# Superconducting spin valve effect in the Co/Pb/Co heterostructure
with insulating interlayers

A. A. Kamashev  Zavoisky Physical-Technical Institute, FRC Kazan Scientific
Center of RAS, 420029 Kazan, Russia  N. N. Garif’yanov  Zavoisky Physical-
Technical Institute, FRC Kazan Scientific Center of RAS, 420029 Kazan, Russia
A. A. Validov  Zavoisky Physical-Technical Institute, FRC Kazan Scientific
Center of RAS, 420029 Kazan, Russia  V. Kataev  Leibniz Institute for Solid
State and Materials Research, Helmholtzstr. 20, D-01069 Dresden, Germany  A.
S. Osin  L. D. Landau Institute for Theoretical Physics RAS, 142432
Chernogolovka, Russia  Ya. V. Fominov  L. D. Landau Institute for Theoretical
Physics RAS, 142432 Chernogolovka, Russia  Laboratory for Condensed Matter
Physics, HSE University, 101000 Moscow, Russia  I. A. Garifullin  Zavoisky
Physical-Technical Institute, FRC Kazan Scientific Center of RAS, 420029
Kazan, Russia

###### Abstract

We report the superconducting properties of the Co/Pb/Co heterostructures with
thin insulating interlayers. The main specific feature of these structures is
the intentional oxidation of both superconductor/ferromagnet (S/F) interfaces.
We study variation of the critical temperature of our systems due to switching
between parallel and antiparallel configurations of the magnetizations of the
two magnetic layers. Common wisdom suggests that this spin valve effect, which
is due to the S/F proximity effect, is most pronounced in the case of perfect
metallic contact at the interfaces. Nevertheless, in our structures with
intentionally deteriorated interfaces, we observed a significant full spin
valve effect. A shift of the superconducting transition temperature $T_{c}$ by
switching the mutual orientation of the magnetizations of the two
ferromagnetic Co layers from antiparallel to parallel amounted to $\Delta
T_{c}=0.2$ K at the optimal thickness of the superconducting Pb layer. Our
finding verifies the so far unconfirmed earlier results by Deutscher and
Meunier on an F1/S/F2 heterostructure with oxidized interlayers [G. Deutscher
and F. Meunier, Phys. Rev. Lett. 22, 395 (1969)] and suggests an alternative
route to optimize the performance of the superconducting spin valves.

##  I Introduction

Models and specific realizations of the superconducting spin valve (SSV) have
been the subject of intensive research over the past 25 years (see, e.g., [1,
2, 3, 4, 5, 6, 7, 8, 9, 10]). The interest in these structures is due to the
possibility to observe and exploit the reciprocal influence of
superconductivity (S) and ferromagnetism (F) on each other’s properties when
they are put into a close contact (see, e.g., [11, 12, 13, 14, 15, 16, 17]).
Moreover, the SSV structures appear as promising devices for applications in
modern superconducting spintronics (see, e.g., [18, 19, 20, 21, 22]). In 1997,
Beasley and coworkers in proposed a theoretical F1/F2/S model of the SSV
structure [1]. Another F1/S/F2 model was developed a little later in 1999 by
Tagirov [2] and Buzdin et al. [3]. In these structures F1 and F2 are metallic
ferromagnetic layers, and S is a superconducting layer. Both models analyze
the penetration of Cooper pairs from the S layer into the F layers under the
action of the exchange field generated by F1 and F2 layers. Those early
theoretical works implied operation principle of the SSV structure based on
the control of the average exchange field acting on the S layer by changing
the mutual orientation of the magnetization vectors of the F layers and thus
suppressing superconductivity to a different degree. Typically, the
superconducting transition temperature $T_{c}$ of the SSV is minimal / maximal
for the parallel (P) / antiparallel (AP) geometry of the two vectors,
respectively. The magnitude of the SSV effect is defined as the difference of
these two temperatures $\Delta T_{c}=T_{c}^{\mathrm{AP}}-T_{c}^{\mathrm{P}}$.
The full SSV effect is realized when $\Delta T_{c}$ is larger than the
superconducting transition width $\delta T_{c}$ in the P and AP
configurations. Several experimental works confirmed the predicted influence
of the mutual orientation of the magnetization vectors of the F layers on
$T_{c}$ in the F1/S/F2 type of structures (see, e.g., [4, 5, 6, 23, 24, 25]).
However, a full switching between the normal and the superconducting state was
not achieved because in these SSVs $\Delta T_{c}$ was always smaller than
$\delta T_{c}$. For the first time, a complete switching between the normal
and superconducting states was observed in the F1/F2/S type of SSV in Ref.
[10].

