Investigation of magnetic interactions in Ba2EuRu1−xCuxO6 using
magnetization and Mössbauer studies
Rakesh Kumar, C. V. Tomy, P. L. Paulose, and R. Nagarajan
Citation: J. Appl. Phys. 97, 10A907 (2005); doi: 10.1063/1.1849711
View online: http://dx.doi.org/10.1063/1.1849711
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JOURNAL OF APPLIED PHYSICS 97, 10A907 共2005兲
Investigation of magnetic interactions in Ba2EuRu1−xCuxO6 using
magnetization and 151Eu Mössbauer studies
Rakesh Kumar and C. V. Tomya兲
Department of Physics, Indian Institute of Technology, Bombay, India, 400 076
P. L. Paulose and R. Nagarajan
Department of CMP&MS, Tata Institute of Fundamental Research, Bombay, India, 400 005
共Presented on 8 November 2004; published online 2 May 2005兲
We report here the magnetization and 151Eu Mössbauer studies in ordered perovskite-type oxides,
Ba2EuRu1−xCuxO6 共x = 0.0, 0.1 and 0.2兲. The parent compound, Ba2EuRuO6, shows a magnetic
ordering at 40 K. Both the magnetization and Mössbauer measurements indicate that the magnetic
ordering temperature is unchanged as Cu partially substitutes Ru. Despite the fact that the ground
state of Eu3+ is a nonmagnetic J = 0 state, the 151Eu Mössbauer resonance spectrum shows a clear
splitting into six lines for T ⬍ TN, indicating a large magnetic hyperfine field 共⬃280 kOe兲. This large
hyperfine field may be arising due to a strong exchange field between the ordered Ru moments
acting on the Eu3+ ions, inducing a magnetic moment for the Eu3+ ions. © 2005 American Institute
of Physics. 关DOI: 10.1063/1.1849711兴
INTRODUCTION
The Rutheno-cuprate family of oxides 共RuSr2RCuO8, R
= Gd, Eu兲 has been studied extensively due to the observation of the coexistence of superconductivity 共Tc
⬃ 20– 40 K兲 and magnetic ordering 共T M ⬃ 130– 150 K兲 with
a ferromagnetic component.1 Numerous experimental results
have led to the belief that the superconductivity originates
from the Cu–O planes and the magnetic order is due to Ru
moments in the Ru–O planes in this layered compound. The
efforts in various laboratories were directed toward understanding the exact nature of the existence of the long range
magnetic ordering of the Ru moments with the superconducting state, without destroying it. These studies have been
augmented by the recent observations of unusual superconducting and magnetic properties in a related family of ordered perovskite-type oxides, A2 Ln Ru1−xCuxO6 共A
= alkaline-earth element; Ln= lanthanide element兲,2,3 in
which the Ln and Ru atoms are regularly ordered. The structural analysis of Sr2YRu1−xCuxO6 共Ref. 4兲 shows that the
partial substitution of Cu for Ru does not create any Cu–O
planes but rather retains the same structure of the parent
compound Sr2YRuO6.5 The absence of Cu–O planes and the
coexistence of magnetic order and superconductivity which
originate from the Y–Ru共Cu兲–O and Sr–O planes, respectively, make these compounds very interesting. We have synthesized and investigated a series of compounds,
Ba2LnRu1−xCuxO6 共Ln= Rare earth兲 in order to examine the
effect of Cu doping on the magnetic and superconducting
properties. Here we report the results of magnetization and
151
Eu Mössbauer studies for Ba2EuRu1−xCuxO6 共0 艋 x
艋 0.2兲.
EXPERIMENT
The Ba2EuRu1−xCuxO6 共x = 0.0, 0.1 and 0.2兲 samples
were prepared by the conventional solid-state reaction
a兲
Electronic address: [email protected]
0021-8979/2005/97共10兲/10A907/3/$22.50
method. The final sintering on pressed pellets was carried out
at 1100 ° C for 24 h in air. All the samples were heated simultaneously in the same furnace to ensure identical heat
treatment. X-ray powder diffraction measurements were performed with X’pert PANalytical diffractometer 共Philips, Holland兲 with Cu-K␣ radiation. The magnetic properties were
measured using a superconducting quantum interference device magnetometer 共Quantum Design, San Diego, CA兲. The
151
Eu Mössbauer spectra 共MS兲 were obtained using a conventional constant acceleration velocity drive and a flow cryostat for temperature variation. The source used was 151SmF3
having a strength of 500 mCi.
RESULTS AND DISCUSSION
X-ray powder diffraction measurements confirmed the
single phase nature of the compounds. The diffraction data
was fitted using Rietveld analysis for a cubic phase having
the space group Fm3̄m. The lattice parameter obtained for
Ba2EuRuO6 is 8.408 Å, in agreement with the reported
value.6 There is no appreciable change in the lattice parameter on substitution of Ru by Cu. The magnetic susceptibility
as a function of temperature is shown in Fig. 1 for
Ba2EuRu1−xCuxO6 samples. The parent compound,
Ba2EuRuO6, shows an antiferromagnetic type magnetic ordering at ⬃40 K, which is consistent with an earlier report.6
The substitution of Cu does not seem to have any effect on
the magnetic ordering, as the ordering temperature almost
remains the same even for the x = 0.2 sample. Since Eu in
these compounds is in the 3+ oxidation state 共confirmed by
the Mössbauer spectra, see below兲, which is a nonmagnetic
J = 0 ground state, the magnetic ordering in this compound
can be assumed to be arising from Ru5+ ions. Neutron diffraction measurements in Ba2YRuO6 共Ref. 5兲 have indeed
shown that the magnetic ordering is due to the Ru moments.
