High-field transport properties of a P-doped BaFe2As2 film on technical substrate

0.89 MB·

by  Kazumasa Iida

High-fieldtransport properties ofa P-doped BaFe As film ontechnical substrate
2 2
KazumasaIida,1,∗ HikaruSato,2 ChiaraTarantini,3 JensHa¨nisch,4 JanJaroszynski,3
HidenoriHiramatsu,2,5 BernhardHolzapfel,4 and HideoHosono2,5
1Department of Crystalline Materials Science,
Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan
2Laboratory forMaterials and Structures, Instituteof Innovative Research,
Tokyo Institute of Technology, Mailbox R3-1,
7 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan
1 3AppliedSuperconductivity Center, National HighMagnetic FieldLaboratory,
0 Florida State University, Tallahassee FL 32310, USA
2 4Karlsruhe Institute of Technology, Institute for Technical Physics,
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
n
a 5Materials Research Center for Element Strategy,
J Tokyo Institute of Technology, Mailbox SE-6,
4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan
6
(Dated:January17,2017)
1

] High temperature (high-Tc) superconductorslikecuprates have superior critical current propertiesin
n magneticfieldsoverothersuperconductors. However, superconductingwiresforhigh-field-magnetap-
o plicationsarestilldominatedbylow-Tc Nb3Sndueprobablytocostandprocessingissues. Therecent
c discoveryofasecondclassofhigh-Tcmaterials,Fe-basedsuperconductors,mayprovideanotheroption
r- for high-field-magnet wires. In particular, AEFe2As2 (AE: Alkali earth elements, AE-122) is one of
p thebestcandidatesforhigh-field-magnetapplicationsbecauseofitshighuppercriticalfield,Hc2,mod-
u erateHc2 anisotropy, andintermediateTc. Herewereportonin-fieldtransportpropertiesofP-doped
s BaFe2As2 (Ba-122)thinfilmsgrownontechnicalsubstrates(i.e.,biaxiallytexturedoxidestemplateson
. metaltapes)bypulsedlaserdeposition.TheP-dopedBa-122coatedconductorsampleexceedsatransport
t
a Jcof105A/cm2at15Tforbothmajorcrystallographicdirectionsoftheappliedmagneticfield,whichis
m favourableforpracticalapplications.OurP-dopedBa-122coatedconductorsshowasuperiorin-fieldJc
- overMgB2 andNbTi,andacomparableleveltoNb3Snabove20T.ByanalysingtheE−J curvesfor
d determiningJc,anon-Ohmiclineardifferentialsignatureisobservedatlowfieldduetofluxflowalong
n thegrainboundaries. However, grainboundariesworkasfluxpinningcentresasdemonstratedbythe
o pinningforceanalysis.
c
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Introduction
The discoveryof Fe-based superconductors(FBS) by Kamihara et al.,[1] broughta hugeimpact to
thephysicscommunity,sincethecompoundconsistsofferromagneticFe,whichhadbeenbelievedtobe
inevitablydetrimentaltotheformationofCooperpairs. Todate,fundamentalquestions,suchasmecha-
nismofCooperpairingandorderparametersymmetry,arestillunderdebate[2]. Ontheotherhand,this
materialclass isattractiveforapplications. Forinstance, AEFe As (AE: Alkaliearthelements, AE-
2 2
122)andFe(Se,Te)possesshighuppercriticalfields(H )exceeding50TandalowH anisotropyclose
c2 c2
to1atlowtemperature[3,4],whichisfavourableforhigh-field-magnetapplications. Furthermore,Ba-
122showslessdeteriorationofcriticalcurrentacrossgrainboundaries(GBs)[5,6]thanYBa2Cu3O7−δ
(YBCO)andBi-basedcuprates. ForCo-dopedBa-122,thecriticalGBmisorientationangle(θ ),where
c
J startstofalloffexponentially,hasbeenreportedtobe6◦−9◦[5,7]. EvenhighangleGBsdonotim-
c
pedethecurrentflowverymuchinsinteredK-dopedBa-122wiresandbulks,ifcleanandwell-connected
GBs are realised[8, 9]. Additionally, Co-doped Ba-122 exhibitsa high tolerance for large densities of
fluxpinningcentresinthesuperconductingmatrix,whichleadstosignificantincreaseincriticalcurrent
density(J )andirreversibilityfield(H )[10].
