arXiv subjects

S. S. Das

Publications and source records attributed to S. S. Das.

8 recordsLinked to original sources

arXiv2026

Hydrogen (deuterium) dynamics and thermal stability in ion-irradiated platinum-hydride thin films synthesized at low temperature

Hydrogen (H) and deuterium (D) interactions with transition metals play a central role in heterogeneous catalysis and hydrogen-related technologies. While H-Pt surface interactions have been extensively studied, direct investigations of hydrogen incorporation and transport in Pt remain limited due to its low solubility. Here, we study H(D) incorporation and desorption dynamics in metastable $PtH(D)_x$ thin films prepared by low-energy ion irradiation, enabling hydrogen loading far above equilibrium concentrations. Nuclear reaction analysis (NRA) reveals a nonuniform hydrogen depth profile with two accumulation regions: the subsurface and the film-substrate interface. Thermal desorption spectroscopy (TDS) exhibits two desorption peaks near 190 and 230 K, consistent with hydrogen release from these sites. Resistance relaxation measurements, analyzed within a two-parallel-channel conduction model, indicate different relaxation kinetics for subsurface and near-interface hydrogen. Arrhenius analysis reveals two thermally activated processes for $PtH_x$ with an average hydrogen concentration of $x = 0.15$, with activation energies of $130 \pm 18$ meV (subsurface) and $164 \pm 26$ meV (near interface). Above 140 K, D exhibits slower relaxation rates with activation energies of $117 \pm 8$ and $121 \pm 7$ meV for $PtD_x$ prepared under the same implantation dose. Within experimental uncertainty, the activation barriers remain comparable, while the prefactors are reduced significantly for D, indicating isotope-dependent attempt frequencies and zero-point energy effects. TDS simulations based on the Polanyi-Wigner formalism reproduce the experimental desorption spectra by resolving subsurface and near-interface contributions, in agreement with the NRA profile. These findings provide insight into hydrogen kinetics in $PtH_x$ for Pt-based catalysis, sensing, and hydrogen-metal interactions.

cond-mat.mtrl-sci↗

arXiv2026

Hydrogen-Induced Sign Reversal in Magnetic Hysteresis Evolution of CoPd Alloys and Co/Pd Multilayers

Hydrogen absorption in magnetic thin film nanostructures can modulate their electronic, magnetic, and transport properties by modifying the electronic structure and lattice strain. However, the influence of composition and nanostructuring on these two competing effects is not well understood. We systematically investigate hydrogen-induced magnetic hysteresis in Co$_x$Pd$_{100-x}$ alloys, [Co(0.1 nm)/Pd(d)]$_{15}$, and [Co(0.2 nm)/Pd(d)]$_{15}$ multilayers, using extraordinary Hall effect characterizations (EHE) in air and 4% H$_2$/N$_2$ mixture. We show that the hydrogen-induced response is not universal, but depends strongly on composition and layer thickness. This reflects competition between Pd-related electronic effects and magnetoelastic anisotropy. In Pd-rich CoPd alloys and Co(0.2 nm)/Pd multilayers, hydrogen initially contracts the hysteresis loops at low Co fractions, followed by loop expansion above x ~ 40%. This contrast results from competition between suppression of Pd-induced magnetization through Pd-4d band filling and hydrogen-driven anisotropic strain that strengthens magnetoelastic anisotropy in Co-rich samples. In contrast, in ultrathin [Co(0.1 nm)/Pd(d)]$_{15}$ multilayers, hydrogen induces a weak, non-monotonic but generally expanding loop behaviour across x = 15-60%, indicating a dominant role of interfacial magnetic connectivity and strain-mediated magnetoelastic anisotropy in the ultrathin limit. Furthermore, we observe a hydrogen-induced reversal of the EHE loop polarity near the crossover regime, reflecting a change in the dominant EHE scattering mechanisms, thus providing an additional degree of magnetic tunability by hydrogen. These results demonstrate that hydrogen can selectively tune the magnetism of CoPd nanostructures via composition-controlled electronic and magnetoelastic effects, offering insights for hydrogen-responsive spintronic and sensing devices.

cond-mat.mtrl-sci↗

arXiv2026

Giant anomalous Hall effect in ultrathin Si/Fe bilayers

Anomalous Hall effect studies on ultrathin Si(50Angstrom)/Fe(t_Fe) bilayers were performed at 300 K. Giant enhancements of about 60 times in saturation anomalous Hall resistivity and 265 times in anomalous Hall coefficient (R_s) were observed upon decreasing the Fe layer thickness t_Fe from 200 to 10 Angstrom. The R_s observed for t_Fe = 10 Angstrom is about three orders of magnitude larger than that of bulk Fe. The scaling law between R_s and longitudinal electrical resistivity (Rho) suggests that the side jump is the dominant mechanism of the anomalous Hall effect. The observed largest Hall sensitivity of 433 Ohm/T surpasses that of the semiconducting GaAs and InAs Hall sensors already reported.

