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Eu Mössbauer and magnetic studies on magnetic superconductor
EuSr2Ru1xCu2þxO8 ð0:1pxp0:25Þ
Rakesh Kumara, C.V. Tomya,, P.L. Pauloseb, R. Nagarajanb
a
Department of Physics, Indian Institute of Technology, Bombay, Mumbai 400 076, India
b
Tata Institute of Fundamental Research, Mumbai 400 005, India
Abstract
Magnetic and 151Eu Mössbauer studies have been carried out on the ruthenocuprate magnetic superconductors,
EuSr2 Ru1x Cu2þx O8 ð0:1pxp0:25Þ. The magnetic ordering temperature (T M ) decreases from 135 K for x ¼ 0:0 to 125 K for x ¼
0:25 and the superconducting transition temperature increases from 19 K (onset for x ¼ 0:0) to 33 K (x ¼ 0:1). The compounds
with xo0 do not show superconductivity down to 2 K but show an increase in T M . The 151Eu Mössbauer studies show that the
Eu ions are in 3+ (4f 6 ; J ¼ 0) and hence in the nonmagnetic state in these compounds. The observation of a broadening in the 151Eu
Mössbauer absorption spectra below the magnetic ordering temperature indicates a small transferred magnetic hyperfine field at the
Eu site.
Keywords: Ruthenates; Mössbauer effect; Superconductivity
The ruthenocuprates are interesting due to the coexistence of superconductivity and magnetic order with
nonzero moment [1]. In these compounds, the Ru–O plane
is believed to be responsible for magnetic ordering and the
Cu–O plane for superconductivity. In spite of considerable
work published by several authors, it is still not clear how
superconductivity co-exists with the long-range magnetic
ordering in these compounds, especially since the magnetic
interaction is mediated through the Cu–O planes. In order
to investigate this system further, we have prepared a series
of compounds, namely, EuSr2 Ru1x Cu2þx O8 ð0:1p
xp0:25Þ and carried out 151Eu Mössbauer studies on
them. The magnetic and transport properties of these
compounds have also been studied.
The EuSr2 Ru1x Cux O8 ð0:1pxp0:25Þ samples were
prepared by the conventional solid-state reaction method
using powders of Ru, SrCO3 , Eu2 O3 and CuO in the
required stoichiometry. The final sintering of the pelletized
powders was carried out in flowing oxygen at 1060 C for 7
days. The X-ray diffraction patterns of all the samples
could be fitted to a tetragonal structure (space group—
P4=mmm, No. 123) with a small trace of SrRuO3 impurity
phase. Resistivity and magnetic susceptibility were measured in the temperature range of 2–300 K. 151Eu
Mössbauer spectra (MS) were recorded at 4.2, 100, 160
and 300 K.
The magnetic ordering temperature (T M ) has been taken
corresponding to the peak in the AC susceptibility data and
the superconducting transition (T c ) as the onset in
resistivity data. The magnetic ordering temperature
(135 K) and the superconducting transition (19 K) of the
parent compound, EuSr2 RuCu2 O8 , obtained from our
magnetic susceptibility [2] and resistivity data (Fig. 1) are
comparable with those reported [3]. On diluting the
magnetic Ru–O sublattice with Cu, the superconducting
transition temperature goes through a maximum (33 K for
x ¼ 0:1), (see inset of Fig. 1) and the magnetic ordering
temperature T M decreases monotonically (see inset of
Fig. 2). Superconductivity was not observed in compounds
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35
80
Tc (K)
Transmission (arb. unit)
ρ (mΩcm)
fit
obs (160 K),
fit
obs (100 K),
fit
obs (4.2 K),
fit
30
0.0
0.1
– 0.05
60
obs (300 K),
25
20
40
0.00
0.05
0.10
x
0.15
0.20
0.25
20
EuSr2Ru0.9Cu2.1O8
-15
-10
0
0
50
100
150
200
250
Fig. 1. Resistivity vs. temperature for EuSr2 Ru1x Cu2þx O8 . Inset shows
the variation of superconducting transition T c (onset) with x.
0
5
EuSr2Ru0.9Cu2.1O8
x
H = 25 Oe, ZFC
-3
M (10 emu/g)
5
0.1
0
145
0.05
140
TM (K)
-5
EuSr2Ru1-xCu2+xO8
15
Fig. 3. 151Eu Mössbauer absorption spectra of EuSr2 Ru0:9 Cu2:1 O8 at
different temperatures.
Table 1
Parameters obtained using a single Lorentzian fit of the
absorption spectra in EuSr2 Ru1x Cu2þx O8
10
10
Velocity (mm/sec)
300
T (K)
-5
151
Eu Mössbauer
T
(K)
Isomer shift
(mm/s)
FWHM
(mm/s)
Intensity
300
160
100
4.2
0.66
0.68
0.70
0.72
2.94
2.99
3.12
3.53
0.062
0.072
0.077
0.075
300
160
100
4.2
0.63
0.68
0.69
0.72
2.98
3.06
3.20
3.50
0.074
0.086
0.089
0.091
135
130
-10
125
0.0
0
50
100
x 0.1
150
0.2
200
T (K)
Fig. 2. ZFC magnetization of EuSr2 Ru0:9 Cu2:1 O8 in 25 Oe. Inset shows
the variation of magnetic ordering temperature T M with x.
with xo0 down to 2 K (Fig. 1). A typical magnetization
curve as a function of temperature (for x ¼ 0:1 compound)
is shown in Fig. 2. The peak at 135 K corresponds to the
magnetic ordering and the diamagnetic signal at 33 K
corresponds to the superconducting transition.
Fig. 3 shows 151Eu Mössbauer absorption spectra of
EuSr2 Ru0:9 Cu2:1 O8 at temperatures 4.2, 100, 160 and
300 K. A single Lorentzian gives a satisfactory fit to the
experimental data indicating the absence of any quadrupole interaction. The parameters obtained from the fit of
the Mössbauer data are given in Table 1. The Isomer shift
values are typical of the ionic Eu3þ state and hence the Ru
ions are expected to be in 5þ state for the charge balancing
in these compounds.
As the temperature is lowered from 300 to 160 K, there is
no appreciable change in the full-width at half-maxima
(FWHM) of the Mössbauer absorption spectra. On further
reducing the temperature from 160 K, the intensity of the
Mössbauer spectra is nearly unchanged but the FWHM
increases. Since Eu3þ is a nonmagnetic ion in nature having
the ground state J ¼ 0, the increase in FWHM may be due
to the small transferred magnetic hyperfine field at the Eu
site arising from the ordered Ru moments.
Neutron diffraction studies of the tetragonal
GdSr2 RuCu2 O8 [4] show Ru moments to be antiferromagnetically ordered with Ru at the corners and Gd at the
body centered position. The investigated compound
EuSr2 RuCu2 O8 is isostructural to its Gd analog and shows
a ferromagnetic behavior in the bulk magnetization,
possibly arising from spin canting [5] of Ru moments,
which may also lead to the nonzero magnetic hyperfine
field at the Eu site.
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The part of the work at IIT Bombay was supported by
CSIR, New Delhi, India.
References
[1] C. Bernhard, et al., Phys. Rev. B 59 (2000) 14099.
[2]
[3]
[4]
[5]
R. Kumar, et al., J. Magn. Magn. Mater. 272–276 (2004) e1073.
R.L. Meng, et al., Physica C 353 (2001) 195.
J.W. Lynn, et al., Phys. Rev. B 61 (2001) R14964.
I. Felner, et al., Physica C 311 (1999) 163.