c irr
Another advantageof Ba-122, in particularP-doped Ba-122, is its inherentlyhigh J . Putzke et al.
c
havereportedontheenhancementofthevortexcoreenergyofthefluxlinesatthequantumcriticalpoint
(QCP)oftheantiferromagneticphase[11]. Indeed,evenmicrostructurallycleanandoptimallyP-doped
Ba-122epitaxialthinfilms,whichwerepreparedbymolecularbeamepitaxy(MBE),exhibitahighself-
fieldJ ofover6MA/cm2 at4.2K[12]. AlthoughexcessmagneticFehasbeenfoundtobeharmfulto
c
superconductivityinFe(Se,Te)[13],Fe-richP-dopedBa-122thinfilmsshowedahigherself-fieldJ of
c
over10MA/cm2 at4.2K, which is the highestvalue everreportedfor FBS[6]. Whereasin the former
case Fe is incorporatedinterstitially[14], in the latter case the Fe may form Fe-containing particles or
regions with differing P-content, both acting as pinning centres[6]. Furthermore, the high J and low
c
anisotropyP-dopedBa-122thinfilmscanbefabricatedbytuningtheprocessingconditionsonly,without
anymodificationofthetargetmaterialusedinpulsedlaserdeposition(PLD)[15].
The aforementioned advantages of P-doped Ba-122 are very suitable for high-field-magnetapplica-
tions. Indeed,P-dopedBa-122thinfilmsontechnicalsubstrateshavebeendemonstratedasFBScoated
conductors[16,17]. Todate,twokindsoftechnicalsubstrateshavebeenemployedforFBScoatedcon-
ductors: Thecube-texturedmetaltapeswithbufferlayers(i.e.,RABiTS)[18]andtheHastelloytapeon
whichbiaxiallytexturedbufferlayersarepreparedbyion-beam-assisted-deposition(IBAD)[19].
IncontrasttoFe(Se,Te)coatedconductors[20,21],transportpropertiesofP-dopedBa-122coatedcon-
ductorsin the presence of extremely high magnetic fields have not yet been reported. Here, we report
on in-field transport properties of a P-doped Ba-122 thin film grown by PLD on metal substrate with
biaxially textured MgO template (IBAD-MgO) in a wide range of temperature and DC magnetic field
upto35T.WeemployIBAD-MgOtemplatewitharelativelylargein-planefullwidthathalfmaximum
(FWHM)value(∆φ =8◦),sinceithasbeendemonstratedbyx-raydiffraction(XRD)andtransmis-
MgO
sionelectronmicroscopy(TEM)thatthetextureofMgOistransferredtotheoverlyingP-dopedBa-122
film, generatingdislocationnetworks[17]. Suchdislocationnetworksenhancethevortexpinningin P-
dopedBa-122[17],sinceθ islessthan9◦[5]. Indeed,in-fieldJ propertiesofourP-dopedBa-122on
c c
IBAD-MgOwith∆φ =8◦ weresuperiortothoseofthefilmonatemplatewith∆φ =4◦[17].
MgO MgO
AhighdensityofthreadingdislocationsisveryeffectiveforimprovingJ forH k cinawiderangeof
c
temperatureandmagneticfieldevenclosetoH .Despitetherelativelylargeθ of6◦−9◦forBa-122,J
irr c c
ofourP-dopedBa-122coatedconductorwithsharpFWHMvaluesofbothin-plane,∆φBa−122 = 5.7◦,
andout-of-planemisorientaion,∆ωBa−122 = 1.2◦ (seeSupplementalFig.S1)is limitedbytheGBs in
the low field regime. However, at high field, it exceeds a transport J of 105A/cm2 at 15T for field
c
appliedin both main crystallographicdirections. Our P-dopedBa-122coated conductorsample shows
superiorin-fieldJ propertiesoverMgB andNbTi,andacomparableleveltoNb Snabove20T.
