Synthesis of Thin Niobium Films on Silicon and Study of Their Superconducting Properties in the Dimensional Crossover Region

Abstract: Niobium films with a thickness of 4–100 nm are synthesized on a silicon substrate under ultrahigh vacuum conditions. Measurements of the electrical resistance show a high superconducting transition temperature Tc in the range of 4.7–9.1 K and record-breaking small transition widths ΔTc in the range of 260–11 m. The dependences of Tc and ΔTc on the magnetic field are investigated, and the superconducting coherence lengths and mean free paths of conduction electrons for different thicknesses of the synthesized films are determined. A significant influence of the magnetic field on ΔTc is found, which reveals the transition from three- to two-dimensional superconductivity at thicknesses below 10 nm. The dependences of Tc and ΔTc on the thickness of the films and the magnitude of the magnetic field are discussed within the framework of existing theories of superconductivity in thin films of superconducting metals

Epitaxial growth and superconducting properties of thin-film PdFe/VN and VN/PdFe bilayers on MgO(001) substrates

© 2020 Mohammed et al. Single-layer vanadium nitride (VN) and bilayer Pd0.96Fe0.04/VN and VN/Pd0.92Fe0.08 thin-film heterostructures for possible spintronics applications were synthesized on (001)-oriented single-crystalline magnesium oxide (MgO) substrates utilizing a fourchamber ultrahigh vacuum deposition and analysis system. The VN layers were reactively magnetron sputtered from a metallic vanadium target in Ar/N2 plasma, while the Pd1-xFex layers were deposited by co-evaporation of metallic Pd and Fe pellets from calibrated effusion cells in a molecular beam epitaxy chamber. The VN stoichiometry and Pd1-xFex composition were controlled by X-ray photoelectron spectroscopy. In situ low-energy electron diffraction and ex situ X-ray diffraction show that the 30 nm thick single-layer VN as well as the double-layer VN(30 nm)/Pd0.92Fe0.08(12 nm) and Pd0.96Fe0.04(20 nm)/VN(30 nm) structures have grown cube-on-cube epitaxially. Electric resistance measurements demonstrate a metallic-type temperature dependence for the VN film with a small residual resistivity of 9 μΩ·cm at 10 K, indicating high purity and structural quality of the film. The transition to the superconducting state was observed at 7.7 K for the VN film, at 7.2 K for the Pd0.96Fe0.04/VN structure and at 6.1 K for the VN/Pd0.92Fe0.08 structure with the critical temperature decreasing due to the proximity effect. Contrary to expectations, all transitions were very sharp with the width ranging from 25 mK for the VN film to 50 mK for the VN/Pd0.92Fe0.08 structure. We propose epitaxial single-crystalline thin films of VN and heteroepitaxial Pd1-xFex/VN and VN/Pd1-xFex (x ≤ 0.08) structures grown on MgO(001) as the materials of a choice for the improvement of superconducting magnetic random access memory characteristics

Iron-implanted epitaxial palladium thin films: Structure, ferromagnetism and signatures of spinodal decomposition

We report on the formation of the dilute Pd1−xFex composites with tunable magnetic properties under ion-beam implantation of epitaxial Pd thin films. Binary Pd1−xFex alloys with a mean iron content x of 0.025, 0.035, and 0.075 were obtained by the implantation of 40 keV Fe+ ions into the palladium films on MgO (0 0 1) substrate to the doses of 0.5 × 1016, 1.0 × 1016 and 3.0 × 1016 ions/cm2, respectively. Structural and magnetic studies have shown that iron atoms occupy regular fcc-lattice Pd-sites without the formation of any secondary crystallographic phase. All the iron implanted Pd films reveal ferromagnetism at low temperatures. The observed multiple ferromagnetic resonances in the implanted Pd1−xFex films indicate a formation of a magnetically inhomogeneous state due to spinodal decomposition into regions, presumably layers, with identical crystal symmetry but different iron contents. The multiphase composite magnetic structure is robust with respect to the vacuum annealing at 770 K, though develops towards well-defined local Pd/Fe ratios

Features of Formation of Cr³⁺ Paramagnetic Centers in Strontium Titanate (SrTiO<inf>3</inf>) Implanted with Chromium Ions

