High-pressure synthesis and physical properties of ruthenate-cuprates RuSr2LnCu2O8 (Ln = Y, Dy, Ho, Er)

Article abstract of DOI:10.1016/S0921-4534(03)01266-8

We report the series of ruthenate-cuprates RuSr₂LnCu ₂O₈ (Ru-1212Ln; Ln = Y, Dy, Ho, Er), which were prepared at 1200-1300 °C under 6 GPa. High-purity sample could be obtained for every system with the 'Ru-poor' starting composition, Ru_0.9Sr ₂LnCu₂O_7.8. It was confirmed that a solid solution of (Ru_1-xCu_x)Sr₂YCu₂O _y exists and its range terminates at x ～ 0.1 without formation of the stoichiometric phase of RuSr₂YCu₂O_y. Many Y-based samples were prepared varying the Ru/Cu ratio and the oxygen contents. According to DC susceptibility measurements, ferromagnetism and superconductivity looked to coexist below approximately 40 K in some samples. However, DC resistivity was always quite high even when a sample showed a large diamagnetism in a low temperature region. These results may suggest phase separation regarding the Ru/Cu ratio in the solid solution of (Ru _1-xCu_x)Sr₂YCu₂O_y, casting doubt for the coexistence of superconductivity and ferromagnetism.

Full text of DOI:10.1016/S0921-4534(03)01266-8

Physica C 398 (2003) 85–94
www.elsevier.com/locate/physc
High-pressure synthesis and physical properties
of ruthenate-cuprates RuSr2LnCu2O8 (Ln ¼ Y, Dy, Ho, Er)
T. Kawashima , E. Takayama-Muromachi
*
Superconducting Materials Center (Namiki Site), Advanced Materials Lab., National Institute for Materials Science,
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Received 10 February 2003; received in revised form 18 April 2003; accepted 8 May 2003
Abstract
We report the series of ruthenate-cuprates RuSr2LnCu2O8 (Ru-1212Ln; Ln ¼ Y, Dy, Ho, Er), which were prepared
at 1200–1300 °C under 6 GPa. High-purity sample could be obtained for every system with the ÔRu-poorÕ starting
composition, Ru0:9Sr2LnCu2O7:8. It was confirmed that a solid solution of (Ru1ꢀxCux)Sr2YCu2Oy exists and its range
terminates at x ꢁ 0:1 without formation of the stoichiometric phase of RuSr2YCu2Oy. Many Y-based samples were
prepared varying the Ru/Cu ratio and the oxygen contents. According to DC susceptibility measurements, ferro-
magnetism and superconductivity looked to coexist below approximately 40 K in some samples. However, DC resis-
tivity was always quite high even when a sample showed a large diamagnetism in a low temperature region. These
results may suggest phase separation regarding the Ru/Cu ratio in the solid solution of (Ru1ꢀxCux)Sr2YCu2Oy, casting
doubt for the coexistence of superconductivity and ferromagnetism.
Ó 2003 Elsevier B.V. All rights reserved.
PACS: 74.72.Jt; 74.62.Dh; 74.25.Ha; 74.25.Fy
Keywords: RuSr2YCu2O8; High-pressure synthesis; DC susceptibility; DC resistivity
1. Introduction
Ru-1212Gd can be substituted by other rare earth
elements. It is known that larger rare earth ele-
ments of Sm and Eu can occupy the Gd-site under
ambient pressure [31]. On the other hand, Tak-
agiwa et al. reported that Y-analogue of RuSr2-
YCu2O8 becomes stable under high-pressure [32].
Ruiz-Bustos et al. also reported other lanthanides
substitution by Ru-1212Gd [28,30]. Namely, high-
pressure condition is effective to introduce a
smaller rare earth element to the Gd-site.
Since the 1212-type compound RuSr2GdCu2O8
(Ru-1212Gd) was reported to show coexistence of
ferromagnetism and superconductivity below 46 K
[1,2], many investigations have been carried out
from various points of view [3–30]. One of our
interests for this system is whether the Gd-site in
* Corresponding author. Tel.: +81-29-860-4510/4674; fax:
+81-29-860-4674.
E-mail address: kawashima.tetsuya@nims.go.jp (T. Kawa-
The Y-phase showed ferromagnetism at 149 K
and superconductivity with zero-resistivity at 23
K. However, Meissner effect was not observed by
shima).