Theories (see, e.g., [13, 15, 16, 26, 27, 28]) predict that at certain
conditions a long-range triplet component (LRTC) in the superconducting
condensate can arise in the S/F bilayer. The generation of the LRTC opens an
additional channel for the leakage of the Cooper pairs from the S into the F
layers in the F1/F2/S SSV at noncollinear configuration of F1 and F2
magnetizations, which significantly suppresses $T_{c}$ and thus should
manifest as a minimum of $T_{c}$ at the orthogonal magnetizations’ geometry
[29]. A large number of theoretical and experimental works have been devoted
to the study of this effect (see, e.g., [29, 30, 31, 32, 33, 34, 35, 36, 37,
38, 39]).

By now, many such SSVs using various elemental metals and alloys have been
studied in sufficient detail and recent results indicate that significant
values of the SSV effect have already been achieved in the F1/F2/S structures
(see, e.g., [35, 36, 39]). Since the principle of an SSV relies on the S/F
proximity effect which is confined to the interface between the S and F
layers, a particular attention was given to the quality of this interface in
terms of its morphology, smoothness, absence of intergrowth, etc. [40], which
defines the mainstream approach in this field. At odds with this approach, a
significant SSV effect of $\Delta T_{c}\sim 0.3$ K in the FeNi/In/Ni
heterostructure with intentionally oxidized F/S interfaces was demonstrated by
Deutscher and Meunier in 1969 [41]. The idea behind the oxidation of the FeNi
and Ni layers was to slightly weaken the S/F proximity effect such that the
superconductivity in the In layer could not be completely destroyed by the
exchange field of the F layers. The authors noted that the thin oxidized
layers became insulating but presumably remained magnetic. In a later
experiment by Li et al. [42], the F layers themselves were insulating by
design. In this special situation, even a very thin additional nonmagnetic
insulating interlayer at the interface immediately suppressed the S/F
proximity effect.

The paradoxical fact that “worsening” of the S/F interface in a metallic
system [41] can yield significant magnitude of the SSV effect is remarkable.

The work by Deutscher and Meunier [41] has never been reproduced and the
research in this direction was not pursued albeit, according to private
communications in the SSV community, some groups attempted but did not succeed
to reproduce that early remarkable result.

In the present work, in order to verify the SSV effect reported by Deutscher
and Meunier for the heterostructure where the superconducting layer was

contacted to the ferromagnets through

thin insulating interlayers and to prove the validity of this concept to other
types of the SSV structures, we investigated the superconducting properties of
the SSV made of completely different F and S materials as compared to those in
Ref. [41]. Specifically, we prepared the multilayers Co1/Pb/Co2 with oxidized
Co1/Pb and Pb/Co2 interfaces following the recipe of Refs. [43, 41]. We
studied the dependence of the magnitude of the SSV effect $\Delta T_{c}$ on
the Pb layer thickness and found that $\Delta T_{c}$ reached 0.2 K for the
optimal thickness, surpassing most of the values previously observed for the
SSVs with perfect metallic contact. We discuss the obtained results in the
context of the existing theoretical models of the S/F proximity effect.

