PHYSICAL REVIEW B 76, 024416 共2007兲
Experimental evidence of magnetization modification by superconductivity
in a Nb/ Ni81Fe19 multilayer
Hong-ye Wu, Jing Ni, Jian-wang Cai, Zhao-hua Cheng, and Young Sun*
State Key Laboratory of Magnetism and Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese
Academy of Sciences, Beijing 100080, People’s Republic of China
共Received 29 November 2006; revised manuscript received 10 April 2007; published 11 July 2007兲
We present experimental evidence of magnetization modification by superconductivity in a ferromagnet/
superconductor heterostructure. By monitoring the magnetization as a function of time with a temperature
change, we demonstrate that the magnetization of the ferromagnet is reduced when the heterostructure is
cooled below the superconducting transition temperature Tc. The magnitude of the reduction increases with
lower cooling temperature. Besides, the M共T兲 curve measured in zero field exhibits irreversibility between
cooling and warming branches above Tc. These results can be qualitatively understood in terms of the mutual
interactions between ferromagnetism and superconductivity.
DOI: 10.1103/PhysRevB.76.024416
PACS number共s兲: 75.75.⫹a, 75.70.Cn, 74.78.Fk
The study of the mutual interaction of superconductivity
and ferromagnetism in ferromagnet/superconductor 共FM/SC兲
heterostructures has attracted considerable attention in recent
years.1–3 On the one hand, ferromagnetism has strong influences on superconductivity, which causes a variety of
interesting phenomena. Experimental work on FM/SC heterostructures has focused dominantly on this direction, including investigations of proximity effects,4–9 artificial flux
pining,10–12 domain wall superconductivity,13–15 etc. On the
other hand, nevertheless, few experiments have been reported on the modification of ferromagnetism by superconductivity. For a long time, people had thought that superconductivity seems to have negligible influence on ferromagnetism because the energy scale for a superconducting transition is much smaller than that for a ferromagnetic ordering.
However, although it can hardly destroy ferromagnetism,
several theoretical works have predicted that superconductivity may modify ferromagnetism to a certain extent.16–19
Experimental demonstration of the influence of superconductivity on ferromagnetism goes far behind theoretical
work. The main obstacle is due to the fact that conventional
magnetization measurements detect the superposed signal
from both the ferromagnet and the superconductor. Therefore, one can hardly distinguish the slight magnetization
change of the ferromagnet from the total magnetization of
the heterostructure. Up to now, there have been only a couple
of experiments that could shed light on this effect. Mühge et
al. investigated Fe/ Nb bilayers using ferromagnetic resonance and the results revealed a decrease of the effective
magnetization below Tc for bilayers with Fe thickness less
than 1 nm.20 However, the analysis of the experimental data
by Garifullin et al. suggests the possibility of the formation
of islands at small thickness of the Fe layer,21 which may
complicate the interpretation of the experimental results. Another experiment was done by Dubonos et al. on submicron
Al/ Ni structures with microscopic Hall probes underneath.22
By monitoring the Hall signal above and below Tc, the authors conclude that the mutual interaction induces a reshuffling of magnetic domains in the ferromagnet.
1098-0121/2007/76共2兲/024416共5兲
Although there have been some signs of this effect, magnetization modification by superconductivity requires more
convincing experimental evidences. In this paper, we present
experimental results to support this effect. By measuring the
magnetization of a FM/SC heterostructure in zero magnetic
field while switching the temperature below and above the
superconducting transition temperature, we clearly demonstrate that the magnetization of the ferromagnetic layers is
reduced by superconductivity.
II. EXPERIMENTS
The
sample
in
this
study
is
a
Nb/ Ni81Fe19 / Nb/ Ni81Fe19 / Nb multilayer, fabricated by using dc magnetron sputtering onto a glass substrate. The base
pressure of the sputtering system is about 3 ⫻ 10−5 Pa. The
film was deposited at an Ar pressure of 0.5 Pa with a field
about 300 Oe in the film plane to induce an easy axis in
Ni81Fe19 共Py兲 layers. The scheme of the heterostructure is
illustrated in the inset of Fig. 1. In this type of structure, each
ferromagnetic layer feels the interaction from two neighboring superconducting layers. In addition, the sum of two
χ (arb. units)
I. INTRODUCTION
2
4
6
T (K)
8
10
FIG. 1. 共Color online兲 Temperature dependence of ac susceptibility of the sample. The inset shows the scheme of the studied
FM/SC heterostructure.
