Enhancement in transition temperature and upper critical field of CeO 0.8F0.2FeAs by yttrium doping

Article abstract of DOI:10.1063/1.3280046

We report significant enhancement in superconducting properties of yttrium substituted Ce_1-xY_xOFFeAs superconductors. The polycrystalline samples were prepared by two step solid state reaction technique. X-ray diffraction confirmed t

Full text of DOI:10.1063/1.3280046

Enhancement in transition temperature and upper critical field of CeO 0.8 F 0.2 FeAs
by yttrium doping
J. Prakash, S. J. Singh, A. Banerjee, S. Patnaik, and A. K. Ganguli
Citation: Applied Physics Letters 95, 262507 (2009); doi: 10.1063/1.3280046
View online: http://dx.doi.org/10.1063/1.3280046
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APPLIED PHYSICS LETTERS 95, 262507 ͑2009͒
Enhancement in transition temperature and upper critical field
of CeO0.8F0.2FeAs by yttrium doping
J. Prakash,1 S. J. Singh,2 A. Banerjee,3 S. Patnaik,2,a͒ and A. K. Ganguli1,a͒
1Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India
2School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India
3UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India
͑Received 11 August 2009; accepted 7 December 2009; published online 30 December 2009͒
We report significant enhancement in superconducting properties of yttrium substituted
Ce1−xYxOFFeAs superconductors. The polycrystalline samples were prepared by two step solid state
reaction technique. X-ray diffraction confirmed tetragonal ZrCuSiAs structure with decrease in both
a and c lattice parameters on increasing yttrium substitution ͑with fixed F content͒. With smaller ion
Y in place of Ce, the transition temperature increased by 6 K. Yttrium doping also lead to higher
critical fields as well as broader magnetization loops, particularly at elevated temperature. © 2009
American Institute of Physics. ͓doi:10.1063/1.3280046͔
Shortly after the discovery of superconductivity in
oxypnictides,1 it was discerned that due to low transition
temperature ͑Tc͒ and the deleterious influence of the grain
boundaries, the possibility of technological application of
these novel materials would be rather limited. The maximum
Tcϳ55 K thus far is reported in a samarium-based
compound2 but this is far below the liquid nitrogen bench-
mark. Moreover the pnictides seem to have all the negatives
of high Tc cuprates particularly with respect to suppressed
in-field intragranular pinning and weak link behavior across
the grain boundaries.3,4 One favorable aspect of oxypnictides
is the high upper critical field ͑Hc2͒ as a consequence of
multiband effects.5 The important question of current rel-
evance is the route to increase the transition temperature and
critical current density in these materials without compro-
mising on high upper critical field parameters. In this letter
we report the successful synthesis of Ce1−xYxO0.8F0.2FeAs by
simultaneous substitution of yttrium for cerium and fluorine
for oxygen and study the superconducting properties in de-
tail. We show that optimal Y addition can lead to significant
increase in the transition temperature ͑Tc͒ and upper critical
field ͑Hc2͒ as well as some signatures of improvement of the
critical current density ͑Jc͒.
netic susceptibility was measured using a tunnel diode based
rf ͑2.3 MHz͒ penetration depth technique.7 Magnetization
hysteresis loops and remanent magnetization of the samples
were measured by a superconducting quantum interface de-
vice magnetometer.
Figure 1 shows the powder x-ray diffraction patterns
for Ce0.6Y0.4O0.8F0.2FeAs and CeO0.8F0.2FeAs. Majority of
the observed reflections could be satisfactorily indexed
on the basis of the tetragonal ZrCuSiAs type structure.