##  II Samples

For our investigation
CoOx(3.5nm)/Co1(3nm)/I1/Pb($d_{\mathrm{Pb}}$)/I2/Co2(3nm)/Si3N4(85nm)
heterostructures with the variable Pb-layer thickness $d_{\mathrm{Pb}}$ in the
range from 60 to 120 nm were fabricated on the high quality single crystalline
substrate MgO (001). Here, Co1 and Co2 are ferromagnetic F1 and F2 layers, I1
and I2 are thin oxide insulating interlayers, Pb is the superconducting layer,
Si3N4 is a protective layer, and CoOx is the antiferromagnetic (AF) bias layer
that fixes the direction of the magnetization of the Co1 layer. The layers
were deposited using an electron beam evaporation (Co, Pb) and AC sputtering
(Si3N4) techniques. The deposition setup had a load-lock station with vacuum
shutters, allowing to transfer the sample holder without breaking the ultra-
high vacuum in the deposition chambers. The load-lock station provides
possibility to oxidize the prepared layers in a controlled atmosphere. This
allows to prepare the AF CoOx layer and fabricate the thin oxide interlayers
(I1 and I2) at the Co1/I1/Pb and Pb/I2/Co2 interfaces. CoOx was prepared by
exposing the metallic Co layer to oxygen atmosphere at 100 mbar during two
hours. Next, Co1 was deposited in the main deposition chamber at the vacuum of
order $10^{-9}$ mbar on top of the CoOx layer. The I1 layer was formed on the
surface of Co1 in the similar way as described above in the oxygen atmosphere
of about $\sim 10^{-2}$ mbar during 60 sec. It was shown in Ref. [44] that
significant partial oxidation of a few nanometers thin metallic Co layer can
be achieved by exposing it to the ambient atmospheric environment implying
that lowering the atmospheric pressure by five orders of magnitude enables one
to oxidize only the surface while not affecting the bulk of the layer. After
that, the Pb layer and subsequent layers of the SSV structure were deposited
at the substrate temperature of $T_{\mathrm{sub}}\sim 150$ K. Such low
$T_{\mathrm{sub}}$ was necessary to obtain a smooth Pb layer [40] and to form
the I2 layer. The similar oxidation procedure was used again to form the I2
layer by exposing the Pb surface to oxygen atmosphere of about $\sim 10^{-2}$
mbar during 30 sec. After that, the Co2 layer was deposited similar to the Co1
layer.

According to Refs. [43, 41] the O2 molecules adsorbed on the surface of the
superconducting Pb layer oxidize the top ferromagnetic Co2 layer during its
deposition thereby forming an insulating magnetic interlayer at the S/F
interface. We consider oxidation of the Pb layer to be unlikely because it was
deposited at a low substrate temperature and exposed to a very low atmospheric
pressure for a very short time, as specified above. According to the
literature, the formation of an oxide on the surface of the Pb film requires
significantly higher temperatures and pressures, and much longer exposition
times (see, e.g., Refs. [45, 46, 47]).

Finally, all samples were covered with a protective Si3N4 layer. The
deposition rates were as follows: 0.5 Å/s for Co1 and Co2, 12 Å/s for Pb, and
1.8 Å/s for Si3N4 films. The final design of the samples is depicted in Fig.
1.

Figure 1: Design of the prepared samples. Cross-dashed areas depict I1 and I2
insulating interlayers (see the text for details).

Based on our previous studies in Refs. [33, 34, 48] we chose the thickness of
the AF CoOx layer to be $d_{\mathrm{CoO}_{x}}=3.5$ nm which is optimal to hold
the direction of the magnetization of the Co1 layer up to the in-plane
external magnetic field strength of $H_{0}^{\mathrm{max}}\sim 1.5$ kOe.
Magnetic studies of samples are presented in Supporting Information LABEL:s1.
We took the same thickness of 3 nm for both Co1 and Co2 layers.

In addition, a control set of the samples with similar thicknesses of the S
and F layers but without insulating interlayers at the Co1/Pb/Co2 interfaces
was prepared for comparison. The list of the studied samples
CoOx(3.5nm)/Co1(3nm)/I1/Pb($d_{\mathrm{Pb}}$)/I2/Co2(3nm)/Si3N4(85nm) with the
variable Pb layer thickness $d_{\mathrm{Pb}}$ is presented in Table 1.

Table 1: List of the studied samples CoOx(3.5nm)/Co1(3nm)/I1/Pb($d_{\mathrm{Pb}}$)/I2/Co2(3nm)/Si3N4(85nm) with the variable Pb layer thickness $d_{\mathrm{Pb}}$. Samples with insulating interlayers |  $d_{\mathrm{Pb}}$ (nm)
---|---
Pb 120 |  120
Pb 100 |  100
Pb 80 |  80
Pb 60 |  60
Pb 40 |  40

##  III Experimental results

Electrical resistivity measurements were carried out with a standard four-
point method in the DC mode. For changing the mutual direction of the
magnetization of the F layers between the P and AP orientations an external
magnetic field of the order $\sim 1\,\mathrm{kOe}<H_{0}^{\mathrm{max}}$ was
always applied in the plane of the sample in all measurements. The strength of
the magnetic field was measured by a Hall probe with an accuracy of $\pm 0.3$
Oe. The sample temperature was monitored by the highly sensitive in the
temperature range of interest carbon Allen–Bradley thermometer. The
temperature measurement error was $\pm\,(5-6)$ mK below 3 K. The
superconducting critical temperature $T_{c}$ was defined as the midpoint of
the transition curve.

