IOP PUBLISHING SUPERCONDUCTOR SCIENCE AND TECHNOLOGY
Supercond. Sci. Technol. 22 (2009) 065015 (5pp) doi:10.1088/0953-2048/22/6/065015
Neutron irradiation of SmFeAsO
1−x
F
x
MEisterer
1
, H W Weber
1
, J Jiang
2
,JDWeiss
2
, A Yamamoto
2
,
A A Polyanskii
2
,EEHellstrom
2
and D C Larbalestier
2
1
Atominstitut der
¨
Osterreichischen Universit¨aten, Vienna University of Technology,
1020 Vienna, Austria
2
National High Magnetic Field Laboratory, Florida State University, Tallahassee,
FL 32310, USA
Received 2 February 2009, in final form 8 April 2009
Published 19 May 2009
Online at stacks.iop.org/SUST/22/065015
Abstract
SmFeAsO
1−x
F
x
was irradiated in a fission reactor by a fast (E > 0.1 MeV) neutron fluence of
4
×10
21
m
−2
. The introduced defects increased the normal state resistivity due to a reduction in
the mean free path of the charge carriers. This leads to an enhancement of the upper critical
field at low temperatures. The critical current density within the grains,
J
c
, increases upon
irradiation. The second maximum in the field dependence of
J
c
disappears and the critical
current density becomes a monotonically decreasing function of the applied magnetic field.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
The discovery of superconductivity in the iron pnictides [1]
is interesting from a theoretical point of view, but this new
class of superconductors could also become important for
applications. The fundamental superconducting parameters
are promising. The transition temperature reaches about
55 K [2–4], which is not as high as in most cuprates but
is significantly higher than in the technologically relevant
superconductors NbTi and Nb
3
Sn or in MgB
2
. The upper
critical field, B
c2
, is extremely large (>50 T) [5–10]and
thermal fluctuations seem to be less important than in the
cuprates [7, 11], where loss free currents are restricted to
fields far below the upper critical field, at least at elevated
temperatures. Zero resistivity was demonstrated at fields close
to
B
c2
(T ) in pnictide single crystals [8].
Irradiation techniques are a powerful tool for assessing
the influence of defects on superconductors, because they
allow one to investigate the same sample prior to and after
the irradiation, which excludes problems of sample to sample
variations. In particular, neutron irradiation was used in
extended studies of the influence of disorder in MgB
2
[12–15],
including the demonstration of the disappearance of two band
superconductivity due to interband scattering at high levels
of disorder [16]. An increase in the upper critical field and
a reduction of its anisotropy were reported and are expected
theoretically. A similar behaviour of
B
c2
was also found in
the A15 compounds [17] and a reduction of anisotropy was
reported for the cuprates (Hg-1201) [18].
The neutron-induced defects are also highly suitable for
investigating flux pinning. This was done successfully in
V
3
Si [19], the cuprates [20, 21]andMgB
2
[22].
The present contribution reports on a first neutron irradi-
ation experiment with the new FeAs based superconductors.
Changes of the reversible and irreversible superconducting
properties were found to be similar to other superconducting
materials.
2. Experimental details
The SmFeAsO
1−x
F
x
sample was prepared at the National High
Magnetic Field Laboratory, Florida State University. The
starting materials of As, Sm, Fe, Fe
2
O
3
and SmF
3
were mixed
and pressed into a pellet, wrapped with Nb foil and sealed in
a stainless steel tube. The sealed sample was heat treated at
1160
◦
C for 6 h in a high temperature isostatic press under
a pressure of 280 MPa. The main phase of the sample is
SmFeAsO
1−x
F
x
, with a grain size of 10–15 μm. The impurity
phases include SmAs, SmOF and FeAs. The size of the sample
used for this study was 1
.1 ×1.7 ×3.7mm
3
.
Neutron irradiation was performed in the central
irradiation facility of the TRIGA-Mark-II reactor at the Atomic
Institute in Vienna. The sample was sealed into a quartz
tube and exposed to the neutron flux for 14 h and 38 min,
corresponding to a fast (
E > 0.1 MeV), thermal (E <
0.55 eV) and total neutron fluence of 4 × 10
21
m
−2
,3.2 ×
10
21
m
−2
and 1.1 × 10
22
m
−2
, respectively [23]. Neutrons
transfer their energy to the lattice atoms by direct collisions.
The transferred energy must exceed the binding energy of
0953-2048/09/065015+05$30.00 © 2009 IOP Publishing Ltd Printed in the UK1
Supercond. Sci. Technol. 22 (2009) 065015 M Eisterer et al
the lattice atom to displace it, thus only fast neutrons lead
to defects. No indirect defect-producing mechanism (e.g. an
induced
α-emission in MgB
2
[14, 24] or a fission reaction in
uranium doped YBa
2
Cu
3
O
7−δ
[25]) exists.
