Spin-valve effects in point contacts to exchange biased Сo₄₀Fe₄₀B₂₀ films
Nonlinear current-voltage characteristics and magnetoresistance of point contacts between a normal metal (N) and films of amorphous ferromagnet (F) Сo₄₀Fe₄₀B₂₀ of different thickness, exchange-biased by antiferromagnetic Mn8₀Ir₂₀ are studied. A surface spin valve effect in the conductance of such...
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Balkashin, O.P. Fisun, V.V. Triputen, L.Yu. Andersson, S. Korenivsk, V. Naidyuk, Yu.G. 2017-06-08T04:38:28Z 2017-06-08T04:38:28Z 2014 Spin-valve effects in point contacts to exchange biased Сo₄₀Fe₄₀B₂₀ films / O.P. Balkashin, V.V. Fisun, L.Yu. Triputen, S. Andersson, V. Korenivski, Yu.G. Naidyuk1 // Физика низких температур. — 2014. — Т. 40, № 10. — С. 1170-1174. — Бібліогр.: 24 назв. — англ. 0132-6414 PACS 72.25.–b, 76.40.Jn, 75.75.–c https://nasplib.isofts.kiev.ua/handle/123456789/119673 Nonlinear current-voltage characteristics and magnetoresistance of point contacts between a normal metal (N) and films of amorphous ferromagnet (F) Сo₄₀Fe₄₀B₂₀ of different thickness, exchange-biased by antiferromagnetic Mn8₀Ir₂₀ are studied. A surface spin valve effect in the conductance of such F–N contacts is observed. The effect of exchange bias is found to be inversely proportional to the Сo₄₀Fe₄₀B₂₀ film thickness. This behavior as well as other magneto-transport effects we observe on single exchange-pinned ferromagnetic films are similar in nature to those found in conventional three-layer spin-valves. Funding by the National Academy of Sciences of Ukraine under project 26/14-Н (NANO) is gratefully acknowledged. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур III Международный семинар по микроконтактной спектроскопии Spin-valve effects in point contacts to exchange biased Сo₄₀Fe₄₀B₂₀ films Article published earlier |
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| title |
Spin-valve effects in point contacts to exchange biased Сo₄₀Fe₄₀B₂₀ films |
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Spin-valve effects in point contacts to exchange biased Сo₄₀Fe₄₀B₂₀ films Balkashin, O.P. Fisun, V.V. Triputen, L.Yu. Andersson, S. Korenivsk, V. Naidyuk, Yu.G. III Международный семинар по микроконтактной спектроскопии |
| title_short |
Spin-valve effects in point contacts to exchange biased Сo₄₀Fe₄₀B₂₀ films |
| title_full |
Spin-valve effects in point contacts to exchange biased Сo₄₀Fe₄₀B₂₀ films |
| title_fullStr |
Spin-valve effects in point contacts to exchange biased Сo₄₀Fe₄₀B₂₀ films |
| title_full_unstemmed |
Spin-valve effects in point contacts to exchange biased Сo₄₀Fe₄₀B₂₀ films |
| title_sort |
spin-valve effects in point contacts to exchange biased сo₄₀fe₄₀b₂₀ films |
| author |
Balkashin, O.P. Fisun, V.V. Triputen, L.Yu. Andersson, S. Korenivsk, V. Naidyuk, Yu.G. |
| author_facet |
Balkashin, O.P. Fisun, V.V. Triputen, L.Yu. Andersson, S. Korenivsk, V. Naidyuk, Yu.G. |
| topic |
III Международный семинар по микроконтактной спектроскопии |
| topic_facet |
III Международный семинар по микроконтактной спектроскопии |
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2014 |
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English |
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Физика низких температур |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| format |
Article |
| description |
Nonlinear current-voltage characteristics and magnetoresistance of point contacts between a normal metal (N)
and films of amorphous ferromagnet (F) Сo₄₀Fe₄₀B₂₀ of different thickness, exchange-biased by antiferromagnetic
Mn8₀Ir₂₀ are studied. A surface spin valve effect in the conductance of such F–N contacts is observed. The
effect of exchange bias is found to be inversely proportional to the Сo₄₀Fe₄₀B₂₀ film thickness. This behavior as
well as other magneto-transport effects we observe on single exchange-pinned ferromagnetic films are similar in
nature to those found in conventional three-layer spin-valves.
