Magnetic characterization of undoped and 15%F-doped LaFeAsO and SmFeAsO compounds

Article abstract of DOI:10.1016/j.jmmm.2009.04.084

In this paper, the magnetic behavior of undoped and 15%F-doped SmFeAsO (Sm-1111) and LaFeAsO (La-1111) samples is presented and discussed. Magnetization measurements are not a simple tool to use for the characterization of the new family of Fe-based super

Full text of DOI:10.1016/j.jmmm.2009.04.084

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Journal of Magnetism and Magnetic Materials 321 (2009) 3024–3030
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Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Magnetic characterization of undoped and 15%F-doped LaFeAsO and
SmFeAsO compounds
Ã
M.R. Cimberle a, , F. Canepa a,c, M. Ferretti b,c, A. Martinelli b, A. Palenzona b,c, A.S. Siri b,d, C. Tarantini e,
M. Tropeano b,d, C. Ferdeghini b
a CNR-IMEM via Dodecaneso 33, 16146 Genova, Italy
b CNR-INFM-LAMIA Corso Perrone 24, 16152 Genova, Italy
c
`
Dipartimento di Chimica e Chimica Industriale, Universita di Genova via Dodecaneso 31, 16146 Genova, Italy
d
`
Dipartimento di Fisica, Universita di Genova via Dodecaneso 33, 16146 Genova, Italy
e National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, the magnetic behavior of undoped and 15%F-doped SmFeAsO (Sm-1111) and LaFeAsO
(La-1111) samples is presented and discussed. Magnetization measurements are not a simple tool to use
for the characterization of the new family of Fe-based superconductors, because magnetic impurities
can be easily formed during the preparation procedure and may affect the magnetic signal. In spite of
this problem bulk magnetization measurements, properly treated, may give very useful information. In
the undoped samples we gathered the main aspects of the physical behavior of the 1111 phase, i.e. the
onset of the spin density wave (SDW), the antiferromagnetic ordering at the Sm sublattice and the
susceptibility increase with increasing temperature above the SDW temperature, and, in addition, we
were able to estimate the Pauli contribution to susceptibility and therein the Wilson ratio both for
LaFeAsO and SmFeAsO compounds, and the amplitude of the jump at the SDW temperature. In the
doped samples, while the presence of magnetic signals due to impurities is dominating in the normal
state, the superconducting behavior may be clearly observed and studied. In particular, in the Sm-1111
superconducting sample the coexistence-competition between superconductivity and antiferromag-
netic ordering of the Sm ions was clearly observed.
Received 16 April 2009
Received in revised form
22 April 2009
Available online 8 May 2009
PACS:
74.70.Dd
75.30.Cr
75.30.Fv
Keywords:
Iron-based oxy-pnictide
Magnetic measurement
Superconductivity
& 2009 Elsevier B.V. All rights reserved.
1. Introduction
remarkable theoretical effort devoted to the comprehension of the
superconductivity mechanism and of the possible coexistence of
superconductivity and magnetism.
The recent discovery of superconductivity at temperatures up
to Tc ¼ 26 K in the iron oxy-pnictide LaFeAs(O1ꢀxFx) [1] has
stimulated a lot of work in this field. In a short period of time
different approaches were attempted to increase the transition
temperature. On the one hand, La was substituted with other rare
earth (RE) with smaller ionic radius (such as Pr, Nd, Ce, Sm, Gd) to
induce chemical pressure [2–4]; on the other hand, the optimal
doping for the superconducting phase was studied and obtained
with a variety of different techniques, i.e. F substitution at the O
site [5], O deficiency [6], and partial substitution on the RE site
with bi- and tetra-valent cationic species [7,8]. By combining
these approaches, Tc was increased up to a considerable value of
about 55 K [2–4,7,9]. Simultaneously, many different structural
and physical characterizations were performed, together with a
These materials present a layered structure with Fe–As layers
alternating to RE–O ones. The parent compound displays a
structural distortion from tetragonal to orthorhombic crystal
symmetry coupled with a spin density wave (SDW) antiferro-
magnetic order which occurs in the Fe sublattice; this ordering is
reflected in all physical properties (i.e. resistivity, specific heat,
Hall effect, and far-infrared reflectance), which all suggest the
opening of an energy gap [9].
After doping, the antiferromagnetic ordering as well as the
structural distortion is suppressed, and the compound exhibits a
metallic behavior down to the superconducting transition tem-
perature. Whether magnetic fluctuations are involved in the
development of superconductivity, it is an open issue that could
give new insight on the analogous problem related to high Tc
superconductors. Moreover, if and what type of magnetism is
present in the doped samples, both in the normal and super-
conducting state, is the object of a lively debate. In addition, when
La is substituted with other RE (Pr, Nd, Sm, and Gd) another source
Ã
Autor para correspondencia. M.R. Cimberle, IMEM-CNR c/o Dipartimento di
Fisica dell’Universita` di Genova-via Dodecaneso 33, 16146 Genova, Italy.
Tel.: +39 0103536448; fax: +39 0103622790.
E-mail address: cimberle@fisica.unige.it (M.R. Cimberle).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.04.084

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of magnetism is introduced, although in the RE–O (charge
reservoir) layers and not in the superconducting (Fe–As) ones.
The study of the oxy-pnictides by magnetic measurements,
commonly used to characterize new materials, is made compli-
cated by the simultaneous presence of many magnetic signals due
to: (1) magnetic impurities that, even if in small amounts, can give
a more or less large signal that is added to the magnetic behavior
of the studied phase, (2) the diamagnetism of the superconduct-
ing phase and (3) the presence of a magnetic rare earth that
substitutes La, whose magnetic signal may be dominant.
For the last reason the feature that, in the undoped samples,
marks the antiferromagnetic ordering due to the onset of the SDW
is rarely shown in the literature by magnetization data. Recently
this feature has been extracted from magnetic measurements on
Ce-, Pr- and Nd-based oxy-pnictides by subtraction of the
paramagnetic signal due to the RE sublattice from the total signal
[10].
In this paper, we present the structural and magnetic
characterization of both undoped and 15%F-doped SmFeAsO and
LaFeAsO samples. In the undoped samples, after the subtraction of
a ferromagnetic background signal due to the impurities, all the
important characteristics of the 1111 phases both for La- and Sm-
based compounds were observed: in particular the paramagnetic
behavior of the Sm ions sublattice was fitted and this allowed to
value both the amplitude of the step at the spin density wave
onset, both the values of the Pauli susceptibility for the phase. In
the doped samples, on the other side, paramagnetic or anti-
ferromagnetic impurities are present, which make uncertain to
extract the magnetic behavior of the samples in the normal state.
However, important information regarding the interplay between
superconductivity and antiferromagnetism of Sm ions sublattice
is obtained and discussed.
2. Sample preparation and characterization
Both undoped and 15%F-doped SmFeAsO and LaFeAsO were
prepared in two steps: (1) synthesis of REAs starting from pure
elements in an evacuated pyrex tube at a maximum temperature
of 550 1C for five days; then (2) synthesis of the oxy-pnictide in
tantalum crucible in evacuated quartz tube by reaction of REAs
with stoichiometric amounts of Fe, Fe2O3 and, for the doped
compounds, FeF2. The sintering step was 30 h at 1250 1C for Sm
phases and at 1150 1C for La phases. Details concerning the
preparation are reported in [11].
Fig. 1. Comparison between the XRPD patterns of the pure and 15%F-doped
LaFeAsO (on the left) and SmFeAsO (on the right) oxy-pnictides; both F-doped
samples are characterized by the presence of the respective rare-earth oxy-fluoride
(arrowed).
The samples were characterized by X-ray powder diffraction
(XRPD) and by Scanning Electron Microscopy (SEM) equipped
with an energy dispersive X-ray microprobe (EDS). Fig. 1 shows
the XRPD patterns of all the samples. The XRPD analysis confirms
the formation of both REFeAsO and REFeAs(O0.85F0.15) phases with
RE ¼ Sm and La. Some spurious peaks are present in the doped
samples, originated by the corresponding rare-earth oxy-
fluorides. In Sm-undoped sample no impurity phase was
detected by X-ray diffraction analysis, whereas SEM–EDS
analysis indicates the presence of FeAs in traces. In the doped
sample by Rietveld refinement of X-ray diffraction data the
presence of both SmOF (4.6%) and FeAs(5.6%) was evaluated.
Both undoped and doped Sm samples were constituted of
connected micrometric crystals, as SEM observation revealed.
After F-substitution the cell size contracts, with a more enhanced
decrease along the c-axis.
pnictide, similarly to what observed in Sm-1111 samples. In
conclusion the doped samples show the presence of the rare-earth
oxy-fluoride, together with iron arsenide compounds. In any case
more than 90% of the samples here presented corresponds to the
right oxy-pnictide phase.
