Magnetic properties of La
0.67
Sr
0.33
MnO
3
Õ YBa
2
Cu
3
O
7
superlattices
P. Przyslupski, I. Komissarov, W. Paszkowicz, P. Dluzewski, R. Minikayev, and M. Sawicki
Institute of Physics, Polish Academy of Sciences, 02 – 668 Warszawa, Poland
Received 8 October 2003; published 29 April 2004
Oxide multilayers composed of ferromagnetic metallic layers of La
0.67
Sr
0.33
MnO
3
LSMO and supercon-
ducting cuprate YBa
2
Cu
3
O
7
YBCO were grown on 100 LaAlO
3
substrates by high-pressure dc sputtering.
We have investigated structural, transport, and magnetic properties of a series of samples in which the layer
thickness of LSMO is fixed at 16 unit cells and that of YBCO varied from 1 to 8 unit cells. Exchange-induced
unidirectional anisotropy in LSMO/YBCO superlattices has been demonstrated. Observation of an exchange
biasing effect in LSMO/YBCO superlattices supports an existence of interlayer exchange coupling between
LSMO layers through YBCO superconducting spacer layers.
DOI: 10.1103/PhysRevB.69.134428 PACS numbers: 74.78.Fk, 75.60.-d, 75.70.Cn
I. INTRODUCTION
Superconducting S and ferromagnetic F multilayers
are interesting from fundamental point of view, since they
offer the appropriate scenario to study competing effects of
superconductivity and ferromagnetism on the scale of their
characteristics lengths. The proximity effect in S/F multilay-
ers is related to new physical phenomena arising from the
interaction between two order parameters. For example, it
was theoretically predicted
1,2
that a magnetic interlayer ex-
change coupling between ferromagnetic layers separated by
d-wave superconductor is possible. The model suggests that
the ground state of the S/F heterostructures changes from
aligned magnetization to antiparallel magnetization with the
period of two unit cells of the c-axis parameter of
YBa
2
Cu
3
O
7
YBCO system. The interplay between super-
conductivity and spin-polarized systems has also potential
applications in the emerging field of spin electronics.
3
The
half metals are suitable materials in this respect. Half metal-
lic behavior has been found experimentally in the manganese
perovskite La
0.67
Sr
033
MnO
3
LSMO.
4
The perovskites are
particularly interesting because of their ability to form high-
quality multilayers
5–9
with high-T
c
superconductors HTSC.
Understanding the interface properties of colossal magne-
toresistance materials CMR and HTSC heterostructures is
required for spin-based devices. The key factor in such stud-
ies is electronic and magnetic quality of the interface region.
Recent elipsometric measurements
10
of far infrared dielectric
measurements provide an evidence of hole charge transfer
from YBCO layers to LCMO layers in YBCO/LCMO super-
lattices. Due to this effect a dramatic depletion of holes for
about 1 nm thick YBCO layer was observed. On the other
hand an increase of holes in LCMO system in spite of nomi-
nal composition La
0.67
Ca
0.33
MnO
3
can drive the LCMO
manganite into antiferromagnetic AF state. In this work we
report and analyze some observations of structural and mag-
netic properties of LSMO/YBCO superlattices. If an effect of
hole charge transfer takes place also in LSMO/YBCO super-
lattices it could transform some of the LSMO layers into
F/AF system. It is known that when F/AF interfaces are
cooled through the Neel temperature T
N
an unidirectional
anisotropy
11
exchange biasing may develop in LSMO lay-
ers. Exchange biasing is manifested as a displacement of the
hysteresis loop along its field axis. Observation of the ex-
change biasing with superconducting spacer layers can indi-
cate a presence of interlayer exchange coupling between
LSMO layers through superconducting YBCO spacer layers.
II. EXPERIMENT
LSMO and YBCO thin films and LSMO/YBCO superlat-
tices were deposited on 100 LaAlO
3
single-crystal sub-
strates by multitarget high pressure sputtering.
6
During the
growth, the temperature of the substrate was set at 770 °C
and the oxygen pressure was kept at 3 mbar. Two targets
with nominal composition of La
0.67
Sr
0.33
MnO
3
and
YBa
2
Cu
3
O
7
were used for deposition. The thickness of dif-
ferent layers was controlled by the deposition times of re-
spective targets. The calibrated deposition rates were 1.6 nm
and 2.6 nm per minute for LSMO and YBCO, respectively.
The off-axis x-ray diffraction measurements Cu K radia-
tion were used for structure characterization. The dc mag-
netization were measured with superconducting quantum in-
terference device SQUID magnetometer in the temperature
range 5 K–300 K in magnetic field up to 0.1 T. Microstruc-
ture properties of samples were examined by transmission
electron microscope TEM JEOL 2000 EX. Samples used
for SQUID measurements were 3 mm3 mm in size. Re-
sistance versus temperature measurements were performed
with four-probe method.
TABLE I. Nominal modulation length, zero resistance tempera-
ture T
c 0
, and the temperature of the onset of diamagnetic moment
T
d0
of the La
0.67
Sr
0.33
MnO
3
/YBCO multilayers.
Sample No. Modulation length T
c 0
K T
d0
K
LaY110 LSMO16 u.c./YBCO1 u.c.
16
LaY105 LSMO16 u.c./YBCO2 u.c.
16
4.2
LaY107 LSMO16 u.c./YBCO3 u.c.
16
28 13
LaY109 LSMO16 u.c./YBCO4 u.c.
16
41 41
LaY106 LSMO16 u.c./YBCO5 u.c.
16
57 57
LaY108 LSMO16 u.c./YBCO6 u.c.
16
63 63
LaY111 LSMO16 u.c./YBCO8 u.c.
16
71 71
PHYSICAL REVIEW B 69, 134428 2004
0163-1829/2004/6913/1344287/$22.50 ©2004 The American Physical Society 69 134428-1