO0.85F0.15 samples is provided and discussed in the current article.


Introduction
The discovery of the new class of oxypnictide superconductors including iron-based LaFeAsO 1-x F x with critical temperature T c at 26 K [1] has provided new impetus to research in the area of high-temperature superconductivity, and has resulted in unravelling of several new issues in the domain of superconducting materials incorporating Fe-As layers as the fundamental structural unit [2][3][4][5][6][7][8]. Such newfangled materials have the general formula REFeAsO, where RE is a rare earth element. The structural unit consists of alternating RE-O and Fe-As layers, rendering charge carriers and conducting planes, respectively. The T c in this class of materials generally depends on (i) the size of the RE 3+ cations, (ii) F substitution on oxygen sites, and (iii) oxygen deficiency in F-free materials. Several reports have shown that T c can be effectively increased to above 55 K by substitution of larger La 3+ ion with RE + cations having smaller radii such as Ce, Pr, Nd, Sm, etc. [2][3][4][5][6][7][8]. The pristine (F free) REFeAsO compounds are nonsuperconducting and show a crystallographic phase transition around 150 K along with a static spin density wave (SDW) like long range ordering of the Fe spins around the same temperature.
These near concomitant structural and the magnetic transitions are confirmed by an anomaly, which is generally seen as a sharp metallic step at around T SDW~1 50 K in the temperature dependence of resistivity ( -T) and also by the hump around the same temperature in the heat capacity [1][2][3][4][5][6][7][8][9][10]. Introduction of carriers, either by O-deficiency or F-doping is observed to simultaneously shift the magneto-structural transitions to lower temperatures and induce superconductivity.
In the REFeAsO system, the negatively charged Fe-As layers that are sandwiched by positively charged insulating RE-O charge reservoir layers are responsible for the conduction and superconductivity [1][2][3][4][5][6][7]9]. Superconductivity can be induced by (i) F substitution at O sites in RE-O layer [1][2][3][4][5][6][7][8][9] and (ii) substitution of 3d metals like Co and Ni at Fe sites [11,12]. The T c 3 of the 3d metal substituted compounds is generally found to be lower and is believed to be due to the enhanced impurity scattering and pair breaking in the conduction layers. In contrast, the F substitution shows the highest T c of up to 55 K. The upper critical field (Hc 2 ) is observed to increase appreciably with lowering of the RE cationic size. For example it has been estimated that Hc 2 ~ 65 T, ~70 T and ~230 T for LaFeAsO 0.9 F 0.1 , PrFeAsO 0.85 F 0.15, and high-pressure fabricated NdFeAsO 0.82 F 0.18 , respectively. For SmFeAsO 0.85 F 0.15, the value of H c2 is well over 200 T [13][14][15][16] . Thus Sm substituted compound appears to be potential candidate for high current/magnetic field applications. The tremendous interest in these compounds is not only due to their high critical temperatures, high upper critical fields, and the ability to sustain high current densities at elevated temperatures but also due to the intriguing presence of well-known magnetic iron (Fe) element in the superconducting Fe-As plane. The micro-structural aspects of the Fe-As and RE-O layers and their impact on the superconducting properties are not very well understood yet.
The current article deals with the emergence of superconductivity in F-substituted SmFeAsO 0.85 F 0.15 with T c~5 6 K, which is the highest to the best of our knowledge for bulk samples prepared by normal two-step synthesis method at low temperature and without invoking high HPHT. The previous works have reported such high T c values in excess of ~55K, with HPHT synthesis process [8,17]. We have studied the correlation between the micro-structural properties and superconducting characteristics such as the T c and the critical current density (J c ) along with the flux pinning behaviour of the bulk samples synthesised at low temperature by a two-step solid state reaction route.

