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Title
The effects of post-growth annealing on the structural and magnetic properties of
BaFe2As2.
Permalink
https://escholarship.org/uc/item/6kj7z7pp
Journal
Journal of physics. Condensed matter : an Institute of Physics journal, 28(11)
ISSN
0953-8984
Authors
Forrest, TR
Valdivia, PN
Rotundu, CR
et al.
Publication Date
2016-03-01
DOI
10.1088/0953-8984/28/11/115702
Peer reviewed
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Journal of Physics: Condensed Matter
PAPER
The effects of post-growth annealing on the
structural and magnetic properties of BaFe
2
As
2
To cite this article: T R Forrest et al 2016 J. Phys.: Condens. Matter 28 115702
View the article online for updates and enhancements.
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1 © 2016 IOP Publishing Ltd Printed in the UK
1. Introduction
The discovery of high-temperature superconductivity in
the layered iron-pnictide materials [1] has generated great
excitement within the strongly correlated electron commu-
nity. While research into the pnictides is still ongoing, one of
the central results has been the intimate connection between
superconductivity, magnetism and the lattice structure. Thus
to explain how superconductivity emerges in these mat-
erials, one must understand the interaction between these
three degrees of freedom. To date superconductivity has been
identied in several groups of pnictides which have related
structures [1–4]. For the majority of these systems, the parent
compounds exhibit a spin-density wave antiferromagnetic
order at low temperatures, although there are some excep-
tions [3, 4]. In most cases the magnetic ordering is either
preceded or accompanied by a structural phase transition, where
the crystal lattice changes from a high temperature tetragonal
to a low temperature orthorhombic structure. Upon chemical
Journal of Physics: Condensed Matter
The effects of post-growth annealing on
the structural and magnetic properties of
BaFe
2
As
2
TRForrest,
1
,
5
, PNValdivia
2
, CRRotundu,
3
,
6
, EBourret-Courchesne
4
and RJBirgeneau
1
,
2
,
3
1
Department of Physics, University of California, Berkeley, CA 94720, USA
2
Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA
3
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
4
Life Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
E-mail: forrest@esrf.fr
Received 7 October 2015, revised 18 December 2015
Accepted for publication 25 January 2016
Published 19 February 2016
Abstract
We investigate the effects of post-growth annealing on the structural and magnetic properties
of BaFe
2
As
2
. Magnetic susceptibility measurements, which exhibit a signal corresponding
to the magnetic phase transition, and high-resolution x-ray diffraction measurements, which
directly probe the structural order parameter, show that annealing causes the ordering
temperatures of both the phase transitions to increase, sharpen and converge. In the as grown
sample, our measurements show two distinct transitions corresponding to structural and
magnetic ordering, which are separated in temperature by approximately 1 K. After 46 days
(d) of annealing at 700 °C, the two become concurrent in temperature. These measurements
demonstrate that the structural phase transition is second-order like when the magnetic and
structural phase transitions are separated in temperature, and rst-order like when the two
phase transition temperatures coincide. This observation indicates that annealing causes
the system to cross a hitherto undiscovered tricritical point. In addition, x-ray diffraction
measurements show that the c-axis lattice parameter increases with annealing up to 30 d, but
remains constant for longer annealing times. Comparisons of BaFe
2
As
2
to SrFe
2
As
2
are made
when possible.
Keywords: iron-pnicitide superconductors, structural phase transition, annealing
(Some guresmay appear in colour only in the online journal)
T R Forrest et al
The effects of annealing on the structural and magnetic properties of BaFe
2
As
2
Printed in the UK
115702
JCOMEL
© 2016 IOP Publishing Ltd
2016
28
J. Phys.: Condens. Matter
CM
0953-8984
10.1088/0953-8984/28/11/115702
Paper
11
Journal of Physics: Condensed Matter
IOP
5
Present address: European Synchrotron Radiation Facility, BP 220,
F-38043 Grenoble Cedex, France
6
Present address: Stanford Institute for Materials and Energy Sciences,
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park,
CA 94025, USA
0953-8984/16/115702+8$33.00
doi:10.1088/0953-8984/28/11/115702
J. Phys.: Condens. Matter 28 (2016) 115702 (8pp)
T R Forrest et al
2
substitution the ordering temperatures of both transitions are
suppressed, and in some cases superconductivity is induced.
