A. A. Kamashev

Superconducting spin-valve effect and triplet superconductivity in CoOx /Fe1/Cu/Fe2/Cu/Pb multilayer

We report magnetic and superconducting properties of the modified spin valve system CoOx/Fe1/Cu/Fe2/Cu/Pb. Introduction of a Cu interlayer between Fe2 and Pb layers prevents material interdiffusion process, increases the Fe2/Pb interface transparency, stabilizes and enhances properties of the system. This allowed us to perform a comprehensive study of such heterostructures and to present theoretical description of the superconducting spin valve effect and of the manifestation of the long-range triplet component of the superconducting condensate.

Peculiarities of performance of the spin valve for the superconducting current

The spin valve effect for the superconducting current based on the superconductor/ferromagnet proximity effect has been studied for a CoOx/Fe1/Cu/Fe2/Cu/Pb multilayer. The magnitude of the effect ΔTc = TcAP − TcP, where TcP and TcAP are the superconducting transition temperatures for the parallel (P) and antiparallel (AP) orientation of magnetizations, respectively, has been measured for different thicknesses of the Fe1 layer dFe1. The obtained dependence of the effect on dFe1 reveals that ΔTc can be increased in comparison with the case of a half-infinite Fe1 layer considered by the previous theory. A maximum of the spin valve effect occurs at dFe1 ∼ dFe2. At the optimal value of dFe1 almost full switching from the normal to the superconducting state when changing the mutual orientation of magnetizations of the iron layers Fe1 and Fe2 from P to AP is demonstrated.

Boosting the superconducting spin valve effect in a metallic superconductor/ferromagnet heterostructure

Superconducting spin valves based on the superconductor/ferromagnet (S/F) proximity effect are considered to be a key element in the emerging field of superconducting spintronics. Here, we demonstrate the crucial role of the morphology of the superconducting layer in the operation of a multilayer S/F1/F2 spin valve. We study two types of superconducting spin valve heterostructures, with rough and with smooth superconducting layers, using transmission electron microscopy in combination with transport and magnetic characterization. We find that the quality of the S/F interface is not critical for the S/F proximity effect, as regards the suppression of the critical temperature of the S layer. However, it appears to be of paramount importance in the performance of the S/F1/F2 spin valve. As the morphology of the S layer changes from the form of overlapping islands to a smooth case, the magnitude of the conventional superconducting spin valve effect significantly increases. We attribute this dramatic effect to a homogenization of the Green function of the superconducting condensate over the S/F interface in the S/F1/F2 valve with a smooth surface of the S layer.

Increasing the performance of a superconducting spin valve using a Heusler alloy

We have studied superconducting properties of spin-valve thin-layer heterostructures CoOx/F1/Cu/F2/Cu/Pb in which the ferromagnetic F1 layer was made of Permalloy while for the F2 layer we have taken a specially prepared film of the Heusler alloy Co2Cr1− xFexAl with a small degree of spin polarization of the conduction band. The heterostructures demonstrate a significant superconducting spin-valve effect, i.e., a complete switching on and off of the superconducting current flowing through the system by manipulating the mutual orientations of the magnetization of the F1 and F2 layers. The magnitude of the effect is doubled in comparison with the previously studied analogous multilayers with the F2 layer made of the strong ferromagnet Fe. Theoretical analysis shows that a drastic enhancement of the switching effect is due to a smaller exchange field in the heterostructure coming from the Heusler film as compared to Fe. This enables to approach an almost ideal theoretical magnitude of the switching in the Heusler-based multilayer with a F2 layer thickness of ca. 1 nm.

Estimate of the Degree of the Spin Polarization of a Ferromagnet from Data on the Superconductor/Ferromagnet Proximity Effect

The superconductor/ferromagnet proximity effect in the Pb/Co2Cr1–xFexAl bilayer systems has been studied. Thin films of the Heusler alloy Co2Cr1–xFexAl have been prepared at different substrate temperatures. It has been established using Andreev spectroscopy of point contacts that the degree of spin polarization of conduction electrons in the Heusler alloy is on the order of 30 and 70% for the films prepared at a substrate temperature of 300 and 600 K, respectively. It has been found that the dependence of the superconducting transition temperature on the thickness of the Pb layer at a fixed thickness of the Heusler layer is determined by the degree of spin polarization of the conduction band in the ferromagnetic layer.

Experimental Investigation of the Role of the Triplet Pairing in the Superconducting Spin-Valve Effect

An important role of the morphology of a superconducting layer in the superconducting spin-valve effect has been established. The triplet pairing induced by the superconductor/ferromagnet proximity effect has been experimentally investigated for samples CoOx/Py1/Cu/Py2/Cu/Pb (where Py = Ni0.81Fe0.19) with a smooth superconducting layer. The optimization of the parameters of this structure has demonstrated a complete switching between the normal and superconducting states with a change in the relative orientation of magnetizations of the ferromagnetic layers from the antiparallel to orthogonal orientation. A pure triplet contribution has been observed for the sample with a permalloy layer thickness at which the superconducting spin-valve effect vanishes. A direct comparison of the experimental data with the theoretical calculation of the temperature of the transition to the superconducting state has been performed for the first time.

Superconducting Spin Valve and Triplet Superconductivity

A review of recent works on spin valves is presented. A number of CoO/Fe1/Cu/Fe2/S multilayers (S = In or Pb) are prepared and studied in detail. The full spin-valve effect for the superconducting current in these heterostructures is realized for the first time; its sign-changing oscillatory behavior is observed, depending on the thickness of the Fe2 layer; and direct evidence of long-range triplet superconductivity is found.