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0921-4534(03)01266-8

86
T. Kawashima, E. Takayama-Muromachi / Physica C 398 (2003) 85–94
DC susceptibility measurement. Moreover, a small
amount of impurity phase was included in the
sample [33]. The coexistence of superconductivity
and ferromagnetism was not, therefore, fully con-
firmed for this phase.
In the present study, we succeeded to prepare a
high-purity Y-based samples under high-pressure
and could measure superconducting and ferromag-
netic properties for the well-characterized samples.
In addition, we synthesized other ruthenate-cup-
rates RuSr2LnCu2O8 (Ln ¼ Dy, Ho, Er) under
high-pressure. Their physical properties are also
reported.
range from 5 to 200 K by field cooling (FC) and
zero field cooling (ZFC) processes. DC electric
resistivity was measured by the standard four-
probe method using the physical property mea-
surement system (quantum design, PPMS) in the
temperature range from 5 to 300 K. Cation ratios
of some selected samples were determined by an
EPMA analyzer (JEOL JXA-8600MX).
3. Results and discussion
3.1. RuSr2YCu2O8 (Ru-1212Y) phase
For the Ln ¼ Y system, we first tested the ideal
starting composition of RuSr2YCu2O8. Since the
weight change of a sample was not observed in the
process of high-pressure synthesis, net oxygen
content of a sample is equal to the initial value in a
starting mixture. However such an initial content
is not exactly equal to the oxygen content of the
Ru-1212Y phase if a high-pressure product is a
multi-phase mixture. We tried oxygen content
analysis by EPMA, but could not obtain a rea-
sonable value since EPMA is less sensitive for a
light element such as oxygen. In this paper,
therefore, oxygen content of a 1212 phase is de-
scribed as a nominal value of a starting mixture.
Strictly speaking, it is a synthesis parameter cor-
responding to the oxygen pressure in the high-
pressure process, rather than the real oxygen
content. However, it would be close to the real
oxygen content when a sample is phase pure.
Fig. 1 shows the powder X-ray diffraction pat-
terns of the high-pressure products treated at 1200
°C [sample (A)] and 1300 °C [sample (B)]. Al-
though small peaks attributed to SrRuO3 and CuO
appeared in their X-ray patterns, main peaks could
be indexed assuming tetragonal system with a ¼
2. Experimental
Starting materials for high-pressure synthesis
were RuO2 (99.9%), SrO2, SrCuO2, Ln2O3 (99.9%;
Ln ¼ Y, Dy, Ho, Er, Tm, Yb, Lu) and CuO
(99.9%). SrCuO2 was obtained by the conventional
solid-state reaction from SrCO3 (99.9%) and CuO
at 1000 °C in air. SrO2 was synthesized through a
solution route; H2O2 (reagent grade) and NH3
(reagent grade) were added into a SrCl2 (reagent
grade) aqueous solution, then SrO2 precipitated
was dried at 150 °C in flowing O2. These materials
were mixed in an agate mortar to obtain starting
mixtures for high-pressure synthesis. About 300
mg of the mixture was sealed in a gold capsule and
allowed to react in a flat-belt-type high-pressure
apparatus at 6 GPa, and at 1200–1300 °C for 3 h,
then quenched to room temperature. The weight
of the gold capsule was checked before and after
the high-pressure run. The weight change was al-
ways less than 0.2 mg against the sample weight of
ꢁ300 mg. High PðO2Þ HIP treatment was carried
out for some selected high-pressure products at
600 °C for 40 h in an atmosphere of approximately
2000 kgf/cm2 by the mixture of argon oxygen gas
(Ar:O2 ¼ 80:20).
ꢀ
ꢀ
3:8192ð8Þ A, c ¼ 11:516ð2Þ A for sample (A) and
ꢀ
ꢀ
a ¼ 3:8168ð7Þ A, c ¼ 11:522ð2Þ A for sample (B).
The phase-purity was better in sample (A) than in
sample (B), suggesting that lower reaction tem-
perature (in this case, 1200 °C) is appropriate to
obtain higher phase purity.