cond-mat.mtrl-sci↗

arXiv2026

Enhancement of anomalous Hall effect in Si/Fe multilayers

Anomalous Hall effect studies were performed at 300 K on Si/Fe multilayers prepared by dc magnetron sputtering. About 60 times enhancement in the saturation Hall resistance and 80 times enhancement in anomalous Hall coefficient are obtained in [Si(50 angstrom)/Fe(tFe)]_20 multilayers when decreasing the Fe layer thickness from 100 Angstrom to 20 Angstrom. The largest anomalous Hall coefficient (Rs) of 1.4 x 10^-7 Ohm m/T was found for t_Fe=20 Angstrom, which is about three orders of magnitude larger than that of pure Fe and Fe/Cr, Al/Fe, Cu/Fe, SiO2/FePt/SiO2 multilayers. The ordinary Hall coefficient R_0 was about two orders of magnitude larger than that of pure Fe. The R_s was found to vary with the longitudinal electronic resistivity, Rho as R_s proportional to (Rho)^2.2, indicating the role of interfaces for the enhancement of the anomalous Hall effect in the multilayers. An increase of Hall sensitivity from 9 mOhm/T to 1.2 Ohm/T is observed on decreasing tFe from 100 Angstrom to 10 Angstrom. The high Hall sensitivity obtained is about three orders of magnitude larger than that of Al/Fe and Cu/Fe multilayers, showing it as an emerging candidate for Hall element for potential applications.

cond-mat.mtrl-sci↗

arXiv2026

Percolation-Driven Magnetotransport due to Structural and Microstructural Evolution in Ultrathin Si/Fe Bilayers

The anomalous Hall effect (AHE) in magnetic nanofilms is highly sensitive to the microstructural and magnetic homogeneity. However, the evolution of the microstructure and morphology near the percolation threshold, and its connection to the resulting magnetic and magnetotransport behavior in low-dimensional magnetic heterostructures, remain poorly understood. In this study, we present a comprehensive analysis of the evolution of the structural, microstructural, and magnetotransport properties of Si/Fe bilayers by varying the Fe layer thickness. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and magnetisation data reveal a percolation-driven transition from a continuous metallic film to percolative network structure of grains when tFe decreases below 30 Angstrom. Transport measurements involving longitudinal resistivity (rho), and the anomalous Hall resistivity (rho_A,h,s) show clear divergence near the percolation threshold. The purely electronic conduction channels (rho) evolve more gradually as compared to the combined electronic and magnetic ones rho_A,h,s. The percolative analysis of the structural, magnetic, and magnetotransport data yields a critical exponent in the range of 0.78 to 1.16, consistent with that of 2D-disordered systems. The AHE scaling relation between the rho_A,h,s and rho reveals a crossover of the AHE mechanism from a mixed intrinsic/side-jump contribution with a minor skew scattering component (n ~ 1.42) in the thick, low-resistive samples (tFe > 30 Angstrom) to a skew-scattering-dominant mechanism (n = 0.62) in the high-resistive films (tFe <= 30 Angstrom). This crossover coincides with the onset of structural and magnetic connectivity between the grains. Furthermore, these findings underscore the interlink between microstructure, morphology, magnetism, and Hall transport under a percolation framework.

cond-mat.mtrl-sci↗

arXiv2022

Resistivity testing of palladium dilution limits in CoPd alloys for hydrogen storage

Palladium satisfies most of the requirements for an effective hydrogen storage material with two major drawbacks: it has a relatively low gravimetric hydrogen density and is prohibitively expensive for large-scale applications. Pd-based alloys should be considered as possible alternatives to a pure Pd. The question is how much one can dilute the Pd concentration in a variety of candidate materials while preserving hydrogen absorption capability. We demonstrate that the resistivity measurements of thin-film alloy samples can be used for a qualitative high-throughput screening and study of the hydrogen-absorbing properties over the entire range of palladium concentrations. Contrary to palladium-rich alloys where additional hydrogen scattering indicates a degree of hydrogen content, the diluted alloy films respond by a decrease of resistance due to their thickness expansion. Evidence of significant hydrogen absorption was found in thin CoPd films diluted to just 20% of Pd.

cond-mat.mtrl-sci↗

arXiv2020

Kinetics of the lattice response to hydrogen absorption in thin Pd and CoPd films

Hydrogen can penetrate reversibly a number of metals, occupy the interstitial sites and cause large expansion of the crystal lattice. The question discussed here is whether the kinetics of the structural response matches hydrogen absorption. We show that thin Pd and CoPd films exposed to a relatively rich hydrogen atmosphere (4% H2) inflate irreversibly, demonstrate the controllable shape memory, and duration of the process can be orders of magnitude longer than hydrogen absorption. The dynamics of the out-of-equilibrium plastic creep is well described by the Avrami - type model of the nucleation and lateral domain wall expansion of the swelled sites.

cond-mat.mtrl-sci↗

arXiv2018

Detection of hydrogen by the extraordinary Hall effect in CoPd alloys

Effect of hydrogen adsorption on the extraordinary Hall phenomenon (EHE) in ferromagnetic CoPd films is studied as a function of composition, thickness, substrate and hydrogen concentration in atmosphere. Adsorption of hydrogen adds a positive term in the extraordinary Hall effect coefficient and modifies the perpendicular magnetic anisotropy with the respective changes in coercivity and remanence of hysteresis loops. Hydrogen sensitive compositions are within the Co concentration range 20% < x < 50% with the strongest response near the EHE polarity reversal point x_0 ~ 38%. Depending on the film composition and field of operation the EHE response of CoPd to low concentration hydrogen can reach hundreds percent, which makes the method and the material attractive for hydrogen sensing.

cond-mat.mtrl-sci↗