Journal of Alloys and Compounds 620 (2015) 210–216

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Solid state synthesis and characterization of bulk b-FeSesuperconductors

http://dx.doi.org/10.1016/j.jallcom.2014.09.0450925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author.

K. Onar a,⇑, M.E. Yakinci a,b

a _Inönü Üniversitesi, Fen Edebiyat Fakültesi, Fizik Bölümü, 44280 Malatya, Turkeyb _Inönü Üniversitesi, Mühendislik Fakültesi, Biyomedikal Mühendisligi Bölümü, 44280 Malatya, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 May 2014Received in revised form 1 September 2014Accepted 5 September 2014Available online 16 September 2014

Keywords:FeSe based superconductorsChalcogenide superconductorsPolycrystalline b-FeSe

Polycrystalline FeSe0.88 was synthesized by solid-state reaction method in sealed quartz tube at differentheat heating cycles. The identification and characterization of FeSe0.88 samples were determined by X-rayDiffraction (XRD), Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDX).The electrical transport, magnetic and thermal transport properties of the obtained samples were alsoinvestigated. The results showed that the sensitivity of resistivity and magnetic susceptibility dependson heating cycles while the structural formation of samples does not. The upper critical field Hc2(0)has been determined with the magnetic field parallel to the sample surface and yielding a maximumvalue of 23.2 T. At the zero field coherence length, n, value was calculated to be 3.33 nm. Calculatedl0Hc2(0)/kBTc rate indicated comparably higher value (3.17 T/K) than the Pauli limit (1.84 T/K) andobtained results were suggested unconventional nature of superconductivity in our samples.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The recent discovery of superconductivity in b-FeSe materialshas attracted a great attention because of having simple crystalstructure and high critical temperature, Tc, up to 37 K at 4 GPa,among the Fe-based superconductors [1,2]. In order to increaseTc and critical current densities of b-FeSe materials, many substitu-tions and/or adding like Al, V, Ti, Co, Mn, Cu, Cr, Ga, Ni, In, Ba andSm introduced to the system [3,4]. Nearly all of the transitionmetals are added into FeSe structure with a certain substitutionratio, generally below 12 at.% [4]. Even single phase materials areprepared but no substantial increase on the Tc was observed. Forhigher substitution levels, over 25 at.%, superconductivity wasgenerally suppressed and well defined semiconducting behaviorappeared [5–8].

Literature survey shows that polycrystalline FeSe materialsmainly have two different phases, i.e.: b-FeSe (stable at room tem-perature) and d-FeSe (stable at high temperature). Together withthese phases, ferromagnetic hexagonal Fe7Se8, monoclinic Fe3Se4,Fe3O4 and pure Fe as an impurity phases can be observed in thesystem when the suitable heating cycles are not applied. Accordingto the obtained results, d-FeSe hexagonal (P63/mmc) structuregrows at high temperatures while b-FeSe phase mainly growsduring cooling stage below certain temperatures and forms as

tetragonal structure with (P4/nmm) crystal symmetry. Recently itwas shown that low temperature b-FeSe phase can easily be sur-rounded by the d-FeSe phase during cooling and prevent or reducethe dissolution of b-FeSe phase to the main matrix [9,10]. In prac-tice, this obviously revealed multiphase samples with no bulksuperconductivity.

For the synthesis of FeSe based materials many well knownmethods can be used and they can be categorized under four maingroups: the solid-state reaction method, the high pressuresynthesis method, the flux method and one step synthesis methods[11–17]. Generally the first and second methods can be used toprepare polycrystalline samples and others can be used for singlecrystal preparation. For the technological applications, which arealso important both for scientists and companies, the thin filmand wire form of the FeSe based materials have been successfullyprepared by many research groups and the results obtained werefound very promising [18–22].

In the present, many research groups produced superconductingb-FeSe materials by using conventional solid-state preparationmethod [4,8]. The most important observation was the differenceon the magnetic and electrical properties which makes difficult tocategorize physical properties. For example, in some cases, particu-larly slightly off-stoichiometric samples, magnetic properties showferromagnetic behavior while resistivity measurements show super-conductivity. However, in contrast to these results, samples showedbulk superconducting properties below their Tzero values [9].

Fig. 1. XRD results of the b-FeSe Superconductors prepared.

K. Onar, M.E. Yakinci / Journal of Alloys and Compounds 620 (2015) 210–216 211

In this work we fabricated FeSe samples by using solid statereaction method and characterized under different magnetic fields.

