Table Of ContentSuperconducting properties of Ba(Fe Ni ) As thin films in high magnetic
1−x x 2 2
fields
Stefan Richter,1,2,a) Fritz Kurth,1 Kazumasa Iida,3 Kirill Pervakov,4 Aurimas Pukenas,2 Chiara Tarantini,5 Jan
Jaroszynski,5 Jens H¨anisch,6 Vadim Grinenko,1,2 Werner Skrotzki,2 Kornelius Nielsch,1,2 and Ruben H¨uhne1
1)Institute for Metallic Materials, IFW Dresden, 01171 Dresden, Germany
2)TU Dresden, 01062 Dresden, Germany
3)Department of Crystalline Materials Science, Nagoya University, Chikusa, Nagoya 464-8603,
Japan
7 4)Department of High Temperature Superconductivity and Superconductor Nanostructures,
1 Russian Academy of Sciences, 119991 Moscow, Russia
0 5)Applied Superconductivity Center, National High Magnetic Field Laboratory,
2 Florida State University, 2031 East Paul Dirac Drive, Tallahassee, Florida 32310,
USA
n
6)Institute for Technical Physics, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1,
a
J 76344 Eggenstein-Leopoldshafen, Germany
8 (Dated: 10 January 2017)
] We report on electrical transport properties of epitaxial Ba(Fe1−xNix)2As2 thin films grown by pulsed laser
n deposition in static magnetic fields up to 35T. The thin film shows a critical temperature of 17.2K and a
o critical current density of 5.7×105A/cm2 in self field at 4.2K while the pinning is dominated by elastic
c
pinning at two-dimensional nonmagnetic defects. Compared to single-crystal data, we find a higher slope of
-
r theuppercriticalfieldforthethinfilmatasimilardopinglevelandasmallanisotropy. Alsoanunusualsmall
p
vortex liquid phase was observed at low temperatures, which is a striking difference to Co-doped BaFe As
u 2 2
thin films.
s
.
t
a
m Fe-based superconductors are intriguing materials for A polycrystalline pellet was prepared from a nominal
both basic research and potential applications. In par- Ba(Fe Ni ) As powder and served as PLD target.
- 0.95 0.05 2 2
d ticular, the electronic phase diagrams show competing The Ni-content of the final target was determined to be
n ground states, which lead to unique normal state prop- inthe overdopedregime(x=0.065±0.015)usingenergy
o erties and unconventional superconductivity1. These dispersiveX-rayspectroscopy(EDX)analysisinaJEOL
c
materials also have the potential to supersede low- scanning electron microscope. CaF (001) was used as
[ 2
temperature superconductors in high-field applications substrate, which has been proven as a suitable template
1 since they combine the advantage of high upper critical for the growth of Ba122 thin films7.
v fieldswithasmallanisotropyatlowtemperatures2. Also X-ray diffraction (XRD) using a Bruker D8 Advance
3
a higher tolerance to low angle grain boundaries com- diffractometerwithCo-K radiationshowsonly(00l)re-
3 α
9 paredtocuprateshasbeenfoundforCo-dopedBaFe2As2 flections of the Ba122 and the substrate (Figure 1a) in-
1 (Ba122)3. Ba122 is among the most studied systems of dicating a c-axis oriented film, together with peaks cor-
0 the Fe-based superconductors since single crystals and responding to a BaF impurity phase.
2
. thin films are relatively easy to grow and several kinds
1 The distribution of this BaF2 phase was investigated
of doping (hole, electron, isovalent, direct/indirect) lead
0 by transmission electron microscopy (TEM) on a cross
7 to superconductivity4. We used Ni as dopant5, which sectionallamellaalongthe[100]direction(preparedwith
1 has two electrons more in the d shell than Fe and will
a focused ion beam technique) using an FEI Tecnai T20
: therefore dope electrons in the system similar to the ex-
v microscope (accelerator voltage=200kV). Precipitates
tensively studied Co substitution. Ni-doped Ba122 was
i with a lateral size of about 150nm were found primar-
X foundtobe aninterestingcandidateforapplicationsdue
ily on the film surface (Figure 1b), i.e. not incorporated
r to inherently strong pinning effects6 leading to a high
a in the superconducting layer. Selected area diffraction
critical current density J .
