APPLIED PHYSICS LETTERS
VOLUME 77, NUMBER 18
30 OCTOBER 2000
Enhanced pinning in a magnetic-superconducting bilayer
A. Garcı́a-Santiagoa)
Departament de Fı́sica Fonamental, Universitat de Barcelona, Diagonal 647, Plta. 3, 08028 Barcelona,
Spain
F. Sánchez and M. Varela
Departament de Fı́sica Aplicada i Òptica, Universitat de Barcelona, Diagonal 647, Plta. 4, 08028 Barcelona,
Spain
J. Tejada
Departament de Fı́sica Fonamental, Universitat de Barcelona, Diagonal 647, Plta. 3, 08028 Barcelona,
Spain
共Received 19 May 2000; accepted for publication 8 September 2000兲
Pinning of vortices in a high-temperature superconductor by the magnetic domain structure of a
highly anisotropic ferromagnet is investigated by means of magnetic measurements in nanoscale
period superconductor/ferromagnet 共SC/FM兲 heterostructures. Two different samples consisting
of highly epitaxial films of YBa2Cu3O7共SC兲 and BaFe12O19共FM兲 are analyzed relative to a
pure superconducting YBa2Cu3O7 film. The irreversibility line obtained in the
magnetic-field-reduced-temperature phase diagram for each heterostructure is found to shift
upwards when compared to the line corresponding to the pure superconducting sample. This effect
is interpreted as an evidence for the enhancement of pinning of vortices in the SC layer by the
magnetic domain structure in the FM layer. © 2000 American Institute of Physics.
关S0003-6951共00兲02644-9兴
The rich variety of static and dynamic properties of
type-II superconductors is mostly due to the interaction between vortices and pinning centers.1 The enhancement of
pinning of vortices is of major importance when considering
the technological applications of these superconductors. The
introduction of artificial pinning centers such as magnetic
particles or dots on the surface of a superconducting film,2–5
thickness modulation,6 micrometer-scale holes,7,8 and columnar defects9,10 has produced some important practical results.
With the appearance of new lithographic techniques it has
also become possible to study the pinning effect of size,
material, and separation of particles deposited on the surface
of a superconductor. The magnetic contribution to the pinning mechanism has been recently established in
experiment.11–13
All the earlier studies of pinning enhancement are based
upon the idea that defects destroy superconductivity locally,
acting as attractors for the normal vortex core. The intrinsic
limitation on the strength of pinning forces due to defects
comes from the fact that they only pin the normal core of
vortices, which is very small in high temperature superconductors 共a typical value for the superconducting coherence
length ␰ is 25 Å; see, for instance, Ref. 1兲. In this case, the
maximum pinning energy equals the condensation energy of
Cooper pairs in the volume of the core, U cp⬃(⌽ 0 /8␲ ␭) 2 ,
where ⌽ 0 is the quantum of magnetic flux and ␭ is the magnetic penetration depth.
Very recently a new pinning mechanism has been
suggested.14 The main theoretical idea is to pin the magnetic
flux of the vortex rather than its core.14–16 In high temperature superconductors the magnetic volume of the vortex is
a兲
Electronic mail: antonio@ubxlab.com
many orders of magnitude larger than the volume of the core
共a typical value for the magnetic penetration depth ␭ is 1500
Å; Ref. 1兲. It has been demonstrated that the pinning of vortices by magnetic inhomogeneities 共domains兲 in
superconductor/ferromagnet 共SC/FM兲 multilayers can be 100
times greater than the pinning by columnar defects.14 A bilayer system formed by a superconductor and a magnetic
layer with perpendicular anisotropy offers the possibility to
test this new idea.
In this letter we present experimental results for the irreversibility line in a SC/FM heterostructure prepared by
pulsed laser deposition. Here SC stands for a YBa2Cu3O7
superconductor 共YBCO兲 and FM stands for barium hexaferrite, BaFe12O19共BFO兲.
The growth of such heterostructure is conditioned by the
different crystal structure of these materials and the strong
chemical interactions between them during the high temperature layer deposition. This problem has been solved by using
a very thin film of ZrO2 –Y2O3 共yttria-stabilized-zirconia,
YSZ兲 as a buffer layer. The YBCO/YSZ/BFO structures
were deposited on YSZ 共001兲 substrates by pulsed laser
deposition using a KrF excimer laser. The heterostructures
were prepared in a single process, by sequentially focusing
the laser beam to about 2 J/cm2 on stoichiometric targets of
BaFe12O19, ZrO2 共mole 9% Y2O3 doped兲 and YBa2Cu3O7.
BFO films, 100 nm thick, were prepared at optimum conditions of 800 °C substrate temperature and 0.1 mbar oxygen
pressure. Afterwards, and prior to the YBCO growth, a
buffer layer was found to be necessary for the YBCO films
to have the required superconducting properties. A YSZ
layer was chosen for that purpose because of its high structural compatibility with both, the BFO bottom film and the
YBCO top film. YSZ has also very low chemical reactivity
0003-6951/2000/77(18)/2900/3/$17.00
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© 2000 American Institute of Physics
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Appl. Phys. Lett., Vol. 77, No. 18, 30 October 2000
FIG. 1. ZFC–FC curves obtained for sample YBCO 共right scale兲 under a
magnetic field of 500 Oe 共solid circles兲 and sample YYBY 100/100 共left
scale兲 under magnetic fields of 500 Oe 共open circles兲 and 1000 Oe 共solid
squares兲.
with both materials, as was confirmed by the preparation of
heterostructures with YSZ buffer layers as thin as 10 nm
thick. The optimal processing parameters 共800 °C, 3
⫻10⫺4 mbar oxygen兲 for the YSZ layers were found to be
coincident with those found in the direct growth on Si 共100兲
substrates.17 The top film in the heterostructure, YBCO, required a higher oxygen pressure of 0.3 mbar during the
growth, but showed optimal superconducting properties at
the same growth temperature used for the other films
共800 °C兲, allowing fast and reproducible deposition. Heterostructures of YBCO and BFO films 共YYBY兲 with relative
thicknesses of 100/100 and 100/80 nm were prepared.
