arXiv subjects

A. Devishvili

Publications and source records attributed to A. Devishvili.

8 recordsLinked to original sources

arXiv2024

Improved performance of polycrystalline antiferromagnet/ferromagnet stack by nitrogen assisted deposition

The deposition process of the IrMn$_3$/Co$_{70}$Fe$_{30}$ bilayer of antiferromagnet/ferromagnet (AF/FM) type was modified by introducing a nitrogen additive in argon plasma during the magnetron sputtering of the Co$_{70}$Fe$_{30}$ layer. This slight modification significantly enhanced the exchange bias energy of the AF/FM bilayer for an IrMn layer thickness ranging from 20{\AA} to 50{\AA} and reduced the coercivity of the FM layer. Calculations indicate that the boost in exchange bias energy and the reduction in coercivity can be attributed to the increased anisotropy energy of the antiferromagnet, resulting in more effective pinning of the ferromagnet by antiferromagnetic grains. The increase in anisotropy is caused by the diffusion of nitrogen from the FM into the AF layer, as established by X-ray diffraction, Neutron Reflectometry, and X-ray Magnetic Circular Dichroism. Our research allows to improve magnetic characteristics of exchange-coupled FM/AF structures through minor modifications in the sputtering process and/or save up to 20% of the costly IrMn$_3$ target by reducing the thickness of the AF layer.

cond-mat.mtrl-sci↗

arXiv2022

Realization of an advanced super-mirror solid-state neutron polarizer for the instrument PF1B at the Institut Laue-Langevin

In this last of a series of three papers on the development of an advanced solid-state neutron polarizer, we present the final construction of the polarizer and the results of its commissioning. The polarizer uses spin-selective reflection of neutrons by interfaces coated with polarizing super-mirrors. The polarizer is built entirely in-house for the PF1B cold neutron beam facility at the Institut Max von Laue - Paul Langevin (ILL). It has been installed in the PF1B casemate and tested at real conditions. The average transmission for the "good" spin component is measured to be >30 %. The polarization averaged over the capture spectrum reaches a record value of Pn ~ 0.997 for the full angular divergence in the neutron beam, delivered by the H113 neutron guide, and the full wavelength band lambda of 0.3-2.0 nm. This unprecedented performance is due to a series of innovations in the design and fabrication in the following domains: choice of the substrate material, super-mirror and anti-reflecting multilayer coatings, magnetizing field, assembling process. The polarizer is used for user experiments at PF1B since the last reactor cycle in 2020.

physics.ins-det↗

arXiv2021

Measurements of the neutron absorption in supermirror coatings

In this work we report on measurements of neutron absorption in supermirror coatings. The measurements were carried out using the SuperADAM instrument at the Institut Laue-Langevin and by measuring the gamma-ray production from m = 3 and m = 4 neutron supermirrors when illuminated by a beam of neutrons. The results provide a valuable validation for recent computational and theoretical work that can be used as input to Monte-Carlo radiation transport calculations for the design of the shielding of neutron scattering instruments.

physics.ins-det↗

arXiv2018

Role of gallium diffusion in the formation of a magnetically dead layer at Y3Fe5O12 / Gd3Ga5O12 epitaxial interface

We have clarified the origin of magnetically dead interface layer formed in yttrium iron garnet (YIG) films grown at above 700°C onto gadolinium gallium garnet (GGG) substrate by means of laser molecular beam epitaxy. The diffusion-assisted formation of a Ga-rich region at the YIG / GGG interface is demonstrated by means of composition depth profiling performed by X-ray photoelectron spectroscopy, secondary ion mass spectroscopy and X-ray and neutron reflectometry. Our finding is in sharp contrast to the earlier expressed assumption that Gd acts as a migrant element in the YIG/GGG system. We further correlate the presence of Ga-rich transition layer with considerable quenching of ferromagnetic resonance and spin wave propagation in thin YIG films. Finally, we clarify the origin of the enigmatic low-density overlayer that is often observed in neutron and X-ray reflectometry studies of the YIG / GGG epitaxial system.

cond-mat.mtrl-sci↗

arXiv2016

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(40nm)/F [F$=$Fe(1,3nm), Co(3nm), Ni(3nm)] was observed using SQUID magnetometry upon cooling below the superconducting transition temperature Tc 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 Tc 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.

cond-mat.supr-con↗

arXiv2015

Neutron reflectometry on highly absorbing films and its application to 10B4C-based neutron detectors

Neutron reflectometry is a powerful tool used for studies of surfaces and interfaces. In general the absorption in the typical studied materials can be neglected and this technique is limited to the measurement of the reflectivity only. In the case of strongly absorbing nuclei the number of neutrons is not conserved and the absorption can be directly measured by using the neutron-induced fluorescence technique which exploits the prompt particle emission of absorbing isotopes. This technique is emerging from soft matter and biology where highly absorbing nuclei, generally in very small quantities, are used as a label for buried layers. Nowadays the importance of highly absorbing layers is rapidly increasing, partially because of their application in neutron detection; a field that has become more and more active also due to the 3He-shortage. In this manuscript we extend the neutron-induced fluorescence technique to the study of thick layers of highly absorbing materials; in particular 10B4C. The theory of neutron reflectometry is a commonly studied topic, however the subtle relationship between the reflection and the absorption of neutrons is not widely known, in particular when a strong absorption is present. The theory for a general stack of absorbing layers has been developed and compared to measurements. This new technique has potential as a tool for characterization of highly absorbing layers. We also report on the requirements that a 10B4C layer must fulfill in order to be employed as a converter in neutron detection.

cond-mat.mtrl-sci↗

arXiv2012

Depth profile of the ferromagnetic order in a YBa$_2$Cu$_3$O$_7$ / La$_{2/3}$Ca$_{1/3}$MnO$_3$ 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.

cond-mat.str-el↗

arXiv2012

Magnetic proximity effect in $\mathbf{YBa_2Cu_3O_7/La_{2/3}Ca_{1/3}MnO_3}$ and $\mathbf{YBa_2Cu_3O_7/LaMnO_{3+δ}}$ superlattices

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa$_2$Cu$_3$O$_7$ (YBCO) and ferromagnetic-metallic (FM-M) La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) or ferromagnetic-insulating (FM-I) LaMnO$_{3+δ}$ (LMO). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the FM-M LCMO and almost absent for FM-I LMO. We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature.

cond-mat.str-el↗