The smallest resulting defects are single displaced atoms
(point defects), but larger defects might also occur. In high
temperature superconductors, the largest defects are so-called
collision cascades [26]. These spherical defects are amorphous
with a diameter of 2–3 nm; the surrounding strain field
enlarges the defect to about 5 nm. Similar defects were
also found in MgB
2
[22, 27]. The actual size, morphology
and density of the defects in SmFeAsO
1−x
F
x
are currently
unknown. However, the defects should be randomly generated,
leading to a homogeneous defect density on a macroscopic
length scale. Self-shielding effects can be neglected, since
the penetration depth of fast neutrons is estimated to be
a few centimetres, which is much larger than the sample
dimensions. Only neutrons of low or intermediate energies are
shielded efficiently because of the large neutron cross section
of samarium at these energies, but the corresponding reactions
or collisions are not expected to produce any defects.
The resistivity was measured at various fixed fields while
cooling at a rate of 10 K h
−1
with an applied current of 10 mA.
Current and voltage contacts were made by silver paste. The
distance between the two voltage contacts was about 1 mm.
The transition temperature at each field,
T
c
(B),was
defined as the temperature where the resistivity drops to
0
.95ρ
n
(T ) (ρ
n
(T ) was extrapolated linearly from its behaviour
between 55 and 60 K). The upper critical field,
B
c2
(T ),was
obtained by inversion of
T
c
(B). The transition width, T ,
is defined by the difference between
T
c
and the temperature
where
ρ(T ) becomes 0.05ρ
n
(T ).
Magnetization loops at various temperatures were
recorded in a commercial 7 T SQUID magnetometer. The field
was always oriented parallel to the smallest sample dimension
(transverse geometry). The ac susceptibility at 33 Hz was
measured with an amplitude of 30
μT at various fields and
temperatures in order to estimate the shielding fraction. The
demagnetization factor was calculated numerically for the
actual sample geometry.
3. Results and discussion
The transition temperature decreases after irradiation, from
53.6 to 53.2 K, which is similar to the decrease found
in YBa
2
Cu
3
O
7−δ
[28] but less than in the thallium based
cuprate superconductors [29] at the same fluence. This
decrease is ascribed to d-wave superconductivity in the
cuprates [30, 31], the reason for the reduction in
T
c
of
SmFeAsO
1−x
F
x
is currently unknown. A moderate amount
of non-magnetic impurities does not reduce
T
c
in isotropic
single band superconductors, thus anisotropy [32–34]or
multiband superconductivity [35–38] are possible candidates
for explaining this decrease. Recently, a suppression of
T
c
by impurity scattering was also predicted for extended s-wave
superconductors [39].
The transition width is rather large (
T = 3.8Kat0T)
and significantly broadens in magnetic fields, as can be seen
Figure 1. Resistive transitions prior to and after neutron irradiation.
The inset shows the zero field transition normalized to the voltage at
55 K. The transition broadens with magnetic field, but no broadening
results from the irradiation. The resistance is significantly enhanced
by irradiation.
in figure 1. It remains unchanged after irradiation, which
confirms the homogeneous distribution of the introduced
defects.
The measured resistivity increases due to the introduction
of the defects, from about 235 to 355
μ cm (∼50%) at 55 K.
We also observe an increase of the phonon contribution to
the resistivity between 50 and 300 K,
ρ := ρ
n
(300 K) −
ρ
n
(55 K), from 470 to 650 μ cm, which is expected
to remain unchanged in single band conductors but can be
altered in multiband conductors [40] (e.g. SmFeAsO
1−x
F
x
).
Alternatively, the increase in
ρ could result from a reduced
connectivity [41] or simply from changes in the distance
between the voltage contacts, which had to be removed for
the irradiation and renewed afterwards. A simple correction
of these effects (keeping
ρ constant) reduces the increase
in resistivity due to the irradiation to about 10%. The
experimental accuracy (distance between voltage contacts)
does not allow a final conclusion about the absolute change of
the resistivity or which scenario applies. A reliable indication
of the change in resistivity is given by
ρ
n
(300 K)/ρ
n
(55 K),
which is independent of possible geometrical errors and which
decreases from 3.03 to 2.83. Thus, we conclude that the
residual resistivity is enhanced. Note that the resistivity
is strongly temperature dependent above
T
c
. Therefore,
the residual resistivity
ρ
0
is smaller than ρ
n
(55 K) but
cannot be assessed. The residual resistivity ratio (RRR
=
ρ
n
(300 K)/ρ
0
) is certainly larger and might be changed much
more significantly due to the irradiation. This is also true for
the relative change in residual resistivity. We emphasize that
the resistivity and the RRR might be influenced by the impurity
phases.