The resistivities of the La- and Sm-based samples, normalized
to their values at 300 K, are reported in Fig. 2(a) and (b),
respectively. The parent samples exhibit well-defined maxima at
the formation of the SDW. These maxima are at T ¼ 147 and 159 K
(while the dr/dT maxima are at T ¼ 131 and 138 K) for Sm- and
La-1111 phases, respectively. Below the SDW temperature the
resistivity decreases. In Sm-1111 phase the resistivity decrease
continues down to the lowest temperature we reached (400 mK):
below about 6 K an enhanced drop is clearly visible. In the
following, we will correlate this drop to the susceptibility
measurements and, in particular, to the establishment of the
antiferromagnetic ordering of the Sm ions sublattice. In La-1111
phase below about 30 K a slight resistivity increase is present:
such behavior has been attributed to carrier localization arising
from SDW gap [12]. The further decrease that we observe below
10 K is difficult to explain: we cannot exclude that can be due to
In LaFeAsO sample metallic iron is detected by SEM–EDS
analysis, inside which some As is dissolved (5%). On the other side
the presence of both LaOF and Fe2As as secondary phases is
detected in the doped sample. Both pure and doped samples are
mainly constituted by aggregated microcrystals of the oxy-

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Fig. 3. Molar susceptibility versus temperature of LaFeAsO and SmFeAsO samples
in the range 2–360 K. The applied field is 30 kOe. Upper-right inset: magnetization
versus field at T ¼ 5 and 300 K for SmFeAsO. Lower-left inset: magnetization
versus field at T ¼ 5 and 300 K for LaFeAsO.
Fig. 2. Resistivity versus temperature measurements of undoped and 15%F-doped
LaFeAsO and SmFeAsO compounds.
lations ( pseudo-gap behavior) of Fe ions in the Fe–As layers
[14,15]. The susceptibility increase at low temperature is probably
due to the presence of traces of spurious phases. The same
hypothesis must be made for the faint peak at T ¼ 350 K, a
temperature rarely covered in measurements reported in the
literature. In our opinion, it marks the presence of the phase
Fe2As, which is known to have an antiferromagnetic transition
right at Tꢁ350 K [16]. We remark that neither X-ray diffraction nor
SEM analyses gave an indication of the presence of Fe2As in this
undoped sample: the magnetic measurement is a more sensitive
instrument in this case. We suggest that an extension of the
temperature range above room temperature could give a useful
indication of the presence of Fe2As.
non-percolative superconductivity in a small sample region,
where some oxygen deficiency is present.
The doped samples are characterized by a nearly linear
decrease in resistivity with temperature. For LaFeAs(O0.85F0.15
sample it results Tc ¼ 26.5 K if defined as the onset of resistivity
/dT: here the
)
and Tc ¼ 21 K if estimated from the maximum of d
r
superconducting transition width is about 101. For SmFeA-
s(O0.85F0.15) sample with the same criteria we obtained Tc ¼ 53.5
and Tc ¼ 51.5 K, respectively.
All the magnetic measurements were performed with a SQUID
magnetometer (MPMS by Quantum Design).
3. Magnetic measurements
For the SmFeAsO sample (upper curve) the main features are
the following: (i) at Tꢁ134 K a local maximum, related to the SDW
onset is present; (ii) at T ¼ 6 K there is a very sharp cusp that we
attribute to the antiferromagnetic ordering of the Sm sublattice, in
agreement with the sharp peak in the specific heat observed by
Tropeano et al.[17] at 4.6 K in the same sample, and by Ding et al.
at 4.6 K [18] in a similar sample, and (iii) a Curie–Weiss like
behavior, related to the presence of paramagnetic Sm, is present. It
is clear that the presence of the magnetic signal from Sm is
dominating, and hides the possible presence of an increase in
magnetization with temperature similar to that observed in La
samples above the SDW transition.
Finally, we point out that, for both the La- and Sm-1111
samples, a small bump is present at about 40 K. Such anomaly has
been already observed in literature [13], and usually attributed to
the presence of some magnetic impurity. The fact that it happens
at the same temperature both in La- and Sm-1111 samples makes
difficult to attribute it to an onset of superconductivity for oxygen
deficiency in some part of the sample.
3.1. Undoped samples
The molar susceptibility of both LaFeAsO and SmFeAsO
samples, measured from 2 K up to 360 K, is shown in Fig. 3. In
these measurements, after a zero field cooling (ZFC) procedure, a
field of 30 kOe was applied.
