Interplay between magnetism and superconductivity in EuFe2-xCoxAs2 studied
by 57Fe and 151Eu Mössbauer spectroscopy
A. Błachowski1, K. Ruebenbauer
* 1, J. Żukrowski2, Z. Bukowski
P. J. W. Moll4, and J. Karpinski4
4 , 3, K. Rogacki3,
1Mössbauer Spectroscopy Division, Institute of Physics, Pedagogical University
PL-30-084 Kraków, ul. Podchorążych 2, Poland
2Solid State Physics Department, Faculty of Physics and Applied Computer Science, AGH
University of Science and Technology
PL-30-059 Kraków, Al. Mickiewicza 30, Poland
3Institute of Low Temperatures and Structure Research, Polish Academy of Sciences
PL-50-422 Wrocław, ul. Okólna 2, Poland
4Laboratory for Solid State Physics, ETH Zurich
CH-8093 Zurich, Switzerland
*Corresponding author: firstname.lastname@example.org
PACS: 74.70.Xa, 75.30.Fv, 76.80.+y
Keywords: iron-based superconductors, spin density waves, transferred hyperfine fields,
Short title: Interplay between magnetism and superconductivity in EuFe2-xCoxAs2
The compound EuFe2-xCoxAs2 was investigated by means of the 57Fe and 151Eu Mössbauer spectroscopy versus
temperature (4.2 – 300 K) for x=0 (parent), x=0.34 – 0.39 (superconductor) and x=0.58 (overdoped). It was
found that spin density wave (SDW) is suppressed by Co-substitution, however it survives in the region of
superconductivity, but iron spectra exhibit some non-magnetic component in the superconducting region.
Europium orders anti-ferromagnetically regardless of the Co concentration with the spin re-orientation from the
a-axis in the parent compound toward c-axis with the increasing replacement of iron by cobalt. The re-
orientation takes place close to the a-c plane. Some trivalent europium appears in EuFe2-xCoxAs2 versus
substitution due to the chemical pressure induced by Co-atoms and it experiences some transferred hyperfine
field from Eu2+. Iron experiences some transferred field due to the europium ordering for substituted samples in
the SDW and non-magnetic state both, while the transferred field is undetectable in the parent compound.
Superconductivity coexists with the 4f-europium magnetic order within the same volume. It seems that
superconductivity has some filamentary character in EuFe2-xCoxAs2 and it is confined to the non-magnetic
component seen by the iron Mössbauer spectroscopy.
The EuFe2As2 is a parent metallic compound of the iron-based superconductors belonging to
the ‘122’ family. It crystallizes within tetragonal unit cell with a small orthorhombic
distortion below ~190 K . A transition to the orthorhombic phase has small hysteresis on
the temperature scale, and it is accompanied by the development of the spin density wave
(SDW) appearing just below transition. SDW has origin in the 3d band of iron and it is
longitudinal wave having propagation direction and magnetic moment aligned with the
crystallographic a-axis [2, 3]. The length of SDW is incommensurate with the lattice period
along the a-axis. SDW is incoherent just below onset of the magnetic ordering and becomes
coherent upon lowering temperature, i.e., at about
Europium is located in the planes perpendicular to the c-axis separating [Fe2As2] layers. It
stays in the divalent
state without orbital contribution to the 4f magnetic moment and
orders magnetically at about K 19 with magnetic moments aligned along the a-axis.
Subsequent europium bearing planes are ordered in the anti-ferromagnetic fashion. Europium
and iron nuclei experience almost axially electric field gradient (EFG) with the principal
component aligned with the c-axis [2, 3].
Superconductivity could be achieved in EuFe2As2 either applying pressure [5-7] or by partial
substitution e.g. either europium by potassium , arsenic by phosphorus , or iron by
cobalt [10, 11]. One can obtain underdoped non-superconducting material, superconductor of
the second type, and finally overdoped material without superconductivity, while increasing
dopant concentration. All these compounds exhibit metallic behavior. SDW order becomes
weaker with the increasing concentration of the dopant, and finally it vanishes in the
overdoped region. The europium magnetic ordering temperature is very weakly perturbed by
the dopant concentration  as long as dopants do not substitute europium itself .
This contribution is concerned with the EuFe2-xCoxAs2 compound investigations by means of
the 57Fe and 151Eu Mössbauer spectroscopy versus temperature and cobalt concentration x.
Single crystals of EuFe2-xCoxAs2 were grown applying tin flux method as described in Refs
[11, 12]. They appeared as single-phase material according to the X-ray diffraction results.
The cobalt concentration x was determined by using EDX analysis. Relative error is estimated
as about 5 % and some overestimation could be expected due to the proximity of the cobalt,
iron and europium fluorescent X-ray lines. Resistivity measurements have been performed
versus temperature on single crystals in a four-point configuration for all dopant
concentrations including parent compound and in the null external magnetic field. Results are
reported here as relative resistivity, i.e., normalized to the resistivity at 300 K for each sample.
Magnetic susceptibility versus temperature and magnetization versus external field for several
temperatures was measured for x=0.37 single crystal.