c 2 3
2
Results
Resistivitymeasurements
The normal-state resistivity ρ (Fig.1a) can be approximatedby ρ = ρ +ATn with an exponent
n n 0
n-value of 1.28, ρ = 3.32×10−2mΩ·cm and A = 8.22×10−5mΩ·cm/K1.28 in the range of 30 <
0
T < 150K in accord with Ref.22. Shibauchiet al. have reportedthat the exponentn is unity at the
quantumcriticalpoint(QCP)oftheantiferromagneticphase,wherethemaximumT isobservedat33%
c
ofPcontentforbulksinglecrystals[23]. Based onthoseresults, weinferthatthePcontentofourBa-
122 thin film on IBAD-MgO is different from the optimal level. Chemical analysis by electron probe
microanalysis revealed a P content of 0.31, high enough to induce superconductivitywith an onset T
c
of 30K for Ba-122single crystal[22]. Thelower T (28.3K) of the P-dopedBa-122coated conductor
c
maybeaconsequenceofepitaxialstrain,sinceMgOsinglecrystallinesubstratesinducein-planetensile
straintoBa-122filmsduetothelatticemismatch[24,25]. ThelatticeparametersaandcofourP-doped
Ba-122coatedconductorsarelocatedbetweenthesinglecrystalsandthinfilmsdepositedonMgOsingle
crystallinesubstrates(Fig.1b).ThecrystallinequalityofIBAD-MgOaffectsmainly∆φ ratherthan
Ba122
∆ω [17],changingtheamountofthein-planestrainandhenceT .
Ba122 c
ThelinearityoftheArrheniusplotsofρ(T,H)forbothmajorcrystallographicdirectionsatacertain
magneticfield(Figs.2aand2b)revealsthermallyactivatedfluxmotionundertheassumptionofalinear
T-dependenceoftheactivationenergy,U (H)[26](Seethemethodsection). ItcanbeseenfromFig.2c
0
thatU (H)forbothH kcandkabarewelldescribedbyHα(1−H/H∗)β above10T,whichhasbeen
0
used for analysing polycrystallineMgB samples by Thompsonet al[27]. H∗ is a characteristic field
2
representingtheirreversibilityfieldat0K[27,28]. TheevaluatedvaluesforH k candk abare48.9T
and59.7T,respectively(forH kcandkabα=0.68and0.64,andβ =1.1and0.94).
Alinearfitforlnρ(H)versusU (H)usinglnρ (H)=lnρ +U (H)/T ,whereρ istheprefactor,
0 0 0f 0 c 0f
yieldsT of26.9KforH k cand27.2KforH k ab,respectively(seeSupplementalFig.S2a). TheT
c c
valuesevaluatedbythis methodare slightlylower thanthe T (see Fig.1a). A plausibleexplanation
c,90
forthisdifferenceistheincreasedtransitionwidth∆T dueto thereducedtexturequalitycomparedto
c
filmsonsinglecrystalsubstratesorsinglecrystalsamples.
H (T) was evaluated from the linear presentationsof Figs.2a and 2b (see SupplementaryFig.S2b
c2
andS2c)applyingaρ = 0.9ρ resistivitycriterion,whereρ isthenormalstateresistivityρ at
n,0.9 n n,0.9 n
28.5K.ShowninFig.2disH forH k candk ab. ThedottedlineinFig.2disthefittingcurveusing
c2
(1−T/T )k. An exponentk of0.9was obtainedforH k ab, whichis farfromthe expectedvalueof
c
0.5forlayeredcompoundslimitedbyPaulipairbreakingatgivenT closetothedimensionalcrossover
temperature[29–31],whichconfirmsthatP-dopedBa-122isa3Dsuperconductor.Becauseofthelackof
lowtemperaturedata,itisnotpossibletofittheH (T)(andH (θ),shownlaterunambiguously)with
c2 c2
apropermodelforFBS[32,33].