Experimental details
Polycrystalline bulk samples with nominal composition SmO 1-x F x FeAs (x=0.0 and 0.15) were synthesized by conventional solid state sintering method using high-purity As, Fe, SmF 3 and Sm 2 O 3 powders all with high purity from Sigma Aldrich as starting materials. To obtain SmAs-Fe 2 As-FeAs powder, Sm, Fe and As were weighed according to the stoichiometric ratio of 1:3:3, mixed and ground thoroughly using mortar and pestle under high purity Ar atmosphere in glove box. The humidity and oxygen content in the glove box was maintained to be less than 1ppm.
The mixed powder was pelletized in disk shape and then encapsulated in an evacuated (10 −3 Torr) silica tube. Then the silica tube embodying the said pellet was heat treated for 12 hrs at 800 C. Further, in the next step, SmAs, Fe 2 As and FeAs were mixed with dehydrated Sm 2 O 3 , Sm 4 and SmF 3 in accordance with (1 + x) Sm + (1 − x) Sm 2 O 3 + xSmF 3 + 3FeAs stoichiometric ratio, where x = 0 for undoped and 0.15 for fluorine doped sample, was compacted and finally heated at 950 C for 72 hrs in continuum with slow heating rate to obtain a sintered pellet. To prevent silica tube from collapsing during the reaction, the tube was filled with high purity Ar gas. The furnace was then allowed to cool in a natural way. The sintered sample was obtained by breaking the quartz tube. The as sintered sample is concrete ceramic-like with dark-coloured surface.
Phase identification and crystal structure investigation were carried out by using powder

Results and discussion
The typical XRD patterns of the as synthesized pristine SmFeAsO and SmFeAsO 0.85 F 0.15 samples are shown in Fig. 1. The Rietveld refinement of X-ray diffraction data collected at room temperature confirmed that all observed reflections are indexed on the basis of tetragonal ZrCuSiAs-type structure with a space group P4/nmm. The simulated pattern shows that the studied samples are nearly single phase except for a weak impurity diffraction peak (marked with asterisk in the XRD pattern) in the F-doped compound. These impurity peaks are assigned to the rare earth oxides. Based on the known structural details Sm and As are taken to be located at the  [19,20]. Generally, this amorphous layer results in the formation of weak link type grain boundaries (GBs) which in turn are detrimental to the intergrain current transport. Some reports have also suggested that most of the GBs of iron-based superconductors are indeed weak link type [20,21] and generally resemble to  sample exhibits a sudden decrease at T anomaly ~150K. The anomaly apparent in the resistivity is known to be due to the collective effect of a crystallographic phase transition from the tetragonal P4/nmm to the orthorhombic Cmma space group around T~150K, and the occurrence of static spin density wave (SDW) instability like magnetic ordering of the Fe spins at a slightly lower 6 temperature of ~140K [5,7,[12][13][14][15][16][17][18]. The resistivity behaviour of SmFeAsO is semiconducting above 150K and step like metallic at lower temperatures. After substitution of 15% O 2by F 1-, the resistivity decreases monotonously with decreasing temperature. This decrease in resistivity suggests that the charge carrier density increases and the anomaly completely disappears with a concomitant appearance of superconductivity at below around T c ( =0)~55 K .Thus, the -T curve of SmFeAsO 0.85 F 0.15 shows the typical metallic behaviour till the superconducting transition is arrived at. The superconducting transition width, ΔT c (T c onset -T c ρ=0 ), is found to be T c 55K has been reported previously but only in materials synthesized through HPHT [8,17].
The normal state resistivity above superconducting transition for the studied SmFeAsO 0.85 F 0.15 sample shows linear dependence on temperature. The blue solid line in Fig. 3(a) shows the fitted resistance plot according to equation ρ = ρ 0 + AT, where ρ 0 is the residual resistivity and A is the slope of the graph. The experimental data fits well in the low temperature range. The linear behaviour of resistivity with temperature deviates above T~250K. The values of ρ 0 and A are found to be as 2.28x10 -7 mΩ-cm and 5.19x10 -8 mΩ-cm/K respectively. This behaviour is quite different from those shown by MgB 2 [22] and other high-T c superconductors [23].