2. Experimental

Polycrystalline FeSe0.88 material was prepared by solid-state reaction method insealed quartz tubes using high purity powders of the iron (99.999% Alfa Aesar) andselenium (99.999% Alfa Aesar) with appropriate stoichiometry of Fe/Se:1/0.88. Ini-tially well mixed raw powders were placed into quartz tube and sealed in the glovebox, then heated up to 550–650 �C with different durations, as can be seen inTable 1. We used constant heating and cooling ramps (2 �C/min) for each heatingcycles. In each step intermediate grinding were applied in glove box under Ar atmo-sphere and then sealed into an evacuated quartz tubes. For the last heating stagesamples were pulverized again and pressed isostatically into a pellet with 60 MPaof pressure in the glove box and re-heated up to 400 �C for 36 or 72 h, Table 1.

After the heat treatment cycles, structural properties of the materials obtainedwere examined by X-ray Powder diffractometer (XRD). A Rigaku RadB DMaxIIdifractometer and Jade 5+ refinement program was used to analyze phase forma-tion. Structural parameters were calculated by using Rietveld refinement and Jade6+ crystal refinement program. The microstructural formations of the samples pre-pared were investigated by using Scanning Electron Microscope (SEM; Leo EVO40VPX) and Energy Dispersive X-ray Spectroscopy (EDX; Bruker 125 eV). The trans-port and magnetic properties of the FeSe polycrystalline samples were carried outusing Physical Properties Measurement System, (Quantum Design; PPMS-9T). 9Tesla magnetic fields were applied during the transport and magnetic measure-ments. The critical current density, Jmag

c , was then calculated by using magnetic hys-teresis loops and the Bean’s model. The thermal transport measurements, Seebeckcoefficient, Thermoelectric Figure of Merit and Thermal Conductivity analysis wereperformed between 1.8 K and 300 K using PPMS-9T. The Tc value of samples weredetermined by the resistivity differentiations dq(T)/dT plot [23].

3. Results and discussions

3.1. X-ray analysis (XRD)

The XRD patterns of the samples prepared with different heattreatment conditions are given in Fig. 1. We realized no big differ-ence between the samples and they all showed same crystalliza-tion behavior, even their peak intensities remained in the samelevel. This indicates that structural evolution of the system wasnot strictly depending on the heat treatment temperature andtime. Despite using 6 different heating cycles, we did not observea single phase structure. Obviously this is the case for all samplesprepared by solid state reaction method in the literature and canbe linked with the large melting temperature difference betweenthe Fe (1521 �C) and Se (221 �C) [24]. According to phase analysis,the b-FeSe phase is the dominant phase all over the samples. But asmall quantity of monoclinic Fe3Se4 phase was identified as animpurity. This is the case for all heating temperature and timecombinations and is thought to be grown during the heating stageof each sample. The possible reaction of this growth is consideredto be FeSe2 ? Fe3Se4. According to calculations, the crystalsymmetry of the samples was found to be tetragonal with thespace group of P4/nmm and crystal parameters were calculatedto be a = 3.76537(6) Å and c = 5.51002(6) Å. These results werewell agreed with the results obtained by various research groupspreviously [25]. We did not identify any oxide peaks as can be seenin some research articles [24–26] and also no any other impuritiessuch as, Fe7Se8, FeSe2 and/or d-FeSe.

Table 1Heating cycles of the samples prepared.

Samplecode

Preparationmethod

I. Heat treatment (�C) Heating time (h) II. He

F1 Solid state 550 12 550F2 Solid state 550 12 550F3 Solid state 600 12 600F4 Solid state 600 12 600F5 Solid state 650 12 650F6 Solid state 650 12 650

3.2. SEM and EDX analysis

Fig. 2a and b shows the surface micrograph of samples heattreated at 550 and 650 �C (sample F1 and F6 respectively) as anexample. Randomly oriented granular formation, which is generalcharacteristic of FeSe based materials prepared with solid-statereaction method, with an average grain size of 3–4 lm wereobtained. All samples have similar microstructural formation, indi-cating that there is no significant effect of thermal treatment onthe morphological evolution, as we obtained on the XRD results.In addition, surface micrograph of samples showed slightly porousformation for all heating cycles and indicating that density of thesamples not high as in the other metallic structures.

Large scale EDX dot mapping result of sample heat treated at650 �C (sample F5) are given in Fig. 3. As seen in Fig. 3 distributionof the FeSe phase on the surface of the sample is homogenous. Wefound no random and/or deficient distribution of Fe and Se atomsor related non superconducting phases on the surface of thesamples. According to EDX analysis, atomic concentration of thesamples were found very close to the nominal composition (with% ±0.1 deviation) and indicating that there exist no problem onthe sample preparation process.

3.3. Electrical transport properties

The resistivity versus temperature graph of all samples preparedare given in Fig. 4. The region around the transition temperaturesare magnified and shown in the inset. All samples were showedsuperconducting transition at around 9 K, which is very close tothe values reported for the single crystals by different researchgroups in the literature [3,9]. We saw no anomalies or unexpectedbehavior such as, poor intergrain connectivity or semiconducting-like behavior, between room temperature and 10 K and all sampleshave showed well defined metallic demeanor, Fig. 4. However,resistivity value of the samples at room temperature shows a

at treatment (�C) Heating time (h) III. Heat treatment (�C) Heating time (h)

24 400 3624 400 7224 400 3624 400 7224 400 3624 400 72

Fig. 2. SEM micrographs of samples heat treated at (a) 550 �C (sample F1) and (b) 650 �C (sample F6).

Fig. 3. EDX dot mapping micrographs of sample heat treated at 650 �C (sample F5).Inserts shows elemental distribution of Fe (red dots) and Se (green dots). (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

Fig. 4. Resistivity versus temperature measurement results of all samples prepared.

Table 2Superconducting properties of b-FeSe superconductors.

Sample No. Tonsetc (K) Tzero

c (K) DT (K) FWHM (K) RRR

F1 9.04 6.01 3.03 5.98 4.13F2 9.01 6.02 2.99 6.09 3.88F3 9.53 6.57 2.96 2.13 4.37F4 9.58 6.59 2.99 1.87 3.87F5 8.98 6.61 2.37 1.61 4.48F6 8.95 6.59 2.36 1.74 3.96

212 K. Onar, M.E. Yakinci / Journal of Alloys and Compounds 620 (2015) 210–216

significant variation. We believed that the heat treatment cycles arequite significant even though we do not see a big difference on thesurface morphology. The best heating cycle was obtained fromsample F5 (see Table 1). The insert in Fig. 4 and Table 2 summarizedthe Tc and Tzero values of samples prepared in this work.

Fig. 5 shows the resistivity transitions under magnetic fields upto 8 T with applied field, H parallel to the sample surface for sam-ple F5 as an example. The insert shows enlarged Tc’s of sample F5under different magnetic field between 1.8 and 12 K. The othersamples were showed very similar trend. The transition curvesare shifted to lower temperatures as the applied field is increasedas expected.

To obtain more information about the quality of samples pre-pared, the full width at half maximum (FWHM) of the dq(T)/dT ver-sus temperature curves were calculated, as in Fig. 6. It wasobserved that FWHM of dq(T)/dT curves slightly increased whenthe final heat treatment time is reduced. The FWHM of the resistivedifferentiation curve, at zero magnetic field for the sample F5(which is the best sample), was found to be 1.61 K, as in Table 2,is slightly higher than that of the single crystal samples preparedby other groups [27–29] and indicating the quality of the sample.In addition, the basic structure of the curves was preserved andno shoulder or any anomalic behavior (such as second or thirdpeaks) were observed. This might be associated with the depinningof vortices as obtained in the superconducting large single crystals[30].

We also calculated the residual resistivity ratio (RRR) value ofthe samples by using the equation qð300 KÞ=qTonset

c , Table 2. Theresults obtained showed that samples heat treated at over 6000C,at the first and second stages, have lower RRR value comparingwith the samples heated at lower than 600 �C. This indicates thatthe effect of heating cycle on the electrical properties. In general,calculated RRR value for our samples (between 3.87 and 4.48) areslightly higher than that of the literature for single crystalline sam-ples [31–33] and lower than the obtained value of McQueen et al.for polycrystalline Fe1.01Se sample, RRR = 10 [9]. This indicates thatour results are in the acceptable level due to the polycrystallinenature of samples and shows the existence of microscopic inhomo-geneities, impurities and/or dislocations which can not be detectedby SEM–EDX or XRD analysis except Fe3Se4 phase.

Fig. 5. Resistivity transitions under different applied magnetic fields for the sampleF5.

Fig. 6. Calculated values and graphs of full width at half maximum of the dq(T)/dTversus temperature of the samples prepared.

Fig. 7. The temperature dependence of upper critical field Hc2(T). The main frame isthe experimental results and insert is the calculated theoretical curves.

K. Onar, M.E. Yakinci / Journal of Alloys and Compounds 620 (2015) 210–216 213

For determination of the upper critical field, Hc2(0), we used 10%Toffset

c , 50% Tmidpointc and 90% Tonset

c criterion and Werthamer–Helfand–Hohenberg (WHH) formula [34], where qonset is the resis-tivity value of midpoint of the transition curve, which is 8.12 K atzero field. The WHH formula is given as:

Hc2ð0Þ ¼ �0:693TcdHc2

dT

� �T¼Tc

ð1Þ

where dHc2/dT is the slope of Hc2(T) at Tc, which is �3.04 for sampleF5. By using Eq. (1) the zero temperature value of upper critical fieldis calculated to be Hc2(0) = 23.2, 17.1 and 9.9 T for 10% Toffset

c , 50%Tmidpoint

c and 90% Tonsetc respectively, Fig. 7. These results are greater

than that (16.3 T) of polycrystal powder [35] but lower than that(33.4 T) of the single crystal samples calculated by the same method[36].

Coherence length along the c-axis, nc, was estimated at 0 Kusing the Ginzburg–Landau (GL) formula:

nc ¼U0

2pHc2

� �12

ð2Þ

where U0 = 2.07 � 10�15 W [37–39]. At the zero field coherencelength n value was calculated to be 3.33 nm. This is slightly highercomparing with previously obtained result for FeSe composition(n = 3.14 nm) by Zhang et al. [38]. We also calculated the

l0Hc2(0)/kBTc value, which is convenient tool to get more informa-tion about the nature of the superconductor materials by usingHc2(0) obtained from the WHH theory. The results obtained showedcomparably higher value (3.17 T/K) than the Pauli limit (1.84 T/K)and suggested unconventional nature of superconductivity in thesample as obtained in the FeSeTe single crystal family [37,38].

3.4. Magnetic properties

Fig. 8 shows the magnetization versus temperature (M�T)curves of samples (F1–F6) measured at 50 Oe where H applied par-allel to the sample surface. Above Tc Sample F1 and F2 showed Pau-li paramagnetic type behavior up to �125 K and then remainunchanged down to �10 K but a sharp drop to diamagnetic phasewas obtained after Tc. A drop, abnormality at around 125 K, isbelieved to be related with the structural phase transformation.This may be due to the impurity phases such as monoclinic and fer-rimagnetic Fe3Se4 or other structural defects that have consider-able effects on the phase transformation as suggested by otherresearch groups [9,11,20,40]. However, sample F3–F6 showed neg-ative ferromagnetic sign down to Tc and then sharp drop wasobtained at Tc. These two different situations obviously suggesteda different spin–spin and spin–orbital effects in the polycrystallinesamples prepared and also indicates the sensitivity of magneticproperties to the heat treatment cycles.

Fig. 9 shows magnetization versus magnetic field (M–H) loopsof sample F5 as an example at 3, 4 and 5 K. The other samplesshowed similar trend. When temperature is increased the shapeof the M–H loop changed. It became narrow and ferromagnetictype M–H curves formed. This suggest that the co-existence of bothmagnetism and superconductivity in our samples due to ferromag-netic nature of F and monoclinic and ferrimagnetic Fe3Se4 impurityphase, as seen in the XRD analysis. The other small amount ofimpurities, may exist in the bulk of the sample in nano scale suchas, Fe7Se8, FeSe2 and/or d-FeSe, where they were not detected inthe XRD and SEM–EDX analysis, may also play a significant roleon this kind of behavior.

The magnetization critical current density, Jmagc , of samples was

calculated from the width of magnetization hysteresis loops andBean’s model. According to Bean’s equation [39]:

Jmagc ¼ 20DM

a 1� a3b

� � A cm�2 ð3Þ

where DM = M+ �M� is the magnetization measured in electro-magnetic units per cubic centimeter and a and b (b > a) are the

Fig. 8. Magnetization versus temperature (M–T) curves of samples (F1–F6) measured at 50 Oe, where H applied field parallel to the samples surface.

Fig. 9. Magnetization versus magnetic field (M–H) loops of sample F5 at 3, 4 and5 K, where H applied field parallel to the samples surface.