The main features for LaFeAsO sample (lower curve) can be
summarized as follows: (i) at T ¼ 152 K a clear decrease in the
susceptibility is observed; (ii) at lower temperatures a minimum
is observable followed by an increase in susceptibility down to the
lowest temperature we reached; (iii) at temperatures greater than
152 K, susceptibility increases with increasing temperature and a
maximum at Tꢁ350 K is seen. This behavior has been already
reported in the literature [5,13]. The susceptibility drop at
T ¼ 152 K is due to the SDW onset and the related antiferromag-
netic ordering of Fe ions, while the susceptibility increase above
152 K is attributed to non-conventional antiferromagnetic corre-

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We remark that for both LaFeAsO and SmFeAsO samples the
temperature-dependent susceptibility signal is superimposed on a
temperature-independent background. We argue that part of this
magnetic background is due to the ferromagnetic impurities
present in our samples. This can be seen from magnetization
versus field measurements at T ¼ 5 and 300 K up to 50 kOe,
displayed in the insets of Fig. 3 for both the samples. Here, a
ferromagnetic signal, sharply increasing at low field and saturat-
ing around 10 kOe, is present in the two compounds. In SmFeAsO,
in addition, there is a clear linear increase in magnetization above
10 kOe, mainly due to the paramagnetic contribution of the Sm
ions (and also to the Pauli paramagnetic signal), while for LaFeAsO
a very small field dependence is observed, and the curves at T ¼ 5
and 300 K nearly overlap. Since the saturation values of the
ferromagnetic components for each sample are nearly the same at
T ¼ 5 and 300 K, we know that the ferromagnetic impurities have
a transition temperature higher than the room temperature. The
extrapolated magnetization values are 62 and 55 emu/mol at
T ¼ 5 and 300 K for La, and 80 and 70 emu/mol at T ¼ 5 and 300 K
for Sm. On the basis of these extrapolated magnetization data,
a linear behavior of the thermal dependence of the ferromagnetic
impurity magnetization has been hypothesized (M(T) ¼ 62.1–
0.024 T and M(T) ¼ 80.2–0.034 T for LaFeAsO and SmFeAsO
samples, respectively,) and subtracted from the measured curves,
yielding the data of molar susceptibility shown in Fig. 4.
These measurements represent well the magnetic behavior of
the 1111 phases. Moreover, following the idea proposed in [10], we
fitted the susceptibility data (between 7 and 100 K) of Sm-1111
phase with a Curie–Weiss-type behavior plus a temperature-
independent term. Unlike in [10], we chose to fit the low-
temperature data, under the reasonable hypothesis that are
dominated, for the temperature-dependent part, by the magnetic
signal of the Sm sublattice.
Therefore, we wrote
w
¼
w0
þ
wSm
(1)
The w0 term covers all the different temperature-independent
contributions (Pauli paramagnetism and Landau diamagnetism of
the conduction electrons, high frequency contribution, diamag-
netic contributions arising from nuclei and electronic inner
shellsy).
It is well known that the Sm presents an anomalous not
Curie–Weiss susceptibility behavior in most of its compounds.
The successful explanation of this fact [19] is that in the Sm3+ ion
the J multiplet intervals are comparable with kT: so, not only the
J ¼ 52 ground state is occupied but also the first excited level J ¼ 72.
As a consequence, the sum of the J levels, carried out up to the
second term and taking the Boltzmann temperature factor into
account, gives the relationship
C1
C2
D
eðꢀ
=TÞ
wSm ¼ x
þ ð1 ꢀ xÞ
(2)
ðT ꢀ WSm
Þ
ðT ꢀ WSmÞ
where x represents the fractional occupation of the two states, ySm
is the paramagnetic Curie temperature, C1 ¼ 0.0903 emu K/mol
and C2 ¼ 1.3452 emu K/mol are the calculated Curie constant
related to the two J states, and
D ¼ E/k is the difference in
temperature between the two states.
As parameters of the fit we obtained w0 ¼ 0.97 ꢂ10ꢀ3 emu/
mol, x ¼ 0.84, ySm ¼ ꢀ59 K and
D ¼ 600 K. The result is presented
in Fig. 4 as a dotted line. We note that the energy gap is in good
agreement with the value 650 K ( ¼ 56 meV) obtained from heat-
capacity measurements on SmFeAsO [20]. By subtracting from the
experimental data at high temperature (T4100 K) the
w values of
the best fit, we obtained the intrinsic susceptibility contribution of
the Fe–As layers, which is shown in the inset of Fig. 4 (upper-right
side). A clear jump at TSDW is emphasized. Its value is about
7 ꢂ10ꢀ5 emu/mol. The LaFeAsO susceptibility jump has about the
same value (6 ꢂ10ꢀ5 emu/mol) and McGuire et al. [10] obtained
comparable values for the other REFeAsO, i.e. ꢁ5 ꢂ10ꢀ5 emu/mol
for CeFeAsO and ꢁ4 ꢂ10ꢀ5 emu/mol for PrFeAsO and NdFeAsO.
This suggests a similar value of the susceptibility jump at the SDW
ordering for all the RE, and supports the correctness of the
estimate of the Sm sublattice susceptibility we did.