To obtain information about upper critical field Hc 2 and the flux pinning properties the temperature dependence of electrical resistivity under applied magnetic field [ρ (T, H)] from 0 to 130kOe for superconducting SmFeAsO 0.85 F 0.15 sample in the superconducting range has been measured and shown in Fig. 3(b). The resistive transition at H=0 gets significant broadening in the applied magnetic field and is observed to become broader with the increasing H. This is generally regarded as a signature of strong vortex motion. This transition broadening with increasing magnetic field H, is a characteristic of type-II superconductors, especially the layered high T c cuprate materials [23]. Further, it is interesting to note that the onset transition temperature (T c onset ) remains nearly invariant with respect to H, the T c (ρ=0) is observed to decrease from ~ 52K to ~42K at H= 130 kOe. This could be due to the granular nature of the Å. This value is smaller than the periodicity of FeAs layer of d=8.5 Å. This clearly suggest a two dimensional behavior in the lower temperature regime.
The temperature derivative of resistivity for the superconducting SmFeAsO 0.85 F 0.15 sample at various applied fields has been shown in inset of Fig. 3 (d). The dρ/dT shows a narrow intense maxima centred at superconducting transition temperature in zero applied fields, which indicate good percolation path of superconducting grains. The broadening of the dρ⁄dT peak increases with applied magnetic field. The broad peak at low temperatures range indicates the intra-grain and inter-grain regimes [23]. It is interesting to note that a clear second peak in dρ⁄dT, usually observed in case of the high T c cuprate materials [23], is absent in the presently studied On the other hand, the broadening in dρ⁄dT peak with applied field, suggestive of weaker inter granular coupling, is relatively higher for SmFeAsO 0.85 F 0.15 than for MgB 2 [22].  [12,27].
To understand the magnetic behaviour comprehensively the temperature and field dependent magnetization measurements have been carried out. Fig. 5(a)   Although the whole-sample current densities are significantly lower than that in randomly grain-oriented bulk of MgB 2 , the grain connectivity seems to be better than that of random polycrystalline high T c cuprates.
In order to understand granular nature of the superconducting sample and in particular to find out the intra and inter grain contributions at high temperatures, the ac susceptibility measurement for the superconducting sample at zero DC bias magnetic fields has been performed [19,29,30]. The real (χ ) and imaginary (iχ ) components of ac susceptibility as function of temperature measured at various AC drive field, ranging from 3Oe to 13Oe at fixed frequency f = 333Hz are presented in Fig. 7. It is clear from the real part of the susceptibility (χ ) that the diamagnetic onset transition temperature is same irrespective to the drive fields and in imaginary (iχ ) components of ac susceptibility two peaks appear in which the higher temperature peak corresponds to the individual superconducting grains i.e. intra granular superconductivity whereas the lower temperatures peak corresponds to the intergranular coupling. The intergranular peak shifts towards lower temperature with increasing ac drive fields and gets broadened, due to the weak coupling between the grains [31].
The temperature dependence of specific heat for the pure and doped samples is shown in To elucidate the non-magnetic contribution of heat capacity and change in entropy for anti-ferromagntic (T N ) ordering of Sm 3+ spins in non superconducting SmFeAsO and superconducting SmFeAsO 0.85 F 0.15 samples, the polynomial interpolation method using equation 11 aT+bT 3 has been used to fit the Cp(T) plot of both samples [10,34]. The fitted and observed C P (T) are close to the T N of Sm is shown in Fig. 8 smaller. We believe that more studies are required to further understand the nature of the GBs and its consequences on the magneto-transport properties.         The real (χ') and imaginary (iχ'') components of AC susceptibility as function of temperature measured at various AC drive field, ranging from 3Oe to 13Oe at 333Hz for SmFeAsO 0.85 F 0.15 sample.