X-ray powder diffraction patterns were ob-
tained by a diffractometer (Philips-PW1800) with
Cu Ka radiation and lattice constants were deter-
mined using the least squares method. DC sus-
ceptibility data were collected by a SQUID
magnetometer (quantum design, MPMS) at a
magnetic field of 1 Oe and in the temperature
According to DC susceptibility data (it is shown
in Fig. 3), both samples showed ferromagnetic
transitions near 150 K. Diamagnetism was seen for

T. Kawashima, E. Takayama-Muromachi / Physica C 398 (2003) 85–94
87
110
103
110
103
RuSr2YCu2O8
213
116
Ru0.9Sr2YCu2O7.8
200
006
a=3.8192(8) Å
c=11.516(2) Å
104
005
116
200
213
114
105
a= 3.817(1) Å
115 211 212
006
102
101
113
104
005
002
c=11.500(7) Å
212
007
114
105
102
113
111
101
002
*
115
211
*
sample (C) before HIP-treatment
(Sr2YRuO6
)
(Sr2YRuO6
)
110
103
sample (A) (treated at 1200 °C)
110
103
(Sr YRuO
)
(Sr2YRuO6
)
(Sr22YRuO
)
6
6
213
116
006
200
104
005
(CuO)
(CuO)
(Sr2YRuO6
)
116
213
200
006
114
105
a=3.8168(7) Å
c=11.522(2) Å
114
105
a= 3.815(1) Å
c=11.503(5) Å
115
101 102
113
007
104
005
212
002
113
111
101
102
211
002
115
*
*
*
sample (C) after HIP-treatment
10 20
sample (B) (treated at 1300 °C)
20 30
30
40
50
60
40
50
60
2θ (degree)
2θ (degree)
Fig. 2. Powder X-ray diffraction patterns of the samples having
a nominal composition, Ru0:9Sr2YCu2O7:8 [sample (C)] before
and after the O2-HIP treatment.
Fig. 1. Powder X-ray diffraction patterns of the samples having
a nominal composition, RuSr2YCu2O8. Sample (A) was treated
at 1200 °C while sample (B) at 1300 °C. Peaks due to SrRuO3
and CuO are labeled by ꢂ.
uted to CuO, all peaks could be indexed by a te-
ꢀ
tragonal cell with a ¼ 3:817ð1Þ, c ¼ 11:500ð7Þ A
sample (B) below 39 K both in the FC and ZFC
data and it looked large enough to assume bulk
superconductivity. On the contrary, only minimal
diamagnetism was observed for sample (A) even in
the ZFC data. These results suggest that higher
reaction temperature (in this case, 1300 °C) is fa-
vorable for increasing a volume fraction of the
superconducting phase. Ferromagnetism and su-
perconductivity seemed to coexist in sample (B).
However, it should be noted that SrRuO3, which is
contained in the both samples as an impurity, has
a ferromagnetic transition near 160 K [34]. To see
the influence of SrRuO3, we synthesized SrRuO3
and measured its DC magnetic susceptibility with
the same measuring conditions. The result revealed
that about 5% contamination of SrRuO3 can ac-
count for the ferromagnetic susceptibilities ob-
served in the samples. The SrRuO3 contents in the
samples (A) and (B) are very roughly estimated to
be 1–2% which may not be small enough to be
ignored. A SrRuO3-free sample is, therefore, nee-
ded to confirm the magnetic properties of Ru-
1212Y.
(see the pattern denoted as ‘‘before HIP-treat-
ment’’), and the SrRuO3 peaks were not observed
at all in the pattern even in an enlarged scale. DC
susceptibility data of this sample is shown in Fig.
3. Ferromagnetic transition was observed near 150
K as well as in the samples (A) and (B). The
magnetic susceptibility at 5 K was slightly (15–
20%) smaller compared with sample (A) which
may be attributed to the removal of SrRuO3. Ac-
cording to these results, we concluded that the
ferromagnetism appearing near 150 K is not due to
We carried out many experiments with various
synthesis conditions, and a SrRuO3-free sample
was obtained with ÔRu-poorÕ starting composition
at 1200 °C. Fig. 2 indicates the powder X-ray
diffraction pattern of the high-pressure product
having a nominal composition Ru0:9Sr2YCu2O7:8
[sample (C)]. Except for a very small peak attrib-
Fig. 3. DC susceptibility data of the sample (A), sample (B)
and sample (C). The arrow part in this figure shows the mag-
netic transition of Sr2YRuO6.

