Magnetization and characteristic fields in Nd0.8Ba 0.2FeAsO0.6F0.4 with binary doping

Article abstract of DOI:10.1016/j.ssc.2011.03.032

In the present work, the iron oxypnictide superconductor of the namely Nd_0.8Ba_0.2FeAsO_0.6F_0.4 is prepared with respect to the observation of distinct magnetization characteristics arising from both electron and

Full text of DOI:10.1016/j.ssc.2011.03.032

Solid State Communications 151 (2011) 956–959
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Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Magnetization and characteristic fields in Nd Ba FeAsO F with
0.8
0.2
0.6 0.4
binary doping
C.Q. Qua,c, C.B. Cai , M.F. Wang , C.Z. Chen , F.Q. Huang
a,∗
c
a
b
a
Research Center for Superconductors and Applied Technologies, Physics Department, Shanghai University, Shanghai 200444, China
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences,
b
c
Shanghai 200050, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present work, the iron oxypnictide superconductor of the namely Nd0.8Ba0.2FeAsO0.6F0.4 is prepared
with respect to the observation of distinct magnetization characteristics arising from both electron and
hole doping. A magnetothermal phase diagram is given for the present iron oxypnictide system, based
on the irreversibility fields (Hirr ) and the upper critical fields (Hc2) obtained from magnetotransport
measurements, as well as the lower critical fields (Hc1) evaluated by magnetization loops at various
temperatures. High Hc2(0) are revealed at low temperature range, which is in consistent with the
observations of comparatively high critical current (Jc ) and the temperature dependent peak effect in
the Jc vs H curves.
Received 29 January 2011
Received in revised form
2
4 March 2011
Accepted 30 March 2011
by P. Chaddah
Available online 8 April 2011
Keywords:
A. high-Tc superconductor
D. Flux pinning and creep
©
2011 Elsevier Ltd. All rights reserved.
1
. Introduction
0.2) [11]. Temperature dependences of electric resistivity and mag-
netic susceptibility reveal that the superconductivity for the stud-
Among the family of oxypnictide based superconductors, the
ied system emerges at x = 0.1, and enhances together with H (0)
c2
RFeAsO1−y (FeAs-1111, R is the rare earth element) exhibits a
as the doping content x increases further. In case of x = 0.2, the su-
relatively high superconducting critical temperature, reaching
perconducting critical temperature reaches as high as 50 K, which
is the first demonstration of superconductivity with a high fluorine
doping induced by both electron and hole doping in this family.
In the present paper, we focus on the sample of x = 0.2, which
5
5 K [1] in the case of electron doping such as SmFeAsO1−xFx.
The parent compounds of both 1111 and 122 type, normally show
the magnetic order phase of spin density wave (SDW), but no
superconductivity [2–4]. They may evolve into superconductors
as a consequence of the normative symmetry breaking of 1111
type SDW due to the element doping, such as SmFe0.9Co0.1AsO [5],
Ca1−xNaxFe2As2 [6], and so on. Noting the iron site doping generally
suppresses the critical transition temperature, and enhances the
disorder of magnetism as well.
exhibits the highest T in Nd Ba FeAsO
c
1−x
x
1−2x 2x
series. A lot of
F
efforts have been made to understand its distinct magnetization
behaviors and magnetothermal phase diagrams, together with
various characteristic magnetic fields. Note that such a special
system with both electron-doping and hole-doping may allow
more breaking of original symmetry in composite and structure,
giving rise to more complicated superconducting and magnetic
behaviors.
Apart from one element or one site substitution, people also
suggest that the superconductivity is tailorable by the second
element doping in other site simultaneously [7–9], such as
Ce1−xGdxFeAsO0.84F0.16 [7], and SmFe1−xRuxAsO0.85F0.1 [8]. It is
reported that the second site of doping could enhance transition
temperature as well as the upper critical field [10].
We recently look into the effect of binary doping on the perfor-
mances of the oxypnictide superconductors, Nd1−xBaxFeAsO1−2xF2x
at a series of doping contents (x = 0.02, 0.05, 0.1, 0.15, and
2. Experimental
Polycrystalline bulk of nominal Nd0.8Ba0.2FeAsO0.6F0.4 was
synthesized by a conventional solid state reaction. Firstly, the
intermediate phases of Fe2As, NdAs and FeO were prepared by the
reactions of the mixed commercial powders of Nd, Fe, As and Fe2O3,
sealed in a vacuum quartz tube, annealed at 600 °C for 6 h and
then 850 °C for 12 h. After that, the pressed pellets were sealed in
a vacuum quartz tube for final annealing at 1160 °C. More details
for sample preparation can be found in the Ref. [12].
∗
Corresponding author.
E-mail address: cbcai@staff.shu.edu.cn (C.B. Cai).
0
038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2011.03.032

