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CHIN. PHYS. LETT. Vol. 26, No. 1 (2009) 017502
Vortex Pinning due to Dynamic Spin–Vortex Interaction in a
Superconductor/Ferromagnet Multilayer *
WU Hong-Ye(吴鸿业), ZOU Tao(邹涛), CHENG Zhao-Hua(成昭华), SUN Young(孙阳)**
State Key Laboratory of Magnetism and Beijing National Laboratory for Condensed Matter Physics, Institute of
Physics, Chinese Academy of Sciences, Beijing 100190
(Received 4 September 2008)
We investigate the mutual interaction between superconductivity and ferromagnetism in a Nb/Ni81Fe19 multilayer
by ac susceptibility measurements. Compared with a pure superconducting Nb film, the critical current density
of the multilayer is apparently enhanced in a low magnetic field region but remains nearly the same in high
magnetic fields, which indicates that a continuous ferromagnetic layer with in-plane magnetization can produce
strong vortex pinning in a low field region. We interpret this unusual vortex-pinning phenomenon as a consequence
of dynamic spin–vortex interaction which induces a spin rotation following vortex movement. In addition, we
propose that this dynamic interaction could be used for spin manipulation via a superconductor.
PACS: 75. 75. +a ,75. 70. Cn ,74. 78. Fk
The study of mutual interactions between su-
perconductivity and ferromagnetism in superconduc-
tor/ferromagnet (SC/FM) heterostructures has at-
tracted considerable attention.[1−4]Initial interest has
been paid to proximity effects that lead to critical
temperature oscillation.[5]Then, vortex pinning phe-
nomena in SC/FM heterostructures have been exten-
sively investigated with respect to both device appli-
cations and fundamental physics.[6−11]In these stud-
ies, the FM layers are usually fabricated into dis-
continuous nanostructures, such as regular arrays of
magnetic dots[6−10]and random arrays of magnetic
nanoparticles.[11]Depending on the size and geome-
try of the array of magnetic dots/particles, a variety
of interesting pinning phenomena have been widely
observed. In those discontinuous nanostructures, vor-
tex pinning is mainly caused by the static interaction
between superconducting vortices and the fixed mag-
netic moments of magnetic dots.
In contrast, vortex pinning in continuous SC/FM
films has received little attention. It was generally
believed that a continuous FM layer with homoge-
neous magnetization would cause negligible pinning
effect because the vortex does not experience a poten-
tial difference. This is true in most cases where the
mutual interaction is not able to influence the spin ori-
entation in the FM layer, i.e., the spin moments are
regarded as fixed at specific sites. However, in cer-
tain circumstances where the mutual interaction may
change the spin structure itself,[11]some new effects
could arise. In this work, we demonstrate for the first
time that the continuous FM layers in an SC/FM het-
erostructure can produce strong vortex pinning effects
when the applied vertical magnetic field is not too
high. Such an unusual pinning phenomenon can be
explained by a dynamic spin–vortex coupling model.
The SC/FM heterostructure in this study is a
Nb/Ni81Fe19/Nb/Ni81Fe19/Nb multilayer fabricated
by dc magnetron sputtering on a glass substrate. The
details of sample preparation can be found in Ref. [12].
The thickness of each Ni81Fe19 (Py) layer is 10nm
so that it has a preferential in-plane easy magneti-
zation direction. Each Nb layer has a thickness of
100 nm which is large enough to ensure that the prox-
imity effect at the Nb/Py interface has negligible in-
fluences on the bulk properties of the superconduc-
tor. For comparison, a pure Nb film with a thickness
of 300 nm was also prepared under the same condi-
tion. Both the multilayer and the Nb film are cut into
discs with 3 mm diameter for ac susceptibility mea-
surements. The ac susceptibility was measured using
a quantum design physical property measurement sys-
tem (PPMS) at 2 and 4 K.
The vortex pinning effect in the SC/FM multilayer
is investigated based on the variation of critical cur-
rent density of the superconductor. We have employed
a method of ac susceptibility measurement to obtain
the critical current density. Ac susceptibility has been
widely used for the determination of the critical cur-
rent density 𝐽𝑐in superconducting materials.[14−16]It
is a non-contact measuring method and thus excludes
the influence of probes. Most experiments are carried
out on disc-shaped superconductors. As illustrated
in the inset of Fig. 2, the experiment is carried out
as follows: an ac field of amplitude ℎ0superimposed
to a dc field 𝐻𝑑𝑐 which is much larger (𝐻𝑑𝑐 ≫ℎ0),
are applied perpendicular to the superconductor sur-
face. In the ac loop, the critical current can be as-
*Supported by the National Natural Science Foundation of China under Grant No 10674170, and the National Basic Research
Programme in China under Grant No 2007CB925003.