88
T. Kawashima, E. Takayama-Muromachi / Physica C 398 (2003) 85–94
SrRuO3 but reflects the magnetism of the Ru-
1212Y phase. In this paper, we sometimes deal
with samples which was not phase pure, but con-
tained SrRuO3 with similar percentages to those in
the samples (A) and (B). However, according to
the aforementioned results, such a small level of
contamination does not affect seriously the quali-
tative discussion given below.
Sample (C) was almost phase pure but did not
show large enough diamagnetism to assume bulk
superconductivity. Because higher synthesis tem-
perature seems favorable to increase the super-
conducting volume fraction, we tried a reaction
temperature of 1300 °C. In addition, we checked
effect of starting oxygen content on the appear-
ance of superconductivity. Fig. 4 shows the pow-
der X-ray diffraction patterns of the high-pressure
1 Oe
Ru0.9Sr2YCu2Oy
y=7.80
y=7.85
y=7.90, sample (D)
y=7.95
10
FC
Tc=37 K
5
0
ZFC
0
50
100
150
200
T (K)
Fig. 5. DC susceptibility data of the samples corresponding to
Fig. 4.
In the y ¼ 7:95 sample, another ferromagnetic-
like transition appeared at 27 K under the FC
measurement. This anomaly was always observed
in high-oxygen-content samples with low phase
purities. We assume that the anomaly is ascribed
to an unknown ferromagnetic impurity phase al-
though it is not identified yet.
Fig. 6 indicates DC electric resistivity data for
the two Ru0:9Sr2YCu2Oy samples with y ¼ 7:8
[sample (C) in Fig. 2] and y ¼ 7:9 [sample (D) in
Fig. 4]. Only minimal diamagnetism was observed
in the former sample (Fig. 3) while fairly large one
in the latter (Fig. 5). However, both samples
showed semiconducting behavior without zero-
samples with the nominal composition of Ru0:9
-
Sr2YCu2Oy (y ¼ 7:80–7.95). Small amounts of
SrRuO3 and CuO were included in all samples.
The impurity content is minimal for y ¼ 7:80 and
increases with increasing the y-value. The lattice
parameters, which are shown in Fig. 4, do not
change substantially with the variation of the
oxygen content. DC susceptibility data of these
samples are shown in Fig. 5. The oxygen content
has a clear effect on the ferromagnetic transition
point near 150 K, i.e., it tends to shift to lower
temperature direction with increasing the y-value.
Below 37 K, a fairly large Meissner effect, as well
as shielding effect, was observed in the high-oxy-
gen-content sample with y ¼ 7:90 [sample (D)].
Ru0.9Sr2YCu2Oy
110
103
213
116
200
a= 3.815(1) Å
c=11.514(3) Å
006
104
005
007
114
105
115
115
002
102
113
113
101
101
004
*
*
*
*
212
*
*
*
*
211
y=7.95
213
116
110
103
a=3.8183(8) Å
c=11.522(3) Å
200
006
104
005
007
211
114
105
002
sample (D)
102 004
212
y=7.90
110
103
200
006
213
116
a= 3.815(1) Å
c=11.507(3) Å
104
005
007
211
114
105
002
115
115
113
004
102
102
101
101
212
y=7.85
213
110
103
200
a=3.8176(9) Å
c=11.515(2) Å
104
005
116
006
114
105
007
211
002
113
004
30
212
y=7.80
20
40
50
60
2θ (degree)
Fig. 4. Powder X-ray diffraction patterns of the sample having
nominal composition, Ru0:9Sr2YCu2Oy (y ¼ 7:80–7.95). Peaks
due to SrRuO3 and CuO are labeled by ꢂ.
Fig. 6. DC resistivity data of sample (C) and sample (D) before
and after the O2-HIP treatment.

T. Kawashima, E. Takayama-Muromachi / Physica C 398 (2003) 85–94
89
8
6
4
2
resistivity until 5 K. Moreover, resistivity values of
both samples were larger by at least three order of
magnitude compared with normal-state resistivi-
ties of typical cuprate superconductors. For the
Ru-1212Gd phase, semiconducting resistivity was
also reported before post annealing in oxygen [35].