C.Q. Qu et al. / Solid State Communications 151 (2011) 956–959
957
Fig. 1. (Color online) X-ray diffraction pattern for the Nd0.8Ba0.2FeAsO0.6F0.4
compound with its lattice parameters.
Fig. 3. (Color online) The magnetization curves M(H) of the present sample of
Nd0.8Ba0.2FeAsO0.6F0.4 at various temperatures from 4 to 80 K.
x = 0.2. The resistivity exhibits the linear behavior above Tc , and
the residual resistance ratio (RRR = R(300 K)/R(Tc ) reaches 5.5,
demonstrating the good quality of the present sample.
The inset of Fig. 2 shows the temperature dependences of
electric resistance in various applied magnetic fields. The super-
conducting transition temperature is determined by using a cri-
terion [13] at which the resistivity becomes negligible (about
−
4
1
0
Ohm). It is found that the transition temperature de-
creases with applied magnetic field, roughly at a slope rate of
.85 K/T, i.e. dTc /dH—0.85 K/T, in contrast to 1 K/T of dTc /dH for
0
SmFe0.85Co0.15AsO [13]. Obviously, the latter is larger than the for-
mer, implying the higher Hc2 and stronger pinning effect probably
existing in the present sample.
T
As well, this value is much lower than that of YBCO (dTc
/
dH—4 K/T) and MgB2 (dTc /dH—1.5 K/T) samples [14,15], suggest-
ing a high value of upper critical field Hc2 in these compounds [16].
One may suppose that the oxypnictides appear as a new class of
high field superconductors with Hc2 surpassing the corresponding
values of Nb Sn and MgB . In reality, the upper critical magnetic
Fig. 2. (Color online) Temperature dependence of the resistivity for the present
sample of Nd0.8Ba0.2FeAsO0.6F0.4, the arrow indicates the critical transition
temperature. The inset shows the temperature dependence of the resistance in
various applied magnetic fields up to 9 T.
3
2
fields of the iron oxypnictides superconductors may surpass the
1
00 T, a benchmark of the high Tc cuprate superconductors. The
values of Hc2 and Hirr for the present sample will be evaluated and
The superconducting properties were measured by using a
physical property measurement system (Quantum Design PPMS)
in magnetic fields up to 9 T. For the magnetization measurements,
rectangular specimens were prepared nearly in the same dimen-
discussed in the section below.
3
.2. Critical field and critical current
3
sion of about 6 × 2 × 1 mm , and the applied magnetic field was
Fig. 3 shows the magnetic hysteresis curves measured at
normal to the long axis direction of the specimen.
temperatures of from 4 to 80 K in the field of −0.5 T
<
H < 0.5 T. The experimental M(H) curves can be understood
as the superposition of a superconducting contribution and a
paramagnetic background emerges above Tc . The arrow indicates
the characteristic magnetic fields, Hc1, which are about 1200 Oe
at 4 K. Note that the peak of magnetization is broadened with the
decreasing of the temperature.
3. Result and discussion
3
.1. Magnetotransport performances
Fig. 1 shows the room temperature X-ray diffraction (XRD)
patterns for the present sample of Nd0.8Ba0.2FeAsO0.6F0.4 alone
with their Rietveld refinement. It is observed that all main peaks
are well indexed based on their basis of space group P4/nmm,
It is nontrivial to notice that the M vs. H is not symmetric,
with the peak deviation from the y axis. This scenario is frequently
observed in iron oxypnictides superconductors, not only in single
crystal [17] but also in polycrystalline samples [13,18,19]. Another
feature is the peak deflection from the origin point [13,20] after
deleting the paramagnetic background determined by M vs. H at
the temperatures above Tc .
◦
◦
although little extra peaks at around 30 and 49 of low intensity
(
marked with ∗). Based on the index, the lattice parameters for
such a compound, is calculated to be a = 3.962(7) and c =
8
.536(0).
Fig. 2 shows the superconducting critical temperature, identi-
Fig. 4 shows the temperature dependence of Hc1. One can find
the value surpasses 0.1 T at 4 K taken above. The values of Hc1
decreasing with the increasing of temperature can be fitted well
fied from the R vs. T curves. It is revealed that the present sample
exhibits the superconductivity 50 K at x = 0.2. Moreover, the su-
perconducting transition width 1T, determined by the tempera-
tures corresponding to 90% and 10% of the normal state resistivity,
decreases with increasing doping content, being as low as 1.86 K at
∗
β
by using the expression of H (T) = H (0) (1 − T/T ) .
irr
irr
c
Based on the magnetotransport measurements (shown in the
inset of Fig. 2), the other two characteristic magnetic fields,