Thetemperaturedependenceoftheirreversibilityfield,H (T)(Fig.2e)wasevaluatedfromρ(T,H)
irr
measurements using a resistivity criterion of ρ = E /J = 1.0−8Ω·cm, where E is the elec-
c c c,100 c
tric field criterion(1µV/cm) fordeterminingJ fromE −J measurementsandJ is the criterion
c c,100
(100A/cm2) for determining H from J (H) measurements (see Supplementary Fig.S2d and S2e).
irr c
TheH dataat0KareestimatedfromtheArrheniusplotsandtheyappeartomatchthelowtempera-
irr
turelimitoftheH datadirectlydeterminedfromtheρ(T,H)usingtheρ criterion. Forcomparison,
irr c
H (T)determinedfromJ (H)isalsoplottedinFig.2eshowingsomedifferenceswiththevalueses-
irr c
timated from ρ(T,H). A plausible reason is a differentfrequencyof the applied currentused in those
investigations[34].
TheangulardependenceofH at20K,whichwasderivedfromρ(H)curvesatconstantangleswith
c2
ρ (Fig.3a)showsaminimumatθ =90◦(H kc)andamaximumatθ =180◦(H kab),asshownin
n,0.9
Fig.3b.Thesingle-bandanisotropicGinzburg-Landau(AGL)theory[35],H (θ)=H (90◦)(sin2(θ)+
c2 c2
3
cos2(θ)/γ2)−0.5 with γ = H (180◦)/H (90◦) (dottedlinein Fig.3b), cannotdescribethemeasured
c2 c2
H (θ)duetothemulti-bandnatureofthismaterial,similarlytoCo-dopedBa-122[28]. A fairlygood
c2
descriptionofthedatais,however,achievedbytheempiricalformulae[28],
−1
δ δ
cosθ
H (θ)=H (90◦)×ǫ(θ,γ,δ), ǫ(θ,γ,δ)= |sinθ|δ+ (1)
c2 c2
γ
(cid:12) (cid:12) !
(cid:12) (cid:12)
(cid:12) (cid:12)
withδ = 1.47andγ = 1.62(solidline). Theparameterγ istheH anisot(cid:12)ropy,(cid:12)whereasδ isameasure
c2
for the ab-peakwidth whose physicalmeaningis still unclear. These two values will be used later for
scalingtheangulardependenceofJ (θ)data.
c
TheangulardependenceofH at20Kderivedusingthesameresistivitycriterionρ =1.0−8Ω·cm
irr c
showsalmostthesametrendasH (θ). UnliketheangulardependenceofJ (seenextsection),noclear
c2 c
peakatθ =90◦(H kc)isobservedinH (θ).
irr
In-fieldcriticalcurrentdensityJc(T,H,θ)
The E −J curves of the P-doped Ba-122 coated conductor sample at 4.2K (Fig.4) show different
behaviour at high and low magnetic fields for both major field directions. Up to 10T they exhibit a
non-Ohmiclineardifferential(NOLD)signature(i.e., E islinearlychangingwithJ in linearscale, see
SupplementalFig.S3),indicativeofJ limitationbyGBs[36]. HereNOLDbehaviourisduetoviscous
c
fluxflowalongtheGBs[37]. Ontheotherhand,NOLDsignatureisalmostabsentabove12.5T,suggest-
ing thatJ is limitedby intra-graindepinningof fluxlines. Thispinningcrossoverfield is observedto
c
decreasewithincreasingtemperature(notshown),whichisconsistentwiththecuprateYBCO reported
inRef.38and39.
Figure5a comparesJ (H)forP-dopedBa-122onIBAD-MgOforH k c at4.2K withP-dopedBa-
c
122onMgOsinglecrystallinesubstrate[15],Fe(Se,Te)onRABiTS[21],YBCOcoatedconductor[40],
MgB [41],NbTi[42,43],andNb Sn[44,45].Pinning-improvedYBCO2nd-generation(2G)tapeshows
2 3
thehighestJ atentiremagneticfields;however,awelltexturedtemplateisnecessary. TheP-dopedBa-
c
122coatedconductorexceedsaself-fieldJ of4MA/cm2andmaintainsahighJ valueof50kA/cm2at
c c
20T.Fortheentirefieldrange,J ofP-dopedBa-122coatedconductorsampleislargerthanforMgB
c 2
and NbTi. Above 20T, the P-doped Ba-122 coated conductor sample shows comparable properties to
Nb Sn. Although lower-field J of P-doped Ba-122 on IBAD-MgO is higher than that of Fe(Se,Te)
3 c
on RABiTS, the latter shows the better performanceat mediumand highfields. Figure5b summarises
J (H)forP-dopedBa-122onIBAD-MgOforbothcrystallographicdirectionsatvarioustemperatures.