Minor amount ͑ϳ10%͒ of Y2O3 was observed as a second-
ary phase for Y-doped sample. The refined lattice parameters
were found to be a=3.9654͑1͒ Å and c=8.5803͑3͒ Å for
Y-doped sample and a=3.988͑3͒ Å and c=8.607͑8͒ Å for
without Y-doped phase. The lattice parameters are smaller
than the parent compound CeOFeAs ͓a=3.996 Å and
c=8.648 Å ͑Ref. 8͔͒ and the reduction in the lattice volume
upon F and Y-doping indicates a successful chemical substi-
tution. Both the lattice parameters for Ce1−xYxOFFeAs
͑x=0.1, 0.2, 0.3, and 0.4͒ decrease with the increase in
yttrium content but in this work we focus only on x=0 and
0.4 compositions. As shown in the inset of Fig. 1, the zero
Polycrystalline samples with nominal compositions of
Ce1−xYxO0.8F0.2FeAs and CeO0.8F0.2FeAs were synthesized
by a two step solid state method6 using high purity Ce, CeO2,
Y2O3, CeF3, and FeAs as starting materials. FeAs was ob-
tained by reacting Fe chips and As powder at 800 °C for 24
h. The raw materials were taken according to stoichiometric
ratio and then sealed in evacuated silica ampoules
͑10−4 torr͒ and heated at 900 °C for 30 h. The powder was
then compacted ͑5 tons͒ and the disks were wrapped in Ta
foil, sealed in evacuated silica ampoules and heated at
1100 °C for 30 h. All chemical manipulations were per-
formed in a nitrogen-filled glove box. The samples were
characterized by powder x-ray diffraction with Cu-K␣ radia-
tion. Resistivity measurements were carried out using a
Cryogenic 8 T Cryogen-free magnet in conjunction with a
variable temperature insert. The inductive part of the mag-
Ce1-xYxO F0.2FeAs
0.8
*
1.5
x=0.4
1.0
0.5
0.0
16 24 32 40 48 56
x=0
&
T(K)
+
20
30
40
50
60
2
θ
FIG. 1. ͑Color online͒ Powder x-ray diffraction patterns of ͑a͒
Ce0.6Y0.4O0.8F0.2FeAs and ͑b͒ CeO0.8F0.2FeAs. The impurity phases are
Fe2As ͑+͒ and CeAs ͑&͒ for samples without Y and Y2O3 ͑*͒ for Y-doped
sample. Inset of figure shows resistivity plot for CeO0.8F0.2FeAs ͑᭺͒ and
Ce0.6Y0.4O0.8F0.2FeAs ͑b͒.
a͒
Authors to whom correspondence should be addressed. Electronic ad-
dresses: spatnaik@mail.jnu.ac.in and ashok@chemistry.iitd.ernet.in.
0003-6951/2009/95͑26͒/262507/3/$25.00
95, 262507-1
© 2009 American Institute of Physics
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262507-2
Prakash et al.
Appl. Phys. Lett. 95, 262507 ͑2009͒
1
0.0
1.2
0.03
H
10
c1
-0.5
0.00
4T
45
1.0
0
-0.03
0T
-1.0
10
4.2 K
0
12 24 36 48
0.0
0.8
µ0H0(T.4)
0.8
0.6
0.4
0.2
T(K)
4
3
2
1
0
-1
10
15 K
30 K
39 42 45 48
T(K)
-2
10
-3
10
35
40
50
0
1
3
4
5
20H(T)
Temperature (K)
µ
FIG. 2. ͑Color online͒ Temperature dependence of the electrical resistivity
of Ce0.6Y0.4O0.8F0.2FeAs under varying magnetic fields ͑0–4 T͒. Upper inset
shows the inductive part of susceptibility as a function of temperature and
the lower inset depicts the temperature dependence of upper critical field
͑Hc2͒ as a function of temperature for Ce0.6Y0.4O0.8F0.2FeAs ͑᭿͒ and
CeO0.8F0.2FeAs ͑᭺͒.
FIG. 3. ͑Color online͒ Magnetic field dependence of magnetization hyster-
esis loop width ͑⌬m͒ at 4.2, 15, and 30 K for Ce0.6Y0.4O0.8F0.2FeAs ͑closed
symbols͒ in comparison with CeO0.8F0.2FeAs ͑open symbols͒. Inset shows
the magnetization hysteresis loop at 4.2
Ce0.6Y0.4O0.8F0.2FeAs and defines procedure for determination of Hc1
K
for polycrystalline
.
field onset transition temperature for x=0.4 is estimated to be
48.6 K as compared to x=0 case where it is 42.7 K.9 Sur-
prisingly for the Ce͑O/F͒FeAs, the Tc is suppressed on in-
creasing external pressure and is lowered to 1.1 K at 265
kbar.10 On the contrary we find that the Tc of Ce͑O/F͒FeAs
enhances on increasing the chemical pressure by substituting
smaller Y3+ ion in place of Ce3+. The onset of diamagnetic
behavior below Tc is confirmed from the inductive part of rf
susceptibility as shown in the upper inset of Fig. 2. The onset
transition temperature ͑Tc͒ determined is 47.5 and 40.5 K for
Ce0.6Y0.4O0.8F0.2FeAs and CeO0.8F0.2FeAs, respectively. This
bulk magnetization shows a slightly lower Tc as compared to
the resistivity data. The residual resistivity ratio ͑RRR
specimen and 18 mg for CeYOFFeAs͒. A clear improvement
in magnetization loop width is observed for CeYOFFeAs
particularly at elevated temperature.