Let us discuss the relatively high positive value of
w0 ¼ 0.97 ꢂ10ꢀ3 emu/mol we obtained from the fit. w0 is mainly
ascribed to the Pauli positive contribution arising from non-
interacting band electrons
wP ¼ 2N(EF)mB2, where N(EF), the density of states at the Fermi
level, is proportional to the effective mass m* of the electrons
through
ð2mnÞ3=2E1F=2
NðEFÞ ¼
(3)
2
4p
For normal metals w0 is around 10ꢀ5C10ꢀ6 emu/mol, which
corresponds to an effective mass m* comparable with the mass of
the free electron. Here, w0 is about one hundred times higher. So,
the possibility that 4f electrons of Sm can hybridize with the
conduction electrons, giving a strong enhancement of carrier
effective mass (heavy fermion state), can be taken into account.
Fig. 4. Molar susceptibility versus temperature corrected for the ferromagnetic
background (see text) of LaFeAsO and SmFeAsO samples in the range 2–360 K. The
dotted line is the fit of Sm ions susceptibility. Upper-right inset: intrinsic
susceptibility contribution of the Fe–As layers. In the lower-left inset
w versus T
data for SmFeAsO at low temperature and different values of the applied magnetic
field are presented (left Y-scale) together with the low-T resistivity data
(continuous line) on the right Y-scale.

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Values of w0 as high as this one or more were found in typical
heavy fermion or concentrated Kondo systems: for example, for
CeCu2Si2 w0 ¼ 6.5 ꢂ10ꢀ3 emu/mol [21] and for CeAl3 w0
¼
3.6 ꢂ10ꢀ3 emu/mol [22]. Recent measurements of heat capacity
on SmOFeAs gave a high value of the electronic coefficient
specific heat: on the same sample here presented it turned out to
g of the
be
g
¼ 42 mJ/K2 mol [17]. The knowledge of
g and w0 allows one to
calculate the Wilson ratio (R) from the relationship R ¼
ð
p
2k2B=3m2BÞðwo
=gÞ. The Wilson ratio, a dimensionless parameter
concerned with correlations among electrons, is equal to or about
1 for metals and normal compounds, and increases as the e–e
interaction increases. We obtain R ¼ 1.80, which implies an
enhanced e–e interaction in SmOFeAs.
Other measurements in literature [18] reported a higher
g
value of 119.4 mJ/K2 mol. With this value an unphysical low value
of R ¼ 0.63 is obtained, which casts some doubt on these data. In
the undoped La-1111 compound an estimate of w0 gives a
maximum value of 5 ꢂ10ꢀ5 emu/mol (see Fig. 4). Using a value
of 3.7 mJ/K2 mol for
g [9], we obtained the maximum value of 1 for
the Wilson ratio, as expected for systems with free electron
character.
Fig. 5. Molar susceptibility versus temperature of LaFeAsO0.85F0.15 and SmFeA-
sO0.85F0.15 compounds. The applied field is 30 kOe. Upper-right inset: molar
susceptibilities in the high-temperature region. Lower-right inset: magnetization
versus field at T ¼ 60 and 300 K for SmFeAsO0.85F0.15. Lower-left inset: magnetiza-
In the left-side inset of Fig. 4 the low-temperature part of
w
versus T data, measured for different values of the magnetic field,
is shown. It is noticeable that no variation of the Sm ions
antiferromagnetic ordering temperature is observed up to the
maximum field of 50 kOe. The independence of also very low
antiferromagnetic ordering temperatures under strong applied
magnetic fields is rare, but it has been already observed in rare
earths and actinides compounds. For example, CeRhIn5 displays
the AF transition at 3.8 K and the same transition is still detected
at the same temperature with a magnetic applied field of 90 kOe
[23]. Another example is PuPd2Sn [24], where both magnetization
and heat-capacity measurements detected the same AF transition
temperature (TN ¼ 11 K) at 0 and 90 kOe applied magnetic fields.
In the same inset the low-temperature resistivity is reported: the
agreement between the susceptibility data, displaying the AF peak
at 6 K, and the resistivity values, where the strong slope change
begins at the same temperature, is remarkable.
tion versus field at T ¼ 60 and 300 K for LaFeAsO0.85F0.15
.
K for La-1111 sample and at T ¼ 60 and 300 K for Sm-1111 sample
are shown in the bottom-left side and bottom-right side insets,
respectively. Different behaviors are observed. For SmFeA-
sO0.85F0.15 at both temperatures the magnetic signal increases
linearly with the field, which indicates that no ferromagnetic
background is present, but the linear slope is different and higher
than that observed in the undoped sample. In the LaFeAsO0.85F0.15
,
on the contrary, a concavity due to the presence of ferromagnetic
impurities is observed both at 30 and 300 K. In particular, at
T ¼ 30 K the continuous variation of slope suggests the presence
of ferromagnetic clusters. Also in this sample the linear part of the
signal is highly increased with respect to the undoped sample.
