51 research outputs found
Pulsed laser deposition growth of heteroepitaxial YBa2Cu3O7/La0.67Ca0.33MnO3 superlattices on NdGaO3 and Sr0.7La0.3Al0.65Ta0.35O3 substrates
Heteroepitaxial superlattices of [YBa2Cu3O7(n)/ La0.67Ca0.33MnO3(m)]x, where
n and m are the number of YBCO and LCMO monolayers and x the number of bilayer
repetitions, have been grown with pulsed laser deposition on NdGaO3 (110) and
Sr0.7La0.3Al0.65Ta0.35O3 (LSAT) (001). These substrates are well lattice
matched with YBCO and LCMO and, unlike the commonly used SrTiO3, they do not
give rise to complex and uncontrolled strain effects due to structural
transitions at low temperature. The growth dynamics and the structure have been
studied in-situ with reflection high energy electron diffraction (RHEED) and
ex-situ with scanning transmission electron microscopy (STEM), x-ray
diffraction, and neutron reflectometry. The individual layers are found to be
flat and continuous over long lateral distances with sharp and coherent
interfaces and with a well-defined thickness of the individual layer. The only
visible defects are antiphase boundaries in the YBCO layers that originate from
perovskite unit cell height steps at the interfaces with the LCMO layers. We
also find that the first YBCO monolayer at the interface with LCMO has an
unusual growth dynamics and is lacking the CuO chain layer while the subsequent
YBCO layers have the regular Y-123 structure. Accordingly, the CuO2 bilayers at
both the LCMO/YBCO and the YBCO/LCMO interfaces are lacking one of their
neighboring CuO chain layers and thus half of their hole doping reservoir.
Nevertheless, from electric transport measurements on asuperlattice with n=2 we
obtain evidence that the interfacial CuO2 bilayers remain conducting and even
exhibit the onset of a superconducting transition at very low temperature.
Finally, we show from dc magnetization and neutron reflectometry measurements
that the LCMO layers are strongly ferromagnetic
Evidence for spin-triplet superconducting correlations in metal-oxide heterostructures with non-collinear magnetization
Heterostructures composed of ferromagnetic La0.7Sr0.3MnO3, ferromagnetic
SrRuO3, and superconducting YBa2Cu3Ox were studied experimentally. Structures
of composition Au/La0.7Sr0.3MnO3/SrRuO3/YBa2Cu3Ox were prepared by pulsed laser
deposition, and their high quality was confirmed by X-ray diffraction and
reflectometry. A non-collinear magnetic state of the heterostructures was
revealed by means of SQUID 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.Comment: 10 pages, 6 figure
Depth profile of the ferromagnetic order in a YBaCuO / LaCaMnO superlattice on a LSAT substrate: a polarized neutron reflectometry study
Using polarized neutron reflectometry (PNR) we have investigated a
YBa2Cu3O7(10nm)/La2/3Ca1/3MnO3(9nm)]10 (YBCO/LCMO) superlattice grown by pulsed
laser deposition on a La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrate. Due to the
high structural quality of the superlattice and the substrate, the specular
reflectivity signal extends with a high signal-to-background ratio beyond the
fourth order superlattice Bragg peak. This allows us to obtain more detailed
and reliable information about the magnetic depth profile than in previous PNR
studies on similar superlattices that were partially impeded by problems
related to the low temperature structural transitions of the SrTiO3 substrates.
In agreement with the previous reports, our PNR data reveal a strong magnetic
proximity effect showing that the depth profile of the magnetic potential
differs significantly from the one of the nuclear potential that is given by
the YBCO and LCMO layer thickness. We present fits of the PNR data using
different simple block-like models for which either a ferromagnetic moment is
induced on the YBCO side of the interfaces or the ferromagnetic order is
suppressed on the LCMO side. We show that a good agreement with the PNR data
and with the average magnetization as obtained from dc magnetization data can
only be obtained with the latter model where a so-called depleted layer with a
strongly suppressed ferromagnetic moment develops on the LCMO side of the
interfaces. The models with an induced ferromagnetic moment on the YBCO side
fail to reproduce the details of the higher order superlattice Bragg peaks and
yield a wrong magnitude of the average magnetization. We also show that the PNR
data are still consistent with the small, ferromagnetic Cu moment of 0.25muB
that was previously identified with x-ray magnetic circular dichroism and x-ray
resonant magnetic reflectometry measurements on the same superlattice.Comment: 11 pages, 7 figure
Influence of La and Mn vacancies on the electronic and magnetic properties of LaMnO₃ thin films grown by pulsed laser deposition
With pulsed laser deposition, we have grown c axis oriented thin films of the nominal composition LaMnO3 (LMO) on LSAT(001) substrates. We find that, depending on the oxygen background pressure during growth, the LMO films contain sizeable amounts of La and/or Mn vacancies that strongly influence their electronic and magnetic properties. Specifically, we show that the Mn/La ratio can be systematically varied from 0.92 at 0.11 mbar to 1.09 at 0.30 mbar of oxygen. The cationic vacancies have markedly different effects that become most pronounced once the samples are fully oxygenated and thus strongly hole doped. All as-grown and thus slightly oxygen-deficient LMO films are ferromagnetic insulators with saturation moments in excess of 2.5 μB per Mn ion, their transport and optical properties can be understood in terms of trapped ferromagnetic polarons. Upon oxygen annealing, the most La-deficient films develop a metallic response with an even larger ferromagnetic saturation moment of 3.8 μB per Mn ion. In contrast, in the oxygenated Mn-deficient films, the ferromagnetic order is strongly suppressed to less than 0.5 μB per Mn ion, and the transport remains insulatorlike. We compare our results with the ones that were previously obtained on bulk samples and present an interpretation in terms of the much stronger disruption of the electronic and magnetic structure by the Mn vacancies as compared to the La vacancies. We also discuss the implications for the growth of LMO thin films with well-defined physical properties that are a prerequisite for the study of interface effects in multilayers
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