Fig. 10. The magnetization critical current density, Jmagc , versus magnetic field of

sample F5.

214 K. Onar, M.E. Yakinci / Journal of Alloys and Compounds 620 (2015) 210–216

dimension of the cross section of the samples and Jmagc is the critical

current density in A cm�2. The Jmagc results showed a field and tem-

perature dependence, Table 2. Fig. 10 shows the field and tempera-ture dependence of Jmag

c of the sample F5, as an example, calculatedfrom the M–H loops given in Fig. 9 and Eq. (3). Jmag

c value decreasedsharply by increasing the applied magnetic field and temperature.This suggest that the weak-link behavior and can be attributed tothe strain generated by the various dislocations, generally locatedat the grain boundaries [41], disordering and also unconventionalpairing mechanism together with the non-oriented nature of thesmall grains [42].

3.5. Thermal transport measurements

Temperature dependences of the Seebeck coefficient (S), thethermal conductivity (j) and the thermoelectric figure of meritfrom 1.8 to 300 K for the samples are presented in Figs. 11a–10c,respectively. From room temperature to 175 K, we obtainedapproximately similar S values for all samples, Fig. 11a. After175 K the S value of the samples were started to change and ataround 120 K a dip was observed for each sample, ranging between

�18 and �38 lV/K depending on the heat treatment. Between 120and 1.8 K it started to increase but the low temperature values ofthe S remain in the negative region and below the room tempera-ture values of each sample. In general, all samples are showed posi-tive S value at around room temperature but then a transition tonegative value was obtained at around 225 K. This indicates somecompetition between hole and electron conduction. After 225 Knegative S values suggest that the charge carriers are electrondominated. However, temperatures between 50 and 75 K smallanomalies were obtained for all samples. This is probably becauseof the different nature of the charge carriers or different spin–spinand/or spin–orbital effects in the samples.

The temperature-dependent thermal conductivity, j, for allsamples are shown in Fig. 11b. The maximum j values, with a peaknear 77 K, were obtained between 4.3 and 13.6 (W/K m) dependingon the different heating cycles. The peak obtained at around 77 K,which is characteristics of metallic structures, occurs possibly dueto equilibrium between the phonon mean free path and a changeon the crystallite size by using different heating cycles as suggestedby other research groups [43]. However, more detailed experi-ments may have done in determining the lattice and phonon con-tributions to the thermal conductivity behavior.

Fig. 11. Effects of heat treatment conditions on the temperature dependences of (a) the Seebeck coefficient (S), (b) the thermal conductivity (j) and (c) the thermoelectricfigure of merit for the samples from 4.2 to 300 K.

K. Onar, M.E. Yakinci / Journal of Alloys and Compounds 620 (2015) 210–216 215

Temperature dependence of thermoelectric figure of meritvalues, which characterize the performance of the samples, werecalculated using the equation [43]:

ZT ¼ S2

qj

!T ð4Þ

where S is the Seebeck Coefficient, j is the conductivity, q is theresistivity and T is the temperature. The result obtained are shownin Fig. 11c. Samples prepared are showed similar trend between 1.8and 300 K and near 130 K all samples reached their own maximum.In general, thermoelectric figure of merit values varied between0.00022 and 0.00032 except sample F2, which showed a very largeincrease, approximately 0.0007. However, results obtained compa-rably small when compared to the most productive metallic mate-rials indicating the effect of various interactions that occur in thesuperconducting samples.

4. Conclusions

The FeSe0.88 bulk polycrystalline superconductors have beenprepared by solid-state reaction method. Structural, Physical andmagnetic properties including thermal transport properties wereinvestigated. The XRD and SEM–EDX results showed that micro-structural evolution of the samples was not strictly depend onthe heat treatment temperature and time. However, in contrary,resistivity and magnetic susceptibility measurements showed var-ious superconducting transitions depending on the heating cycles.

High upper critical field, Hc2(0), was found to be 23.2 T. At the zerofield coherence length n value was calculated to be 3.33 nm. Thel0Hc2(0)/kBTc value, which is convenient tool to get more informa-tion about the nature of the superconductor materials showedcomparably higher value (3.17 T/K) than the Pauli limit (1.84 T/K). The obtained results were suggested unconventional nature ofsuperconductivity in the sample. The Jc value of samples is stilllow but it is in the limit of bulk applications.

References

[1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130 (2008)3296.

[2] M.K. Wu, M.J. Wang, K.W. Yeh, Sci. Technol. Adv. Mater. 14 (2013) 014402.[3] M.K. Wu, F.C. Hsu, K.W. Yeh, T.W. Huang, J.Y. Luo, M.J. Wang, H.H. Chang, T.K.

Chen, S.M. Rao, B.H. Mok, C.L. Chen, Y.L. Huang, C.T. Ke, P.M. Wu, A.M. Chang,C.T. Wu, T.P. Perng, Physica C 469 (2009) 340.

[4] B.H. Mok, Cryst. Growth Des. 9 (2009) 3260.[5] E.L. Thomas, W. Wong, D. Phelan, J.N. Millican, J. Appl. Phys. 105 (2009)

073906.[6] K.W. Yeh, C.T. Ke, T.W. Huang, T.K. Chen, Y.L. Huang, P.M. Wu, M.K. Wu, Cryst.

Growth Des. 9 (2009) 4847.[7] Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, Y. Takano, J. Phys. Soc. Jpn. 78

(2009) 074712.[8] S.B. Zhang, H.C. Lei, X.D. Zhu, G. Li, B.S. Wang, L.J. Li, X.B. Zhu, W.H. Song, Z.R.

Yang, Y.P. Sun, Physica C 469 (2009) 1958.[9] T.M. McQueen, Q. Huang, V. Ksenofontov, C. Felser, Q. Xu, H. Zandbergen, Y.S.

Hor, J. Allred, A.J. Williams, D. Qu, J. Checkelsky, N.P. Ong, R.J. Cava, Phys.Rev. B79 (2009) 014522.

[10] C.E.M. Campos, De J.C. Lima, T.A. Grandi, K.D. Machado, P.S. Pizani, Solid StateCommun. 123 (2002) 179.

[11] Y. Takano, Sci. Technol. Adv. Mater. 13 (2012) 050301.

216 K. Onar, M.E. Yakinci / Journal of Alloys and Compounds 620 (2015) 210–216

[12] Z. Gao, L. Wang, Y. Qi, D. Wang, X. Zhang, Y. Ma, Supercond. Sci. Technol. 21(2008) 105024.

[13] Z. Gao, L. Wang, Y. Qi, D. Wang, X. Zhang, Y. Ma, H. Yang, H. Wen, Supercond.Sci. Technol. 21 (2008) 112001.

[14] H. Hiramatsu, T. Katase, T. Kamiya, M. Hirano, H. Hosono, Appl. Phys. Express 1(2008) 101702.

[15] E. Choi, S. Jung, N.H. Lee, Y. Kwon, W.N. Kang, D.H. Kim, M. Jung, S. Lee, L. Sund,Appl. Phys. Express 2 (2009) 083004.

[16] Y.L. Chen, Y.J. Cui, Y. Yang, Y. Zhang, L. Wang, C.H. Cheng, C. Sorrell, Y. Zhao,Supercond. Sci. Technol. 21 (2008) 115014.

[17] A.S. Sefat, M.A. Mcguire, B.C. Sales, R. Jin, J.Y. Howe, D. Mandrus, Phys. Rev. B 77(2008) 174503.

[18] K. Togano, A. Matsumoto, H. Kumakura, Solid State Commun. 152 (2012) 740–746.

[19] Y. Ma, Supercond. Sci. Technol. 25 (2012) 113001.[20] T. Ozaki, K. Deguchi, Y. Mizuguchi, Y. Kawasaki, T. Tanaka, T. Yamaguchi, S.

Tsuda, H. Kumakura, Y. Takano, Supercond. Sci. Technol. 24 (2011) 105002.[21] Y. Mizuguchi, H. Izawa, T. Ozaki, Y. Takano, O. Miura, Supercond. Sci. Technol.

24 (2011) 125003.[22] Z. Gao, Y. Qi, L. Wang, D. Wang, X. Zhang, C. Yao, Y. Ma, Supercond. Sci. Technol.

24 (2011) 065022.[23] M. Fang, H. Wang, C. Dong, Q. Huang, J. Phys.: Conf. Ser. 449 (2013) 012015.[24] J.L.P. Júnior, F.C. Serbena, A.R. Jurelo, J. Supercond. Nov. Magn. 24 (2011) 1437–

1441.[25] P.H. Zhao, W. Yan, J.Y. Yang, Y.L. Han, G. Aldica, V. Sandu, P. Badica, J.C. Nie, J.

