arXiv subjects

I. A. Garifullin

Publications and source records attributed to I. A. Garifullin.

17 recordsLinked to original sources

arXiv2025

Superconducting spin valve effect in Fe/Si$_3$N$_4$/Pb/Si$_3$N$_4$/Fe heterostructures

The structures of the superconducting spin valve (SSV) Fe/Si$_3$N$_4$/Pb/Si$_3$N$_4$/Fe (where Si$_3$N$_4$ is a dielectric insulating layer of controlled thickness) were investigated. The dependence of the magnitude of the SSV effect on the thicknesses of the superconducting (S) and insulating (I) layers was studied. Optimization of the S and I layer thicknesses enabled a complete switching between the normal and superconducting states when the mutual orientation of the magnetizations of the ferromagnetic (F) layers changed from antiparallel to parallel. A maximal SSV effect value of 0.36\,K was achieved in an external magnetic field of 1\,kOe. These results demonstrate that SSV structures with tunable S/F interface transparency controlled by insulating interlayers are promising for achieving a significant magnitude of the effect. This opens new avenues for the development of such systems and their potential applications in spintronic devices.

cond-mat.supr-con↗

arXiv2024

Expanding the operational temperature window of a superconducting spin valve

To increase the efficiency of the superconducting spin valve (SSV), special attention should be paid to the choice of ferromagnetic materials for the F1/F2/S SSV multilayer. Here, we report the preparation and the superconducting properties of the SSV heterostructures where Pb is used as the superconducting S layer. In the magnetic part of the structure, we use the same starting material, the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al$_{y}$, for both F1 and F2 layers. We utilize the tunability of the magnetic properties of this alloy, which, depending on the deposition conditions, forms either an almost fully spin-polarized half-metallic F1 layer or a weakly ferromagnetic F2 layer. We demonstrate that the combination of the distinct properties of these two layers boosts the generation of the long-range triplet component of the superconducting condensate in the fabricated SSV structures and yields superior values of the triplet spin-valve effect of more than 1 K and of the operational temperature window of the SSV up to 0.6 K.

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arXiv2024

Superconducting spin valve effect in the Co/Pb/Co heterostructure with insulating interlayers

We report the superconducting properties of the Co/Pb/Co heterostructures with thin insulating interlayers. The main specific feature of these structures is the intentional oxidation of both superconductor/ferromagnet (S/F) interfaces. We study variation of the critical temperature of our systems due to switching between parallel and antiparallel configurations of the magnetizations of the two magnetic layers. Common wisdom suggests that this spin valve effect, which is due to the S/F proximity effect, is most pronounced in the case of perfect metallic contact at the interfaces. Nevertheless, in our structures with intentionally deteriorated interfaces, we observed a significant full spin valve effect. A shift of the superconducting transition temperature $T_c$ by switching the mutual orientation of the magnetizations of the two ferromagnetic Co layers from antiparallel to parallel amounted to $ΔT_c =0.2$ K at the optimal thickness of the superconducting Pb layer. Our finding verifies the so far unconfirmed earlier results by Deutscher and Meunier on an F1/S/F2 heterostructure with oxidized interlayers [https://doi.org/10.1103/PhysRevLett.22.395, G. Deutscher and F. Meunier, Phys. Rev. Lett. 22, 395 (1969)] and suggests an alternative route to optimize the performance of the superconducting spin valves.

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arXiv2019

Superconducting spin-valve effect in heterostructures with ferromagnetic Heusler alloy layers

We report a comparative analysis and theoretical description of the superconducting properties of two spin-valve-valve structures containing the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al$_{y}$ as one of two ferromagnetic (F1 or F2) layers of the F1/F2/S structure, where S stands for the superconducting Pb layer. In our experiments we used the Heusler alloy layer in two roles: as a weak ferromagnet on the place of the F2 layer and as a half-metal on the place of the F1 layer. In the first case, we obtained a large ordinary superconducting spin-valve effect $ΔT_c$ assisted by the triplet superconducting spin-valve effect $ΔT_c^{trip}$. In the second case, we observed a giant magnitude of $ΔT_c^{trip}$ reaching 0.5 K. An underlying theory based on the solution of the Usadel equations using Kupriyanov-Lukichev boundary conditions with arbitrary material parameters for all layers and arbitrary boundary parameters for all interfaces is presented in Appendix. We find a good agreement between our experimental data and theoretical results.