© 2020, Pleiades Publishing, Ltd. Abstract: We present results of structural (XPS) and magnetic resonance (EPR) studies of single-crystal plates of strontium titanate (SrTiO3) heavily doped with chromium utilizing ion implantation technique. It was shown that the temperature of the oxide matrix during ion implantation (300 or 900 K) significantly affects the valence state of the implanted chromium impurity (Cr0 or Cr3+) and the depth profiles of its distribution in the SrTiO3 matrix. Using the EPR method, it was established that as a result of implantation of chromium impurity at an elevated substrate temperature during irradiation, the dominating color centers in the surface implanted oxide layer appear in the form of trivalent chromium ions in a cubic environment with a g-factor g = 1.976 and the value of the hyperfine coupling constant with the magnetic nucleus of 53Cr isotope, A = 16.2 × 10–4 cm–1. From a comparison of these data with parameters for the cubic Cr3+ impurity center in the SrTiO3 single crystal, it was concluded that this center is localized in the structural positions of titanium. Upon implantation of a chromium impurity in SrTiO3 at room temperature of the substrate, in addition to cubic Cr3+ centers, new signals were detected in the EPR spectra, apparently associated with the formation of oxygen vacancies in the implanted oxide. However, the number of these additional centers does not change significantly even after high-temperature heat treatment of samples in air at temperatures up to 900 K. It is noted that signals of this type also appear in the spectra of SrTiO3 samples obtained as a result of hot implantation and then subjected to high-temperature thermal annealing

ELECTRICAL PROPERTIES OF TITANIUM NITRIDE FILMS SYNTHESIZED BY REACTIVE MAGNETRON SPUTTERING

259-259A use of the four-probe resistance measurements as a tool for characterization of a quality of titanium nitride thin films deposited by the reactive dc magnetron sputtering will be discussed in the report. Few series of ~ 50 nm thick films on various substrates as fused silica, monocrystalline silicon and magnesium oxide have been deposited with several degrees of freedom (substrate temperature, magnetron chamber atmosphere and working pressure etc.) varied in a wide range. Electrical resistivity correlation with the films properties will be reported

Epitaxial thin-film Pd<inf>1-x</inf>Fe<inf> x</inf> alloy: a tunable ferromagnet for superconducting spintronics

© 2020, Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature. Thin epitaxial films of the palladium-rich Pd1−xFex alloy were synthesized and extensively studied as a tunable ferromagnetic material for superconducting spintronics. The (001)-oriented MgO single-crystal substrate and the composition range of x = 0.01–0.07 were chosen to support the epitaxial growth and provide the films with magnetic properties spanning from very soft ferromagnet for memory applications to intermediately soft and moderately hard for the programmable logic and circuit biasing, respectively. Dependences of the saturation magnetization, Curie temperature and three magnetic anisotropy constants on the iron content x were obtained for the first time from the analyses of the magnetometry and ferromagnetic resonance data. The experimental results were discussed based on existing theories of dilute ferromagnetic alloys. Simulation of the hysteresis loops within the Stoner-Wohlfarth model indicates the predominant coherent magnetic moment rotation at cryogenic temperatures. The obtained results were compiled in a database of magnetic properties of a palladium-iron alloy in a single-crystal thin-film form considered as a material for superconducting spintronics

Analysis of nanostructured cobalt ion beam-modified Ge surface for high capacity Li-ion battery anodes by X-ray photoelectron spectroscopy

© 2020 Published under licence by IOP Publishing Ltd. The results of the study of nanostructured germanium (Ge), intended for its subsequent use as electrodes of lithium-ion batteries are presented. Single-crystal germanium plate was irradiated by cobalt ions with an energy of 40 keV in the dose 4×1016 Co+ / cm2 for the nanostructuring. The changes in the elemental composition occurring during the electrochemical lithiation of manufactured germanium electrode were investigated by X-ray photoelectron spectroscopy (XPS)

Formation of Pores in Thin Germanium Films under Implantation by Ge⁺ Ions

© 2020, Pleiades Publishing, Ltd. Abstract: Results are presented of a study of the morphology of germanium films nanostructured by ion implantation. Film samples were grown by magnetron sputtering in an ultrahigh-vacuum installation and then irradiated with 40 keV Ge+ ions at fluences in the range of (1.8–8) × 1016 ions/cm2. Scanning electron microscopy demonstrated that vacancy complexes with diameters of ~50–150 nm are gradually formed in the bulk of implanted germanium with increasing implantation fluence. After a certain implantation fluence is reached, the complexes emerge on the surface, thereby forming a developed surface profile of the irradiated films