In doped La-1111 compound it has been shown that the
intrinsic susceptibility of Fe ions exhibits a linear increase with
temperature [13,15], as we observed in the undoped compound
above the SDW transition. Therefore, we hypothesize that the
Curie–Weiss type behavior, observed in the normal state of the
LaFeAsO0.85F0.15 sample, could be due to the presence of
paramagnetic impurities.
Finally, we underline that the overall magnetic behaviors are in
agreement with the resistivity curves presented in Fig. 2. In both
1111 compounds the high T magnetic orderings (SDW) correspond
to resistivity decreases at comparable temperatures.
3.2. Doped samples
In SmFeAsO0.85F0.15 sample a Curie–Weiss-type behavior is
expected, due to the presence of magnetic Sm ions: but, as
previously outlined, the Curie–Weiss-type signal increased by
about a factor two compared to the undoped one (see Figs. 4 and 5
for the comparison), which indicates that also in this sample a
great part of the magnetic signal is spurious and paramagnetic.
In an effort to study in more detail the spurius phases present
in our doped samples we prepared and measured SmOF and FeAs,
while magnetic contributions eventually arising from Fe2As and
FeF2 were estimated from literature data (FeF2 is a magnetic
compound used as precursor in the sample preparation). SmOF
exhibits a paramagnetic susceptibility with a room-temperature
value of about 10ꢀ3 emu/mol and FeAs exhibits a signal of the
order of 10ꢀ3 emu/mol nearly constant with temperature. Fe2As is
known to order antiferromagnetically at T ¼ 353 K [16]; on the
contrary, the detailed magnetic characterization performed on
FeF2 [25,26] reveals that this compound is antiferromagnetic
below TN ¼ 78 K and the Curie–Weiss behavior presented above is
3.2.1. Normal state
In Fig. 5, molar susceptibility versus temperature
measurements are presented for doped La- and Sm-1111
compounds from T ¼ 2 K up to T ¼ 360 K. The applied magnetic
field was 30 kOe for ZFC and FC procedure.
The main features in Fig. 5, common to both the samples, are
the following: (i) a clear decrease in magnetization, indicating the
superconducting transition, can be seen at Tcꢁ19 and Tcꢁ50 K for
La- and Sm-1111 samples, respectively, (ii) above Tc a monotonous
decrease in magnetization with increasing temperature (Curie–
Weiss type) is observed up to about 330 K, after which a slight
increase in magnetization with a faint maximum at Tꢁ350 K
occurs (the up-right-side inset shows an enlargement of the molar
susceptibilities in the high-temperature region, where the max-
imum at about T ¼ 350 K is evidenced) and (iii) no feature related
to the SDW is observed. For Sm-1111 compound only, a complex
behavior in ZFC–FC measurements on the superconducting side
may be observed, which we will discuss in the next paragraph.
The field dependence of the magnetization measured at
T ¼ 30 K (just above the superconducting transition) and T ¼ 300
ascribed to an iron magnetic moment of 4.7 mB
.
On the basis of the afore-mentioned quantitative Rietveld
analysis we subtracted a contribution of 4.6% SmOF and 5.6% FeAs

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from the experimental data. In addition, since the peak in both
samples at T ¼ 350 K, (inset of Fig. 5) suggested to us the presence
of Fe2As, we tried to calculate its contribution. However, the
literature data on this phase [16] are affected by the presence of
ferromagnetic impurities, thus preventing any quantitative esti-
mate. Finally, we hypothesized that the strong paramagnetic
signal present in both LaFeAsO0.85F0.15 and SmFeAsO0.85F0.15
samples could be ascribed to FeF2. However, the estimate of
FeF2 concentration to simulate the experimental susceptibility
curve correctly, leads to an unrealistic content higher than 6%, but
this phase has not been detected neither by XRPD nor by SEM
analysis. It seems that probably the contribution of some other
spurious phases, in concentration lower than XRPD or SEM
threshold detectability level, must be taken into account. In
conclusion the normal state susceptibility of the doped 1111
phases could not be extracted.
error. Such phenomenology may be understood bearing in mind
that Sm ion sublattice orders antiferromagnetically at low
temperature, as we have observed in the parent sample (see
Fig. 3). Such ordering has been evidenced by Tropeano et al. [17] in
both SmFeAsOF and SmFeAsO0.85F0.15 samples by specific heat
measurements. In particular, in SmFeAsO sample a peak at
T ¼ 4.6 K is observed, while in the superconducting one the peak
is shifted to T ¼ 3.7 K. Below this temperature superconductivity
and RE lattice antiferromagnetism coexist. The ZFC and FC curves
originated at T ¼ 2 K show less irreversibility (i.e. are nearer)
because, starting from a temperature just below TN and passing at
TN through the maximum of magnetization of the Sm sublattice,
the superconductor feels a higher internal magnetic field: that is
the magnetic ordering affects the superconducting properties. The
slightly increasing FC signal observed from 2 K up to 5 K depends
on the competition between the superconducting FC signal
and the magnetic signal of Sm ions sublattice. To study in
more detail the evolution of the competition-coexistence
between superconductivity and magnetism we performed FC
measurements at magnetic field higher than 30 kOe.
