Magnetic properties of polycrystalline KClO3 -doped
Y(1 0:2x)Ba(2 0:2x)Kx Cu3 Oy (x = 0
0.40) superconductors
Michael R. Koblischka, Anjela Veneva, and Masato Murakami
Superconductivity Research Laboratory, Internarional Superconductivity Technology Center
16-25 Shibaura 1-chome, Minato-ku, Tokyo 105-0023, Japan
The eect of KClO3 -addition on the magnetic properties of polycrystalline hightemperature superconductors with nominal composition Y(1 0:2x) Ba(2 0:2x) Kx Cu3 Oy (x = 0 0.40) was investigated. DC susceptibility as function of temperature was measured in eld-cooled
cooling and eld-cooled warming modes in magnetic elds in the range 10 mT 0 Ha 7 T
on KClO3 -doped and pure YBCO samples. From the Meissner curves measured at a eld of 10
mT, we deduce that the samples are single-phase. However, the low-temperature behaviour of the
samples with KClO3 additions at large applied elds is dierent from pure YBCO.
Abstract:
Key words:
YBCO HTSC, KClO3 -addition, magnetic properties, ux pinning
INTRODUCTION
Investigating the inuence of the dierent additions on the magnetic and electrical properties and
microstructure of the high-Tc superconductors (HTSC) is of interest, as much for the understanding of the intergranular phenomena, as for solving technological problems. A special interest is
associated with doping of YBa2 Cu3 O7 Æ (YBCO) superconducting materials by alkali-chlorinecontaining additions, which improve the superconducting properties [1{4]. A Tc of 96 K has been
observed for Y1 Ba2 x Kx Cu3 Oy samples (x = 0.2) with KCl [4]. The aim of this work is to investigate the eect of KClO3 -addition on the magnetic properties of polycrystalline YBCO HTSC.
Here, we present eld-cooled cooling (FCC) and -warming (FCW) measurements of KClO3 -doped
and pure YBCO HTSC in magnetic elds up to 7 T.
EXPERIMENTAL
Polycrystalline KClO3 -doped YBCO high-temperature superconductors (HTSC) with nominal
composition Y(1 0:2x) Ba(2 0:2x) Kx Cu3 Oy (x = 0 0.40) were synthesized by a solid state reaction in air [3]. Powders of Y2 O3 , BaCO3 , CuO, all with a purity 99.99 % and KClO3 (more
than 99.9 %), were mixed, pressed into pellets and heated at 880 Æ C for 24 h. The reacted pellets
were reground, pressed again and sintered at 915 Æ C for 20 h. They were annealed at 600 Æ C for
14 h, before being cooled to room temperature. A detailed study on the thermal behavior of the
samples was performed by dierential thermal analysis (DTA) on the starting mixtures. The phase
content, microstructure and morphology of the crystalline grains were characterized using X-ray
powder diraction (XRD) and scanning electron microscopy (SEM) coupled with an electron probe
microanalyzer (EPMA).
Temperature scans in both FCC and FCW modes were carried out in various elds between 10
mT and 7 T. The measurements were performed in a commercial SQUID magnetometer (Quantum
Design model XL), enabling to measure in a continuous temperature sweep mode with a controlled
temperature sweep rate dT=dt = 35 mK/min; the datapoints are recorded in steps of 50 mK in
(a)
(b)
0
-1
YBCO pure
7T
χ DC [10
χ DC [10
-3
]
-3
]
0
-2
6T
7T
5T
6T
4T
5T
3T
4T
3T
2T
1T
0.5 T
2T
-4
1T
0.5 T
-2
YBCO doped
20
40
60
80
100
120
T [K]
20
40
60
80
100
120
T [K]
Fig. 1. (a): Temperature scans (FCC) of the DC susceptibility DC of a pure YBCO sample in applied
elds between 0.1 T and 7 T; the FCW data are omitted for clarity. Note that all curves are monotonuous
in contrast to NdBa2 Cu3 O7 Æ crystals [5] in all applied elds. Further, at low temperatures there is an
upturn of DC due to the paramagnetic contribution of Cu. (b): The same experiments on the KClO3 doped sample with x = 0.30. Again, all curves are monotonous, but at low T , their behaviour is dierent
from the pure sample as marked by arrows.
the transition region. All DC (T ) curves are measured between 1.7 K and 120 K. Note that the
temperature sweep is not halted for taking a datapoint as in a conventional SQUID magnetometer.