To study the SSV effect the samples were cooled down from room to low
temperatures in a magnetic field of the order 5 kOe (field cooling procedure)
applied in the sample plane. This field aligns magnetization of both F layers
and the magnetization vector of the Co1 layer is getting fixed in the
direction of the applied field and remains biased by the AF CoOx layer after
the reduction of the field strength to its operational value of $H_{0}=1$ kOe
independent of the subsequent direction of the field vector [33, 34, 48]. At
this field value the temperature dependence of the resistivity $R(T)$ was
recorded for the P and AP configurations of the magnetizations of the Co1 and
Co2 layers by appropriate rotation of the magnetization of the Co2 layer by
the external magnetic field.

Fig. 2 depicts the superconducting transition curves for the samples Pb 100
(a) and Pb 60 (b) at P ($H_{0}=+1$ kOe) and AP ($H_{0}=-1$ kOe) orientations
of the Co1 and Co2 layers’ magnetization, respectively. The magnitude of the
SSV effect for the sample Pb 100 amounts to $\Delta T_{c}=0.07$ K whereas for
the sample Pb 60 it rises up to $\Delta T_{c}=0.2$ K. Obviously, the sample Pb
60 demonstrates the full SSV effect since, as is evident from Fig. 2(b), in
this case $\Delta T_{c}>\delta T_{c}$.

Figure 2: Superconducting transition curves for the samples Pb 100 (a) and Pb
60 (b) at P ($H_{0}=+1$ kOe) ($\bigcirc$) and AP ($H_{0}=-1$ kOe) ($\square$)
orientations of the Co1 and Co2 layers’s magnetizations, respectively.

Fig. 3 shows the dependence of $\Delta T_{c}$ and of $T_{c}$ on the thickness
of the Pb layer $d_{\mathrm{Pb}}$ for the whole set of samples with insulating
interlayers. $\Delta T_{c}$ increases and $T_{c}$ decreases approximately
linearly with decreasing $d_{\mathrm{Pb}}$. The maximum magnitude of the SSV
effect $\Delta T_{c}=0.2$ K is reached at the minimum thickness of the
superconducting layer $d_{\mathrm{Pb}}=60$ nm.

Figure 3: The dependence of the magnitude of the SSV effect $\Delta T_{c}$
(left vertical scale) and of the superconducting critical temperature
$T_{c}^{\mathrm{P}}$ for the parallel orientation of the magnetizations of the
Co1 and Co2 layers (right vertical scale) on the Pb layer thickness
$d_{\mathrm{Pb}}$.

Notably, in the control set of the structures with similar parameters, but
without the insulating interlayers, superconductivity was not observed down to
the lowest temperature of the experimental setup of 1.4 K for all Pb
thicknesses.

##  IV Discussion

###  IV.1 Phenomenology

Our results demonstrate a significant SSV effect in the heterostructures with
insulating interlayers at the F1/S/F2 interfaces and finally verify the
earlier observation by Deutscher and Meunier [41]. The S-layer thickness
appears to be an essential parameter for observing the full SSV effect. As
$d_{\mathrm{Pb}}$ decreases, the value of $\Delta T_{c}$ increases and reaches
its maximum of 0.2 K at $d_{\mathrm{Pb}}=60$ nm (Fig. 3). The decrease of
$T_{c}$ as a function of $d_{\mathrm{Pb}}$ is approximately linear down to
$d_{\mathrm{Pb}}=40$ nm. A sharp drop of $T_{c}$ is expected at smaller
thicknesses of the S layer due to the size effects (see, e.g., [49]).
Apparently, the inverse S/F proximity effect becomes more pronounced as
$d_{\mathrm{Pb}}$ decreases, despite the existence of insulating interlayers.
The here obtained value of $\Delta T_{c}$ at the optimal thickness of the Pb
layer is twice as high compared to those found before in Refs. [30, 33, 34,
48] for the structures with elemental metallic ferromagnetic layers but
without insulating interlayers.

