arXiv subjects

D. G. Merkel

Publications and source records attributed to D. G. Merkel.

8 recordsLinked to original sources

arXiv2017

Alloy formation at the bottom and top Fe interfaces of Ti/Fe/Ti trilayers

The chemical mixing at the Fe-on-Ti and Ti-on-Fe, i.e. the bottom and top interfaces of Fe in atomically deposited layers, were studied experimentally and by molecular dynamics simulations of the layer growth. The basic structure and chemical composition of the layers were explored in cross-sections using transmission electron microscopy. The calculations show a concentration distribution along the layer growth direction which is atomically sharp at the Ti-on-Fe interface for the (001) and (110) crystallographic orientations of the Fe layer, while it varies over a few atomic layers for Fe(111) substrate and at the Fe-on-Ti interface for all studied substrate orientations. Conversion electron Mössbauer spectroscopy and X-ray reflectometry measurements indicate an even larger asymmetry of the bottom and top Fe interface in Ti/Fe/Ti trilayers grown over Si(111) substrate by vacuum evaporation.

cond-mat.mes-hall↗

arXiv2016

Magnetic state of Nb(1-7nm)/Cu$_{30}$Ni$_{70}$(6nm) superlattices revealed by Polarized Neutron Reflectometry and SQUID magnetometry

We report results of a magnetic characterization of [Cu$_{30}$Ni$_{70}$(6nm)]$_{20}$ (x=1-7nm) superlattices using Polarized Neutron Reflectometry (PNR) and SQUID magnetometry. The study has shown that the magnetic moment of the structures growths almost linearly from H = 0 to H$_{sat}$ = 1.3kOe which is an indirect evidence of antiferromagnetic (AF) coupling of the magnetic moments in neighbouring layers. PNR, however, did not detect any in-plane AF coupling. Taking into account the out-of-plane easy axis of the Cu$_{30}$Ni$_{70}$ layers, this may mean that only the out-of-plane component of the magnetic moments are AF coupled.

cond-mat.mes-hall↗

arXiv2012

GINA - A Polarized Neutron Reflectometer at the Budapest Neutron Centre

The setup, capabilities and operation parameters of the neutron reflectometer GINA, the recently installed "Grazing Incidence Neutron Apparatus" at the Budapest Neutron Centre, are introduced. GINA, a dance-floor-type, constant-energy, angle-dispersive reflectometer is equipped with a 2D position-sensitive detector to study specular and off-specular scattering. Wavelength options between 3.2 and 5.7 Å are available for unpolarized and polarized neutrons. Spin polarization and analysis are achieved by magnetized transmission supermirrors and radio-frequency adiabatic spin flippers. As a result of vertical focusing by the five-element (pyrolytic graphite) monochromator the reflected intensity from a 20x20 mm sample has doubled. GINA is dedicated to studies of magnetic films and heterostructures, but unpolarized options for non-magnetic films, membranes and other surfaces are also provided. Shortly after its startup, reflectivity values as low as 3x10-5 have been measured on the instrument. The facility is now open for the international user community, but its development is continuing mainly to establish new sample environment options, the spin analysis of off-specularly scattered radiation and further decrease of the background.

physics.ins-det↗

arXiv2012

Controlling exchange coupling strength in NixCu100-x thin films

Thickness (dF) and concentration (x) dependence of the Curie temperature of NixCu100-x(dF) ferromagnetic alloy layers (x =0.55,0.65, dF =[3nm÷12nm]) being in contact with a vanadium layer was studied. The Curie temperature of the ferromagnetic layers depends on the thickness when it is comparable with the interface layer between the F and the vanadium layers, which is attributed to the proximity coupling of the interface region with the rest of the F layer. The present study provides valuable information for fabrication of samples with controlled exchange coupling strength for studies of superconductor/ferromagnet (S/F) proximity effects.

