We report on a study of the structural, magnetic and superconducting properties of Nb(25nm)/Gd(d f )/Nb(25nm) hybrid structures of a superconductor/ ferromagnet (S/F) type. The structural characterization of the samples, including careful determination of the layer thickness, was performed using neutron and X-ray scattering with the aid of depth sensitive mass-spectrometry. The magnetization of the samples was determined by SQUID magnetometry and polarized neutron reflectometry and the presence of magnetic ordering for all samples down to the thinnest Gd(0.8nm) layer was shown. The analysis of the neutron spin asymmetry allowed us to prove the absence of magnetically dead layers in junctions with Gd interlayer thickness larger than one monolayer. The measured dependence of the superconducting transition temperature Tc(d f ) has a damped oscillatory behavior with well defined positions of the minimum at d f =3nm and the following maximum at d f =4nm; the behavior, which is in qualitative agreement with the prior work (J.S. Jiang et al, PRB 54, 6119). The analysis of the Tc(d f ) dependence based on Usadel equations showed that the observed minimum at d f =3nm can be described by the so called "0" to "π" phase transition of highly transparent S/F interfaces with the superconducting correlation length ξ f ≈ 4nm in Gd. This penetration length is several times higher than for strong ferromagnets like Fe, Co or Ni, simplifying thus preparation of S/F structures with d f ∼ ξ f which are of topical interest in superconducting spintronics.

We have used spin-polarized neutron reflectometry to investigate the magnetization profile of superlattices composed of ferromagnetic Gd and superconducting Nb layers. We have observed a partial suppression of ferromagnetic (F) order of Gd layers in [Gd(dF )/Nb(25nm)]12 superlattices below the superconducting (S) transition of the Nb layers. The amplitude of the suppression decreases with increasing dF . By analyzing the neutron spin asymmetry we conclude that the observed effect has an electromagnetic origin -the proximity-coupled S layers screen out the external magnetic field and thus suppress the F response of the Gd layers inside the structure. Our investigation demonstrates the considerable influence of electromagnetic effects on the magnetic properties of S/F systems.Artificial heterostructures with alternating superconducting (S) and ferromagnetic (F) layers are currently attracting great attention due to a diverse set of proximity effects [1-5], including the Larkin-Ovchinnikov-Fulde-Ferrell phase, π-phase superconductivity and triplet pairing. These effects show how ferromagnetism influences the superconducting properties of the S/F heterostructures. Converse proximity effects in which superconductivity influences ferromagnetism have received less attention. Such magnetic proximity effects are expected in systems where the F and S transition temperatures, T F and T c , are comparable, which is the case for heterostructures of cuprate high-T c superconductors and ferromagnetic manganates [6][7][8][9], and for some bulk compounds [10][11][12]. However, because of the chemical and electronic complexity of these materials, simple model systems for magnetic proximity effects are highly desirable.

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Peculiarities of the formation of a neutron enhanced standing wave in the structure with a thin highly absorbing layer of gadolinium are considered in the article. An analogue of the poisoning effect well known in reactor physics was found. The effect is stronger for the Nb/Gd/Nb system. Despite of this effect, for a Nb/Gd bilayer and a Nb/Gd/Nb trilayer placed between Al 2 O 3 substrate and Cu layer, it is shown theoretically and experimentally that one order of magnitude enhancement of neutron density is possible in the vicinity of the Gd layer. This enhancement makes it possible to study domain formation in the Gd layer under transition of the Nb layer(s) into the superconducting state (cryptoferromagnetic phase).