It is interesting to note that there is no appreciable change in
the zero field-cooled and field-cooled data, indicating the ab-
97, 10A907-1
© 2005 American Institute of Physics
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10A907-2
J. Appl. Phys. 97, 10A907 共2005兲
Kumar et al.
FIG. 1. Magnetic susceptibility as a function of temperature for
Ba2EuRu1−xCuxO6. The inset shows the M vs H curves obtained at T
= 1.8 K.
sence of any canting of spins, in contrast to that observed in
Sr2EuRu1−xCuxO6.7 This is further confirmed by the magnetization versus magnetic field data below the magnetic ordering temperature, which show only a linear increase in magnetization as a function of the magnetic field 共inset Fig. 1兲.
The magnetic susceptibility shows a deviation from the
Curie–Weiss type behavior below ⬃190 K, which may be
attributed to the crystalline electric field 共CEF兲 effect. Heat
capacity measurements are in progress to ascertain the nature
of the CEF effect in these compounds. An increase in the low
temperature susceptibility 共Curie tail兲 is observed in all the
samples. This may be arising from an impurity phase, which
is not detected within the resolution of our x-ray measurements or due to the site disorder 共see below兲 already present
in these compounds.
The Mössbauer spectra 共MS兲 of all the samples were
taken at 4.2, 20, 35, 40, 45, 50, and 300 K. Figure 2 shows
the spectra obtained at 4.2 K for all the samples. The 151Eu
MS at 4.2 K show a well split six lines and retains it as the
FIG. 3.
151
Eu Mössbauer spectra for Ba2EuRu1−xCuxO6 at T = 40 K.
temperature is increased. The splitting decreases and collapses to single line at the magnetic ordering temperature
TN = 40 K, as shown in Fig. 3. The MS below 40 K were
fitted using a least-squares fit method for a single magnetic
hyperfine field. The fit was not exactly matching with the
data near the center of the spectrum. To obtain an exact fit,
an additional single line nonmagnetic component was added
in the fitting. Thus the MS at 4.2 K consists of two subspectra, magnetic and nonmagnetic. The nonmagnetic contribution was found to 6.8% for Ba2EuRuO6 共see Table I兲. The
nonmagnetic contribution can arise either from a small trace
of Eu2O3 as impurity or the Eu ions going to wrong sites
共site disorder兲, where the Eu ions may not be involved in the
strong exchange interaction. This may also be the reason for
the Curie tail observed in the magnetization measurements at
low temperatures. The solid lines in Fig. 2 correspond to the
fitting for magnetic and nonmagnetic subspectrum and the
dotted line is the resultant. The MS, which is a single line
above 40 K, have been fitted with a single Lorentzian and
gives a positive Isomer shift with respect to EuF3 indicating
Eu ions to be in 3+ state. No appreciable change in the
isomer shift is observed as the temperature is varied. A quadrupole interaction term is found to be negligible in all the
TABLE I. Various parameters obtained from the fit to the Mössbauer spectra
for Ba2EuRu1−xCuxO6.
FIG. 2. 151Eu Mössbauer spectra for Ba2EuRu1−xCuxO6 at T = 4.2 K. The
solid lines are fit to magnetic and nonmagnetic spectrum, while the dashed
line is the resultant fit.
Parameters
T 共K兲
x = 0.0
x = 0.1
x = 0.2
Hyperfine
field
共kOe兲
Disorder
共%兲
4.2
20
35
4.2
20
35
4.2
20
35
278
277
233
6.8
9.6
21.7
1.25
1.28
1.33
285
282
242
2.8
3.4
4.9
1.30
1.32
1.32
280
274
238
11.5
14.7
15.5
1.27
1.32
1.32
Isomer
shift
共mm/s兲
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10A907-3
J. Appl. Phys. 97, 10A907 共2005兲
Kumar et al.
samples at all the temperatures. The observed hyperfine field
共⬃280 kOe兲 at the Eu nucleus at 4.2 K is nearly as large as
that seen in a divalent Eu system 共e.g., 330 kOe at 4.2 K for
EuS兲. This large magnetic hyperfine field on the Eu3+ site is
very rare since the ground state of Eu3+ is a nonmagnetic J
= 0 state. The large hyperfine field indicates a strong exchange interaction from the Ru moments. In the presence of
such a strong exchange interaction, the excited states of Eu3+
may overlap with its ground state, which thereby produces
non zero magnetic hyperfine field at the Eu nucleus.
Another interesting observation is that for x = 0.1, the
disorder decreases 共hyperfine field increases兲 and then increases for x = 0.2 共hyperfine field decreases兲. This may be
linked to the observation that in Sr2YRu1−xCuxO6, superconductivity exists only for intermediate concentrations 共x
⬃ 0.1兲. It is interesting to investigate the origin of superconductivity in this class of materials, which survives even in
the presence of such strong hyperfine fields. Different heat
treatments are in progress to examine whether superconductivity can be obtained in Ba2EuRu1−xCuxO6. It will be interesting to study how the exchange interactions modify the
hyperfine fields in the presence of superconductivity.
In conclusion, we have shown that in Ba2EuRu1−xCuxO6,
it is possible to substitute Cu up to 20%. All these compounds orders magnetically at the same temperature TN
⬃ 40 K. The Cu substitution appears not to affect TN. Mössbauer spectra above TN show that Eu is in the 3+ state. Below TN, the Mössbauer spectra are split into six lines, indicating a large hyperfine field. Due to the exchange
interactions between the ordered Ru moments, a surprisingly
large magnetic hyperfine field is produced at Eu nucleus.
ACKNOWLEDGMENT
This work is supported by a grant from CSIR, New
Delhi, India.
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