Transition-metal Films Research Articles

Antiferromagnetic coupling across nonmagnetic transition-metal films alloyed with ferromagnetic elements

Antiferromagnetic coupling across nonmagnetic transition-metal films alloyed with ferromagnetic elements

Surface effects in ferrimagnetic TbFe rare-earth–transition-metal thin films and nanoparticles

Surface effects in ferrimagnetic TbFe rare-earth–transition-metal thin films and nanoparticles

Formation of Laser-Induced Periodic Structures on Thin Films of Transition Metal and Semiconductor Nitrides

Formation of Laser-Induced Periodic Structures on Thin Films of Transition Metal and Semiconductor Nitrides

Thickness and composition effects on atomic moments and magnetic compensation point in rare-earth transition-metal thin films

Rare-earth (RE) transition-metal (TM) thin films have exhibited great promise for spintronic applications; however, the variation of their magnetic properties with composition and thickness has yet to be adequately quantified. In this paper, we investigate the atomic magnetic moments of few-nanometer-thick GdCo and TbCo via x-ray magnetic circular dichroism and find a significant decrease in both RE and Co average moments with increasing RE concentration, consistent with a local Co environment model description. We observe a shift in compensation composition to higher RE concentrations as the thickness of GdCo and TbCo films decreases $<10$ nm. Based on the dependence of the saturation magnetization $({M}_{s})$ on temperature and thickness, we posit the existence of an intrinsic unalloyed RE dead layer at room temperature that reduces the effective RE concentration in the remaining alloyed region, resulting in significant, nonzero ${M}_{s}$ in thin films at higher RE concentrations than is observed in thick films. We demonstrate a simple model for RE-TM films that accurately describes ${M}_{s}$ as a function of both RE content and film thickness. In addition, the atomic $g$ factors of GdCo and TbCo are found to vary significantly with RE concentration, largely deviating from their elemental values over the bulk of composition space.

Orbital correlations in ultrathin films of late transition metals

We develop a two-orbital Hubbard model of electron correlations in ultrathin (111)-oriented fcc films of late transition metals such as Co and Ni. Our model indicates that the Mott-Hund's interaction results in ferromagnetic nearest-neighbor orbital correlations. Frustration associated with the mismatch between orbital and crystal symmetries prevents orbital ordering, resulting in the orbital liquid state. This state can be manifested in phenomena involving spin-orbit coupling, such as magnetic anisotropy.

Particle-hole asymmetric lifetimes promoted by nonlocal spin and orbital fluctuations in SrVO3 monolayers

The two-dimensional nature of engineered transition-metal ultrathin oxide films offers a large playground of yet to be fully understood physics. Here, we study pristine ${\mathrm{SrVO}}_{3}$ monolayers that have recently been predicted to display a variety of magnetic and orbital orders. We find that nonlocal magnetic (orbital) fluctuations lead to a strong (weak to moderate) momentum differentiation in the self-energy, particularly in the scattering rate. In the one-band 2D Hubbard model, momentum selectivity on the Fermi surface (``$k={k}_{F}$'') is known to lead to pseudogap physics. Here instead, in the multiorbital case, we evidence a differentiation between momenta on the occupied (``$k<{k}_{F}$'') and the unoccupied side (``$k>{k}_{F}$'') of the Fermi surface. Based on the dynamical vertex approximation, and introducing a ``binaural fluctuation diagnostics'' tool, we advance the understanding of spectral signatures of nonlocal fluctuations. Our work calls to (re)examine ultrathin oxide films and interfaces with methods beyond dynamical mean-field theory and may point to correlation-enhanced thermoelectric effects.