Supercond. Nov. Magn. 25 (2012) 1781–1785.[26] P. Diko, V. Antal, V. Kavec�ansky, Ch. Yang, I. Chen, Physica C 476 (2012) 29–31.[27] I. Felner, I. Nowik, M.I. Tsindlekht, Z. Ren, X. Shen, G. Che, Z. Zhao arXiv:

805.2794, 2008.[28] I. Nowik, I.F.H.U. Jerusalem, V.P.S. Awana, A. Vajpayee, H. Kishan, J. Phys.:

Condens. Matter 20 (2008) 292201.[29] H. Mukuda, N. Terasaki, H. Kinouchi, M. Yashima, Y. Kitaoka, S. Suzuki, S.

Miyasaka, S. Tajima, K. Miyazawa, P. Shirage, H. Kito, H. Eisaki, A. Iyo, J. Phys.Soc. Jpn. 77 (2008) 093704.

[30] R.A. Zargar, A. Pal, A.K. Hafiz, V.P.S. Awana, J. Supercond. Nov. Magn. 27 (2014)897–901.

[31] T. Taen, Y. Tsuchiya, Y. Nakajima, T. Tamegai, Phys. Rev. B 80 (2009) 092502.[32] B.C. Sales, A.S. Sefat, M.A. McGuire, R.Y. Jin, D. Mandrus, Phys. Rev. B 79 (2009)

094521.[33] T.J. Liu, X. Ke, B. Qian, J. Hu, D. Fobes, E.K. Vehstedt, H. Pham, J.H. Yang, M.H.

Fang, L. Spinu, P. Schiffer, Y. Liu, Z.Q. Mao, Phys. Rev. B 80 (2009) 174509.[34] V.P.S. Awana, Anand Pal, Arpita Vajpayee, Monika Mudgel, H. Kishan,

Mushahid Husain, R. Zeng, S Yu, Y.F. Guo, Y.G. Shi, K. Yamaura, E. Takayama-Muromachi, J. Appl. Phys. 107 (2010) 09E128.

[35] F.C. Hsu, J.Y. Luo, K.W. Yeh, T.K. Chen, T.W. Huang, P.M. Wu, Y.C. Lee, Y.L.Huang, Y.Y. Chu, D.C. Yan, M.K. Wu, Proc. Natl. Acad. Sci. USA 105 (2008)14262.

[36] U. Patel, J. Hua, S.H. Yu, S. Avci, Z.L. Xiao, H. Claus, J. Schlueter, V.V. VlaskoVlasov, U. Welp, W.K. Kwok, Appl. Phys. Lett. 94 (2009) 082508.

[37] D. Velasco Soto, F.J. Rivera Gomez, C.R. Santillan Rodriguez, R.J. SaenzHernandez, M.E. Botello Zubiate, J.A. Matutes Aquinoa, J. Appl. Phys. 113(2013) 17E138.

[38] S.B. Zhang, Y.P. Sun, X.D. Zhu, X.B. Zhu, B.S. Wang, G. Li, H.C. Lei, X. Luo, Z.R.Yang, W.H. Song, J.M. Dai, Supercond. Sci. Technol. 22 (2009) 015020.

[39] Q.P. Ding, S. Mohan, Y. Tsuchiya, T. Taen, Y. Nakajima, T. Tamegai, Supercond.Sci. Technol. 24 (2011) 075025.

[40] K.O. Funtov, C.R. Lin, Y.J. Siao, C.C. Wang, I.S. Lyubutin, S.S. Starchikov, T.V.Dmitrieva, International Conf. On the Applications of the Mössbauer Effects,Croatia, 1–6 September 2013.

[41] G. Deutscher, Appl. Phys. Lett. 96 (2010) 122502.[42] S. Lee, J. Jiang, J.D. Weiss, C.M. Folkman, C.W. Bark, C. Tarantini, A. Xu, D.

Abraimov, A. Polyanskii, C.T. Nelson, Y. Zhang, S.H. Baek, H.W. Jang, A.Yamamoto, F. Kametani, X.Q. Pan, E.E. Hellstrom, A. Gurevich, C.B. Eom, D.C.Larbalestier, Appl. Phys. Lett. 95 (2009) 212505.

[43] E.L. Thomas, W. Wong-Ng, D. Phelan, J.N. Millican, J. Appl. Phys. 105 (2009)073906.