To analyze relative contribution from inter and intra-
granular regions to ⌬m͑H͒ and consequently to the corre-
sponding critical state current density, in Fig. 4 we show the
remanent moment as a function of increasing applied field.
The samples were studied ͑at T=5 K͒ under cycles of in-
creasing magnetic field followed by removal of the field and
measurement of remanent moment.3,13 When external field is
applied, first the flux enters into the intergranular region fol-
lowed by grain penetration above lower critical field Hc1.
The remanent moment equals the product of local current
density multiplied by current loop size and therefore a single
step variation mR versus H curve indicates predominant role
of intragrain supercurrents. We also observe that the rema-
nent moment deviates from zero at ϳ7 and 20 mT for
CeO0.8F0.2FeAs and Ce0.6Y0.4O0.8F0.2FeAs, respectively. This
implies some improvement in intergrain pinning as well in
the Y doped sample. In the inset of Fig. 4, we plot Hc1͑T͒
derived from the departure from linearity in low field M-H
loop ͑elucidated in the inset of Fig. 3͒. The technique is
=␳300/␳ ͒ is estimated to be 6.6 and 3.5 for CeO0.8F0.2FeAs
50
and Ce0.6Y0.4O0.8F0.2FeAs, respectively. We emphasize that
the onset of superconductivity in Ce0.6Y0.4O0.8F0.2FeAs rep-
resents the highest transition temperature in Ce-based oxyp-
nictides synthesized at ambient pressure.
The temperature dependence of the resistivity under
varying magnetic fields is shown in Fig. 2 for sample
Ce0.6Y0.4O0.8F0.2FeAs. Using a criterion of 90% of normal
state resistivity, the estimated Hc2-T phase diagram
for both samples is shown in lower inset of Fig. 2. The
Werthamer Helfand Hohenberg formula,11 gives Hc2͑0͒=
−0.693 Tc͑dHc2/dT͒T=Tc. The slope of dHc2/dT estimated
from the Hc2-T plots is Ϫ2.67 and Ϫ1.45 T/K for Y-doped
and without Y-doped samples, respectively. Correspondingly,
we find Hc2͑0͒ϳ90 T and 43 T for Ce0.6Y0.4O0.8F0.2FeAs
and CeO0.8F0.2FeAs respectively. Evidently, we are able to
significantly increase upper critical field by simultaneous
doping of yttrium and fluorine.12
0.012
3
2
0.009
1
0
0
10 20 30
T(K)
0.006
0.003
0.000
The inset in Fig. 3 shows a characteristic magnetization
loop for the sample Ce0.6Y0.4O0.8F0.2FeAs. Both samples
showed asymmetric M͑H͒ loop similar to polycrystalline La
iron oxypnictides.3 The loop width ͓⌬m͑H͔͒ is small, which
is a general feature of oxypnictides and could be due to ei-
ther poor intergrain connectivity or weak intragranular
pinning.3 The magnetic field dependence of ⌬m at 4.2, 15,
and 30 K for both samples are compared in Fig. 3. The
samples were of different shapes and we have normalized
⌬m by the weight of the specimen ͑14.8 mg for CeOFFeAs
T=5K
-3
-2
-1
0
10
10
10
10
µ0H(T)
FIG. 4. ͑Color online͒ The field-dependent remanent magnetization at 5 K
for both samples. The inset shows the temperature dependence of lower
critical field ͑Hc1͒.
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262507-3
Prakash et al.
Appl. Phys. Lett. 95, 262507 ͑2009͒
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mates Hc1, but there is definite relative increase in the lower
critical field for the Y doped sample.
In summary, we have demonstrated substantial enhance-
ment in transition temperature, critical fields, and magnetiza-
tion loop widths by simultaneous doping of Y in place of Ce
and F in place of O in semimetal CeOFeAs. This multitudi-
nous beneficial effect is caused by three independent param-
eters; higher chemical pressure, selective tuning of interband
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