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arXiv2019

Superconducting switching due to triplet component in the Pb/Cu/Ni/Cu/Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$ spin-valve structure

We report the superconducting properties of the Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$/Cu/Ni/Cu/Pb spin-valve structure which magnetic part comprises the Heusler alloy layer HA = Co$_2$Cr$_{1-x}$Fe$_x$Al$_y$ with a high degree of spin polarization (DSP) of the conduction band and the Ni layer of variable thickness. We obtained that the separation between the superconducting transition curves measured for the parallel ($α= 0^\circ$) and perpendicular ($α= 90^\circ$) orientation of the magnetizations of the HA and Ni layers reaches up to 0.5 K ($α$ is an angle between the magnetization of two ferromagnetic layers). For all studied samples the dependence of the superconducting transition temperature $T_c$ on $α$ demonstrates a deep minimum in the vicinity of the perpendicular configuration of magnetizations. This suggests that the observed minimum and the corresponding full switching effect of the spin valve is caused by the long-range triplet component of the superconducting condensate in the multilayer. Such a large effect can be attributed to a half-metallic nature of the HA layer which in the orthogonal configuration efficiently draws off the spin-polarized Cooper pairs from the space between the HA and Ni layers. Our results indicate a significant potential of the concept of the superconducting spin-valve multilayer comprising a half-metallic ferromagnet recently proposed by A. Singh et al., Phys. Rev. X 5, 021019 (2015) in achieving large values of the switching effect.

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arXiv2018

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

We have studied superconducting properties of the spin-valve thin layer heterostructures CoO$_x$/F1/Cu/F2/Cu/Pb where the ferromagnetic F1 layer was standardly made of Permalloy whereas for the F2 layer we have taken a specially prepared film of the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al 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 almost ideal theoretical magnitude of the switching in the Heusler-based multilayer with the F2 layer thickness of $\sim 1$\,nm.

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arXiv2017

Two-dimensional electron gas at the interface of the ferroelectric-antiferromagnetic heterostructure Ba_0.8Sr_0.2TiO_3/LaMnO_3

The temperature dependence of the electrical resistivity of the heterostructures consisting of single crystalline LaMnO$_3$ samples with different crystallographic orientations covered by the epitaxial ferroelectric Ba$_{0.8}$Sr$_{0.2}$TiO$_3$ film has been studied. Results obtained for the heterostructure have been compared with the electrical resistivity of the single crystalline LaMnO$_3$ without the film. It was found that for the samples with the films where the polarization axis is perpendicular to the crystal surface the electrical resistivity strongly decreases, and at the temperature below ~160 K undergoes the insulator-metal transition. Ab-initio calculations were also performed for the structural and electronic properties of the BaTiO$_3$/LaMnO$_3$ heterostructure. Transition to the 2D electron gas at the interface is shown.

cond-mat.str-el↗

arXiv2016

Isolation of proximity-induced triplet pairing channel in a superconductor/ferromagnet spin valve

We have studied the proximity-induced superconducting triplet pairing in CoO$_x$/Py1/Cu/Py2/Cu/Pb spin-valve structure (where Py = Ni$_{0.81}$Fe$_{0.19}$). By optimizing the parameters of this structure we found a triplet channel assisted full switching between the normal and superconducting states. To observe an "isolated" triplet spin-valve effect we exploited the oscillatory feature of the magnitude of the ordinary spin-valve effect $ΔT_c$ in the dependence of the Py2-layer thickness $d_{Py2}$. We determined the value of $d_{Py2}$ at which $ΔT_c$ caused by the ordinary spin-valve effect (the difference in the superconducting transition temperature $T_c$ between the antiparallel and parallel mutual orientation of magnetizations of the Py1 and Py2 layers) is suppressed. For such a sample a "pure" triplet spin-valve effect which causes the minimum in $T_c$ at the orthogonal configuration of magnetizations has been observed.

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arXiv2016

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.

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arXiv2015

Superconducting spin valve effect and triplet superconductivity in CoO$_x$/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.

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arXiv2014

EPR study of superconductors

Historical review on the studies of the electron paramagnetic resonance in superconductors performed in the period from 1970 to 1990 at the Kazan Physical Technical Institute of the Russian Academy of Sciences (group of Dr. E. G. Kharakhash'yan) in collaboration with Kazan State University (group of Prof. B. I. Kochelaev) and with the Institute for Physical Problems of Russian Academy of Sciences (group of Prof. N. E. Alekseevskii) is presented. We have observed for first time electron paramagnetic resonance of impurities in a type II superconductor; found indication for a long-range exchange interaction between magnetic impurities arising due to the superconducting correlations; observed the magnetic ordering of impurities in the superconducting state; and, finally, we found one of the first evidences for heterogeneity of the 1:2:3 high-$T_c$ superconductor which is its natural property

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arXiv2013

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 CoO_x/Fe1/Cu/Fe2/Cu/Pb multilayer. The magnitude of the effect $ΔT_c$ = T_c^{AP} - T_c^{P}, where T_c^{P} and T_c^{AP} 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 d_{Fe1}. The obtained dependence of the effect on d_{Fe1} reveals that $ΔT_c$ 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 d_{Fe1} ~ d_{Fe2}. At the optimal value of d_{Fe1}, 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.