The thickness of the films was measured by stylus profilometry. X-ray diffraction measurements in a four circle
diffractometer with CuK ␣ radiation revealed that
the BFO and YBCO films are single-oriented, with 共001兲
out-of-plane orientation. The in-plane analysis showed
that the films are epitaxial with the relationship
共001兲YBCO关 100兴 储 共0001兲BFO关 11– 20兴 储 共001兲YSZ关 010兴 . A
detailed study on crystal properties, chemical composition,
and surface morphology of the films will be published elsewhere.
Electrical transport properties were measured as a function of temperature between 30 and 300 K, using the four
contact method. These measurements produced superconducting transition temperatures of 90, 88, and 87.5 K for
samples YBCO, YYBY 100/100, and YYBY 100/80, respectively. The zero field cooled 共ZFC兲 and field cooled 共FC兲
magnetization curves as a function of temperature, and the
isothermal magnetization curve as a function of magnetic
field were obtained using a commercial superconducting
quantum interference device magnetometer.
First we measured the superconducting properties of a
pure 100 nm YBCO film deposited onto a LaAlO3 substrate,
as well as the magnetic properties of a pure 100 nm BFO
film deposited onto YSZ, to have a reference prior to the
investigation of the heterostructures. Figure 1 shows the
ZFC–FC curves obtained in the first sample for a field of 500
Oe 共solid circles兲. For a given magnetic field, the temperature at which the curves merge determines the corresponding
point on the irreversibility line 共IL兲 in the magnetic-fieldtemperature phase diagram. Figure 2 shows in a reduced
temperature scale the IL extracted from these measurements
Garcı́a-Santiago et al.
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FIG. 2. Irreversibility lines 共in a reduced temperature representation兲 extracted from ZFC–FC data for samples YBCO 共solid circles兲, YYBY 100/
100 共solid squares兲, and YYBY 100/80 共open triangles兲.
and those obtained at higher fields for this sample 共solid
circles兲.
The magnetic properties of the pure 100 nm BFO film
were inferred from the isothermal magnetization curve recorded at different temperatures. The hysteresis cycles thus
obtained showed that the easy axis of the magnetization is
perpendicular to the plane of the film. The coercive and anisotropy fields are 6 and 10 kOe, respectively, and turn out to
be approximately constant between 5 and 100 K.
The ZFC and FC curves for the YYBY 100/100 heterostructure obtained for magnetic fields of 500 and 1000 Oe
are shown in Fig. 1 as open circles and solid squares, respectively. ILs derived for this sample and sample YYBY 100/80
are shown in Fig. 2 as solid squares and open triangles, respectively. Isothermal magnetization measurements above
the superconducting transition temperature revealed that both
the magnetic anisotropy and the coercive field of the BFO
layer in the heterostructure are similar to those obtained for
the pure BFO film. As it is seen from Fig. 2, the ILs for the
heterostructures are shifted upwards with respect to the IL
for the pure YBCO film, indicating that pinning is more intense in the former that in the latter.
We shall discuss these results within the scope of the
theoretical approach suggested by Bulaevskii et al.14 The effective magnetic field B in the YBCO layer is a sum of the
applied magnetic field H and the demagnetizing field 4 ␲ M
in the BFO layer: B⫽H⫹4 ␲ M . Magnetic domain walls are
pinned if the applied field is lower than the coercive field,
which is of the order of 5 kOe for the BFO layer. However,
a stripe magnetic domain structure, and consequently pinning of vortices due to these domains,14 should be present
until the BFO film is saturated, that is up to 10 kOe. The
stripe domain structure modulates the effective magnetic
field through the YBCO layer, creating the pinning potential
for vortices. The strength of such potential depends on the
absolute value of the magnetization, which is constant at low
temperature. Therefore, one should expect this potential to
remain almost unchanged below the superconducting transition temperature if the applied field is not too high. In fact,
this is what has been observed in our heterostructures: the IL
of the bilayer lies above the IL of a pure YBCO layer until
10 kOe, the maximum field in our measurements. At each
temperature, the effectiveness of the pinning enhancement
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Garcı́a-Santiago et al.
Appl. Phys. Lett., Vol. 77, No. 18, 30 October 2000
associated with the magnetic domains can be estimated as
the ratio of the irreversibility fields for the heterostructure
and the pure YBCO film. The maximum enhancement obtained in our experiments is close to the factor of 2.
This increase in the pinning potential should result in a
100% increase of the critical current. In the case of the pure
YBCO film, the pinning potential is due to defects, either
naturally existing or artificially created. The typical pinning
barrier for a unit cell due to defects U cp is of order 1000 K.14
In the case of the pinning potential created in the YBCO
layer by magnetic domains of the BFO layer, its maximum
value can be estimated from U mp⬃⌽ 0 M 0 d s , where M 0 is
the saturation magnetization of the BFO layer, and d s is the
thickness of the YBCO layer.14 The values of these parameters in our samples give a constant barrier of 10 000 K in
the entire temperature range. Thus, in principle, the critical
current in the heterostructure might be increased by a factor
of 10 with respect to the pure superconducting sample.
This work was made possible by CICYT Project Nos.
IN96-0027 and MAT99-0984, and CIRIT Project No.
1996PIRB-00050. A.G.S. thanks Ministerio de Educación y
Cultura for Contract No. 72532275 DLDTT 共Project No.
PB1996-0169兲.
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