Competition of magnetism and superconductivity in underdoped (Ba1-xKx)Fe2As2

Competition of magnetism and superconductivity in
underdoped (Ba1−xKx)Fe2As2
Marianne Rotter1, Marcus Tegel1, Inga Schellenberg2 , Falko
M. Schappacher2, Rainer Po¨ttgen2, Joachim Deisenhofer3, Axel
Gu¨nther3, Florian Schrettle3, Alois Loidl3, and Dirk Johrendt1
1Department Chemie und Biochemie der Ludwig-Maximilians-Universita¨t Mu¨nchen,
Butenandtstr- 5-13 (Haus D), 81377 Mu¨nchen, Germany
2Institut fu¨r Anorganische und Analytische Chemie, Universita¨t Mu¨nster,
Corrensstrasse 30, D-48149 Mu¨nster, Germany
3Experimentalphysik V, Center for Electronic Correlations and Magnetism, Institute
for Physics, Augsburg University, D-86135 Augsburg, Germany
E-mail: johrendt@lmu.de
Abstract.
Polycrystalline samples of underdoped (Ba1−xKx)Fe2As2 (x ≤ 0.4) were
synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific
heat and 57Fe-Mo¨ssbauer-spectroscopy. The structural phase transition from
tetragonal to orthorhombic lattice symmetry shifts towards lower temperatures,
becomes less pronounced at x = 0.1-0.2 and is no longer present at x = 0.3. Bulk
superconductivity is observed in all samples except (Ba0.9K0.1)Fe2As2 by resistivity
and magnetic susceptibility measurements. Specific heat data show a broad SDW phase
transition in (Ba0.9K0.1)Fe2As2, which is hardly discernible in (Ba0.8K0.2)Fe2As2. No
SDW anomaly is found in the specific heat of optimally doped (Ba0.6K0.4)Fe2As2,
where C changes by 0.1 J/K at Tc = 37.3 K. 57Fe-Mo¨ssbauer-spectra show full magnetic
hyperfine field splitting, indicative of antiferromagnetic ordering at 4.2 K in samples
with x = 0-0.2, but zero magnetic hyperfine field in samples with x = 0.3. The
spectra of (Ba0.9K0.1)Fe2As2 and (Ba0.8K0.2)Fe2As2 in the phase transition regions
are temperature-dependent superpositions of magnetic and non-magnetic components,
caused by inhomogeneous potassium distribution. Our results suggest the co-existence
of AF magnetic ordering and superconductivity without mesoscopic phase separation in
the underdoped region and show unambiguously homogeneous superconducting phases
close to optimal doping. This is in contrast to recently reported results about single
crystal (Ba1−xKx)Fe2As2.
PACS numbers: 74.70.Dd, 61.50.Ks, 74.81.-g, 33.45.+x
Submitted to: New J. Phys.
ar
X
iv
:0
81
2.
28
27
v1
[
co
nd
-m
at.
su
pr
-co
n]
1
5 D
ec
20
08