**Email: youngsun@aphy.iphy.ac.cn
c
○2009 Chinese Physical Society and IOP Publishing Ltd
017502-1
CHIN. PHYS. LETT. Vol. 26, No. 1 (2009) 017502
sumed to be constant, and only determined by 𝐻𝑑𝑐,
i.e., 𝐽𝑐=𝐽𝑐(𝐻𝑑𝑐). A maximum in the out-of-phase
component of the first harmonic of the ac susceptibil-
ity 𝜒′′ can be found when the ac profile reaches the
centre of the sample. This fact allows one to determine
𝐽𝑐(𝐻𝑑𝑐) from ℎ0and the dimension of the sample. It
has be shown that a maximum in 𝜒′′(ℎ0) appears when
the relation
ℎ0= 0.971𝐽𝑐𝑑, (1)
is satisfied,[14,15]where 𝑑is the thickness of the super-
conducting disc.
In our situation, the multilayer sample is not a
single superconducting disc but three discs separated
by two thin Py layers. However, ac susceptibility of
the sample shows negligible out-of-phase component
above the superconducting transition temperature 𝑇𝑐,
which suggests that the thin Py layers have no direct
contribution to out-of-phase component of ac suscep-
tibility. In the superconducting state, the vertical flux
lines produced by the superconducting vortices should
penetrate through all the layers because the FM lay-
ers are thin compared to the penetration depth of Nb.
Thus, the multilayer can be regarded as a single super-
conducting disc with a thickness of 300 nm in the ac
susceptibility measurements and Eq.(1) still applies
for the multilayer.
Fig. 1. Temperature dependence of the in-phase ac sus-
ceptibility of the Nb/Ni81Fe19 multilayer and a pure Nb
film. The inset shows the 𝑀−𝐻loop of the multilayer at
10 K with field perpendicular to the film surface.
Figure 1shows the in-phase ac susceptibility of
the multilayer and a pure Nb film. The superconduct-
ing transition temperature is 8 K for both the sam-
ples. The sharpness of the transition as well as the
high transition temperature suggests the good quality
of the superconductors. Although the transition in
the Nb film is slightly sharper than that in the multi-
layer, the difference is little, which proves that the thin
Py layers have little influence on the bulk properties
of the superconductor. The inset of Fig. 1shows the
𝑀−𝐻loop of the multilayer at 10K with applied field
perpendicular to the film surface. The magnetization
changes linearly with magnetic field and saturates at
about 1 T, which suggests that the anisotropy field in
the out-of-plane direction is about 1 T.
Then we performed a series of ac susceptibility
measurements to obtain the critical current density
of the superconductor. The experimental procedure
is the same as that described above. The sample was
cooled down in zero field and both the dc and ac field
were applied perpendicular to the sample surface. Fig-
ure 2shows several typical curves of the out-of-phase
component 𝜒′′ of ac susceptibility measured with dif-
ferent ac fields at 4 K. As is expected, there is a max-
imum in each curve. The peak becomes pronounced
and shifts to lower fields as the amplitude of ac field in-
creases. At each maximum point, we can apply Eq. (1)
and obtain the critical current density in a correspond-
ing dc field. By varying the amplitude of ac field from
0.125 up to 17 Oe (this range is limited by the PPMS
equipment), we can obtain the critical current density
𝐽𝑐as a function of dc magnetic field.
0.0 0.5 1.0 1.5 2.0 2.5
χ'' (arb. units)
μ0H (T)
T=4 K
h
0
=
0.125 Oe
h
0
=
0.5 Oe
h
0
=
2 Oe
h
0
=
4 Oe
h
0
H
dc
d
Fig. 2. Out-of-phase component of ac susceptibility mea-
sured with different ac fields as a function of applied dc
magnetic field for the multilayer at 4K.
As shown in Fig. 3, the obtained 𝐽𝑐of the mul-
tilayer is in the order of 107–109A/m2, which is a
reasonable scale for the superconductor Nb. For com-
parison, the critical current density of a pure Nb film
is also measured in the same way at 4K. When the
two sets of 𝐽𝑐are plotted together in Fig.3, we find
an interesting relation between them. In the low field
region, 𝐽𝑐of the SC/FM multilayer is much higher
than that of the Nb film. The difference diminishes
with increasing dc field and reaches zero at a crossover
field. Above the crossover field the multilayer and the
Nb film have nearly the same 𝐽𝑐values within experi-
mental resolution. In order to confirm this interesting
relation further, we have performed the same experi-
ments at 2 K and obtained 𝐽𝑐for both the multilayer
and the Nb film. As plotted in Fig. 3, the same fea-
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CHIN. PHYS. LETT. Vol. 26, No. 1 (2009) 017502
ture is observed at 2 K except that the critical current
density at 2 K is much higher. In the low field region,
𝐽𝑐of the multilayer is much higher than that of the
Nb film. Above a crossover field, they are nearly the
same.