c
At intermediate fields J for the two directions is comparable, indicative of the presence of correlated
c
pinningalongthec-axis.
ByanalysingthepinningforcedensityF =µ H×J ,informationonvortexpinningcanbeobtained.
p 0 c
In general, the normalised pinning force, f = F /F , is plotted as a function of reduced field
p p p,max
h = H/H atagiventemperatureforhigh-T superconductors. However,weplotf asafunctionof
1 irr c p
h = H/H ,whereH isthefieldatwhichF showsthemaximum[46–49],sinceJ couldnotbe
max max p c
measureduptoH atalltemperatures.AscanbeseeninFig.5c,thef curvesatdifferenttemperatures
irr p
forH kcalmostfallontoamastercurveintherangeof0<h<3describedby
25 h
f = h0.5(1− )2 (2)
p
16 5
Thisformulaisanalogoustohp(1−h )q (p = 0.5andq = 2)foundbyDew-Hughes[50]forpinning
1 1
4
byplanardefectssuchasGBandtwinboundaries,andbyKramerforlinedefectarrays[51]. Inhigh-T
c
superconductorswithextremelyshortcoherencelengthsξ,afurtherclassificationofthedefectsizewith
respecttoξisnecessary.IthasbeenrecentlyfoundbyPaturietal.thattheexponentpis0.5irrespective
ofq fora defectsizeoftheorderofξ andespeciallyfordislocationsinundopedYBCO films[49]. On
thecontrary,pincreasestowards1withincreasingdefectsize. Thisconfirmsthefindingthatpinningin
oursampleisdominatedbythedislocationswithnano-size.Here,itshouldbenotedthatasignofNOLD
signature does not contradictGB pinning. In fact it has been reported for YBCO that the dislocations
inGBscanworkasvortexpinningcentres[52,53]. Thefluxpreferentiallyflowsacrossthedislocation
coresintheGBplane,whichexplainstheE−J curveswithNOLDsign.
Abrikosov-Josephsonvortices(AJV)arepresentinlow-angleGBsinbothYBCO[54]andFBS.Unlike
Josephsonvortices(JV),AJV have normalcoresandcan be trappedbyflux pinning. Furthermore,the
presence of an interaction betweenAbrikosovvortices(AV) in the grainand AJV at the GBs has been
experimentallyfoundinRef.55: anincreaseinpinningpotentialforAVleadstotheenhancementofthe
pinningpotentialforAJV.
ForH k ab thef curvesat both10and 15K followwellthe GBpinningline (redsolidline) upto
p
16T (correspondingto h = 2 and 3.2 in Fig.5d, respectively). In contrast, f at 20K neither follows
p
theGBpinningnorpoint-likepinning(redsolidandbluedashedlines,respectively)inhighfieldregime,
althoughthef curveliesontheGBpinninglinebelowh < 2. Similarly,thef curveat4.2Kfollows
p p
theGBpinningcurveuptoh < 1.5andthenapproachesthepoint-likepinningcurvebeyondh > 1.5.
Hence, differentlyfrom the H k c case, the dominantpinningmechanism for H k ab is varyingwith
temperatureandfieldstrength.
The angular dependence of the critical current density, J (θ) (Fig6a-d), shows two distinct peaks:
c
a relatively sharp peak at H k ab and a broad maximum at H k c, which arises from the network
of threading dislocations comprising the low-angle GBs[17]. Surprisingly, the c-axis peaks [J (90◦)]
c
remainvisibleevenclosetoH atalltemperatures.Unlikesinglebandsuperconductors,theanisotropy
irr
ofcoherencelength,γ =ξ /ξ ,andpenetrationdepth,γ =λ /λ ,ofFBSexhibitoppositebehaviour
ξ ab c λ c ab
withtemperature[56]. ForanoptimallydopedBa-122system,γ > γ holdsatalltemperature. Inthis
λ ξ
caseevenoccasionaluncorrelateddefectsslightlylargerthanξyieldastrongc-axispinning[57].Suchan
effectincombinationwiththreadingdislocationsalongthec-axismayenhanceenormouslytheaverage
pinningpotentialforappliedfieldsparalleltothec-axis.
ShowninFig.6eisthescalingbehaviourofJ (θ)asafunctionoftheeffectivefield[i.e.,ǫ(θ,γ,δ)×
c
µ H]at20K.Hereδ = 1.47andγ = 1.62wereusedasobtainedbytheH (θ)fit. Ascanbeseen,all
0 c2
J (θ)curvescollapseontoamastercurveinawideangularrangearoundH k ab. Differencesbetween
c
the master curve and the measured J (H) for H k c are correlated pinning contributions. Here we
c
emphasisethattheJ peakatθ = 180◦ isfullydeterminedbytheelectronicanisotropyat20Kandno
c
intrinsicpinningorpinningbyplanardefectsisobserved.
Discussionsandconclusions
InordertorealiseFBScoatedconductors,highJ valueswithlowanisotropyinhighfieldsareneces-
c
sary. J ofourP-dopedBa-122coatedconductornearlyreachedthepracticallevelof∼0.1MA/cm2 at
c
15Tforanyappliedfielddirectionsat4.2K[seeFig.5a)],whichshowssuperiorpropertiesoverMgB
2
and NbTi. Above 20T the level of J is comparable to Nb Sn. Additionally, the intrinsic anisotropy
c 3
estimatedat20KfromtheH dataisbelow2. Moreover,thecorrelateddefectsincreaseJ forH k c
c2 c
substantiallysuppressingtheeffectiveJ anisotropy.
c
Asstatedabove,theinequalityofξ andλanisotropyincombinationwithalargedensityofthreading
dislocationsalongthec-axissignificantlyenhancestheaveragepinningpotential. Itisworthmentioning
thatthepopulationofthreadingdislocationscanbecontrolledbytheprocessingconditionsonly,without
5
anymodificationofthePLDtarget[15].
Compared to optimally P-doped Ba-122 films on MgO single crystal substrates by MBE[12] and
PLD[15], the level of J of the P-doped Ba-122 coated conductor still needs to be improved. Film
c
stoichiometryespeciallyforPcontentshouldbecontrolledprecisely. Asstatedbefore,thePcontentof
ourBa-122filmslightlydiffersfromtheoptimallevel,wheretheQCPcausesasharpmaximumforthe
vortexcoreenergy[11]. As a consequence,the slightdeviationfromthe optimalPlevelin oursample
resultsinalowervortexcoreenergy,whichdirectlyreducesJ .
c
Unlike in electron and hole dopedBa-122 systems, aliovalentdisorderthat contributesto pinningin
the Co or K cases is absent in P-doped Ba-122. However, J can be further enhanced by introducing
c
growth defects (e.g. intragrain dislocations since the PLD processing conditions strongly affect their
density[15]) andartificial structures(e.g. nanoparticles). Moreover,the thermalconductivityof single
crystalline MgO is different from that of IBAD-MgO template, which infers the optimum deposition
temperaturemaychange.
TheintroductionofartificialpinningcentresiseffectiveforfurtherimprovementofJ . Infact,Miura
c
etal.havereportedtheintroductionofBaZrO intoP-dopedBa-122matrix[58]inanalogytotheaddi-
3
tionofBaZrO toYBCO.Hence,acombinationoftheintroductionofartificialpinningcentresandthe
3
precisecontrolofPcontentwillyieldbetterperformingP-dopedBa-122coatedconductors.