This observation is not trivial as it apparently contradicts the paramount
prerequisite of the S/F proximity effect of having a perfect metallic contact
between the S and F layers. It is plausible that oxide insulating interlayers
remain magnetic as suggested in Refs. [43, 41]. They may play a dual role of
attenuating the influence of the metallic ferromagnetic layer on the S layer
that completely suppresses superconductivity in our control fully metallic
Co1/Pb/Co2 stacks and at the same time maintaining some kind of the proximity
effect that enables switching between the normal and superconducting states.

At the same time, the nontrivial S/F proximity effect in our system originates
from metallic F layers. This is evidenced by the fact that similar control
structures with reduced thickness of the Co layers (2 nm instead of 3 nm) did
not show any spin-valve effect.

Note that the switching on/off superconductivity in the trilayer EuS/Al/EuS
where EuS is a ferromagnetic insulator has been demonstrated by Li et al.
[42]. This type of system is different from metallic-type structures of Ref.
[41] and the present work, in which only very thin oxidized interfaces are
insulating.

Figure 4: Modeling results for the $\Delta T_{c}(d_{\mathrm{Pb}})$ and
$T_{c}^{\mathrm{P}}(d_{\mathrm{Pb}})$ dependences, main figure and inset,
respectively. The parameters of the model are as follows:
$\xi_{\mathrm{S}}=41$ nm, $\xi_{\mathrm{F}}=12$ nm, $h=0.035$ eV,
$\gamma=0.093$, and $\gamma_{b}=0.48$ (see the text and Ref. [50] for the
exact definitions).

###  IV.2 Theoretical analysis

The modeling of the observed significant SSV effect in heterostructures with
intentionally deteriorated S/F interfaces is rather challenging due to an
increased complexity of these interfaces as compared to the ideal metallic
contacts between the layers. Though experimentally the formation of an
insulating interlayer by oxidation appears to be a doable task, their
characteristics, such as thickness, exact composition and physical properties,
cannot be sufficiently well controlled at present. Therefore, in the following
we will discuss if the main tendencies of the SSV effect with regard to the
thickness of the superconducting Pb layer (Fig. 3) could be at least
qualitatively captured by theory.

The proximity-effect theory suitable for the description of $T_{c}$ in
symmetric F1/S/F2 structures was formulated in Ref. [50]. Applying this theory
to our experimental data, under the above-mentioned somewhat ambiguous
conditions we still can achieve qualitative agreement with experiment, which
captures the main tendencies demonstrated by the data. In the case of the
$\Delta T_{c}(d_{\mathrm{Pb}})$ dependence, see Fig. 4, the theory
demonstrates the nonmonotonicity of the dependence and approximate position of
the maximum. This maximum is expected since the spin-valve effect should be
suppressed both in the limit of very thin and very thick S layer, with a
maximal value at some thickness $d_{S}$ of the order of the coherence length
$\xi_{S}$. At the same time, the quantitative agreement between theory and
experiment is not good, as expected. The same is true in the case of the
$T_{c}^{\mathrm{P}}(d_{\mathrm{Pb}})$ dependence plotted in the inset to Fig.
4. The theoretical model predicts a sharp decline in $T_{c}$ at a
$d_{\mathrm{Pb}}$ value close to 40 nm, but the measurements suggest a
smoother dependence of the critical temperature possibly extending to lower
temperatures. The model also suggests an asymmetric peak in $\Delta
T_{c}(d_{\mathrm{Pb}})$, whereas a more symmetric peak is observed in Fig. 3
(at the same time, the left side of the peak is steeper than the right one
both in theory and in experiment). Finally, the model indicates a rapid
convex-type decay of $\Delta T_{c}(d_{\mathrm{Pb}})$ to the right of the peak,
whereas the experimental data suggests a linear dependence. At the same time,
due to limited number of experimental points, we cannot exclude a nonlinear
behavior around the peak in $\Delta T_{c}(d_{\mathrm{Pb}})$ (in order to check
this, points in the thickness range between 40 and 60 nm would be required).

The fitting parameters given in the caption to Fig. 4 were obtained as
follows. The coherence lengths in the S and F materials, $\xi_{\mathrm{S}}$
and $\xi_{\mathrm{F}}$, we estimated from the residual resistivities of the
materials. The values of the exchange energy $h$, the materials-matching
interface parameter $\gamma$, and the interface resistance parameter
$\gamma_{b}$ were chosen in order to provide correct position of the $\Delta
T_{c}$ maximum and acceptable overall values of this quantity (cf. Fig. 3).
The same values were then employed to plot the theoretical curve for
$T_{c}^{\mathrm{P}}(d_{\mathrm{Pb}})$. The value of $\gamma$ is consistent
with the values of $\xi_{\mathrm{S}}$ and $\xi_{\mathrm{F}}$.

