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].
In the layered S/F hybrids superconducting condensate penetrates through the S/F interface into a ferromagnetic
layer, and the pairing wave function not only decays deep into the F metal, but simultaneously oscillates, as the result of
the influence of the exchange field of ferromagnetic layer on the cooper pairs [5]. 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 [6]. A single
mode solution in Ref. [6] for the critical fields was further developed in [7] 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 Cu41Ni59/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 Fig.1 RBS of the trilayer sample (inset - sketch)
method. The base pressure in the “Leybold Z400” vacuum system was about
2×10-6 mbar. Pure argon (99.999%, “Messer Griesheim”) at a pressure of 8×10 -3 mbar was used as sputter gas. 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 [8,9]. 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 wires of
50 μm in diameter were then attached to the
strips by ultrasonic bonding for four-probe
resistance measurements [10] 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)).
Figure 2 displays the temperature dependencies Fig. 2 Temperature dependence of the critical magnetic fields: (a)
of the critical magnetic fields: perpendicular (a) and perpendicular and (b) parallel
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.
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