However, resistivity itself was quite low (about 20
mX cm at largest) compared with our samples
(about 7 X cm at largest). Takagiwa et al. reported
that their Ru-1212Y sample showed zero-resistiv-
ity after O2-HIP post-treatment [32]. Lattice
sample (C)
0
8
sample (D)
6
4
2
0
ꢀ
parameters of their sample (a ¼ 3:843 A and
ꢀ
c ¼ 11:555 A) are greatly different from ours
suggesting a different cation and/or oxygen com-
position.
We also tested the HIP treatment with oxygen
pressure of 400 atm for sample (C) and sample (D)
and the zero-resistivity was indeed attained in the
both samples as shown in Fig. 6. However, as
shown in Fig. 2 for sample (C), the Ru-1212Y
phase was decomposed to Sr2YRuO6 and CuO
after the HIP treatment. The same decomposition
reaction occurred in sample (D), as well. More-
over, Fig. 3 indicates that the superconducting
volume fraction did not increase by the HIP
treatment. (The anomaly near 27 K in the FC data
of the HIP treated sample is due to the magnetic
transition of Sr2YRuO6 [36].)
The aforementioned experimental data suggest
strongly that the samples were inhomogeneous
with a mixed state of superconducting and insu-
lating regions and that the decrease of the resis-
tivity after the HIP treatment was due to some
physical change in grain boundary. To check this
suspicion, we carried out EPMA analysis for
sample (C) and sample (D) without the HIP
treatment. According to microscope observations,
the grain sizes of the samples were 5–10 lm. We
selected about 30 large grains for each sample and
analyzed their cation compositions by focusing the
electron beam at the center of a grain. The average
cation compositions of both samples were roughly
identical with the ideal composition of RuSr2-
YCu2Oy. However, after more careful analysis, it
was found that Ru/Cu ratio is distributed in a
different way in the two samples. Fig. 7 shows the
number of grains observed as a function of the Ru/
Cu ratio. In sample (C), which showed minimal
0.40
0.45
0.50
0.55
Ru/Cu
Fig. 7. EPMA Ru/Cu ratio for sample (C) and sample (D)
before the O2-HIP treatment.
diamagnetism, the Ru/Cu ratio looks to obey a
normal distribution curve around the point of Ru/
Cu ꢃ 0.5. On the other hand, the Ru/Cu ratio
distributed more widely in sample (D) for which
fairly large diamagnetism was observed. Its dis-
tribution histogram suggests existence of Ru-poor
grains in the sample and formation of solid solu-
tion of (Ru1ꢀxCux)Sr2YCu2O8 [(Ru,Cu)-1212Y] as
well as previous reports for Ru-1212Gd and Ru-
1212Eu [25,27].
Generally, broadening of (0 0 L) peaks in the
powder X-ray diffraction pattern is observed when
formation of solid solution is occurred. To inves-
tigate the relation of peak width and the physical
properties, we checked the peak sharpness and
width of (0 0 L) peaks of sample (C) and sample
(D). However, we could not detect any difference
of peak width between both X-ray diffraction
patterns.
In order to confirm the existence of the solid
solution, we tested (Ru1ꢀxCux)Sr2YCu2O8ꢀx (x ¼
0:1–0.3) nominal compositions, at 6 GPa and 1300
°C. Although we do not show their X-ray patterns
here, the phase purities of these samples were
quite high and all peaks could be indexed with

90
T. Kawashima, E. Takayama-Muromachi / Physica C 398 (2003) 85–94
tetragonal system. With increasing the Cu content,
the c-dimension tends to shrink. However degree
(Ru0.9Cu0.1)Sr2YCu2Oy
1 Oe
10
8
ꢀ
of the shrinkage is not so high (about 0.01 A per
y=7.9
y=8.0
y=8.1
10% Cu substitution), and in addition, the a-di-
mension is almost independent of the Cu content.
From these results, the presence of the Ru/Cu solid
solution was confirmed but it was also found that
the deviation of the Ru/Cu ratio from the stoi-
chiometric one is not easily detected by the X-ray
diffraction measurement.
FC
6
4
2
0
ZFC
-2
-4
As a next step, we examined effect of oxygen
content on the phase formation and supercon-
ductivity for the samples with x ¼ 0:1 and 0.2. Fig.