The archetypal class of pnictide superconductors are derived
from parent compounds with the chemical formula: MFe
2
As
2
(M = Ca, Sr, Ba), and are commonly known as the 122 com-
pounds. For this structural class, superconductivity has been
identied with substitution on all three atomic sites [2, 5–9],
or through the application of hydrostatic pressure [10, 11].
In addition, superconductivity has also been induced by the
application of epitaxial strain in thin lms of BaFe
2
As
2
[12].
There has already been a signicant amount of research con-
ducted into the nature of the structural and magnetic phase
trans itions in these compounds. For CaFe
2
As
2
and SrFe
2
As
2
,
the consensus is that the phase transitions are concurrent
in temper ature, and can therefore be thought of as a single
rst-order magnetostructural transition [13–15]. The case
of BaFe
2
As
2
is more complex however; there have been
reports of these phase transitions being both rst [16] and
second-order, or at least nearly so [17]. Of particular rele-
vance is the previous research conducted by the authors of
this paper [18], where high resolution heat capacity, resis-
tivity and x-ray diffraction measurements were taken on a
BaFe
2
As
2
single crystal, that had been wrapped in tantalum
foil and annealed in a low pres sure argon atmosphere at 700
°C. Results showed that anomalies observed in the resis-
tivity and heat capacity were raised in temperature from
135.4 K to 140.2 K after 30 d of annealing. Measurements
of the sample’s residual resistivity ratio (RRR) showed a
consistent improvement with annealing time. This result was
conrmed by a later work where BaFe
2
As
2
single crystals
were annealed in a BaAs powder [19]. Returning to our pre-
vious research [18], heat capacity measurements on the as
grown sample showed a peak in C/T that is consistent with
a rst-order transition. Furthermore, the 30 d of annealing
caused the peak to increase and sharpen in temperature.
To corroborate this interpretation, high resolution x-ray dif-
fraction measurements of the structural phase transition were
performed on the sample after 30 d of annealing. The x-ray
measurements showed that upon cooling the orthorhombic
distortion initially appeared as a continuous splitting of the
tetragonal reection. At a slightly lower temperature this
continuous splitting was interrupted by the abrupt appear-
ance of a second set of orthorhombic reections, whose
positions remain approximately constant. As the sample
was cooled further, the rst set of reections were rapidly
suppressed; their positions continued to move outwards,
but never merged with the second set of reections. This
important result suggests that the structural phase trans-
ition in 30 d annealed BaFe
2
As
2
begins as second-order,
but evolves into a rst-order transition upon lowering of
the temperature. It was speculated that the driving force
behind this evolution is the formation of the antiferromagn-
etic order [18]. This unusual structural phase transition
was conrmed in an as grown sample of BaFe
2
As
2
by Kim
et al [20], who also conclusively showed that the discontin-
uous jump in the structural phase transition’s order parameter
does indeed correspond to the appearance of the antiferro-
magnetic order. Thus over a small temperature range it is
proposed that BaFe
2
As
2
exists in a phase which has an ortho-
rhombic structure, but is paramagnetic. Furthermore, Kim
et al performed similar x-ray measurements of the structural
order para meter in Ba(Fe
1−x
Co
x
)
2
As
2
and Ba(Fe
1−x
Rh
x
)
2
As
2
.
For the Co-substituted compound, a tricritical point wherein
the two phase transitions become completely separated and
the magn etic phase transition becomes second-order, was
identied at a doping level of
≈x 0.22
. This result was later
conrmed by separate resistivity measurements [21].
There is therefore a subtle difference in the structural
and magnetic ordering temperatures in BaFe
2
As
2
, although
they are still strongly coupled to one another. This has been
demonstrated by mean-eld calculations of the free energy,
which show that the structural phase transition is not due to
an intrinsic structural instability, but instead arises from mag-
netoelastic coupling to an Ising-nematic degree of freedom of
the magnetic ground states [20]. What is more, the inclusion
of an anharmonic term to the elastic free energy was shown
to reproduce the phase diagram of Ba(Fe
1−x
Co
x
)
2
As
2
by
solely changing the material’s elastic properties. Specically,
an increase in the bare shear modulus leads to a suppression
of the two phase transition temperatures and causes them to
diverge. Initially the structural transition is second-order and
the magnetic transition is rst-order, with its onset causing
the discontinuity in the structural order parameter. Upon a
further increase of the bare shear modulus, the magnetic trans-
ition also becomes second-order, creating a tricritical point.
Interestingly, the model also predicts that reducing shear
modulus will cause the two phase transitions to increase in
temperature, nally becoming concurrent and rst-order [20].
Since annealing offers a clean method to change the structural
and magnetic ordering temperatures, and possibly their sepa-
ration, it affords a good opportunity to search for changes in
the coupling between the two phase transitions. Furthermore,
it is important to study the limiting behaviour of extended
annealing periods, as it might allow for the access of regions of
the BaFe
2
As
2
phase diagram that have not yet been measured.
In addition, while it has been shown that annealing has
a substantial effect on the structural and magnetic ordering
in BaFe
2
As
2
, it is still not understood what effect it has at
the microscopic level. For example, is the main effect of
annealing to remove defects and other impurities which act
as a chemical dopant? Or does annealing cause changes in the
effective pressure within the crystal?
In order to answer these questions, we have undertaken a
number of x-ray diffraction and magnetic susceptibility meas-
urements on a single crystal of BaFe
2
As
2
that was annealed
for a cumulative period of 46 d. In addition, high resolution
x-ray diffraction measurements of the structural phase trans-
ition in as grown, and after 30 and 46 d of annealing were
also taken. Since neutron diffraction is unable to resolve the
jump in the structural phase transition [17], these high resolu-
tion x-ray diffraction measurements offer a method to accu-
rately determine the separation in ordering temperatures.
A full description of these measurements is given in section2,
while the experimental results are shown in section3. Finally
this work is discussed in section4, and the conclusions are
given in section5.
J. Phys.: Condens. Matter 28 (2016) 11570 2
T R Forrest et al
3
2. Experimental details
A single crystal of BaFe
2
As
2
was grown using the self-ux
method [22]. In order to prevent the absorption of oxygen
during the annealing process, the crystal was wrapped in tan-
talum foil (a getter that readily combines with oxygen), and
placed inside a quartz tube that was lled with argon gas.