Underdoped (Ba1−xKx)Fe2As2 2
1. Introduction
The discovery of superconductivity (SC) in iron arsenides [1, 2] with transition
temperatures (Tc) up to 55 K [3] has attracted an enormous interest in the scientific
community [4]. Besides the outstanding physical properties of this new class of
superconducting materials, scientists found fresh hope that iron arsenides may help
to finally solve the mystery of high-Tc superconductivity. But prior to this long-term
objective, many fundamental issues of the iron arsenides need to be clarified. Among
them, the structural and magnetic phase diagrams with respect to doping, reflecting the
interplay between superconductivity and magnetism, are discussed controversially.
In both the LaFeAsO (1111) and BaFe2As2 (122) families, superconductivity
evolves from poor metallic parent compounds with quasi two-dimensional tetragonal
crystal structures, which are subject to orthorhombic lattice distortions below certain
temperatures (To). Static long-range antiferromagnetic (AF) ordering emerges with Ne´el
temperatures (TN) well below To in LaFeAsO [5], but very close to To in BaFe2As2 [6].
The structural and magnetic transitions of the parent compounds are strongly affected
by doping of the FeAs layers either with electrons or holes, and superconductivity
appears at certain doping levels. For the underdoped phases in the transition zone,
it has been reported that SC and AF ordering is either separated or co-existing. Also
the overlap of the orthorhombic distortion with the SC and AF areas in the phase
diagrams are still not clear, neither in the 1111 nor in the 122 systems.
The first phase diagram of LaFeAsO1−xFx, constructed by µSR data, showed a
sharp-cut vertical border between the SC and the orthorhombic AF phases at x =
0.045 [7]. But neutron diffraction experiments showed, that although the magnetic
ordering vanishes around x ≈ 0.04, the orthorhombic lattice still exists at least to x =
0.05, where superconductivity has already emerged [8]. This is in line with the recently
published neutron study of CeFeAsO1−xFx, where AF ordering disappears exactly before
SC emerges, but the orthorhombic lattice persists extensively into the SC dome up to
x ≈ 0.1 [9]. Similar results have been reported for SmFeAsO1−xFx from µSR experiments
[10] and structural investigations using synchrotron radiation [11]. Thus at the moment
all signs are that in the case of the 1111-family, static AF order is completely suppressed
before SC emerges, but the orthorhombic lattice co-exists with superconductivity and
the temperature difference between To and TN increases with the doping level. This
behavior of the 1111-superconductors is strongly reminiscent to the monolayer high-Tc
cuprates. For instance in La2−xSrxCuO4, the AF order is well separated from the SC
state, but the orthorhombic phase exists largely in the superconducting dome [12].
In the 122-family, co-existence of the orthorhombic structure with SC has been
first published for (Ba1−xKx)Fe2As2 up to x ≈ 0.2 (Tc ≈ 26 K) by X-ray powder
diffraction [13]. Following neutron diffraction experiments also showed orthorhombic
symmetry and long-range AF ordering co-existing up to x = 0.3 (Tc < 15 K) [14]. The
different shapes of the superconducting domes Tc(x) of (Ba1−xKx)Fe2As2 may be due to
different synthesis conditions. However, the x values in Ref. [13] are determined from

Underdoped (Ba1−xKx)Fe2As2 3
X-ray data by Rietveld refinements, whereas only the nominal compositions are given
in Ref. [14]. Since diffraction methods provide the mean structural information on a
rather long spatial scale, short-range phase inhomogeneities are averaged. Thus one may
understand the observed co-existence of SC with AF ordering in (Ba1−xKx)Fe2As2 by
phase-separation in magnetic non-superconducting and non-magnetic superconducting
mesoscopic domains. Local probes like µSR and 57Fe-Mo¨ssbauer-spectroscopy can
provide more accurate information. Recently, three reports about µSR experiments,
each conducted with almost optimally doped superconducting (Ba1−xKx)Fe2As2 single
crystals, concluded consistently phase separations into SC and AF domains. The non-
magnetic superconducting volume fractions were found to be ≈ 30% [15], 40% [16], and
25% [17]. In the latter report, the lateral scale of of the inhomogeneities were estimated
to 65±10 nm by magnetic force microscopy (MFM) imaging. However, the onset of
AF ordering in the superconducting crystals were detected at ≈ 70-80 K irrespective of
different doping levels.
In the present paper, we report on a detailed study of the structural and magnetic
transitions of polycrystalline underdoped (Ba1−xKx)Fe2As2 (x ≤ 0.4). The samples
were characterized by magnetic susceptibility and specific heat measurements. The
crystal structures and chemical compositions were determined by Rietveld refinements
of x-ray powder patterns. Detailed temperature-dependent 57Fe-Mo¨ssbauer-spectra were
recorded in order to detect the evolution of magnetic ordering on a local spatial scale.
2. Experimental
2.1. Sample preparation
Samples of (Ba1−xKx)Fe2As2 with x = 0.1, 0.2 and 0.3 were prepared by heating
stoichiometric mixtures of the elements (all purities > 99.9 %) in alumina crucibles
enclosed in silica tubes under an atmosphere of purified argon. In order to minimize
the loss of potassium by evaporation at elevated temperatures, the gas volume in the
crucibles was reduced by alumina inlays. The mixtures were heated slowly (50 K/h) to
873 K, kept at this temperature for 15 h and cooled down to room temperature. The
reaction products were homogenized in an agate mortar and annealed at 923 K for 15 h.
After cooling, the samples were homogenized again, pressed into pellets and sintered at
1023 K for 15 h. The obtained black crystalline powders are stable in air for weeks. The
Ba:K ratios were checked by EDX and chemical analysis (ICP-AAS), which resulted in
the nominal composition within 5%.
2.2. X-ray structure determination
Phase purity was checked by X-ray powder diffraction using a Huber G670 Guinier
imaging plate diffractometer (Cu-Kα1 radiation, Ge-111 monochromator), equipped
with a closed-cycle He-cryostat. Rietveld refinements of all diffractograms were
performed with the TOPAS package [18] using the fundamental parameters approach