Mössbauer absorbers for the 57Fe spectroscopy were prepared in the powder form in the same
manner as described in Ref. . Absorbers for 151Eu spectroscopy were made in the same
way, albeit some of them contained about twice as much material per unit area. 57Fe spectra
were collected using the same equipment and procedures as described in Ref. . 151Eu
spectra were collected applying 151SmF3 source kept at room temperature and a scintillation
detector. Spectra were processed within the transmission integral approximation by using
applications from the MOSGRAF-2009 suite . Iron spectra with SDW component were
K 189 SDW is already fully coherent .
processed by GMFPHARM application treating the electric quadrupole interaction in the first
order approximation. Remaining spectra were processed by GMFP application and the full
Hamiltonian was diagonalized in both nuclear states. The europium hyperfine anomaly was
accounted for. Spectral shifts are reported versus room temperature α-Fe or versus room
temperature 151SmF3 source, respectively.
The temperature evolution of the resistivity is given in Figure 1 for various cobalt
concentrations x. Upon entering the SDW state, the
metallic behavior could be changed significantly
due to the partial gapping of the Fermi surface
leading to the decrease of the carrier concentration.
On the other hand, the spin scattering is reduced
owing to the increasing spin order. The first
mechanism leads to the upturn, while the second to
the downturn of the resistivity with lowering of the
temperature. A reduced carrier concentration is
clearly seen for x=0.34 sample. Additionally, the
divalent europium magnetic ordering is seen as
much less pronounced kink on the resistivity due to
further reduction of the spin scattering. This kink
could be seen for the parent compound and for the
overdoped x=0.58 sample, as otherwise it is
masked by the much stronger effect due to the
development of the superconductivity. The effect
of the europium ordering on the resistivity is much
lesser for the overdoped material in comparison
with the parent compound. Hence, one can
conclude that the coupling between 4f and
conduction electrons is weak. The zero resistance
in the superconducting state has been observed
only for x=0.39 sample.
Figure 1 Relative resistivity (normalized to the
resistivity at 300 K for each sample) plotted versus
temperature for all samples investigated. The
current was applied in the a-b plane. Blue arrows
indicate change of the slope due to the SDW
development. Red arrow shows change of the slope
due to the europium magnetic ordering in the
parent compound. Note that zero resistance is
obtained only for
drop of the resistivity is observed for
temperature to the superconducting state. Insets
show expanded regions of the low temperatures.
sample. The significant
samples due to development of the
s T denotes transition
Results of the magnetic measurements are shown in Figure 2. A magnetic susceptibility
shows some small diamagnetic deviation at 10 kOe and below 5 K for x=0.37 sample
indicating that part of the sample is in the superconducting state. This feature is particularly
pronounced for the zero-field-cooled (ZFC) state of the material. A significant hump observed
for the susceptibility (x=0.37) measured in the zero field (see, Figure 2a) is due to the
magnetic ordering of divalent europium. The presence of such hump is an indication that the
anti-ferromagnetic order of the europium atoms (observed in the parent compound) is
perturbed by cobalt replacing iron . The ac magnetic susceptibility data obtained for
x=0.37 sample at 200 K yield the effective magnetic moment
the Bohr magneton) in good agreement with the value obtained from the saturation
magnetization at low temperature (see, Figure 2b). Such an effective moment is due to the
divalent europium with ) 1(
, where the atomic giro-magnetic factor amounts
and the respective spin of the atomic shell equals
magnetization at low temperature (see, Figure 2b) yields magnetic moment
ordered state being in fair agreement with the susceptibility data. Results of the electric and
magnetic measurements strongly suggest filamentary character of the superconductivity.
µ stands for
. The saturation
µ 2 . 6
Figure 2 Section (a) shows real part of the magnetic susceptibility
temperature T for the crystal with composition
was applied along the c-axis. The ac field of
applied in the same direction. Results were obtained for zero-field-cooled (ZFC) and field-
cooled states. The inset shows expanded vertical scale for
behavior below 5 K is more pronounced for
Section (b) shows magnetization M loops obtained at 2, 20 and 100 K versus external field
H for the sample with composition 370x
. The field H was applied along the c-axis. For
the lowest temperature of 2 K magnetization M saturates in the field of about 3 kOe at the
value of 83 emu/g corresponding to the magnetic moment in the ordered state of
magnetons per chemical formula.
χ′ plotted versus
and 5 , 0
. A dc field of
Oe and frequency
kOe 10 and 5 =
kOe 10 =
and it was obtained in the ZFC state.
2 . 6 Bohr
Figure 3 57Fe Mössbauer spectra of EuFe2-xCoxAs2 for various concentrations x obtained at
room temperature (RT), 80 K and 4.2 K. Contribution due to the non-magnetic (NM)
component is shown in the lower right corner of each spectrum. Amplitudes
SDW fields are shown in blue. Transferred field on iron for the SDW component is shown in
green on the left together with the angle
transferred fields for the SDW component. Transferred field on iron for the NM component is
shown in green on the right together with the angle θ.
Figure 3 shows selected 57Fe spectra obtained versus temperature and cobalt concentration x.
Spectra were fitted with the SDW model  in the magnetically ordered region and with a
quadrupole doublet otherwise. The shape of SDW and corresponding hyperfine magnetic field
distributions are shown in Figure 4. For all spectra obtained at 4.2 K, i.e., below onset of the
europium magnetic order one has to take into account transferred hyperfine field on iron.
Essential results are summarized in Table I and in Figure 3. For x=0 (parent compound)
results have been already published . There is no non-magnetic component below transition
to the SDW-state. SDW approaches quasi-rectangular shape close to saturation. The
transferred field on iron due to the magnetic ordering of europium is undetectable.
tα . For x=0.39 it is impossible to separate SDW and