No averaging of the signal is performed, and the scan length is 1 cm. This procedure ensures a large
number of datapoints even in a sharp superconducting transition. More details of the measurement
procedure are given by Koblischka et al. [5].
RESULTS AND DISCUSSION
After the second heat treatment the XRD data showed that in all cases the KClO3 -doped samples
are pure 123 phase materials with only an indication of traces of CuO for the sample with x =
0.40. We would like to emphasize that the X-ray diraction patterns of the doped samples do
not contain any such extra lines within the detection limit of the X-ray method (normally the
detection limit of XRD for impurities is estimated around 4-5 %, so it is not possible to detect
minor impurities). And, no chlorine or other impurity phases related with chlorine or potassium
are found in the KClO3 -doped samples. We have carried out detailed microstructural studies and
SEM-EPMA revealed traces of potassium and chlorine in the grain boundaries of the samples with
addition [3]. The values of the lattice parameters of KClO3 -doped samples are comparable to the
data published for the desired YBCO orthorhombic structure [6]. The inuence of KClO3 on the
superconducting properties depends on the concentration of the addition in the initial batch. AC
susceptibility data of these samples are given by Veneva et al. [3]. The grain size of the pure YBCO
sample is 28 m; that of the KClO3 -doped samples 40 m.
In Fig. 1 (a), DC of the pure YBCO sample is plotted for various applied elds; in (b) the same
experiment is shown for the KClO3 -doped sample with x = 0.30. All DC curves are monotonous,
and no secondary step in the transitions can be observed in elds above 4 T as in some YBCO single
crystals and all NdBa2 Cu3 O7 Æ samples investigated in a similar way [5]. This demonstrates that
the polycrystalline YBCO samples with their relatively small grains do not contain oxygen vacancy
clusters providing a spatial distribution of Tc , in contrast to bulk samples. At low temperatures
(T < 15 K), all FCC curves of the pure sample exhibit a clear upturn of the DC curves, which
is due to the paramagnetic moment of Cu2+ . These are located in the Cu-O-chains [7], and their
number depends on the oxygenation state. In stark contrast to this behaviour, the doped sample in
(b) exhibits a downturn of the FCC/FCW curves towards more negative (= diamagnetic) values,
which develops with increasing applied eld. This may point to a change in the copper valence, or,
more likely, to the fact that the grain boundary regions become more strongly superconducting.
In case that the coupling strength of the grains increases, shielding currents can ow on a larger
length scale [8], which results in an increased diamagnetic signal. This latter explanation is in
accordance with previous results obtained from AC susceptibility studies [3], where it was concluded
that the KClO3 addition leads to a modication of the coupling strength between the grains and
to an alteration of the pinning eectiveness for the intergranular ux lines. It was shown in
Ref. [3] that the grain connectivity is indeed improved by the KClO3 -addition, if small amounts of
KClO3 -additions (x = 0.20 0.30) are employed. This stronger superconducting signal may then
completely mask the paramagnetic contribution of the copper.
CONCLUSIONS
We have presented FCC/FCW measurements on pure YBCO and KClO3 -doped samples in elds
between 10 mT and 7 T. All DC curves are found to be monotonous, but the doped samples
exhibit a clear downturn to more diamagnetic values at low temperatures. This indicates that the
increasing KClO3 -addition leads to a modication of the coupling strength of the grains.
Acknowlegdments.
This work is partially supported by New Energy and Industrial Technology Development Organization (NEDO). AV and MRK gratefully acknowledge support from the
Japanese Science and Technology Agency (STA).
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