The results are presented in Fig. 7. For the sake of clarity, we
show the magnetization instead of susceptibility: thus, the
measurements at different fields are spaced and better visible.
The increase of the magnetic field produces a less-pronounced
diamagnetism and makes the antiferromagnetic peak of Sm ions
more and more visible. The position of the peak agrees with the
maximum observed in the specific heat measurements.
In conclusion, Sm antiferromagnetism and superconductivity
appear to coexist in SmFeAsO0.85F0.15 sample at low temperature.
Since antiferromagnetic ordering of the RE ions bearing magnetic
moments in iron pnictides has been observed for other RE
(Nd,Ce,Pr,Gd), this coexistence is a general character of this new
class of superconducting compounds, as of many other classes of
superconducting and magnetic materials (RERh4B4, borocarbides,
ruthenocuprates, y). On the one hand, we recall that super-
conductivity and RE magnetism are located in different planes of
the unit cell in SmFeAs(O1ꢀxFx). On the other hand, the problem of
the magnetism in the FeAs planes, where superconductivity sets
in, is totally an open question.
3.2.2. Superconducting state
As it may be seen in Fig. 5 the behavior of LaFeAsO0.85F0.15
sample below the superconducting transition temperature Tc is
the standard diamagnetic one (with ZFC and FC curves nearly
coincident, which indicates a large reversibility region in the H–T
phase diagram), while the behavior of the SmFeAsO0.85F0.15
sample is quite unusual. In Fig. 6, an enlargement of Fig. 5 in
the low-temperature region is shown for the data relative to the
doped Sm-1111 sample, to observe its superconducting behavior
in detail: the four curves correspond to ZFC–FC measurements
made starting from T ¼ 2 K (open symbols) or from T ¼ 5 K (filled
symbols). At Tꢁ50 K magnetization begins to decrease, owing to
the diamagnetic signal of the superconducting state, but, as
temperature is lowered, a minimum occurs after which the signal
increases. The ZFC and FC measurements are coincident from Tc
down to about the temperature of the minimum (ꢁ25 K), which
indicates also for this sample a large zone of reversibility in the
H–T phase diagram. At lower temperatures ZFC and FC curves
separate, but measurements performed starting from 2 or 5 K do
not overlap, showing different behaviors that depend on the
measurement starting temperature. In the FC curves a slightly
higher value is observed for the measurement that started from
5 K. Moreover, the curve starting from 2 K shows a faint maximum
at 4 K. The ZFC curves show the expected shielding, but with a
higher susceptibility values for the measurement that started
from 2 K. These differences are small but outside the experimental
Fig. 6. Molar susceptibility versus temperature of SmFeAsO0.85F0.15 in the low-
temperature region. Open symbols: ZFC–FC measurements performed starting
from 2 K. Filled symbols: ZFC–FC measurements performed starting from 5 K. The
applied field was 30 kOe.
Fig. 7. FC magnetization versus temperature of SmFeAsO0.85F0.15 in the low-
temperature region for different applied fields starting from 2 K (filled symbols)
and 5 K (open symbols).

ARTICLE IN PRESS
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M.R. Cimberle et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3024–3030
4. Conclusions
[5] T. Nomura, S.W. Kim, Y. Kamihara, M. Hirano, P.V. Sushko, K. Kato, M. Takata,
A.L. Shluger, H. Hosono, arXiv: (2008) 0804.3569.
[6] Z. Ren, G.C. Che, X.L. Dong, J. Yang, W. Lu, W. Yi, X.L. Shen, Z.C. Li, L.L. Sun,
F. Zhou, Z.X. Zhao, Europhys. Lett. 83 (2008) 17002.
[7] C. Wang, L. Li, S. Chi, Z. Zhu, Z. Ren, Y. Li, Y. Wang, X. Lin, Y. Luo, X. Xu, G. Cao,
Z. Xu, Europhys. Lett. 83 (2008) 67006.
We have prepared and characterized undoped and 15%F-doped
La- and Sm-1111 compounds.
In the undoped samples we estimated the temperature-
independent Pauli paramagnetic susceptibility and therein the
Wilson ratio: in La-1111 phase the obtained value indicates a
nearly free electron character, while in Sm-1111 compound a
heavy fermions character is observed. Moreover, in this last
sample, by subtracting the Sm paramagnetism from the total
signal, we estimated the amplitude of the susceptibility step at
the SDW temperature that turned out to be comparable with that
of the La one. In the doped samples the superconducting behavior
could be observed, and for Sm-1111 phase the competition-
coexistence of superconductivity and antiferromagnetic ordering
of Sm ions sublattice was clearly detected.