024416-1
©2007 The American Physical Society
PHYSICAL REVIEW B 76, 024416 共2007兲
WU et al.
10K
6K
10
M/M0
1.01
0
1
-5
M/MS
-4
M (10 emu)
5
5K
6K
7K
8K
10K
H=0 Oe
1.02
-10
0.99
-60 -40 -20
0
20 40 60
0
H (Oe)
-5000
0
H (Oe)
15 K
0
-1
-10000
15 K
1.00
5000
10000
10
20
30
40
Time (min)
FIG. 2. 共Color online兲 M-H curves above and below Tc. The
inset shows the low field part of the M-H loop above Tc. The
applied field is in the film plane along the easy axis of the Py layers.
ferromagnetic layers makes it easier to detect the magnetization change. The thickness of each Nb and Py layer is
100 nm and 10 nm, respectively. The dc magnetization was
measured using a superconducting quantum interference device 共SQUID兲 magnetometer down to 5 K. The ac susceptibility was measured using a physical property measurement
system 共PPMS兲 down to 2 K. The ac field has a frequency of
2 kHz and amplitude of 5 Oe.
III. RESULTS AND DISCUSSION
The main panel of Fig. 1 shows the ac susceptibility of the
sample with applied field parallel to the sample surface. The
superconducting transition temperature Tc is 8.0 K and the
transition is sharp, which suggests a good quality of the Nb
layers. In Fig. 2, we show the M共H兲 curves at temperatures
above and below Tc. The field is applied parallel to the film
surface and along the easy axis of the Py layers. The diamagnetic signal from the glass substrate has been deducted.
Above Tc, the magnetization is only from the Py layer which
is a very soft ferromagnet. The low field part of the M共H兲
curve is plotted in the inset of Fig. 2, which shows a squarelike loop with a coercive field Hc ⬇ 5 Oe. Below Tc, the magnetization is a superposition of a ferromagnetic magnetization and a type-II superconductor magnetization. The
coexistence of soft ferromagnetism and superconductivity
below Tc 共8 K兲 makes the Nb/Py multilayer very suitable for
the study of the mutual interactions. In fact, many experimental works in this field have employed the Nb/Py system
as the studied samples. For instance, Rusanov et al.15 studied
the “domain-wall superconductivity” in the Nb/Py bilayers
and Stamopoulos et al.23 investigated the influence of ferromagnetism on superconductivity in the Nb/Py bilayers and
trilayers.
In order to investigate the influence of superconductivity
on ferromagnetism, we designed a new experiment of magnetization measurement. The idea is based on the fact that
the magnetization of a ferromagnet 共Py in this work兲 is
nearly constant for a small temperature change 共a few K兲 at
very low temperature. First, we measure the magnetization at
FIG. 3. 共Color online兲 Normalized magnetization 共M / M 0兲 as a
function of time with a temporary temperature change. The sample
is first magnetized to saturation at 15 K in 100 Oe. Then, the field is
removed and the magnetization is measured as a function of time.
M 0 is the initial magnetization at 15 K in zero field. After
15 minutes the temperature is changed to a lower temperature
共5 – 10 K兲 and the magnetization is measured for 10 minutes. Finally, the temperature is returned to 15 K and the magnetization is
measured for another 15 minutes. The final magnetization is reduced after a temporary cooling into the superconducting state.
a temperature slightly above the superconducting transition
temperature Tc, which only measures the magnetization of
the ferromagnetic layers. Then we lower the temperature to
let the Nb layers enter into the superconducting state. If superconductivity has an influence on ferromagnetism, the
magnetization of the Py layers may have a detectable
change. When temperature is raised back to the initial temperature above Tc, the measured magnetization is equivalent
to that of the FM layers in the superconducting state due to
the small temperature change. Therefore, by comparing the
initial and final magnetization at the same temperature above
Tc, one can tell whether the magnetization of the FM layers
is modified in the superconducting state.
Figure 3 shows the magnetization measurements with a
temperature change. In each measurement, the sample is first
magnetized to saturation at 15 K in a magnetic field of
100 Oe. Then, the field is removed and the magnetization is
measured as a function of time. After 15 minutes, the temperature is changed to a lower temperature 共5 – 10 K兲 and the
magnetization is measured for 10 minutes. Finally, the temperature is returned to 15 K and the magnetization is measured for another 15 minutes.