A decrease in the mean free path of the charge carriers
is expected to result in an increase of the upper critical
field. Indeed, the slope
−dB
c2
/dT increases after irradiation
(figure 2), but this cannot compensate for the small reduction
(
∼0.4 K) of the transition temperature in the investigated field
range (
15 T). However, the enhanced slope indicates that the
2
Supercond. Sci. Technol. 22 (2009) 065015 M Eisterer et al
Figure 2. Change of the upper critical field after neutron irradiation.
upper critical field will be enhanced at low temperatures. The
slope is nearly constant above 6 T (
−dB
c2
/dT = 7.5TK
−1
)
before the irradiation and a positive curvature can be observed
up to 15 T after the irradiation, with a maximum slope of
10.7 T K
−1
between 12 and 15 T. This corresponds to an
enhancement by about 40%. Note that this enhancement
is incompatible with d-wave superconductivity, for which
impurity scattering is predicted to decrease
−dB
c2
/dT [42].
The hysteresis loop at 5 K prior to and after neutron
irradiation is plotted in the upper panel of figure 3.
The paramagnetic background, which arises most probably
from impurity phases, slightly decreases, but the hysteresis
increases. The latter indicates an increase in the critical
current densities, which are estimated from the Bean model
in the lower panel. Zero intergranular currents (see below)
were assumed and cuboidal grains with typical dimensions
of 10
μm, as found by backscattered electron imaging. The
solid and open symbols refer to the unirradiated and irradiated
state, respectively. The critical current density,
J
c
, increases
more significantly at higher temperatures and the ‘fishtail’
effect [5, 43] disappears after neutron irradiation, i.e.
J
c
decreases monotonically with field. This is strikingly similar
to the changes found in single crystalline [20, 44]andmelt
textured [45, 46] cuprate superconductors and indicates that
pinning in the unirradiated sample is between weak (ordered
flux line lattice) and strong (disordered flux line lattice)
pinning. Thus, an order–disorder transition takes place at
intermediate fields which leads to the second peak (or fishtail
effect) in the magnetization curve [22, 47–50]. The neutron-
induced defects are strong enough to deform the flux line
lattice plastically over the whole field range. However, the
defects resulting from neutron irradiation are usually not the
most efficient pinning centres. For instance, the critical current
densities in neutron-irradiated cuprates are approximately one
order of magnitude smaller than the highest reported values
(in films [51], or in single crystals containing columnar
defects [52]). Thus, the critical currents should be more
efficiently improved in SmFeAsO
1−x
F
x
by the addition of
stronger pinning centres.
The interpretation of the irreversible magnetic moment,
m
irr
,intermsofJ
c
is quite delicate, since it is a
Figure 3. The width of the hysteresis loop increases after irradiation
(upper panel) and the ‘fishtail’ effect disappears (lower panel).
Figure 4. Optical (left) and magneto-optical (right) image of a
reference sample at 6 K and 60 mT after zero field cooling. No
global Bean profile is established, showing that the intergranular
currents are negligible.
priori unknown, whether m
irr
is predominantly generated by
(intergranular) currents flowing around the whole sample or
by (intragranular) currents shielding individual grains only.
We observed a pronounced magnetic granularity by magneto-
optical imaging, with no global Bean profile (figure 4, right).
Thus, it is safe to assume that the irreversible magnetic moment
is made up almost entirely by intragranular currents. This
behaviour is in marked contrast to that seen in a high pressure
treated Sm-1111 sample where clear evidence was seen of the
global current flow [43, 53].
This is also demonstrated by ac susceptibility measure-
ments. Diamagnetic shielding was nearly perfect in the present
3
Supercond. Sci. Technol. 22 (2009) 065015 M Eisterer et al
Figure 5. The ac susceptibility decreases rapidly with increasing
magnetic field, since the grains decouple.
sample at zero magnetic field. The data in figure 5 refer
to 5 K. The susceptibility was calculated by assuming a
demagnetization factor of 0.58. The slightly smaller value at
zero applied dc field (
−0.83 instead of −1 in the ideal case) can
result either from a residual magnetic field or from geometrical
imperfections. The ac susceptibility decreases by a factor of
nearly 2 by applying a dc field of only 60 mT and remains
constant at higher fields. The rapid decrease at low fields
reflects the decoupling of the individual grains and the small ac
field penetrates the whole sample along the grain boundaries
at fields above about 60 mT, when the intergranular currents
become negligible.
4. Conclusions
The introduction of disorder by fast neutron irradiation
enhances the normal state resistivity and the upper critical
field at low temperatures. Efficient flux pinning centres
are introduced, which enhance
J
c
. Like in the cuprates,
the second maximum (‘fishtail’) in the field dependence of
the critical current disappears, implying that the irradiation-
induced defects distort the flux line lattice plastically at all
magnetic fields.
Acknowledgments
We are grateful to A Gurevich, M Putti, C Tarantini and
F Kametani for discussions. Work at the NHMFL was
supported by NSF, the State of Florida and AFOSR.
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