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arXiv2012

Evidence for Triplet Superconductivity in a Superconductor-Ferromagnet Spin Valve

We have studied the dependence of the superconducting (SC) transition temperature on the mutual orientation of magnetizations of Fe1 and Fe2 layers in the spin valve system CoO_x/Fe1/Cu/Fe2/Pb. We find that this dependence is nonmonotonic when passing from the parallel to the antiparallel case and reveals a distinct minimum near the orthogonal configuration. The analysis of the data in the framework of the SC triplet spin valve theory gives direct evidence for the long-range triplet superconductivity arising due to noncollinearity of the two magnetizations.

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arXiv2011

Manifestation of New Interference Effects in Superconductor/Ferromagnet Spin Valve

Superconductor/ferromagnet (S/F) spin valve effect theories based on the S/F proximity phenomenon assume that the superconducting transition temperature Tc of F1/F2/S or F1/S/F2 trilayers for parallel magnetizations of the F1- and F2-layers (TcP) are smaller than for the antiparallel orientations (TcAP). Here, we report for CoOx/Fe1/Cu/Fe2/In multilayered systems with varying Fe2-layer thickness the sign-changing oscillating behavior of the spin valve effect ΔTc=TcAP-TcP. Our measurements revealed the full direct spin valve effect with TcAP>TcP for Fe2-layer thickness dFe2<1 nm and the full inverse (TcAP =1 nm. Interference of Cooper pair wave functions reflected from both surfaces of the Fe2-layer appear as the most probable reason for the observed behavior of ΔTc.

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arXiv2010

Full spin switch effect for the superconducting current in a superconductor/ferromagnet thin film heterostructure

Superconductor/ferromagnet (S/F) proximity effect theory predicts that the superconducting critical temperature of the F1/F2/S or F1/S/F2 trilayers for the parallel orientation of the F1 and F2 magnetizations is smaller than for the antiparallel one. This suggests a possibility of a controlled switching between the superconducting and normal states in the S layer. Here, using the spin switch design F1/F2/S theoretically proposed by Oh et al. [Appl. Phys. Lett. 71, 2376 (1997)], that comprises a ferromagnetic bilayer separated by a non-magnetic metallic spacer layer as a ferromagnetic component, and an ordinary superconductor as the second interface component, we have successfully realized a full spin switch effect for the superconducting current.

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arXiv2008

Experimental Observation of the Inverse Proximity Effect in Superconductor/Ferromagnet Layered Structures

We have studied the nuclear magnetic resonance (NMR) of 51V nuclei in the superconductor/ferromagnet thin film heterostructures Ni/V/Ni and Pd{1-x}Fe{x}/V/Pd{1-x}Fe{x} in the normaland superconducting state. Whereas the position and shape of the NMR line in the normal state for the trilayers is identical to that observed in a single V-layer, in the superconducting state the line shape definitely changes, developing a systematic distortion of the high-field wing of the resonance line. We consider this as the first experimental evidence for the penetration of ferromagnetism into the superconducting layer, a phenomenon which has been theoretically predicted recently and dubbed the inverse proximity effect.

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arXiv2006

Proximity Effects in Ferromagnet/Superconductor Heterostructures

We review the present status of the experimental and theoretical research on the proximity effect in heterostructures composed of superconducting (S) and ferromagnetic (F) thin films. First, we discuss traditional effects originating from the oscillatory behavior of the superconducting pair wave function in the F-layer. Then, we concentrate on recent theoretical predictions for S/F layer systems. These are a) generation of odd triplet superconductivity in the F-layer and b) ferromagnetism induced in the S-layer below the superconducting transition temperature $T_{c}$ (inverse proximity effect). The second part of the review is devoted to discussion of experiments relevant to the theoretical predictions of the first part. In particular, we present results of measurements of the critical temperature $T_{c}$ as a function of the thickness of F-layers and we review experiments indicating existence of odd triplet superconductivity, cryptoferromagnetism and inverse proximity effect.

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