As shown in Fig. 3, after removing the field at 15 K, the
remanence magnetization decays very slightly with time.
When temperature is changed to 10 K 共above Tc兲 and then
back to 15 K subsequently, the magnetization remains nearly
the same. This proves the presumption that the magnetization
of the ferromagnetic Py layers is essentially constant for a
small temperature change at very low temperatures. In strong
contrast, when temperature is changed from 15 K to a temperature below Tc, there is a positive jump in magnetization
whose magnitude increases with lower temperature. The increase of magnetization below Tc is consistent with recent
studies.8,14 Stamopoulos et al. studied LaCaMnO / Nb and
FePt/ Nb multilayer and bilayer hybrids and observed that
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EXPERIMENTAL EVIDENCE OF MAGNETIZATION…
1.06
1.06
5K
10K
(a)
H=0 Oe
1.04
M/M0
H=100 Oe
1.04
1.02
M/M0
1.00
0.98
1.02
1.06
15 K
15 K
(b)
H=100 Oe
1.04
0
10
20
Time (min)
30
M/M0
1.00
40
1.02
1.00
FIG. 4. 共Color online兲 Normalized magnetization 共M / M 0兲 as a
function of time with a temporary temperature change in a constant
100 Oe field. M 0 is the initial magnetization at 15 K. The final
magnetization is identical to the initial magnetization.
the magnetization of the superconductor increases below Tc
and follows that of the ferromagnetic layer, which indicates
that the superconductor behaves ferromagnetically coupled
to the ferromagnetic layer in low applied magnetic field.8,14
As discussed in Refs. 8 and 14, the increase of magnetization
below Tc results from the diamagnetism of the superconductor in response to the stray field of neighboring ferromagnetic layers. Since the diamagnetism of a superconductor is
stronger at lower temperature, the magnetization jump increases with lower cooling temperature.
The most interesting result in Fig. 3 is that after temperature returns to 15 K the magnetization does not restore but is
apparently lower than the initial state before changing temperature. The magnetization difference 兩⌬M兩 between the initial and final state grows with decreasing temperature and
seems to saturate below 6 K. Therefore, these results prove
that superconductivity do have a detectable influence on ferromagnetism. We note that the magnetization is not constant
when temperature is changed exactly to Tc 共8 K兲. This could
be due to a little overcooling when lowering and stabilizing
the temperature.
In the above experiments, the magnetization is measured
in a zero magnetic field so that any changes of magnetization
can only be due to the mutual interaction itself between the
superconductor and ferromagnet. For comparison, we also
performed another experiment in which a constant 100 Oe
field is applied all of the time instead of being removed. As
shown in Fig. 4, when temperature changes from 15 to 10 K
共above Tc兲 and then back to 15 K, the magnetization is
nearly constant, the same as that in Fig. 3. When temperature
changes from 15 to 5 K 共below Tc兲, the magnetization shows
a positive jump as observed in the zero-field case. However,
when temperature returns to 15 K, the final state is identical
to the initial state, i.e., no magnetization reduction is observed in a constant 100 Oe field. Since a 100 Oe field surely
exceeds the saturation field of Py layers, it acts as an external
parameter that stabilizes the magnetic state of the FM layers
against any contribution that comes from the adjacent superconductor.
In order to further confirm the magnetization modification
by superconductivity, we have measured magnetization as a
0.98
4
6
8
10
12
14
16
T (K)
FIG. 5. 共Color online兲 共a兲 Normalized magnetization 共M / M 0兲 as
a function of temperature in zero field. The sample is first magnetized to saturation in a 100 Oe field at 15 K. Then, the field is cut
off and the magnetization is measured with cooling and warming
cycles in zero field. The warming branch is below the cooling
branch above Tc, indicating that the magnetization of the FM layers
is reduced by superconductivity. 共b兲 Normalized magnetization
共M / M 0兲 as a function of temperature in a constant 100 Oe. The
cooling and warming branch is identical. M 0 is the initial magnetization at 15 K.
function of temperature with cooling and warming cycles
across Tc. At first, the sample is magnetized to saturation in a
100 Oe field at 15 K. Then, the field is cut off and the magnetization is measured with cooling down to 5 K in zero
field. After that the magnetization is measured with warming
up to 15 K. As shown in Fig. 5共a兲, in the cooling branch the
magnetization has a sudden increase below Tc, which is consistent with previous reports in Refs. 8 and 14. In the warming branch, the magnetization is reversible below Tc. However, there is an apparent difference between the cooling and
warming branches above Tc. The warming branch is lower
than the cooling branch, which means that the magnetization
of the FM layers is reduced in the superconducting state. The
irreversibility in the M共T兲 cycle further confirms the results
in Fig. 3. For comparison, we also measured the M共T兲 cycle
in a constant 100 Oe field. As shown in Fig. 5共b兲, the cooling
branch is identical with the warming branch in all temperatures. This result is consistent with Fig. 4 as 100 Oe is
enough to stabilize the magnetic state of the ferromagnetic
Py layers.