AnattempttofabricatealonglengthP-dopedBa-122coatedconductorhasstartedquiterecently. As
a result, a 15cm long P-doped Ba-122 coated conductorhas been realised by PLD using a reel-to-reel
system[16]. AlbeittheresultantP-dopedBa-122showedasmallself-fieldI of0.47mA(corresponding
c
toaJ of4.7×104A/cm2)at4.2K,animprovementofI isforeseenbyapplyingtheaforementioned
c c
methods.
Insummary,wehaveinvestigatedin-fieldtransportpropertiesofaP-dopedBa-122thinfilmgrownby
PLDontechnicalsubstrateinawiderangeoftemperatureandDCmagneticfieldupto35T.TheP-doped
Ba-122 coated conductor exceeds a transport J of 105A/cm2 at 15T for both major crystallographic
c
directionsoftheappliedfield. Additionally,theJ peaksforH k cremainvisibleevenclosetoH at
c irr
alltemperaturesbytheenhancedvortexpinningduetothecombinationoflargepopulationofthreading
dislocationsandtheinequalityofξ andλanisotropy. ThisleadstoalowerJ anisotropy. Byanalysing
c
pinningforcedensities,weestablishedthattheGBpinningcontributionisdominantforH k c,whereas
forH k ab, thedominantpinningisvaryingwithtemperature. Theresultsobtainedthroughthisstudy
areconsideredpromisingforfuturehigh-field-magnetapplicationsofAE-122systems.
METHODS
GrowthoftheP-dopedBa-122filmandstructuralcharacterisation
TheP-dopedBa-122thinfilmof185 nmthicknesswasgrownbypulsedlaserdepositiononanIBAD-
MgOHastelloymetal-tapesubstratesuppliedbyiBeamMaterials,Inc[59]. Thestackingstructureofthe
IBAD-MgOsubstrateasshowninref.17consistsoffirstaplanarisingbottombed-layeramorphousY O
2 3
ontheHastelloy,secondabiaxiallytexturedMgOlayerformedbyIBAD,andatophomoepitaxialMgO
layer.TheIBAD-MgOsubstratewithalargein-planedistributionangleof∆φ =8◦wasinvestigated
MgO
becausehigherJ with isotropicpropertiescan beachievedcomparedto the film onthe well in-plane-
c
aligned IBAD-MgO metal-tapes(i.e., ∆φ = 4◦)[17]. A polycrystallineBaFe (As P ) disk
MgO 2 0.65 0.35 2
wasusedasthePLDtarget.Weemployedahighergrowthtemperatureof1200◦CthanforoptimisedP-
dopedBa-122filmsonMgOsingle-crystalsubstrates(1050◦C)[15],sincethePconcentrationincreases
with increasinggrowthtemperaturefora giventargetcomposition. As expected,a higherPconcentra-
tion closer to the optimum P concentration than in previous studies was achieved[15, 17]. The other
growthparameters[e.g.,theexcitationsourceandthelaserfluenceofthesecondharmonics(wavelength:
6
532nm)ofaNd-dopedyttrium-aluminum-garnetpulsedlaser and3J/cm2, respectively]werethesame
asreportedinRef.15.
To determine the crystalline phases, ω-coupled 2θ scan X-ray diffraction measurements were per-
formed. The asymmetric 103 diffraction of the P-doped Ba-122 film was measured to confirm the in-
planecrystallographicfour-foldsymmetrywithoutin-planerotationaldomains. Thecrystallinityofthe
filmwascharacterisedonthebasisofthefullwidthsathalfmaximum(FWHMs)oftheout-of-plane004
(∆ω)andthein-plane200rockingcurves(∆φ). TheresultsofthoseXRDmeasurementscanbefoundin
SupplementaryInformationFig.S1. Thechemicalcompositionwas determinedwith anelectron-probe
microanalyser.TheaccelerationvoltageoftheelectronbeamwasoptimisedwhilemonitoringtheNiKα
spectrumtoavoidthematrixeffectfromtheNi-containingHastelloymetal-tapes.
In-planetransportmeasurements
A small bridge of 15µm width and 500µm length was patterned by photolithography, followed by
ion-beametching. Auelectrodeswith50nmthicknesswereformedbysputteringandlift-off. Transport
propertiesusingtheresultantbridgeweremeasuredbyastandardfour-probemethod.