8 shows the powder X-ray diffraction patterns of
the x ¼ 0:1 samples with y ¼ 7:9–8.1. The phase
purity was lower and SrRuO3 was formed in the
high oxygen content samples with y ¼ 8:0 and 8.1.
The length of c-axis extended with the increase of
the oxygen content, which may suggest the intro-
duction of excess interstitial oxygen atoms. DC
susceptibility data of these samples are shown in
Fig. 9. With increasing the oxygen content, ferro-
magnetic transition near 150 K shifted to low
temperature direction and the magnetic moment
due to the ferromagnetism become smaller. Similar
tendency was observed in the Ru0:9Sr2YCu2Oy
samples as described before (see Fig. 5). Under the
field-cooling condition, another ferromagnetic-like
transition appeared in the y ¼ 8:1 sample. Similar
phenomenon was also observed in the Ru0:9Sr2-
0
50
100
150
200
T (K)
Fig. 9. DC susceptibility data of the samples corresponding to
Fig. 8.
Fig. 10. DC resistivity of the samples corresponding to Fig. 8.
YCu2Oy sample with the high oxygen content (see
Fig. 5).
110
(Ru0.9Cu0.1)Sr2YCu2Oy
103
213
116
212
200
006
a= 3.815(1) Å
c=11.521(4) Å
007
211
104
005
114
105
Fairly large diamagnetism was observed in the
y ¼ 7:9 sample in spite of its small oxygen content.
However, zero-resistivity was only observed in the
y ¼ 8:1 sample in the DC resistivity measurements
as shown in Fig. 10. Normal-state resistivity values
were quite high in the three samples, in particular,
in the y ¼ 7:9 one. In order to confirm the sam-
ple reproducibility, we prepared another y ¼ 7:9
sample with the same synthesis conditions to ob-
tain essentially the same susceptibility and resis-
tivity data.
113
*
*
102
101
101
101
*
115
*
002
*
y=8.1
110
103
213
200
a= 3.815(1) Å
c=11.516(3) Å
006
116
104
005
007
211
113 114
105
102
115
*
002
*
212
y=8.0
110
103
104
005
006
213
a= 3.816(1) Å
c=11.514(3) Å
116
200
007
211
115
114
105
002
102
113
004
*
212
y=7.9
10
20
30
40
50
60
2θ (degree)
Fig. 8. Powder X-ray diffraction patterns of the sample having
nominal compositions, (Ru0:9Cu0:1)Sr2YCu2Oy (y ¼ 7:9–8.1).
Peaks due to SrRuO3 and CuO are labeled by ꢂ.
Fig. 11 shows the powder X-ray diffraction
patterns of x ¼ 0:2 samples with y ¼ 7:8–8.1.

T. Kawashima, E. Takayama-Muromachi / Physica C 398 (2003) 85–94
91
110
103
(Ru0.8Cu0.2)Sr2YCu2Oy
200
213
a= 3.808(1) Å
c=11.514(7) Å
104
006
*
116
212
007
005
114
113 105
102
102
*
*
101
101
101
101
002
*
* *
211
*
115
y=8.1
110
103
200
213
a= 3.813(1) Å
c=11.513(3) Å
006
114
104
005
116
007
115
211
113
113
113
002
*
105
212
y=8.0
110
103
200
200
213
006
a= 3.816(1) Å
c=11.512(3) Å
104
005
116
007
115 211
114
105
102
102
*
002
212
y=7.9
110
103
213
116
a= 3.815(1) Å
c=11.504(5) Å
104
005
006
007
115 211
114
105
002
*
212
y=7.8
10
20
30
40
50
60
2θ (degree)
Fig. 11. Powder X-ray diffraction patterns of the sample having
nominal compositions, (Ru0:8Cu0:2)Sr2YCu2Oy (y ¼ 7:8–8.1).
Peaks due to SrRuO3, CuO and unknown impurity phases are
labeled by ꢂ.
Fig. 13. DC resistivity of the samples corresponding to Fig. 11.
They were almost single-phase 1212-type samples,
though SrRuO3 and other unknown impurities
were formed in the y ¼ 8:1 sample. The c-dimen-
sion tends to extend with the increase of the oxy-
gen content, similarly to the case of x ¼ 0:1.