The results presented in Figs. 3 and 5 clearly demonstrate
that the magnetization of the ferromagnetic layers can be
reduced by superconductivity. In the following, we give a
qualitative discussion on the underlying mechanism. The interactions between the FM and SC layers may be illustrated
in Fig. 6 by either a macroscopic or microscopic view. Macroscopically, the ferromagnetic layers have an in-plane magnetization which is initially constant with time. In the superconducting state, due to the Meissner effect the stray field of
the ferromagnet magnetization induces screen currents inside
the superconducting layers. In turn, the screen currents produce an effective magnetic field acting on the ferromagnet,
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PHYSICAL REVIEW B 76, 024416 共2007兲
WU et al.
(a)
FIG. 6. 共Color online兲 Illustration of the interactions between the FM and SC layers. In a macroscopic view, the stray field of the FM layer induces screen currents inside the SC layers, which
in turn produce an effective magnetic field acting
on the FM layer, as demonstrated in 共a兲. Microscopically, the interaction between the SC and
FM layers is attributed to that between a set of
magnetic moments in FM layers and their dipole
images in SC layers. 共b兲–共d兲 represent different
states with the maximal, intermediate, and minimal energy.
(b)
(c)
(d)
as indicated by the dotted lines in Fig. 6共a兲. Since this effective field is opposite to the magnetization direction of the FM
layers, it may drive spin rotation away from the original
direction. As a result, the magnetization of the FM layers is
reduced.
In a microscopic view, one can attribute the interaction
between the SC and FM layers to that between a set of magnetic moments in FM layers and their dipole images in SC
layers. The interaction energy between two magnetic mo␮
ments can be expressed as E = 4␲0r3 关M1 · M2 − 3共M1 · e兲
⫻共M2 · e兲兴, where r is the distance between the two moments
and e is the unit vector along r. Apparently, the interaction
energy depends on the relative orientation between the moments. When the moments are parallel, as the configuration
in Fig. 6共b兲, there will be a maximal energy. The moments in
FM layers are initially set in the easy direction and parallel to
the interface as in the configuration in Fig. 6共b兲. As temperature changes below Tc, dipole images appear in SC layers
that are parallel to the interface. This state is not stable because of the maximal interaction energy. Therefore, the moments in FM layers tilt out of the plane to a state as in the
configuration in Fig. 6共c兲. As a result, the net magnetization
along the easy axis direction is reduced.
It is well known that the distance between the dipole images and the FM /SC interface is related to the penetration
depth ␭ of the superconductor. As the penetration depth becomes smaller at lower temperature, the distance between the
moments decreases. Consequently, the interactions between
the ferromagnet and superconductor increase, which may explain why the magnetization change 兩⌬M兩 increases with
lower cooling temperature. As the interaction energy between two dipoles decays rapidly with their distance as r−3,
only moments near the interface are expected to have significant tilt angle due to energy distance relations. We note that
although the magnetization change induced by superconductivity is well detectable, the relative change 兩 ⌬MM0 兩 is small
共⬃1 % 兲. This also indicates that the magnetization modification by superconductivity could mainly happen at the FM/SC
interface region. This view could be tested in the future by
studying the dependence of the relative change on the FM
layer thickness. In Ref. 22, spontaneous magnetization
changes were previously observed in mesoscopic FM/SC
structures. The authors propose that the superconducting
screening can lead to the change in a distribution of magnetic
domains and consequently causes a magnetization change.
Although this could be also true in our case, we would rather
leave it as an open question, since macroscopic measurement
of magnetization cannot tell the microscopic details.
IV. CONCLUSION
In summary, we have studied the influence of superconductivity on ferromagnetism in a Nb/Py multilayer. Above
the superconducting transition temperature Tc 共8 K兲, the
magnetization of the sample is constant as temperature
changes from 15 K to 10 K, which indicates that the magnetization of the ferromagnetic layers is essentially constant for
a small temperature change at very low temperature. Based
on this fact, we performed a series of magnetization measurements in zero field with a temporary cooling below Tc.