What is unexpected in the above fitting parameters, is a rather small value of
the interface resistance parameter $\gamma_{b}=0.48$. In the tunneling limit,
one can estimate $\gamma_{b}\sim t_{b}^{-1}l_{\mathrm{F}}/\xi_{\mathrm{F}}$ in
terms of the effective interface transparency $t_{b}\ll 1$ (while
$l_{\mathrm{F}}$ is the mean free path in the F material). The transparency
values are not directly measurable. At the same time, it is known that in the
case of conventional tunnel junctions with insulating interfaces of thickness
from 10 to 30 atomic layers, the order of magnitude of $t_{b}$ varies between
$10^{-3}$ and $10^{-5}$ [51]. In that case, we would expect larger values of
$\gamma_{b}$ than the one resulting from our fitting. However, we have checked
that larger $\gamma_{b}$ notably suppresses sensitivity of $T_{c}$ to the
presence of the F layers (the inverse proximity effect becomes strongly
suppressed). The obtained small value of $\gamma_{b}$ points at small
thickness of the tunneling barriers in our junctions. Note that this
correlates with observations by Deutscher and Meunier [41], who concluded that
according to the resistance measurements, the barriers in their experiment
were “much thinner than in a conventional tunneling junction”.

While the theory [50] assumes symmetric F1/S/F2 structure, our samples may
actually be asymmetric from the point of view of the interface transparencies.
The oxidation times of the two interfaces were different in our samples and
our fabrication procedure was such that the oxidation affected different
materials (first, Co1 was oxidized, then Pb) at different temperatures.
However, generalization of our theory to the case of two different
$\gamma_{b}$ parameters is still expected to suppress the proximity effect
almost completely in the case of two tunneling interfaces (while already one
tunneling interface with $t_{b}\ll 1$ should effectively “detach” the
corresponding F layer and thus suppress the effect of rotating magnetization
on $T_{c}$).

A possible reason for not too small transparencies following from the fitting
is that the insulating layers in our samples are actually very thin (a few
atomic layers). Another possibility is that the resulting oxides are not good
insulators but possess finite conductivity or that metallic shortcuts are
present inside the insulating layers. Finally, in contrast to our theory [50]
assuming nonmagnetic insulating barriers, the interfaces could be magnetically
active [52], which would introduce additional degrees of freedom into the
problem (in particular, the interfaces could then nontrivially behave under
the action of rotating magnetic field). Further experiments with better
control of the insulating interfaces are clearly needed in order to clarify
the role of the oxidized interfaces.

##  V Conclusions

In summary, we have investigated superconducting properties of the Co1/Pb/Co2
SSV heterostructures with thin oxide insulating interlayers formed at the
Co1/Pb and Pb/Co2 interfaces. We found the optimal thickness of the
superconducting Pb layer for the realization of the full superconducting spin
valve effect with the magnitude $\Delta T_{c}=0.2$ K. Our finding finally
substantiates the results of the earlier work by Deutscher and Meunier [41]
where a surprisingly large SSV effect was found for the F1/S/F2 structures
with insulating interlayers. It is remarkable that the here obtained value of
$\Delta T_{c}$ significantly exceeds those of many of the multilayers prepared
of elemental metallic ferromagnets and superconductors where special care was
taken to achieve a perfect metallic contact at the S/F interface in order to
enhance the S/F proximity effect.

At the same time, for the spin valve effect, the key role is played not by the
strength of proximity effect but rather by sensitivity of the system to the
variation of the relative magnetizations. Our strategy was to achieve a
“fragile” superconductivity which is sensitive to this kind of control. To
this end, we have realized systems with such parameters that superconductivity
is completely suppressed in the limit of perfect metallic interfaces. The role
of the insulating interface layers is then to restore superconductivity in the
system. This fragile “restored” superconductivity turns out to be indeed very
sensitive to the configuration of the F part of the structure.

Our findings thus call for further exploration of this promising route to
improve the operational parameters of the superconducting spin valves by
advancing the preparation technologies and developing of underlying theories.

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