Electric-field driven stability control of skyrmions in an ultrathin transition-metal film

AbstractTo realize future spintronic applications with magnetic skyrmions—topologically nontrivial swirling spin structures—it is essential to achieve efficient writing and deleting capabilities of these quasi-particles. Electric-field assisted nucleation and annihilation is a promising route, however, the understanding of the underlying microscopic mechanisms is still limited. Here, we show how the stability of individual magnetic skyrmions in an ultrathin transition-metal film can be controlled via external electric fields. We demonstrate based on density functional theory that it is important to consider the changes of all interactions with electric field, i.e., the pair-wise exchange, the Dzyaloshinskii–Moriya interaction, the magnetocrystalline anisotropy energy, and the higher-order exchange interactions. The energy barriers for electric-field assisted skyrmion writing and deleting obtained via atomistic spin simulations vary by up to a factor of three more than the variations of the interactions calculated from first-principles. This surprising effect originates from the electric-field dependent size of metastable skyrmions at a fixed magnetic field. The large changes in lifetimes allow the possibility of electric-field assisted thermally activated writing and deleting of skyrmions.

Thermal evolution of yttrium tetraboride thin films – A candidate for high temperature applications

The common drawback of ceramic transition-metal boride films, the inherent brittleness, limits their practical applications under thermal load. Yttrium tetraboride (YB4), having a high melting temperature, chemical inertness, high hardness, and low Young's modulus of elasticity in the bulk form, is a promising candidate for thin films capable of withstanding the demanding mechanical conditions at high temperatures. Here, we report the evolution of the chemical composition, nanostructure, and mechanical properties of sputtered, slightly overstoichiometric YB4.7 thin films after vacuum annealing up to 1300 °C. The as deposited YB4.7 thin films have an amorphous character with hardness of 23.4 GPa and low elastic modulus of 280.7 GPa. Annealing at 800 °C leads to partial crystallization of the films and formation of tetragonal YB4 and cubic YB6 phases in an amorphous matrix. These structural transformations are accompanied by a 12% increase in hardness up to 26.4 GPa at 1000 °C while the elastic moduli remain relatively low by approx. 300 GPa. The cube-corner indents revealed a ductile behavior of the as deposited films, while crack propagation along grain boundaries was observed after annealing at 800 °C. The observed mechanical behavior provides the opportunity to introduce the YB4.7 films into multilayer systems with harder but significantly more brittle transition-metal diborides to reduce crack propagation and achieve better overall thermal shock resistance.

Microscopic theory of light-induced ultrafast skyrmion excitation in transition metal films

Magnetic skyrmions are topological excitations of great promise for compact and efficient memory storage. However, to interface skyrmionics with electronic devices requires efficient and reliable ways of creating and destroying such excitations. In this work, we unravel the microscopic mechanism behind ultrafast skyrmion generation by femtosecond laser pulses in transition metal thin films. We employ a theoretical approach based on a two-band electronic model, and show that by exciting the itinerant electronic subsystem with a femtosecond laser ultrafast skyrmion nucleation can occur on a 100 fs timescale. By combining numerical simulations with an analytical treatment of the strong s–d exchange limit, we identify the coupling between electronic currents and the localized d-orbital spins, mediated via Rashba spin–orbit interactions among the itinerant electrons, as the microscopic and central mechanism leading to ultrafast skyrmion generation. Our results show that an explicit treatment of itinerant electron dynamics is crucial to understand optical skyrmion generation.

Ionic Liquid Gating Control of Oxygen Vacancies in the La0.8Ba0.2MnO3 Ultrathin Films

In this work, we show ionic liquid field-effect of La0.8Ba0.2MnO3 (LBMO) untrathin films prepared by the sol-gel method. When applying a positive gate voltage (V g) in vacuum, it is found that the film resistivity continuously increases tenfold within hours or minutes depending on the magnitude of V g, accompanied with disappearance of the low-temperature metallic transport behavior. In contrast, the film resistivity changes little in air or under a negative applied V g. Such a difference reveals that the increased resistivity is related to the oxygen depletion in the films under the positive V g, especially at the grain boundaries. After removing the positive V g in vacuum, the room-temperature resistivity begins to fall off and the low-temperature metallic state is partially restored in several tens hours, suggesting oxygen diffusion from the relatively oxygen-rich grains to the seriously oxygen-deficient grain boundaries. Furthermore, the oxygen content in the oxygen-deficient films can be almost fully restored in ten minutes by external annealing (200 °C) or Joule self-heating (21 mA) the films in air. These experimental findings provide an additional method in tuning oxygen vacancies in transition-metal oxide films.