[8] H.H. Wen, G. Mu, L. Fang, H. Yang, X. Zhu, Europhys. Lett. 82 (2008) 17009.
[9] J. Dong, H.J. Zhang, G. Xu, Z. Li, G. Li, W.Z. Hu, D. Wu, G.F. Chen, X. Dai, J.L. Luo,
Z. Fang, N.L. Wang, Europhys. Lett. 83 (2008) 27006.
[10] M.A. McGuire, R.P Hermann, A.S. Sefat, B.C. Sales, R. Jin, D. Mandrus,
F. Grandjean, G. Long, arXiv: 0811.0589v1.
[11] A. Martinelli, M. Ferretti, P. Manfrinetti, A. Palenzona, M. Tropeano,
M.R. Cimberle, C. Ferdeghini, R. Valle, M. Putti, A.S. Siri, Supercond. Sci.
Technol. 21 (2008) 095017.
[12] C. Hess, A. Kondrat, A. Narduzzo, J.E. Hamann-Borrero, R. Klingerer. J. Werner,
G. Behr, B. Buchner, arXiv:0805.0264
[13] R. Klingerer, N. Leps, I. Hellmann, A. Popa, C. Hess, A. Kondrat, J. Hamann-
Borrero, G. Behr, V. Kataev, B. Buchner, arXiv: 0808.0708v1.
[14] G.M. Zhang, Y.H. Su, Z.Y. Weng, D.H. Lee, T. Xiang, arXiv: 0809.3874v2.
[15] T. Imai, K. Ahilan, F. Ning, MA. McGuire, A.S. Sefat, R. jin, B.C. Sales,
D. Mandrus, arXiv: 0810.0305v1.
[16] M. Yuzuri, R. Tahara, Y. Nakamura, J. Phys. Soc. Jpn. 48 (1980) 1937.
[17] M. Tropeano, A. Martinelli, A. Palenzona, E. Bellingeri, E. Galleani d’Agliano,
T.D. Nguyen, M. Affronte, M. Putti, Phys. Rev. B 78 (2008) 094518.
[18] L. Ding, C. He, J.K. Dong, T. Wu, R.H. Liu, X.H. Chen, S.H. Li, Phys. Rev. B 77
(2008) 180510.
Acknowledgments
[19] J.H. Van Vleck, In: The Theory of electric and magnetic susceptibilities, Oxford
University Press, fifth ed., 1959.
[20] P.J. Baker, S.R. Giblin, F.L. Pratt, R.H. Liu, G. Wu, X.H. Chen, M.J. Pitcher,
D.R. Parber, S.J. Clarke, S.J. Blundell, arXiv:0811.2494v1.
Financial support of the ‘‘Compagnia di S. Paolo’’ and of the
Italian Foreign Affair Ministry (MAE)—General Direction for the
Cultural Promotion and Cooperation is acknowledged.
[21] W. Lieke, U. Rauchschwalbe, C.D. Bredl, F. Steglich, J. Aarts, F.R. de Boer, J. Appl.
Phys. 53 (1982) 2111.
[22] K. Andres, J.E. Graebner, H.R. Ott, Phys. Rev. Lett. 35 (1975) 1779.
[23] B.E. Light, S.K. Ravhi, A.L. Cornelius, P.G. Pagliuso, J.L. Sarrao, Phys. Rev. B 69
(2004) 024419.
[24] K. Gofryk, D. Kaczorowski, J-C. Griveau, N. Magnani, R. Jardin, E. Colineau,
J. Rebizant, F. Wastin, R. Caciuffo, Phys. Rev. B 77 (2008) 014431.
[25] J.W. Stout, L.M. Matarrese, Rev. Mod. Phys. 25 (1953) 338.
[26] G.K. Wertheim, D.N.E. Buchanan, Phys. Rev. 161 (1967) 478.
References
[1] Y. Kamihara, T. Watanabe, M. Hirano, H.J. Hosono, Am. Chem. Soc. 130 (2008)
3296.
[2] X.H. Chen, T. Wu, G. Wu, R.H. Liu, H. Chen, D.F. Fang, Nature 453 (2008) 761.
[3] G.F. Chen, Z. Li, D. Wu, G. Li, W.Z. Hu, J. Dong, P. Zheng, J.L. Luo, N.L. Wang,
Phys. Rev. Lett. 100 (2008) 247002.
[4] Z. Ren, W. Lu, J. Yang, W. Yi, X. Shen, Z. Li, G. Che, X. Dong, L. Sun, F. Zhou,
Z.X. Zhao, Europhys. Lett. 82 (2008) 57002.