Magnetic susceptibility data of the x ¼ 0:2 samples
are shown in Fig. 12. The appearance of the fer-
romagnetism was affected by the oxygen content in
a very complicated way. In the y ¼ 7:9 sample, the
ferromagnetism was, compared with the y ¼ 7:8
one, suppressed drastically with a low transition
temperature of approximately 120 K and a small
ferromagnetic moment. When the oxygen content
was increased to 8.0 and 8.1, the ferromagnetism
was recovered again. The additional ferromag-
netic-like transition at 27 K was observed in the
y ¼ 8:0 and 8.1 samples.
Diamagnetic moments were observed both in
the ZFC and FC measurements for all samples
and superconducting volume fraction at 5 K
tends to increase with increasing the oxygen con-
tent. The superconducting transition temperature
also tends to raise with the oxygen content from
43 K in y ¼ 7:8 to 65 K in y ¼ 8:1. As shown in
Fig. 13, normal-state resistivity decrease dras-
tically in the high-oxygen content samples and
zero-resistivity was attained in x ¼ 8:0 and 8.1.
Nevertheless, the normal-state resistivity values of
these two samples were at least one order of
magnitude higher than those of normal cuprate
superconductors.
The aforementioned experimental results can be
summarized as follows:
(1) There is a solid solution of (Ru1ꢀxCux)Sr2-
YCu2Oy and its range terminates at x ꢁ 0:1
without formation of the stoichiometric phase
of RuSr2YCu2Oy.
(2) Oxygen content y in the (Ru1ꢀxCux)Sr2-
YCu2Oy phase is also variable.
(3) Superconductivity is largely affected by the
oxygen content y. Superconducting transition
Fig. 12. DC susceptibility data of the samples corresponding to
Fig. 11.

92
T. Kawashima, E. Takayama-Muromachi / Physica C 398 (2003) 85–94
temperature tends to be raised with increasing
the oxygen content of starting mixture.
3.2. RuSr2LnCu2O8 (Ln ¼ Dy, Ho, Er) phase
(4) Ferromagnetism tends to be suppressed by in-
creasing the Cu content, x.
In order to obtain RuSr2LnCu2O8 (Ln ¼ Dy,
Ho, Er, Tm, Yb, Lu) phases, we first tested the
stoichiometric compositions at 1200 °C and 6
GPa. Only for Ln ¼ Dy, Ho and Er, the 1212-type
phases were formed, while other systems were
crystallized in simple perovskite structures. In the
samples of Ln ¼ Dy, Ho and Er, SrRuO3 was in-
cluded as an impurity phase. However, the im-
purity phase disappeared by starting from the
Ru-poor compositions as well as in the Ln ¼ Y
system. Fig. 14 shows the powder X-ray diffrac-
tion patterns of the Ru0:9Sr2LnCu2O7:8 (Ln ¼ Dy,
(5) Influence of oxygen content on the ferromag-
netism is not always simple. In the case of
the x ¼ 0:2 samples, ferromagnetic behavior
changes in a complicated way as a function
of the y-value.
(6) High-pressure samples show high normal-state
resistivities even when they have large diamag-
netic susceptibilities at low temperatures.
Through the present experiment, we cannot
confirm the coexistence of ferromagnetism and
superconductivity for the Y-based system. On the
contrary, the present results seem to cast doubt for
the coexistence. The normal-state resistivity is al-
ways very high even when a sample shows a large
diamagnetic susceptibility. The apparent coexis-
tence of superconductivity and ferromagnetism
may be explained by assuming a phase separation
in a sample regarding the Ru/Cu ratio, i.e., super-
conductivity may be ascribed to Ru-poor grains
with large x values while the ferromagnetism to Ru-
rich grains with small x values which have semi-
conducting nature. However, two parameters of
the Ru/Cu ratio and the oxygen content affect the
superconductivity and ferromagnetism, and it
makes the system quite complex. We need further
studies for the final conclusion.
110
103
Ru0.9Sr2LnCu2O7.8
213
a= 3.810(1) Å
c=11.505(4) Å
200
116
104
005
006
114
105
101
100
002
113
202 115 211 007
Ln=Er
102
102
004
004
200
006
110
103
213
116
a= 3.813(1) Å
c=11.499(5) Å
104
005
101
100
114
002
113
113
202 115 211
007
007
105
Ln=Ho
110
103
200
006
213
116
a= 3.820(1) Å
c=11.512(4) Å
104
005
101
100
114
105
211
002
202 115
50
102 004
30
Ln=Dy
20
40
60
2θ (degree)
Fig. 14. Powder X-ray diffraction patterns of the sample having
nominal composition, Ru0:9Sr2LnCu2O7:8 (Ln ¼ Dy, Ho, Er).
Fig. 15. DC susceptibility data of the samples corresponding to Fig. 14. Insert in (c) indicates DC resistivity of the Ln ¼ Er sample.

T. Kawashima, E. Takayama-Muromachi / Physica C 398 (2003) 85–94
93
Ho, Er) samples. Single-phase 1212 samples were
obtained for the three systems.
tivity for the Y-based system. On the contrary, the
present results seem to cast doubt for the coexis-
tence because the normal-state resistivity is always
very high even when a sample shows a large dia-
magnetic susceptibility. The apparent coexistence
of superconductivity and ferromagnetism may be
explained by assuming a phase separation in a
sample regarding the Ru/Cu ratio for the solid
solution of (Ru1ꢀxCux)Sr2YCu2Oy.
For Ln ¼ Dy, Ho and Er systems, single-phase
1212 samples were obtained with the Ru-poor
starting composition as well as in the Ln ¼ Y sys-
tem. According to DC susceptibility measure-
ments, every sample exhibited a ferromagnetic
transition near 170 K and only Ln ¼ Er sample
showed diamagnetism below 43 K. However, it
had high resistivity and zero-resistivity was not
attained.
DC susceptibility data are shown in Fig. 15 for
the three single-phase samples. All samples ex-
hibited ferromagnetic transitions near 170 K and
only the Ln ¼ Er sample showed diamagnetism
below 43 K. However, in DC resistivity data of the
Ln ¼ Er sample (insert in Fig. 15(c)), zero-resis-
tivity was not observed until 5 K though resistivity
tended to decrease below 23 K. Moreover, resis-
tivity value was quite high like the Ln ¼ Y system.
We cannot confirm the coexistence of ferromag-
netism and superconductivity for the Ln ¼ Er
system as well as for the Ln ¼ Y system.
4. Conclusion
We prepared the 1212-type ruthenate-cuprates
RuSr2LnCu2O8 (Ln ¼ Y, Dy, Ho, Er) at 1200–
1300 °C under 6 GPa. For the Ln ¼ Y phase, high-
purity sample was obtained with the Ru-poor
starting composition, Ru0:9Sr2YCu2O7:8, treated
at 1200 °C. According to DC susceptibility mea-
surements, it showed the ferromagnetism near 150
K but with minimal diamagnetic susceptibility. On
the other hand, the sample prepared at 1300 °C
Acknowledgements
The present authors express their sincere thanks
to Drs. M. Akaishi and S. Yamaoka (NIMS) for
helpful suggestions on the high-pressure synthesis,
to A. Watanabe (NIMS) for HIP-experiments and
to K. Kosuda (NIMS) for EPMA measurement.
This work was supported by the Multi-Core Pro-
ject and the COE project organized by the Minis-
try of Education, Culture, Sports, Science and
Technology, Japan.
with a higher oxygen content, Ru0:9Sr2YCu2O7:9
,
underwent ferromagnetic transition near 150 K
followed by diamagnetic transition at 37 K. Both
samples did not show zero-resistivity and had
quite high resistivity values compared with nor-
mal cuprate superconductors. After O2-HIP post-
treatment, these samples showed zero-resistivity
but decomposition of the 1212 phases occurred.
The EPMA analysis suggested existence of
Ru-poor grains in the samples and formation of
solid solution of (Ru1ꢀxCux)Sr2YCu2O8. Indeed,
we could obtain the single-phase sample of
(Ru1ꢀxCux)Sr2YCu2Oy in the range of x ¼ 0:1–0.3
at 1300 °C. Superconductivity and ferromagnetism
were largely affected by the Ru/Cu ratio and the
oxygen content y. Superconducting transition
temperature tends to be raised with increasing the
oxygen content while ferromagnetism tends to be
suppressed by increasing the Cu content. In spite
of testing many samples, we could not confirm the
coexistence of ferromagnetism and superconduc-
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