Underdoped (Ba1−xKx)Fe2As2 4
as reflection profiles (convolution of appropriate source emission profiles with equatorial
and axial instrument contributions as well as crystallite microstructure effects). In
order to obtain crystal structures inclusive of the Ba:K ratios, all profile contributions
were refined freely, but in order to obtain accurate lattice parameter changes, all
profile contributions were refined at room temperature and held constant for all other
temperatures (except for the lorentzian strain contribution). All diffractograms were
measured without an internal standard, so the absolute lattice parameters might be
slightly offset. In all cases, an empirical 2θ-dependent intensity correction for different
absorption lengths arising from the Guinier geometry setup was applied.
2.3. Magnetic susceptibility and specific heat
The magnetic properties were studied using a commercial SQUID magnetometer
(Quantum Design MPMS-5) with external magnetic fields up to 50 kOe. The heat
capacity was measured in a Quantum Design Physical Properties Measurement System
for temperatures from 2 K to 300 K.
2.4. Mo¨ssbauer spectroscopy
A 57Co/Rh source was available for the 57Fe-Mo¨ssbauer spectroscopy investigations.
The samples were placed in thin-walled PVC containers at a thickness of about 10
mg Fe/cm2. The measurements were run in the usual transmission geometry in the
temperature range from room temperature to 4.2 K. The source was kept at room
temperature. The total counting times per spectrum ranged between 5 h and 1 day.
3. Results and Discussion
3.1. Crystal structures and phase transition
The crystal structures were determined by x-ray powder diffraction. Fig. 1 shows the
x-ray powder patters of (Ba1−xKx)Fe2As2 (x = 0.1, 0.2, 0.3) at 300 K with Rietveld-fits
and the difference lines. Crystallographic data and selected bond lengths and angles at
300 K and 10 K, respectively, are compiled in Table 1. The temperature dependencies of
the a and b lattice parameters are shown in Fig. 2. In line with Ref. [13], the parameter a
of the tetragonal phase decreases with the doping level x, while c increases (not shown).
The tetragonal-to-orthorhombic phase transition is strongly affected by the potassium
content. The transition temperatures To decreases to ≈ 100 K at x = 0.2 and is no longer
visible at x = 0.3. Also the magnitude of the distortion, expressed by the differences
between a and b at 10 K, decreases from 0.73% (x = 0) to 0.70% (x = 0.1) to 0.49%
(x = 0.2). Thus with increasing potassium doping levels, the structural transition of
BaFe2As2 is shifted towards lower temperatures and also less pronounced. It is no longer
present at x = 0.3 (see Fig. 2).

Underdoped (Ba1−xKx)Fe2As2 5
2 0 4 0 6 0 8 0 1 0 0
0
1
2
3
4
5
0
1
2
3
4
0
1
2

2 � ( d e g r e e s )

Inten
sity (
104 c
ounts
)
1 0 0 % B a 0 . 9 K 0 . 1 F e 2 A s 2
1 0 0 % B a 0 . 8 K 0 . 2 F e 2 A s 2


9 1 % B a 0 . 7 K 0 . 3 F e 2 A s 2 9 % F e A s
Figure 1. (Color online) X-ray powder patterns of (Ba1−xKx)Fe2As2 (Crystallo-
graphic x = 0.13, 0.20, 0.24) with Rietveld profile fits and difference lines.
3.2. dc resistivity
The temperature dependence of the dc resistivity of (Ba1−xKx)Fe2As2 is shown in Fig. 3.
At the lowest doping concentration (x = 0.1), the typical SDW anomaly is still vis-
ible, but shifted towards lower temperatures and less pronounced than in undoped
BaFe2As2 [6]. We observe a drop of the resistance below 3 K, associated with a super-
conducting transition, even though zero resistance could not be reached at 1.8 K. The
curvature of the resistivity of (Ba0.8K0.2)Fe2As2 is still reminiscent to a SDW anomaly,
but smeared over a larger temperature range between ≈ 120 K and 70 K. The super-
conducting transition at 24 K is rather broad (≈ 4 K), but zero resistance is clearly