|
| issn |
0132-6414 |
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https://nasplib.isofts.kiev.ua/handle/123456789/119673 |
| citation_txt |
Spin-valve effects in point contacts to exchange biased Сo₄₀Fe₄₀B₂₀ films / O.P. Balkashin, V.V. Fisun, L.Yu. Triputen, S. Andersson, V. Korenivski, Yu.G. Naidyuk1 // Физика низких температур. — 2014. — Т. 40, № 10. — С. 1170-1174. — Бібліогр.: 24 назв. — англ. |
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| first_indexed |
2025-11-26T17:39:54Z |
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2025-11-26T17:39:54Z |
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1850765987838689280 |
| fulltext |
Low Temperature Physics/Fizika Nizkikh Temperatur, 2014, v. 40, No. 10, pp. 1170–1174
Spin-valve effects in point contacts to exchange biased
Сo40Fe40B20 films
O.P. Balkashin1, V.V. Fisun1, L.Yu. Triputen1, S. Andersson2, V. Korenivski2,
and Yu.G. Naidyuk1
1B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine
47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: Balkashin@ilt.kharkov.ua
2Nanostructure Physics, Royal Institute of Technology, Stockholm, SE-10691, Sweden
Received June 2, 2014, published online August 21, 2014
Nonlinear current-voltage characteristics and magnetoresistance of point contacts between a normal metal (N)
and films of amorphous ferromagnet (F) Сo40Fe40B20 of different thickness, exchange-biased by antiferromag-
netic Mn80Ir20 are studied. A surface spin valve effect in the conductance of such F–N contacts is observed. The
effect of exchange bias is found to be inversely proportional to the Сo40Fe40B20 film thickness. This behavior as
well as other magneto-transport effects we observe on single exchange-pinned ferromagnetic films are similar in
nature to those found in conventional three-layer spin-valves.
PACS: 72.25.–b Spin polarized transport;
76.40.Jn Metal-to-metal contacts;
75.75.–c Magnetic properties of nanostructures.
Keywords: point contacts, spin valve, spin transfer torque, exchange bias, CoFeB.
Interest in the study of layered systems ferromagnet/an-
tiferomagnet (F/AF) is due to some open fundamental
questions as to the mechanisms of exchange bias as well as
applications of such structures in spintronic devices [1] ex-
ploiting the giant magnetoresistance effect [2]. Exchange
bias was first discovered in microparticles of cobalt oxide
CoO [3], manifested as a field-shift of the hysteresis loop
with respect to H = 0. Nowadays, this effect is widely used
for pinning the magnetization of one of the layers of a spin
valve (SV) [4], which typically consists of two ferromag-
netic films separated by a normal metal, F1/N/F2. In the
antiparallel configuration of the magnetization of the F1,2
layers, the resistance of the SV is high, and is lowered on
switching into the parallel configuration. This change in
the resistance in response to an external field gives the SV
its key functionality as a field sensor. For pinning the di-
rection of the magnetization of the ferromagnetic layer the
following procedure is used: at a temperature above the
Neel temperature TN of the antiferromagnet, but below the
Curie temperature TC of the ferromagnet, the AF is mag-
netically disordered while F is magnetically ordered. As
the temperature is decreased below TN, with a suitably high
magnetic field applied to the sample (to achieve saturation
of F), the AF becomes magnetically ordered, with its inter-
face spins aligned by the ferromagnet. This preferred spin
direction in the AF, induced by the field-cooling, acts as
magnetic bias when the F-magnetization is switched by a
reversing magnetic field. The AF-bias results in a shift of
the hysteresis loop of F proportional to the interfacial ex-
change force, with its quantifying parameter, the so-called
exchange-bias field Нex, defined as
eх ( ) / 2L RH H H= − + . (1)
Here HL и HR are the magnetic field values at which
the magnetization reversal in F occurs.
The growing interest in magnetic nanostructures based
on multicomponent amorphous alloys is due to the fact that
the realization of the effect of spin transfer torque in such
systems [5] requires smaller switching currents as compar-
ed with single-element ferromagnets [6]. In this regard,
amorphous alloy CoFeB is a promising candidate for ap-
plications in devices based on the giant magnetoresistance
effect due to its high electrical resistance and low magnetic
anisotropy [7,8], which are beneficial to functional para-
meters of spintronic nanostructures. In particular, a tunnel
junction CoFeB/MgO/CoFeB with epitaxial films of mag-
© O.P. Balkashin, V.V. Fisun, L.Yu. Triputen, S. Andersson, V. Korenivski, and Yu.G. Naidyuk, 2014
mailto:Balkashin@ilt.kharkov.ua
Spin-valve effects in point contacts to exchange biased Сo40Fe40B20 films
nesium oxide has been demonstrated to have giant (up to
one thousand percent) magnetoresistance, between its par-
allel and antiparallel magnetic configurations [9], which is
due to a nearly full spin asymmetry of the electron trans-
port through the CoFeB/MgO interface.
The key magnetic properties for a variety of spintronic
devices are the coercive field of the magnetic material, its
saturation magnetization, weak magnetostriction, all pre-
sent in CoFeB, which is why the material is widely used in
research and industrial applications. The alloy is amor-
phous and therefore can optimally grow on almost any
substrate, avoiding lattice mismatch between the under-
and over-layers and minimizing other defects in the stack.
In our papers [10–13] it was shown that in nonmagnet-
ic-ferromagnetic single-interface tip-surface point contacts,
with the ferromagnet being polycrystalline Co, the magne-
to-transport exhibits many of the characteristic of tradi-
tional pillar-like F1/N/F2 spin-valves (with two magnetic
interfaces, having parallel and antiparallel spin configura-
tion). Our analysis of the experiments resulted in a spin-
transport model for a single F/N interface in the point con-
tact region [11,13]. The model proposes that an atomically
thin surface layer of spins in cobalt is magnetically rotated
with respect to the spins in the bulk, forming an atomically
thin domain wall. It is well known that in narrow contacts
between two ferromagnetic electrodes, the width of a mag-
netic domain wall can be sufficiently small, commensurate
with the size of the narrowing [14]. Such a thin domain
wall allows to have orientation of the surface layer magnet-
ization different from the bulk. As a result, in a simple po-
int contact, with a single ferromagnetic film, two magnetic
subsystem are realized — the bulk of the film and its sur-
face layer with different anisotropy and coercivity values,
forming a spatial structure of a “surface spin-valve” [11].
In [12], analyzing the dependence of the magnetoresis-
tance of the surface SV based on an exchange biased
Co/FeMn film, we found that the exchange bias shifts not
only the inner transitions of the magnetoresistance loop,
corresponding to the magnetization reversal of the bulk of
the Co film, but also the external transitions coming from
the reversal of the surface layer. Thus, despite the weak
coupling between the surface layer and the bulk of the film,
the coupling is sufficient to offset the switching of the sur-
face layer.
The aim of this work was to determine the effect of the
thickness of the ferromagnetic layer on the magnitude of
the exchange bias acting on the bulk of the film and
the surface layer, using point contacts based on amorphous
Co40Fe40B20. It is known from the literature [15,16] that
increasing the thickness of the ferromagnetic film should
result in diminishing the effect of the exchange bias, func-
tionally according to ~1/t, where t is the thickness of
the ferromagnet.
For this study AF/F exchange biased bilayers were used,
with AF being Mn80Ir20 and F is Co40Fe40B20. The ex-
change bias was set in the standard way: by slowly cool-
ing the sample in a magnetic field of 350 Oe, from tempe-
rature higher than the blocking temperature of the AF
(Tb ~ 513 K for Mn80Ir20) to room temperature.
The studied samples were multilayer films deposited on
oxidized Si substrates and schematically are shown in
Fig. 1. A seed layer of Ta was first deposited on to the sub-
strate, followed by copper and antiferromagnetic Mn80Ir20,
for exchange-biasing the Co40Fe40B20 overlayer. The struc-
ture was capped with a 3 nm thick copper layer to protect
the ferromagnet against oxidation. All layers were deposit-
ed in-situ, without breaking the vacuum, for ensuring high-
quality interfaces. The tip was made out of a thin copper
wire, sharpened into to a needle, first mechanically, then
by chemical etching in HNO3.
The measurements of the differential resistance dV/dI(V)
of the point contacts as a function of the applied voltage
bias were made using the traditional method of synchronous
detection of the amplitude of a modulated ac-signal. The resis-
tance of the studied point contacts was in the range 7–30 Ω,
and their radii, estimated using Maxwell's formula [17],
were 35–10 nm. This estimate used the typical value of the
resistivity of Co78Fe11B11 of 100 μΩ·cm [18]. External
magnetic field was applied parallel to the film plane. All
measurements were made at 4.2 K.
Figure 2 shows the dependence of the differential re-
sistance dV/dI(V) in a magnetic field, where a peak is seen
at the negative bias polarity. The peak position shifts to-
ward higher currents with increasing magnetic field. A si-
milar effect, the “dynamic SV effect”, was observed in
point contacts based on cobalt [10,19], and was attributed
to the excitation of a stationary precession of the magneti-
zation vector due to a transfer of the spin angular momen-
tum of the transport electrons to the angular momentum of
the ferromagnetic lattice. Thus, the dynamic SV effect is
also present in amorphous magnetic films, in our case
Fig. 1. (Color online) Schematic of a tip-surface N/F/AF point con-
tact. The layer thicknesses are shown in parentheses in nanometers.
Cu needle
Cu(3)
Co Fe B ( )40 40 20 t
Mn Ir (10)80 20
Cu(100)
Та(5)
Substrate
Cap
Multilayer AF+F
Low Temperature Physics/Fizika Nizkikh Temperatur, 2014, v. 40, No. 10 1171
O.P. Balkashin, V.V. Fisun, L.Yu. Triputen, S. Andersson, V. Korenivski, and Yu.G. Naidyuk
Co40Fe40B20. Notice that coefficient k (see inset in Fig. 2)
has the value corresponding to the lower bound for the
same coefficient measured on crystalline cobalt point con-
tacts [19]. Hence we can conclude that the excitation of the
single-interface SV effects in Co40Fe40B20 requires lower
driving current densities than in the case of pure cobalt.
Figure 3 shows the dependence of dV/dI(V) and magne-
toresistance dV/I(V = 0,H) of point contacts to Co40Fe40B20
films of various thickness (20, 9, 6, 3 nm). It is known for
SV [5] that sweeping the magnetic field results in magne-
toresistance that has the form of two rectangle-like mean-
ders or loops. Figures 3(b),(d),(f),(h) show our measured
magnetoresistance as two such loops, most clearly seen for
the CoFeB film thickness of 20 nm. The switching ob-
served in the magnetoresistance loop is between two stable
spin configurations in the point contact core, and by analo-
gy with traditional three-layer SV, between the parallel and
antiparallel states of the single-interface spin valve in our
case. Comparing the dV/dI of Figs. 3(a) and 3(b), where a
clear rectangle-shaped hysteresis is observed, one can no-
tice that the magnitude of the resistance change in dV/dI(V)
and dV/dI(H) is essentially the same. This indicates that the
effect of the electron spin-transfer torque and the effect of
the magnetic field is on the same magnetic system, in the
core of the point contact as that is what determines the
measured resistance.
The lack of apparent hysteresis in the dV/dI(V) for
smaller thicknesses (t = 3, 6, 9 nm) may be due to different
values of the exchange-pinning strength at the surface lay-
er (switching layer). For example, the curve in Fig. 3(g)
does show hysteresis with, however, significantly washed
out transitions between the low- and high-resistance states.
It is possible that in this particular case, the coercivity of
the magnetic surface layer is large such that the spin trans-
fer from the electron current is not sufficient for reversing
the layer’s magnetization. Additional to the washed out
switching in thinner films, the corresponding change in the
resistance is about 0.1%, which is several times smaller
than that for 20 nm. The rather sharp peaks of the respec-
tive R-H curves are similar to the dependence characteristic
of anisotropic magnetoresistance. Indeed, as shown in [20],
the anisotropic magnetoresistance has a typical value of
0.15% for the composition Co73.8Fe16.2B10. It is therefore
possible that our thinner films exhibit predominantly aniso-
tropic magnetoresistance. It may be informative to note
that for the thinner films the typical point contact size ex-
ceeds the film thickness, so the ferromagnet occupies a
smaller portion of the contact core and may contribute less
to the measured resistance.
Figure 4 shows the dependence of the exchange bias
field, calculated using equation (1) for several contacts,
versus the inverse of the thickness of Co40Fe40B20. The
values of the characteristic fields of the magnetization re-
versal, HB (for bulk) and HS (surface) are determined from
the R-H curves as positions of the switching boundaries
(inner and outer) on the H-axis (Figs. 3(b),(d),(f),(h), for
both directions of the field.
As can be seen from Fig. 4, with decreasing thickness
of the ferromagnet, the exchange field increases. This is
consistent with the literature data [15,16] and is due to the
fact that the thinner ferromagnet is more strongly “pinned”
by the antiferromagnet and, as a consequence, it requires
larger values of the applied field for the magnetization re-
versal. In addition, the field strength must be sufficient to
Fig. 2. (Color online) Spin-transfer peak in dV/dI(I) for a typical
point contact of Сo40Fe40B20 (t = 20 nm)–Cu, for applied mag-
netic field in the range 0 to 3.5 T. The negative bias corresponds
to the current direction flow from the film into the tip (electrons
from the tip into the magnetic film). The inset shows the peak
position as a function of the magnetic field magnitude.
–4 –2 0 2
18.0
18.4
18.8
19.2
0 1 2 3 4–2
–1
dV
/d
I,
Ω
0
3.5 T
H T,
I,
m
A
I, mA
= –1– I kH
= 0.26 mA/Tk
Fig. 3. (Color online) dV/dI(V) and the corresponding magnetoresist-
ance dV/dI(H) for contacts Сo40Fe40B20 (t)–Cu (t = 3, 6, 9, 20 nm).
Arrows indicate the direction of the respective sweep, from left to
right and from right to left. Text legends show the film thickness,
t, value of the radii of the contacts, r, estimated using the Max-
well formula [16], and the scale of the magnetoresistance in %.
8.60
8.65
11.56
11.58
18.89
18.90
–0.4 –0.2 0 0.2 0.4
18.66
18.69
8.7
9.0
19.5
21.0
11.6
12.0
–80 –40 0 40 80
19
20
0.4%
0.1%
0.2%
0.1%
(d)
3 nm
6 nm
9 nm
20 nm
(a)
0.4%
(f)
(h)
r = 15 nm
r = 22 nm
r = 15 nm
(b)t = 20 nm
r = 30 nm
( )с
t = 9 nm
(e)
t = 6 nm
(g)
t = 3 nm
Voltage, mV Field, T
dV
/d
I,
Ω
dV
/d
I,
Ω
1172 Low Temperature Physics/Fizika Nizkikh Temperatur, 2014, v. 40, No. 10
Spin-valve effects in point contacts to exchange biased Сo40Fe40B20 films
overcome the self-pinning by the surface mentioned above.
It was also found that the surface layer also is affected by
the exchange bias, because the outer switching field is
asymmetric with respect to H = 0. Apparently, the magnet-
ic subsystem of the film and the surface layer have a mag-
netic coupling. Similar field offsets were observed for ex-
change-biased films of polycrystalline cobalt [12].
In some cases, dV/dI(H) showed inverse magnetoresis-
tance (high resistance at high field), illustrated in Fig. 5, in
contrast to the more common positive MR (low resistance
at high field). Figure 6 shows schematically a transport
model for anisotropic magnetoresistance, where MR inver-
sion is possible if the current is predominantly in the plane
of the film. We also note that a qualitatively similar picture
arises in the case of perpendicular magnetic anisotropy,
i.e., when the favored direction of the magnetization in
the ferromagnetic film is orthogonal to its plane. Perpen-
dicular magnetic anisotropy is usually characteristic of very
thin ferromagnetic films due to the effect of the interfaces,
however, in [21] it has been observed in Сo40Fe40B20
films with thicknesses up to 172 nm. This was attributed to
the presence of special (300) texture in the film. In our
case, the cause of the perpendicular anisotropy can poten-
tially be the mechanical stress produced by the tip in the
point contact core region [23]. Thus, the observed inver-
sion of the magnetoresistance can be due to a significant
in-plane current flow and/or perpendicular magnetic aniso-
tropy in the contact region. It may be informative to men-
tion that no MR inversion was observed in point contacts
to exchange-biased cobalt films, Со/Fe50Mn50 [12]. One
more potential reason for the inversed MR may be a change
of the sign of the spin asymmetry coefficient β of the cur-
rent from positive to negative, which is observed for Cr, V,
or Mn impurities in ferromagnets (see Table 1 and Fig. 10
of [23]). In our case, the latter is possible due to the pres-
ence of a Mn-rich layer in the stack.
Conclusion
We have studied the differential resistance dV/dI(V) and
magnetoresistance dV/dI (H, V = 0) of point contacts based
on films of amorphous ferromagnetic Сo40Fe40B20 of dif-
ferent thickness, exchange-biased by antiferromagnetic
Mn80Ir20. We have observed a single-interface spin-valve
Fig. 4. (Color online) Exchange-bias field Hex for
Сo40Fe40B20/Mn80Ir20 versus the thickness of the ferromagnetic
film: circles correspond to the surface layer of the film, triangles
– to the bulk of the film. Straight lines: linear fits to the corre-
sponding data points.
0 1 2 3 4
20
40
60
80
surface
bulk
H
ex
, m
T
10/ , nm t
–1
Fig. 5. dV/dI(V) and the corresponding magnetoresistance dV/dI(H)
for two contacts Сo40Fe40B20 (t)–Cu (t = 9, 20 nm), exhibiting
inverse magnetoresistance. The scale in percent near dV/dI(H)
shows the magnitude of the MR.
–0.1 0 0.1
24.35
24.40
–50 0 5024
26
28
14.08
14.10
14.12
14
15
Field, T
20 nm
9 nm9 nm
Voltage, mV
0.15%
0.2%
(c) (d)
(b)20 nm
(a)
dV
/d
I,
Ω
dV
/d
I,
Ω
dV
/d
I,
Ω
dV
/d
I,
Ω
Fig. 6. Schematic of a N/F tip-surface point contact. The ferro-
magnetic film is shown as a gray rectangle. Bold arrows show the
direction of the magnetization M, and the dashed arrows repre-
sent the current flow lines in the contact. It is known that aniso-
tropic magnetoresistance is smaller when the current flows per-
pendicular to M [22]. Two graphs in the middle illustrate the an-
isotropic magnetoresistance behavior with increasing magnetic
field: (a) and (b) is the case of the current flow predominantly
perpendicular to the film plane, and (c) and (d) is the case of the
dominating contribution from the current component in the film
plane. It is assumed that M has an out of plane component in both
cases.
=>
=>
H 0<
H > 0
H0
H < 0
H > 0H0
(a) (b)
(c) (d)
Low Temperature Physics/Fizika Nizkikh Temperatur, 2014, v. 40, No. 10 1173
O.P. Balkashin, V.V. Fisun, L.Yu. Triputen, S. Andersson, V. Korenivski, and Yu.G. Naidyuk
effect, characteristic of the conventional three-layer
nanopillars F1/N/F2. The strength of the exchange bias in
the bulk as well as at the surface of CoFeB is inversely
proportional to the thickness of the ferromagnetic film,
which demonstrates that point contacts to multicomponent
amorphous single-layer ferromagnetic films can behave
similar to the traditional multi-layer spin valves. We addi-
tionally observe an unusual magnetoresistance inversion in
the studied amorphous exchange-biased system and dis-
cuss its possible origins.
Funding by the National Academy of Sciences of
Ukraine under project 26/14-Н (NANO) is gratefully
acknowledged.
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Conclusion
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