By comparing the initial and final magnetization at the same
temperature above Tc, we found that the magnetization of the
ferromagnetic layers is reduced in the superconducting state.
In addition, the M共T兲 curves with cooling and warming
cycles also prove the magnetization modification by superconductivity. These results can be qualitatively explained in
terms of the mutual interactions between superconductivity
and ferromagnetism. Although the microscopic picture and
the exact physics are still unclear at this stage, this work
provides a solid evidence for the possibility of magnetization
modification by superconductivity. As more efforts will be
devoted to this topic in future studies, spin manipulation via
a superconductor could become a promising approach and
may have important applications in spintronics.
ACKNOWLEDGMENTS
This work is supported by the National Natural Science
Foundation of China under Grant No. 10674170 and the
State Key Project of Fundamental Research in China.
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13 Zhaorong
*Corresponding author; youngsun@aphy.iphy.ac.cn
A. I. Buzdin, Rev. Mod. Phys. 77, 935 共2005兲.
2
I. F. Lyuksyutov and V. L. Pokrovsky, Adv. Phys. 54, 67 共2005兲.
3 I. A. Garifullin, J. Magn. Magn. Mater. 241, 570 共2002兲.
4 C. L. Chien and Daniel H. Reich, J. Magn. Magn. Mater. 200, 93
共1999兲.
5 J. Y. Gu, C.-Y. You, J. S. Jiang, J. Pearson, Ya. B. Bazaliy, and S.
D. Bader, Phys. Rev. Lett. 89, 267001 共2002兲.
6
Y. Obi, M. Ikebe, and H. Fujishiro, Phys. Rev. Lett. 94, 057008
共2005兲.
7 Ion C. Moraru, W. P. Pratt, Jr., and Norman O. Birge, Phys. Rev.
Lett. 96, 037004 共2006兲.
8 D. Stamopoulos, N. Moutis, M. Pissas, and D. Niarchos, Phys.
Rev. B 72, 212514 共2005兲.
9
C. Monton, F. de la Cruz, and J. Gumpel, Phys. Rev. B 75,
064508 共2007兲.
10 J. I. Martın, M. Velez, A. Hoffmann, Ivan K. Schuller, and J. L.
Vicent, Phys. Rev. Lett. 83, 1022 共1999兲.
11
O. M. Stoll and M. I. Montero, J. Guimpel, Johan J. Akerman,
and Ivan K. Schuller, Phys. Rev. B 65, 104518 共2002兲.
12
Young Sun, M. B. Salamon, K. Garnier, and R. S. Averback,
Phys. Rev. Lett. 92, 097002 共2004兲.
1
Yang, Martin Lange, Alexander Volodin, Ritta Szymczak, and Victor V. Moshchalkov, Nat. Mater. 3, 793 共2004兲.
14
D. Stamopoulos and M. Pissas, Phys. Rev. B 73, 132502 共2006兲.
15 A. Yu. Rusanov, M. Hesselberth, J. Aarts, and A. I. Buzdin, Phys.
Rev. Lett. 93, 057002 共2004兲.
16
A. I. Buzdin and L. N. Bulaevskii, Sov. Phys. JETP 67, 576
共1988兲.
17 L. N. Bulaevskii and E. M. Chudnovsky, Phys. Rev. B 63,
012502 共2000兲.
18 F. S. Bergeret, K. B. Efetov, and A. I. Larkin, Phys. Rev. B 62,
11872 共2002兲.
19 F. S. Bergeret, A. F. Volkov, and K. B. Efetov, Phys. Rev. B 69,
174504 共2004兲.
20
Th. Mühge, N. N. Garif’yanov, Yu. V. Goryunov, K. Theis-Bröhl,
K. Westerholt, I. A. Garifullin, and H. Zabel, Physica C 296,
325 共1998兲.
21
I. A. Garifullin, J. Magn. Magn. Mater. 240, 571 共2002兲.
22
S. V. Dubonos, A. K. Geim, K. S. Novoselov, and I. V. Grigorieva, Phys. Rev. B 65, 220513 共2002兲.
23
D. Stamopoulos, E. Manios, and M. Pissas, Phys. Rev. B 75,
014501 共2007兲.
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