ThetemperaturedependenceoftheresistivityoftheP-dopedBa-122coatedconductorshowsaT
c,90
of28.3K(Fig.1a), whichisabout3KlowerthanthatoftheoptimallyP-dopedBa-122singlecrystals.
T is defined as the intersection between the steepest slope of the superconducting transition and a
c,90
90% reduction of the fit of the normalstate resistivity using ρ = ρ +ATn. On the other hand, the
n 0
onsetT isdefinedastheintersectionbetweenthefitcurveasstatedaboveandthesteepestslopeofthe
c
superconductingtransition.ThedifferencebetweenT andtheonsetT isnegligible.
c,90 c
TheactivationenergyU (H) forvortexmotionwas evaluatedbythetemperaturedependenceofthe
0
resistivity measurementsin various field strengths up to DC 35T at the National High Magnetic Field
Laboratory,Tallahassee,FL,USA.Accordingtothemodelofthermallyactivatedfluxflow[26],theslope
oflinearfityieldsthepinningpotentialforvortexmotionatgivenfields(Fig.2c).Ontheassumptionthat
U(T,H) = U (H)(1 −T/T ), both equations, lnρ(T,H) = lnρ (H)− U (H)/T and lnρ (H) =
0 c 0 0 0
lnρ +U (H)/T ,areobtained,whereρ isaprefactor.
0f 0 c 0f
InordertofurtherunderstandtheH anisotropyforaP-dopedBa-122coatedconductorsample,the
c2
angulardependenceof the magnetoresistivitywas measuredat 20K. Using the same constantcriterion
ρ forevaluatingH ,theangulardependentuppercriticalfield[H (θ)]wasderived(Fig.3b).
n,0.9 c2 c2
Acriterionof1µV/cmwasemployedforevaluatingJ . InJ measurement,themagneticfieldwas
c c
always appliedin the maximumLorentzforceconfiguration. Low-fieldmeasurementswere performed
inaQuantumDesignphysicalpropertymeasurementsystem(PPMS)inmagneticfieldsupto16T.For
highfieldmeasurementsuptoDC35T,theexperimentswereconductedattheNationalHighMagnetic
FieldLaboratory,Tallahassee,FL,USA.
Acknowledgement
A portion of this work was performed at the National High Magnetic Field Laboratory, which was
supportedbyNationalScienceFoundationCooperativeAgreementNo. DMR-1157490,andtheStateof
Florida.TheworkatTokyoInstituteofTechnologywassupportedbytheMinistryofEducation,Culture,
Sports, Science and Technology (MEXT) through Element Strategy Initiative to Form Core Research
Center. K.I.acknowledgessupportbytheJapanSocietyforthePromotionofScience(JSPS) Grant-in-
AidforScientificResearch(B)GrantNumber16H04646. H.HiwasalsosupportedbyJSPSforYoung
Scientists(A)GrantNumber25709058,JSPSGrant-in-AidforScientificResearchonInnovativeAreas
Nano Informatics(GrantNumber 25106007),and Supportfor TokyotechAdvancedResearch (STAR).
7
We acknowledge support by Deutsche Forschungsgemeinschaftand Open Access Publishing Fund of
KarlsruheInstituteofTechnology.
Authorscontribution
K.I., C.T., J.H., H.S. and H.Hi. designed the study and wrote the manuscripttogether with J.J. and
H.Ho. Thinfilmspreparation,structuralcharacterisationsandmicrobridgefabricationswerecarriedout
byH.S.andH.Hi. K.I.,C.T.,J.H.andJ.J.conductedhighfieldtransportmeasurements. C.T.,H.S.and
H.Hi. performedlowfieldtransportmeasurements. K.I.,C.T.,H.Hi,andH.Ho. supervisedtheprojects.
Allauthorsdiscussedtheresultsandimplicationsandcommentedonthemanuscriptatallstages.
ADDITIONALINFORMATION
Theauthorsdeclarenocompetingfinancialinterests.Correspondenceandrequestsformaterialsshould
beaddressedtoK.I.
∗ Correspondenceto:[email protected]
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