A. Devishvili

Self-assembled iron oxide nanoparticle multilayer: x-ray and polarized neutron reflectivity

We have investigated the structure and magnetism of self-assembled, 20 nm diameter iron oxide nanoparticles covered by an oleic acid shell for scrutinizing their structural and magnetic correlations. The nanoparticles were spin-coated on an Si substrate as a single monolayer and as a stack of 5 ML forming a multilayer. X-ray scattering (reflectivity and grazing incidence small-angle scattering) confirms high in-plane hexagonal correlation and a good layering property of the nanoparticles. Using polarized neutron reflectivity we have also determined the long range magnetic correlations parallel and perpendicular to the layers in addition to the structural ones. In a field of 5 kOe we determine a magnetization value of about 80% of the saturation value. At remanence the global magnetization is close to zero. However, polarized neutron reflectivity reveals the existence of regions in which magnetic moments of nanoparticles are well aligned, while losing order over longer distances. These findings confirm that in the nanoparticle assembly the magnetic dipole-dipole interaction is rather strong, dominating the collective magnetic properties at room temperature.

Depht Profile of the ferromagnetic order in a YBa2Cu3O7/La2/3Ca1/3MnO3 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.

Polarized neutron reflectivity from monolayers of self-assembled magnetic nanoparticles

We prepared monolayers of iron oxide nanoparticles via self-assembly on a bare silicon wafer and on a vanadium film sputter deposited onto a plane sapphire substrate. The magnetic configuration of nanoparticles in such a dense assembly was investigated by polarized neutron reflectivity. A theoretical model fit shows that the magnetic moments of nanoparticles form quasi domain-like configurations at remanence. This is attributed to the dipolar coupling amongst the nanoparticles.

Laterally patterned spin-valve superlattice: Magnetometry and polarized neutron scattering study

The magnetization reversal of magnetic multilayers with spin-valve like characteristics, patterned into an array of parallel stripes, was structurally and magnetically analyzed, in detail, via x-ray scattering, magnetometry, and polarized neutron reflectivity. Each stripe contains a multiple repetition of the layer sequence [Fe/Cr/Co/Cr]. X-ray and neutron scattering maps of the patterned multilayer show rich details resulting from the superposition of Bragg peaks representing the lateral in-plane periodicity and the out-of-plane multilayer period. Detailed analysis of specular and off-specular polarized neutron intensity was used to ascertain the antiparallel alignment of the Co and Fe magnetization within the kink region of their combined hysteresis loop between the coercive fields of Fe and Co layers. This includes also an examination of domain formation and inter- as well as intra-stripe correlation effects upon magnetization reversal. Our combined study shows that the shape induced anisotropy via patterning is capable of overriding the four-fold crystal anisotropy but is unable to eliminate the ripple domain state of the Co layers, already present in the continuous multilayer.

Magnetic Proximity Effect inYBa2Cu3O7/La2/3Ca1/3MnO3andYBa2Cu3O7/LaMnO3+δSuperlattices

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa2Cu3O7 and ferromagnetic-metallic La0.67Ca0.33MnO3 or ferromagnetic-insulating LaMnO(3+δ). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La0.67Ca0.33MnO3 and almost absent for ferromagnetic-insulating LaMnO(3+δ). We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature.

Magnetic Proximity Effect in YBa 2 Cu 3 O 7 / La 2 / 3 Ca 1 / 3 MnO 3 and YBa 2 Cu 3 O 7 / LaMnO 3 + δ Superlattices

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa2Cu3O7 and ferromagnetic-metallic La0.67Ca0.33MnO3 or ferromagnetic-insulating LaMnO(3+δ). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La0.67Ca0.33MnO3 and almost absent for ferromagnetic-insulating LaMnO(3+δ). We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature.

Magnetic proximity effect in

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa2Cu3O7 and ferromagnetic-metallic La0.67Ca0.33MnO3 or ferromagnetic-insulating LaMnO(3+δ). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La0.67Ca0.33MnO3 and almost absent for ferromagnetic-insulating LaMnO(3+δ). We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature.