UPPER CRITICAL FIELD IN TRILAYER STRUCTURES
FERROMAGNET-SUPERCONDUCTOR-FERROMAGNET (FSF).
ANTROPOV Evgheni
Institute of Electronic Engineering and Nanotechnologies “D. Ghiţu” ASM, Chisinau, MD2028, Moldova
Reviewer: Morari R., dr.
Keywords: critical magnetic fields, FSF-trilayer, spintronics, superconductivity, nanostructures.
The upper critical magnetic field Bc2 of an isotropic type-II superconductor generally obeys linear temperature
dependence in the vicinity of the superconducting transition temperature Tc [1]. Prepared metallic multilayers (ML)
consisting of alternating superconducting (S) and normal metal (N), or even of two different superconductors S and 𝑆 ′ ,
show nonlinear Bc2(T) dependences (see review [2] and references herein). Among the layered superconducting systems,
the superconductor-ferromagnet S/F metallic hybrids attract a special attention because of perspectives of applications in
superconducting spintronics [3,4].
Investigation of the critical magnetic fields of the layered S/F hybrids demonstrates that magnetic field penetration
not only into the superconducting, but also into the ferromagnetic layers should be taken into account to describe
correctly the critical magnetic fields [5]. A single mode solution in Ref. [5] for the critical fields was further developed in
[6] applying a multimode solution of the superconductivity equations for SF-bilayers and SF-multilayers. As far as
superconducting valve core is represented by FSF-trilayer, in this report we consider the temperature dependence of the
critical magnetic fields for the trilayer FSF structure, both experimentally, studying Cu 41Ni59/Nb/Cu41Ni59 trilayers, and
theoretically by calculating the critical fields within the Usadel equations formalism.
Trilayer samples Cu41Ni59/Nb/Cu41Ni59 were grown on an atomically
smooth surface of silicon substrate Si (1 1 1) by magnetron sputtering method.
Layer of ferromagnet was grown in a wedge-like shape, which allows to create a
series of samples of different thickness on the same substrate. The first and last
layers were a capturing silicon thin film for protection the main F/S/F structure
from oxidation in atmosphere [7,8]. Samples of equal width (about 2.5 mm)
were cut perpendicular to the wedge gradient to obtain a batch of F/S/F strips,
with varying Cu41Ni59 layer thickness dF, for Bc2(T,dF) measurements. Aluminum Fig.1 RBS of the trilayer sample (inset - sketch)
wires of 50 μm in diameter were then attached to the strips by ultrasonic bonding
for four-probe resistance measurements [9] The sketch of whole F/S/F structure is given in Figure 1.
The critical field measurements were performed
in a 4He cryostat equipped with a superconducting
solenoid providing fields up to 9 T. The temperature
was controlled in the range of 1.5–10 K with an
accuracy of 1 mK. The resistivity measurements were
performed with an AC bridge using the conventional
four-probe method. The critical magnetic fields were
measured in two geometries: the field is perpendicular
⊥
to the sample surface (𝐵𝑐2
(𝑇)), and the field is
∥
parallel to the sample plane (𝐵𝑐2
(𝑇)).
Fig.2 Temperature dependence of the critical magnetic fields:
Figure 2 displays the temperature dependencies of the (a) perpendicular and (b) parallel
critical magnetic fields: perpendicular (a) and parallel (b) to the
sample plane for the FSF series with the niobium layer thickness 𝑑𝑁𝑏 ≈ 15.5 𝑛𝑚. As can be seen from the figure, they
show non-monotonic behavior with variation of the ferromagnetic Cu41Ni59 layer thickness, which needs a special
consideration.
Thе work was supported by the A.v.Humboldt Foundation Institutspartnerschaften-grant “Nonuniform
superconductivity in layered SF-nanostructures Superconductor/Ferromagnet”, and Moldavian State Program Grant
11.836.05.01A.
References
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[9] ZDRAVKOV V., SIDORENKO A., OBERMEIER G., GSELL S., SCHRECK M., MÜLLER C., HORN S.,
TIDECKS R., and TAGIROV L.R. Physical Review Letters 97, 057004 (2006).