Control of superconductivity by means of electric-field-induced strain in superconductor/piezoelectric hybrids

The controlled modification of superconductivity by any means, specifically in hybrid systems, has attracted much interest in the recent decades. Here, we present experimental data and phenomenological modeling on the control of T C of superconducting (SC) Nb thin films, with thickness 3 nm ≤ dNb≤50 nm, under the application of in-plane strain, S(E ex ) induced by an external out-of-plane electric field, E ex to piezoelectric (PE) single crystals, namely, (1-x)Pb(Mg 1/3 Nb 2/3 )O 3 -xPbTiO 3 (PMN-xPT), with x = 0.27 and 0.31. We report experimental modification of T C of Nb by E ex , accurately described by a phenomenological model that incorporates the constitutive relation S(E ex ) of PMN-xPT. The systematic experimental-phenomenological modeling approach introduced here is generic and paves the way for an understanding of the underlying physical mechanisms in any SC/PE hybrid

Modulation of the properties of thin ferromagnetic films with an externally applied electric field in ferromagnetic/piezoelectric/ferromagnetic hybrids

In many cases, technological advances are based on artificial low-dimensional structures of heterogeneous constituents, thus called hybrids, that when come together they provide stand-alone entities that exhibit entirely different properties. Such hybrids are nowadays intensively studied since they are attractive for both basic research and oncoming practical applications. Here, we studied hybrids constituted of piezoelectric (PE) and ferromagnetic (FM) components in the form FM/PE/FM, ultimately aiming to provide means for the controlled modulation of the properties of the FM electrodes, originating from the strain imposed to them by the PE mediator when an electric field is applied. The PE component is in single crystal form, 0.71Pb(Mg1/3Nb 2/3)O3-0.29PbTiO3 (PMN-PT), while the FM outer layers are Cobalt (Co) in thin film form. Detailed magnetization measurements performed under variation of the electric field applied to PMN-PT demonstrated the efficient modulation of the properties of the Co electrodes at low temperature (coercive field modulation up to 27% and saturation magnetization absolute modulation up to 4% at T = 10 K for electric field not exceeding 6 kV/cm). The modulation degree faints upon increase of the temperature, evidencing that the thermal energy eventually dominates all other relevant energy scales. Candidate mechanisms are discussed for the explanation of these experimental observations. The results presented here demonstrate that commercially available materials can result in quantitatively noticeable effects. Thus, such elemental Co/PMN-PT/Co units can be used as a solid basis for the development of devices. 2013 AIP Publishing LLC

Superconducting thermomagnetic instabilities tuned through electric-field-controlled strain in Nb/PMN-PT/Nb hybrids

Electric-field-controlled piezoelectric strain has been used, recently, to modify the superconducting properties in a new class of piezoelectric/superconducting (PE/SC) hybrids. Here, we investigate the appearance of thermomagnetic instabilities (TMIs) and the respective modification of the critical current density (J C ) through the application of electric field (E ex ) in PE/SC hybrids. Specifically, the SC nanolayers are Nb (thickness, d SC = 20 nm) deposited on both surfaces of PE macroscopic crystals of (1-x)Pb(Mg 1/3 Nb 2/3 )O 3 -xPbTiO 3 (PMN-PT) with optimum composition x = 0.31 (thickness, d PE = 0.5-0.8 mm). The appearance of TMIs and the modification of J C by E ex is studied for two PMN-PT crystals of drastically different surface roughness (Sa). In the case of the PMN-PT crystal with low Sa (on the order of a few tenths of nm) TMIs are absent so that J C does not change under the variation of E ex . On the contrary, in the case of the PMN-PT crystal with high Sa (on the order of a few hundreds of nm) E ex induces TMIs in the Nb nanolayers. Specifically, the number of TMIs exhibits a non-monotonic increase on E ex , thus causing a non-monotonic degradation of J C . These experimental data are interpreted in terms of the variation of both volume strain and surface roughness on E ex . This work highlights practical means to control the current-carrying capability of SC nanolayers through strain provided by PE substrates

Deterioration of exchange bias in CoO-Co bilayers by the roughness of the ZnO substrates

The Exchange Bias (EB) effect is observed at the interface of Antiferromagnet/Ferromagnet (AF/FM) structures and depends on the interface roughness (IR). Until today, only low IR values, usually below 10 nm, have been investigated. Here we investigate an extended range of IR through controlling the surface roughness (SR) of the employed substrates. We employ CoO/Co bilayers (thickness within 10-60 nm), a classic AF/FM structure that exhibits intense EB. ZnO was employed as the substrate in both film and bulk forms, enabling us to vary the SR up to 840 nm. Our data reveal a strong relative decrease, ranging within 20-65%, of both the shift HshiftEB and coercive HcEB fields upon increase of SR (IR), for both parallel and normal magnetic field-sample configurations. For the explanation of these findings we propose that in thin AF/FM structures deposited on rough substrates the local magnetization, Mf of the FM is ‘locked’ mainly in-layer due to shape anisotropy, thus it is forced to follow the morphologically rough landscape of the substrate. This imposes misalignment between Mf, that is ‘directionally random’, and Hex, that is ‘directionally oriented’. This weakens the biasing potential of Hex on Mf and reduces the relative macroscopic parameters Hshift EB amd Hc EB