L. Mustafa

Evidence for spin-triplet superconducting correlations in metal-oxide heterostructures with noncollinear magnetization

Heterostructures composed of ferromagnetic La0.7Sr0.3MnO3, ferromagnetic SrRuO3, and superconducting YBa2Cu3O6+x were studied experimentally. Structures of composition Au/La0.7Sr0.3MnO3/SrRuO3/YBa2Cu3O6+x were prepared by pulsed laser deposition, and their high quality was confirmed by x-ray diffraction and reflectometry. Anoncollinear magnetic state of the heterostructureswas revealed by means of superconducting quantum interference device magnetometry and polarized neutron reflectometry. We have further observed superconducting currents in mesa structures fabricated by deposition of a second superconducting Nb layer on top of the heterostructure, followed by patterning with photolithography and ion-beam etching. Josephson effects observed in these mesa structures can be explained by the penetration of a triplet component of the superconducting order parameter into the magnetic layers.

Magnetic proximity effect at the interface between a cuprate superconductor and an oxide spin valve

A heterostructure that consists of the YBa2Cu3O7–δ cuprate superconductor and the SrRuO3/La0.7Sr0.3MnO3 ruthenate/manganite spin valve is investigated using SQUID magnetometry, ferromagnetic resonance, and neutron reflectometry. It is shown that a magnetic moment is induced due to the magnetic proximity effect in the superconducting part of the heterostructure, while the magnetic moment in the composite ferromagnetic interlayer is suppressed. The magnetization emerging in the superconductor coincides in order of magnitude with the results of calculations taking into account the induced magnetic moment of Cu atoms because of orbital reconstruction at the interface between the superconductor and the ferromagnet, as well as with the results of the model taking into account the variations in the density of states at a distance on the order of the coherence length in the superconductor. The experimentally obtained characteristic penetration depth of the magnetic moment in the superconductor considerably exceeds the coherence length of the cuprate superconductor, which indicates the predominance of the mechanism of induced magnetic moment of Cu atoms.

On the explanation of the paramagnetic Meissner effect in superconductor/ferromagnet heterostructures

An increase of the magnetic moment in superconductor/ferromagnet (S/F) bilayers V(40 nm)/F (, Co(3 nm), Ni(3 nm)) was observed using SQUID magnetometry upon cooling below the superconducting transition temperature T C in magnetic fields of 10 Oe to 50 Oe applied parallel to the sample surface. A similar increase, often called the paramagnetic Meissner effect (PME), was observed before in various superconductors and superconductor/ferromagnet systems. To explain the PME effect in the presented S/F bilayers a model based on a row of vortices located at the S/F interface is proposed. According to the model the magnetic moment induced below T C consists of the paramagnetic contribution of the vortex cores and the diamagnetic contribution of the vortex-free region of the S layer. Since the thickness of the S layer is found to be 3-4 times less than the magnetic-field penetration depth, this latter diamagnetic contribution is negligible. The model correctly accounts for the sign, the approximate magnitude and the field dependence of the paramagnetic and the Meissner contributions of the induced magnetic moment upon passing the superconducting transition of a ferromagnet/superconductor bilayer. (Less)

Structural, Magnetic, and Superconducting Characterization of the CuNi/Nb Bilayers of the S/F Type Using Polarized Neutron Reflectometry and Complementary Techniques

Structural, magnetic, and superconducting properties of S/F bilayers Nb/Cu 40Ni 60 deposited on silicon substrate have been characterized using polarized neutron reflectometry and complementary techniques. The study allowed to determine real thicknesses of the S and F layers as well as the r.m.s. roughness of the S/F interfaces. The latter does not exceed 1 nm, showing the high quality of the S/F interface. Using SQUID and a mutual inductance setup, we determined the superconducting transition temperatures of the samples, which are in agreement with the literature data. Using of polarized neutron reflectometry (PNR) for the single S layer allowed to determine the screening length λ of the superconducting layer, λ = 120 nm. This value is higher than the London penetration depth for pure niobium which may indicate that the superconductor is in the dirty limit. PNR and SQUID studies of magnetic properties of the CuNi layer have shown the presence of ferromagnetism in all investigated samples.

Spin transport in epitaxial magnetic manganite/ruthenate heterostructures with an LaMnO3 layer

Epitaxial La0.7Sr0.3MnO3/LaMnO3/SrRuO3 (LSMO/LMO/SRO) heterostructures with an LMO layer 0–35 nm thick are grown by laser ablation on an NdGaO3 substrate at a high temperature. X-ray diffraction and transmission electron microscopy demonstrate sharp interfaces and epitaxial growth of the LSMO and SRO layers in the heterostructures at an LMO layer thickness of 0–35 nm. SQUID measurements of the magnetic moment of the heterostructures with an LMO layer and the data obtained with reflectometry of polarized neutrons show that the manganite LMO layer is a ferromagnet at a temperature below 150 K and strongly affects the magnetic moment of the heterostructures at low temperatures. The magnetoresistance of the mesostructure created from the heterostructure using lithography and ion etching decreases with increasing LMO layer thickness and weakly depends on the direction of an applied magnetic field. If the LMP layer is absent, a negative magnetoresistance is detected; it is likely to be caused by a negative magnetization of the SRO layer.

Evolution of non-collinear magnetic state of exchange biased ferromagnet/normal metal/ferromagnet/superconductor heterostructure in magnetic field studied by polarized neutron reflectometry

By using waveguide enhanced polarized neutron reflectometry we have characterized the magnetic state of exchange biased CoOx(20 nm)/Co(4 nm)/Nb(5 nm)/Co(2 nm)/Nb(25 nm)/Al2O3 system. Measurement allowed to determine the dependence of the inclination angles of magnetic moment of the both Co layers as a function of applied field. According to the measurement the soft Co(2 nm) layer magnetization turns towards external field in magnetic fields as small as 20 Oe. In contrast direction of magnetic moment of Co(4 nm) layer cannot be altered in magnetic fields as high as 2 kOe.

Ferromagnetic State of LaMnO3 Interlayer in La0.7Sr0.3MnO3/LaMnO3/SrRuO3 Heterostructures

Magnetic state of epitaxial heterostructures La0.7Sr0.3MnO3/LaMnO3/SrRuO3 (LSMO/LMO/SRO) was studied by SQUID magnetometer, ferromagnetic resonance (FMR) and polarized neutron reflectometry (PNR) techniques. The fit of PNR and FMR data give us the magnetization of each layer, while the SQUID gives the total magnetization of our structures. The magnetic moment of the LMO layer has the magnetization 4πMLMO=4.2kGs (2.4 mB/Mn) at T=140K according to our PNR data fitting.

Triplet superconductivity in oxide ferromagnetic interlayer of mesa-structure

We present experimental data on Nb-Au/La0.7Sr0.3MnO3/SrRuO3/YBa2Cu3O7-δ mesa- structure with in plane linear size 10÷50 μm. The mesa-structures were patterned from the epitaxial heterostructures fabricated by pulsed laser ablation and magnetron sputtering. Superconducting critical current was observed for mesa-structures with the interlayer thicknesses up to 50 nm. In the mesa-structures with just one, either La0.7Sr0.3MnO3 or SrRuO3 interlayer with a thickness larger than 10 nm no superconducting current was observed. The registered superconducting current for the mesa-structures with a thinner interlayer is attributed to pinholes. Obtained results are discussed in terms of superconducting long-range triplet generation at interfaces of superconductor and a composite ferromagnet consisting of ferromagnetic materials with non-collinear magnetization.