Fig. 3. Critical current density 𝐽𝑐as a function of applied
dc magnetic field at 2 and 4 K. 𝐽𝑐of the Nb/Ni81Fe19 mul-
tilayer is much larger than that of a pure Nb film in the low
field region, suggesting a significant vortex pining effect.
The enhancement of critical current density in the
low field region indicates that there is a strong vortex
pining effect in the multilayer sample. Therefore, for
the first time our results demonstrate that continu-
ous FM layers can produce significant vortex pining
effects. These results raise two interesting questions:
(i) how does a continuous FM layer produce signifi-
cant pining effect to a superconducting vortex? and
(ii) why does the pining effect only happen in the low
field region? In the following, we try to answer these
questions by considering a dynamic interaction model.
Initially, the FM layers have a strong in-plane easy
direction so that spins lie in the plane. When a ver-
tical magnetic field (𝐻𝑐1< 𝐻 < 𝐻𝑐2) is applied to
the film, vortices are created in the SC layers. The
flux lines generated by the superconducting vortices
penetrate through all the SC and FM layers and are
slightly distorted in the thin FM layers. The spins
in the FM layers suffer an inhomogeneous magnetic
field generated by the vortices. If the central field at
the vortex core is large enough, the spins under the
vortex centre would be driven out of plane. We can
make an estimate of the average field in a vortex core
based on the relation 𝐵= Φ0/𝜋𝜆2, where Φ0is flux
quantum and 𝜆is the penetration depth. By using
𝜆= 40 nm for Nb, we obtain 𝐵= 0.64 T. The maxi-
mum field at the vortex centre should be larger than
the average field, and thus is comparable with the out-
of-plane anisotropy field of the FM layers. Therefore,
the inner field in the vortex is able to induce a local
spin rotation out of plane.
When the vortex moves in the superconductor, the
spin rotation in response to the vortex core would
propagate in the FM layer to follow the vortex move-
ment. This process makes some spins ahead the vortex
rotate to the out-of-plane direction, and some spins
behind the vortex turn back to the in-plane direction,
which causes an extra energy loss and gives a viscos-
ity to vortex movement. Therefore, the spin-tilted-up
regions in the continuous FM layer can act as effective
pinning centres to the vortices. Unlike the magnetic
dot pinning centres at specific sites in most previous
studies, the pining centres here move following the
vortices they pin. We term this kind of interaction
between the spin and the superconducting vortex as a
dynamic spin–vortex coupling.
(a) Low field
(b) High field
SC
FM
SC
1.0
0.0
SC
FM
SC
1.0
0.0
Fig. 4. Illustration of the spin–vortex configuration and
the magnetic field distribution in (a) the low field region
and (b) the high field region.
The dynamic spin–vortex coupling is effective only
in the low field region where the vortices are far from
each other so that the magnetic field is well patterned
(Fig. 4(a)). In the high-field region, the density of vor-
tices is very high so that the magnetic field distribu-
tion becomes much smooth (Fig. 4(b)) and the lowest
field in the superconductor could be higher than the
anisotropy field. As a result, all the spins in the FM
layers are driven to the vertical direction. In this situ-
ation, the FM layer has null pinning effect because the
vortex experiences no potential difference as it moves.
Experimentally, the multilayer and the pure Nb film
have the same 𝐽𝑐values in the high-field region.
The above model can qualitatively explain the ob-
served experimental results. On the one hand, the
spin–vortex coupling produces strong pinning to vor-
tices. On the other hand, this dynamic interaction
017502-3
CHIN. PHYS. LETT. Vol. 26, No. 1 (2009) 017502
induces a spin rotation and propagation in the FM
layer. This evokes an idea of spin manipulation using
superconducting vortices. In fact, Berciu et al.[17]have
recently proposed the principle of manipulating spin
and charge in diluted magnetic semiconductors (DMS)
using superconducting vortices. Their numerical sim-
ulations on an SC-DMS bilayer structure suggest that
the inhomogeneous magnetic field of superconducting
vortices can create local spin and charge textures in
the DMS quantum well. With further understanding
of the spin–vortex interaction in various SC/FM het-
erostructures, spin manipulation via a superconductor
could become promising in the future.
In conclusion, we have demonstrated that the crit-
ical current density of an SC/FM multilayer is ap-
parently enhanced in a low magnetic field region but
remains nearly the same in high magnetic fields, which
indicates a strong vortex pinning effect in a low field
region. This kind of pinning effect is due to the dy-
namic coupling between spins and superconducting
vortices. Since the dynamic spin–vortex interaction
would induce a spin rotation propagating in the FM
layer, we propose that such an interaction could be
used for spin manipulation via a superconductor.
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