c confirmed them as BaF . The BaF is formed presum-
2 2
Here we report on thin films of Ba(Fe1−xNix)2As2 ablyduetoareactionofexcessBawithfluorinediffusing
with a thickness of 100nm which have been prepared
out of the substrate. Besides these precipitates, the film
by pulsed laser deposition (PLD) using a KrF excimer
showsacleanmicrostructure. Planardefects,whichwere
laser (λ=248nm, pulse duration 25ns, energy density reportedoccasionallyforCo-dopedBa122PLDfilms8–10
3J/cm2, repetition rate 7Hz). The deposition was per-
are almost absent in our film. A partly amorphous reac-
formed under ultra high vacuum conditions (base pres-
tionlayeratthefilm-substrateinterfaceofabout5-10nm
sure 10−9mbar) at a deposition temperature of 750◦C.
thickness was observed (Figure 1c) similar to Co-doped
Ba122 films on CaF 7. It is likely caused by beam dam-
2
age during the lamella preparation. XRD texture mea-
surements (data not shown) reveal the epitaxial relation
a)Electronicmail: [email protected] (001)[110]Ba122k(001)[100]CaF . The full width at half
2
2
(a)
(a) H||ab
(b)protecon BaF2 (c) (b) H|| c
layer
Ba122
reacon
Ba122 layer
CaF2 100 nm CaF2 10 nm
FIG. 1. Microstructural properties of the investigated Ni-
dopedBa122film: a)XRD2θscanshowingthec-axisoriented
growth of the Ba122. A detected BaF impurity phase is
2
marked with blue stars. b) In TEM, BaF impurities are
2
found as precipitates on the film surface (black arrow). c)
A reaction layer of 5-10nm thickness at the film-substrate
interface was observed. TheBa122 phaseabovethisreaction FIG.2. Superconductingtransitioninappliedmagneticfields
layershowsagoodcrystallinequalitywithoutanyobservable for: a) H kab and b) H kc.
defects.
is commonly observed in other Ba122 systems due to a
maximum(FWHM)ofthe(103)φscan(∆φ=0.93◦)and small Ginzburg number. The observed T value is lower
of the (004)rockingcurve (∆ω=0.71◦) (both values not thanresults on optimally dopedsingle crycstals16 but fits
corrected for instrument broadening and geometrical ef- welltodataforslightlyoverdopedmaterialinagreement
fects)indicateagoodcrystallinequalityofthesupercon- withthemeasuredNi-contentofthetarget. However,the
ducting layer and are comparable to the results for Co- directcomparisonofT valuesbetweensinglecrystalsand
c
dopedBa122thinfilmsonCaF27. Forthedetermination thin films might be problematic due to complex effects
ofthec-axislength,thepeakpositionsofthe2θscanand of strain on the phase diagram. It has been shown for
the Nelson Riley extrapolation11 were used, while the a- Co-doped Ba122, that compressive strain can even lead
axis was extractedfrom reciprocalspace maps measured to higher transition temperatures in slightly overdoped
in a high resolution X-ray diffractometer. With lattice thinfilmscomparedtooptimallydopedsinglecrystals15.
parametersc=13.031±0.002˚A anda=3.949±0.001˚A,
A 90% criterion at 17.3K was used for the evalua-
the Ba122 unit cell is elongated along the c-direction in tion of the upper critical field H and the irreversibil-
c2
the thin film in comparison to single crystals with sim- ity field H was evaluated at ρ=10−4mΩcm (which is
ilar doping (c=12.986˚A a=3.963˚A for x=0.065)12,13. irr
roughly 0.5% of the normal state resistivity at 30K).