The tube was then evacuated using a combined roughing
and turbo pump set-up. The pumping was continued until a
pres sure level of less than
×
−
110
5
mbar was displayed, after
which the tube was immediately sealed. The sample was then
annealed at 700 °C for cumulative periods of 22, 30 and 46
d. X-ray diffraction measurements of the room temperature
lattice parameters, and magnetic susceptibility measurements
of the phase transition(s), were taken on the sample as grown
and after each annealing period. The magnetic susceptibility
measurements were taken with a Quantum Design Magnetic
Property Measurement System (MPMS), with a magnetic
induction of 5T, parallel to the sample’s a-b crystallographic
plane. The susceptibility was recorded upon cooling through
the phases transition(s) at intervals of 0.25 K. The x-ray dif-
fraction measurements of the (0 0 4), (0 0 8) and (0 0 12)
reections were taken using a X’Pert pro four-circle diffrac-
tometer, with a monochromated Cu-
α
1
source. High resolu-
tion x-ray diffraction measurements of the structural phase
transition were taken at beamline 7-2, Stanford Synchrotron
Radiation Laboratory, on the sample as grown and after 30 and
46 d of annealing. The diffraction measurements were taken
with an x-ray photon energy of 16 keV, in a vertical scattering
geometry. Transverse scans (rocking curves) of the (0 0 8)
reection in as grown and after 30 d of annealing, gave a full
width at half maximum (FWHM) in θ of ⩽ 0.1°, indicating a
good quality of the surface mosaic. Transverse scans (rocking
curves) of the same reection after 46 d of annealing gave
a FWHM of ∼0.25°. In order to study the structural phase
trans ition, high resolution reciprocal space scans along the
[1 1 0] direction were used to measure the splitting of an
(H H L) tetragonal reection into the (H 0 L) and (0 H L)
orthorhombic reections. (It was not possible to measure the
splitting of a (H H 0) reection as the crystal natural cleaves
with the [0 0 1] direction as the surface normal.) For the as
grown and 30 d annealed samples, the splitting of the (3318)
T
reection was measured. For the 46 d annealed sample, scans
along the [1 1 0] direction across (3 3 18)
T
reection did not
show a clean single peak in the tetragonal phase. Since this
scan is not completely perpendicular to the crystal mosaic arc,
it is likely that the degradation in crystal mosaic quality of the
46 d annealed sample is responsible for this effect. By contrast
the (2 2 1 6)
T
reection, where the [1 1 0] reciprocal lattice
scan is more perpendicular to the crystal mosaic arc, showed
single peak in the tetragonal phase. Therefore the splitting
of this reection was measured in the 46 d annealed sample.
It should be noted that previous measurements of the structural
phase transition in BaFe
2
As
2
studied the splitting of (2 2 L)
or (1 1 L) type reections [18, 20]. Therefore measurements
on the (2 2 16) will still be able to resolve the unusual splitting
of the peak, if it is present. These reciprocal lattice scans were
taken as the temperature was decreased from 160 K to a base
temperature of 80 K. Close to the structural phase transition,
measurements were taken at temperature steps of 0.1 K. A rest
period of at least 4 min preceded each scan, this ensured that
the measured temperature uctuations were less than 0.025 K.
3. Experimental results
The process of annealing causes the crystals to dull. To deter-
mine if this effect is indicative of a change in chemical compo-
sition, wavelength dispersive x-ray spectroscopy measurements
were taken after each annealing period. Results showed that
annealing produced no change in the Ba:Fe:As elemental ratio
and did not lead to the inclusion of Tantalum.
3.1. Annealing induced changes in the magnetic
susceptibility
In order to examine the effects of annealing on the magnetic
properties of BaFe
2
As
2
, magnetic susceptibility measure-
ments were taken on a single crystal sample as grown and
after each annealing period. Figure 1(a) shows these mea-
surements across the phase transition(s), where the results
have been normalized to allow for an easy comparison. The
gureshows a step in the magnetic susceptibility that is indic-
ative of a phase transition(s). Annealing raises the temper-
ature of the phase transition(s) from 135.3 K in the as grown
sample to 139.8 K after 46 d of annealing. The trans ition
temperature(s) for the sample after 46 d of annealing is slightly
lower than the value previously reported in a 30d annealed
sample (140.2 K) [18]. However the important point is that
post growth annealing raises the temperature of the phase
Figure 1. (a) χ versus T of the phase transitions for a BaFe
2
As
2
sample, as grown and after 22, 30 and 46 d of annealing.
The data has been normalized to be 0 at 130 K and 1 at 145 K.
(b)
()()χ∂ ∂TT/
versus T.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
normalized χ (arb. units)
(a)
A.G.
22 days
30 days
46 days
130 131 132 133 134 135 136 137 138 139 140 141 142 143 144
0
2
4
6
8
10
12
14
Temperature (K)
d(χT)/dT (me.m.u./mol)
(b)
J. Phys.: Condens. Matter 28 (2016) 11570 2