9
58
C.Q. Qu et al. / Solid State Communications 151 (2011) 956–959
Fig. 4. (Color online) The temperature dependence of Hc1 evaluated from M(H).
The inset shows the phase diagram characterized by the temperature dependences
of Hc2, Hirr and Hc1.
Fig. 6. (Color online) The field dependence of the critical current density at 20 K
estimated from magnetization curves. Peak effect is marked with arrow.
Fig. 7. (Color online) The critical current density vs. the applied magnetic field up
to 5000 Oe at several given temperatures. It is revealed that the peak position shifts
towards the low magnetic field with increasing the temperature, until it disappears
near the Tc .
Fig. 5. (Color online) The extended temperature dependences of Hc1, Hirr and Hc2
at a wide range of temperatures, based on the values fitted by the formula of
2
∗
β
1−(T/Tc )
Hirr(T) = Hirr(0) (1 − T/Tc
)
and the G–L formula of Hc2(T) = Hc2(0) 1
.
2
+(T/Tc )
1
62.5 T. This value is very close to the 162.8 T deduced by the
irreversibility fields (Hirr) and upper critical fields (Hc2) are deter-
mined by the criterion of 10% and 90% of normal state resistance,
respectively. Noting that similar evaluation is widely applied to
YBaCuO, MgB2, the Fluorin doped LaFeAsO and Strontium doped
PrFeAsO polycrystalline samples [2,21,22]. The inset shows the re-
lations between the reduced temperature and characteristic mag-
netic fields including Hc1, Hirr and Hc2. It is revealed that the Hirr(T)
curve is close to the Hc2(T) curve, implying that the strong flux pin-
ning effect hinders the dissipating resistance due to flux jumping.
To further investigate the various characteristic magnetic fields
and their relation, temperature dependences are taken into ac-
count through the whole temperature range below the critical
transition temperature. The results are shown in Fig. 5, where the
Hirr(0) and Hc2(0) are evaluated by the formula below, respec-
tively.
Werthamer–Helfand–Hohenberg theory, the second formula. This
suggests that the present sample exhibits good quality, regardless
of lower than corresponding values for iron oxypnictides 42 622
family [23] and single step synthesized NdFeAsO0.8F0.2 [24]. How-
ever, the slope of dHc2(T)/dT|
T
c
for the present sample is much
larger than that of hole doped Pr1−xSrxFeAsO [2], and the electron
doped LaFeASO1−xFx [21,22].
Also one can get Hirr(0) of 146 T from the third formula with
β = 1.42 which corresponds to the value (β) of YBaCuO [25,26],
and this value is higher than that of Sr(Fe1−xCox 2As2−y with Hirr(0)
)
of 70 T, but the value of β is consistent, while remaining lower than
that (β ∼ 2) predicted by the melting theory [27].
To further understand the pinning characteristic of present
sample, the critical current density in various magnetic fields is
studied as well. Fig. 6 shows the magnetic field dependence of
2
1
1
− (T/Tc
+ (T/Tc
)
magnetization critical current (J ) estimated from magnetization
curves and the conventional critical state model, i.e. Bean model.
c
Hc2(T) = Hc2(0)
(1)
2
)


The center deviated peaks occur for all the magnetization curves.
As marked by an arrow, a typical peak appears near 3.2 T in the
case of the applied temperature 20 K.
Fig. 7 shows the magnetic Jc derived from the magnetic
hysteresis vs. the applied magnetic field up to 5000 Oe at several
given temperatures. Noting that the maximum applied field is
lower than that of at 20 K in Fig. 6, to look into the low field
dHc2
Hc2(0) = −0.693Tc
(2)
(3)
dT
T=Tc
∗
β
Hirr(T) = Hirr(0) (1 − T/Tc ) .
The first one follows the Ginzburg–Landau theory, with which
the value of upper critical field Hc2(0) is evaluated to be

C.Q. Qu et al. / Solid State Communications 151 (2011) 956–959
959
effect purposely. It is obvious that a distinct peak is present, and
its position shifts towards the low magnetic field with increasing
the temperatures, until it disappears near the Tc . This should be
reasonable as the irreversibility fields decreases, and thus the
pinging effect occurs at a relatively low field with increasing
temperatures.
Shanghai Leading Academic Discipline Project (No. S30105).
Also the support from the Knowledge Innovation Program
of the Chinese Academy of Sciences (Physical Properties and
Mechanism of Iron based Superconductors), the Open Project of
State Key Laboratory of Functional Materials for Informatics and
Shanghai Fundamental Research (No. 10JC1415800) is gratefully
acknowledged.
As for the origination of peak effects, one may attribute it
to the flux pinning, either from the coexisting weak magnetic
heterogeneous phases which are probably most effective at
the intermediate fields or from the high density defects or
nonsuperconducting phases being enhanced at matching induced
magnetic fields. For the other oxypnictide based superconductors,
there are few reports with regard to the peak effect. At present it
is unclear whether or not this is due to the peak effect as observed
in most bulks of high Tc cuprates [28].
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This work is partly sponsored by the Science and Technology
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