cond-mat.mtrl-sci↗

arXiv2011

Tayloring neutron optical performance of Fe/Si multilayers

Reflected neutron beam originating from a crystal monochromator contains higher order wavelength contribution. Creating multilayer mirror structures with custom reflectivity curves including a monochromatic polarized neutron beam is a challenge in modern neutron optics. In this work we present the study of three types of magnetron-sputtered aperiodic Fe/Si layer structures with the purpose of higher harmonic suppression. First, an approximate sinusoidal profile was achieved directly by carefully controlling the evaporation parameters during sputtering process. Second, we implemented a random distribution of the layer thicknesses in which the layer structure of the sample was derived from a prescribed simulated spectrum. Third, a quasi-sinusoidal rounded bilayer scattering length profile was attempted to achieve by 0, 5, 10, 27 and 270x1e15 /cm2 fluence of 350 keV neon irradiation starting from a sharp bilayer profile. Besides the highest fluence, which considerably decreased the overall bilayer contrast, we found an improvement of the monochromatisation with increasing irradiation fluence by the decrease of the total intensity of higher order reflections from 11.1% to 2.2% while the intensity of the first order Bragg peak decreased from 80% to 70%. The polarizing efficiency also increased with increasing fluence of Ne+ irradiation from 78.8% to 90.7%. We were able to achieve 92% and 82% reflectivity of the random multilayer structure and sinusoidal profile sample with 87.6% and 97.1% polarizing efficiency, respectively.

cond-mat.mtrl-sci↗

arXiv2011

Self-Diffusion of Iron in L10 FePd films Upon He-irradiation (alternative title: An alternative for diffusion studies in isotopic FePd multilayers)

The continuous need towards improving the capacity of magnetic storage devices requires materials with strong perpendicular magnetic anisotropy. FePd, CoPd and their Co(Fe)Pt counterparts very attractive candidate for such purposes. The magnetic properties of these films are largely dependent on the orientation and local distribution of the L10 FePd phase fraction which are mainly controlled by diffusion processes and involve diffusion paths of a few angstroms. Highly ordered as well as disordered epitaxial isotope-periodic 57FePd/natFePd (001) thin films were prepared by molecular-beam epitaxy on MgO(001) substrate. Short range diffusion of different phases in FePd thin film induced by room temperature 130 keV He+ irradiation was investigated at fluences up to 30\times1015 ions/cm2. Conversion electron Mössbauer spectroscopy and synchrotron Mössbauer reflectivity experiments showed that the inter-atomic diffusion across 57FePd/natFePd interface occur mainly via the iron rich regions. The ratio of the diffusion length in the L10, fcc and iron rich structure are 1:1.4:5.6 respectively. Assuming that the diffusion coefficient in the fcc phase is between the diffusion coefficient of the L10 phase in the crystallographic c direction and perpendicular to it, the diffusion coefficient in the c-direction of the L10 phase is found more than 1.9 times lower than in the a-b plane.

cond-mat.mtrl-sci↗

arXiv2011

Self-Diffusion of Iron in L10 FePd film - as revealed by reflectometric methods

L10 (CuAu(I)-type) ordered FePd as well as FePt and CoPt, due to their high magnetic anisotropy, are candidate materials for future ultra-high density magnetic recording. Atomic diffusion governs the structural relaxation and associated changes in the physical and magnetic properties in these alloys. Such processes involve diffusion paths of a few angstroms. In developing modern storage devices it is indispensable to understand these processes. Neutron reflectometry (NR) and Synchrotron Mössbauer Reflectometry (SMR) are suitable non destructive methods to study self-diffusion in a chemically homogenous isotope multilayer with diffusion lengths of the order of a few angstroms. From the diffusion parameters obtained from NR and SMR evaluation we determined activation energies (Q) and preexponent factors (D0) for the distinct environment in the FePd samples. We obtained the highest activation energy (1.82\pm0.38 eV)and lowest preexponential factor((5.76\pm0.35)*10-14 m2/s) for the ordered L10 as oppose to the iron rich regions were the activation energy (1.48\pm0.26 \pmV) found to be lowest and the preexponential factor the highest ((1.01\pm0.61)*10-13 m2/s).

cond-mat.mtrl-sci↗

arXiv2011

Neutron Reflectometer with Polarization Option at the Budapest Neutron Centre

The ever increasing need for product advancement and miniaturization keeps thin film assemblies, membranes, magnetic and non-magnetic multilayer and patterned heterostructures in the limelight of material science and technological development. A number of thin film and surface characterization methods have emerged recently to meet the new challenges. The increased interest in magnetic thin film analytical instruments - mainly triggered by the discovery of the giant magnetoresistance and related phenomena1 - resulted in a boom of PNR studies as well as of the construction of a number of new neutron reflectometers with polarization option at neutron sources all over the world. Here we report on the design, construction and operation parameters and first example uses of the "Grazing Incidence Neutron Apparatus" (GINA) a recently installed neutron reflectometer at the Budapest Neutron Centre (BNC), Hungary.

physics.ins-det↗