This originates from compressive strain in the film due
The data shown in Figure 3 are in good agreement with
to the smaller a-axis parameter and larger thermal ex-
recent pulsed field measurements for overdoped single
pansion coefficient of the CaF2 substrate14,15. crystals16. The H data were fitted using a two-band
c2
Transportpropertieshavebeenmeasuredusingafour- modelforacleansuperconductorincludingparamagnetic
point technique on a bridge of 2mm length and 90µm effects18. The fit suggests an upper critical field of 45T
width prepared by laser cutting. Measurements were and a smaller anisotropy than in overdoped single crys-
performed in static magnetic fields up to 35T in max- tals with a similar critical temperature16. The slope of
imum Lorentz force configuration at the National High the upper critical field -dH /dT=8.5T/K (Hkab) near
c2
Magnetic Field Laboratory. The critical temperature T ishigherforthefilmthanforsinglecrystalswithsimi-
c
T , evaluated at 90% of the normal resistance is 17.3K lardopinglevelandmorecomparabletooptimallydoped
c
(Fig.2). The superconducting transition is sharp with crystals16. This might be explained with the presence of
∆T =T -T <1K.The broadening of the transition epitaxialstraininthefilmleadingtoachangeintheband
c c90 c10
with increasing external field is relatively small, which structure, which results in smaller Fermi velocities15,19.
3
(a)
(a)
(b)
(b)
FIG. 4. Critical current density for 4.2K. a) The angular
FIG.3. Uppercriticalfieldandirreversibilityfielddatafora)
Hkaband b)Hkc. Blue dotsshowH using a90% criterion dependence of Jc shows a maximum for Hkab. The inset
c2 shows the scaling over an effective field (2)20. b) Magnetic
and the blue lines represent two-band fits of these data. For
comparison, results from Wanget al.16 for single crystals are fielddependanceforHkab(orangetriangels)andHkc(purple
shown for x=0.054 (green squares) and x=0.06 (magenta diamonds). The inset shows the normalized pinning force
triangels). The stars show H determined from theresistiv- density and fitsusing equation (3).
irr
itydata(darkgrey),thefitofthepinningforcedensity(light
grey) and the fit of the activation energy (grey). Open sym-
tion.
bols are extrapolated from Arrhenius plots. The inset shows
the activation energy for both field orientations. The solid The linear dependence of Arrhenius plots of the re-
lines are fitsusing eq. (1)17. sistivity (not shown here) indicates thermally activated
motion of the flux lines above H . The pinning poten-
irr
tial U extracted from the Arrhenius slopes is shown in
0
the inset of figure 3b. The solid lines represent fits with
Interestingly,atlowtemperaturesthe irreversibilityfield
the phenomenological function8,17:
almost seems to reach the H line as its natural limi-
c2
tation for both field directions. This would mean that
pBhaa(Fsee1a−txlNowix)t2eAmsp2etrhaitnurfielsmisnschoonwtraastsmtoalrlevsourlttsexfolriqCuiod- U0 ∝H−a 1− HH b (1)
dopedfilms8. However,oneshouldtakeintoaccountthat (cid:18) irr(cid:19)
H valuesatlowtemperaturescouldbeadditionallyen- We achieved the best fits with parameters a=0.4,
c2
hanced due to the formation of the Fulde-Ferrel-Larkin- b=1.0, µ H =45T for Hkab and a=0.7, b=1.0,
0 irr
Ovchinnikov (FFLO) state18. This possibility was not µ H =42T for Hkc.
0 irr
takenintoaccountintheH analysisinthepresentwork The critical current density J is shown in figure 4.
c2 c
due to a lack of experimental data at low temperatures. The J at 4.2K in self field exceeds 5.7×105A/cm2.
c
Future investigations in high fields will clarify this ques- This is lower than J values for differently doped Ba122
c
4
has been observed before in Co-doped8 and P-doped22
Ba122 films at low temperatures. This might be related
tothesmallfilmthickness,whichrestrictsthebendingof
vortices. For Hkab, the smaller q value is in qualitative
agreementwiththeresultsoftheactivationenergy,since
the exponents of the activation energy (1) and the pin-
ningforcedensity(3)shouldfulfilltherelationsa≈1−p,
b≈q17. For Hkc these equations are not fulfilled, which
has also been observed for Co-doped films8.
The anisotropy γ of the critical fields and critical cur-
rent density is shown in figure 5. For H and H it
c2 irr
is calculated by γ = Hab where Hab and Hc represent
Hc
the critical field for the respective field orientation. The
data points and the two-bandfit for H suggest a small
c2
anisotropy close to 1 at low temperatures. This agrees
well with the anisotropy extracted from the activation
energyfits. Suchasmallanisotropydespiteofthehighly
FIG. 5. The upper critical field and the activation energy anisotropiccrystalstructureisakeyfeatureofiron-based
show a small anisotropy close to 1 at low temperatures. In superconductors25. The Jc anisotropy follows the Hirr
comparison,theanisotropyofthecriticalcurrentdensityand anisotropy and both are noticeably increased compared
theirreversibility field is noticeably increased. to the H anisotropy. This indicates that the incorpo-
c2
rateddefects inthefilmaffectthe H anisotropy,differ-
irr
entiating it from the intrinsic H anisotropy.
c2
films21,22,presumablyduetothelowerT . Alsomagneti-
c The enhancement of the H anisotropy is typically
sationmeasurementsonoptimallydopedsingle crystals6 irr
observed in Fe-based superconductors by introduction
showed a higher Jc while it was highest for Hkc, i.e. an of pinning centers8,26, which is in contrast to cuprate
inversion of the J -anisotropy occured. This inversion
c high-T superconductors,wheretheadditionofnanopar-
c
was not observed for our thin films.
ticlesoftenleedstoareductionintheH andJ -scaling
irr c
Figure 4a shows the angular dependance of Jc at anisotropy27.
T =4.2K.OnlyonemaximumforHkabisobserved. The
In summary, we reported the epitaxial growth of
data can be scaled over an effective field20:
Ba(Fe1−xNix)2As2 thin films,investigatedthe structural
and superconducting properties and evaluated their per-
formance in high magnetic fields. The films show a
cos2(θ)
Heff =H ·ssin2(θ)+ γ2 (2) uTpcpvearluceritoifcaalbfioeultd1a7tKT anindcaonmpenahriasnoncedtossloinpgeleofcrtyhse-
c
tals with similar doping level. The enhanced slope
Ananisotropyvalueofγ=1.8wasfoundforthecritical
-dH /dT might be explained by the presence of com-
current density at 4.2K. c2
pressive strain in the film. The critical current den-
The field dependence of J is shown in figure 4 b).
c sity reaches 5.7×105A/cm2 at 4.2K in self field which
TheinsetshowsthenormalizedpinningforcedensityF ,
p is smaller than the best results achieved for Co-doped
which was fitted with the Dew Hughes model23:
Ba122 thin films21. The pinning is dominated by elastic
p q pinning at grain boundaries or growth domain bound-
H H
Fp ∝ H 1− H (3) aries and the anisotropy at low temperatures is small
(cid:18) irr(cid:19) (cid:18) irr(cid:19) similar to results for other Fe based superconductors.
The respective fit parameters for Hkab and Hkc are Furthermore, we found the vortex liquid region of the
p=0.56,q=1.32andp=0.48,q=1.67. Thecorrespond- vortex-matter phase diagram for Ni-doped films to be
ingH valuesareshowninfigure3. The p-valuesofthe rather small which is surprisingly different to Co-doped
irr
pinning force fits for both field directions suggest, that Ba122 films.
the pinning is influenced by elastic effects of the vortex This work was supported by the German research
lattice and vortex interactions. This contribution to the foundation (DFG) within the research training group
pinning is significant, if the size of the pinning centers GRK 1621. A portion of this work was performed at
r is smaller than or comparable to the coherence length the National High Magnetic Field Laboratory, which
ξ24. For Hkc, the fit is close to (p=0.5, q=2), which is supported by National Science Foundation Cooper-
canbeattributedtopinningattwo-dimensionalnonmag- ative Agreement No. DMR-1157490 and the State of
neticdefects23. Inourfilmthesedefectscouldbe growth Florida. ThisworkwasalsopartiallysupportedbyJSPS
domain boundaries or small-angle grain boundaries, al- Grant-in-Aid for Scientific Research (B) grant number
thoughtheyhavenotbeendirectlyconfirmedinTEMin- 16H04646and the DFG grant number GR4667/1-1.
vestigations. For Hkab,the exponent q is smaller,which
5
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