Tetragonal distortion in magnetron sputtered bcc-W films with supersaturated carbon

Carbon has a low solid solubility in bcc tungsten at equilibrium. However, metastable supersaturated solid solutions can be synthesized with magnetron sputtering. Here, we present a systematic study on the phase stability and mechanical properties of such supersaturated W–C solid solutions. Θ–2θ scans show a split of the 200/020 and the 002 peaks for supersaturated films which is explained by a tetragonal distortion of the bcc structure. This split increases with increasing C content and is maximized at 4 at.% C, where we observe an a/b axis of 3.15–3.16 Å and a c-axis of 3.21–3.22 Å. We performed first-principles calculations of lattice parameters, mixing enthalpies, elastic constants and polycrystalline elastic moduli for cubic and tetragonal W–C solid solutions. Calculations show that tetragonal structure is more stable than the bcc supersaturated solid solution and the calculated lattice parameters and Young’s moduli follow the same trends as the experimental ones as a function of C concentration. The results suggest that supersaturated films with lattice distortion can be used as a design approach to improve the properties of transition metal films with a bcc structure.

Thermoelectric coefficient of nanoscale transition metals films

We study the thermoelectric coefficient of island transition metal films as a function of temperature and film thickness. We show that the thermoelectric coefficient value is determined by the properties of the film metal element and is different for different metals at dimensionless conductance value g>1 and g<1. The main factor in the behavior difference of the thermoelectric coefficient as a function of temperature is the individual characteristics of the Fermi surface of each metal. Dependence of the thermoelectric coefficient on the film thickness is also determined by the Fermi surface shape change. Keywords: Island film, Thin film, Thermopower, Thermoelectric coefficient, Conductivity.

Термоэдс наноразмерных пленок переходных металлов

We study the thermoelectric coefficient of island transition metal films as a function of temperature and film thickness. We show that the thermoelectric coefficient value is determined by the properties of the film metal element and is different for different metals at dimensionless conductance value g>1 and g<1. The main factor in the behavior difference of the thermoelectric coefficient as a function of temperature is the individual characteristics of the Fermi surface of each metal. Dependence of the thermoelectric coefficient on the film thickness is also determined by the Fermi surface shape change.

Boosting pH-Universal Hydrogen Evolution of FeP/CC by Anchoring Trace Platinum

To improve the electrocatalytic properties for hydrogen evolution reactions, strategies need to be adopted, such as increasing specific surface area and active site, as well as decreasing interface energy. Herein, we report the preparation of FeP on carbon cloth using a two-step process of hydrothermal and phosphating. Otherwise, to utilize the excellent catalytic performance of Pt and decrease consumption of Pt, the hyperdispersed Pt nanoparticles for the sake of modifying transition-metal phosphides film were designed and fabricated. Finally, 3D FeP-Pt/CC was successfully prepared by means of electro-deposition using three electrodes. The crystalline structure, surface morphology and elemental composition of the synthesized samples have been investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). The XRD results show that the as-prepared products are of orthorhombic FeP structure, and EDS results indicate that there exist Pt elements in 3D FeP-Pt/CC. The electrocatalytic performances were evaluated by, such as linear scan voltammetry, tafel plots and electrochemical impedance spectroscopy on electrochemical workstations. These results show that the FeP-Pt/CC exhibit a current density of 10 mA·cm−2 at an over-potential of 58 mV for HER in 0.5 M H2SO4, which is very close to the values of 20%Pt/C which was previously reported. FeP-Pt/CC has excellent durability.

Wafer-Scale Epitaxy of Flexible Nitride Films with Superior Plasmonic and Superconducting Performance.

Transition-metal nitrides (e.g., TiN, ZrN, TaN) are incredible materials with excellent complementary metal-oxide semiconductor compatibility and remarkable performance in refractory plasmonics and superconducting quantum electronics. Epitaxial growth of flexible transition-metal nitride films, especially at the wafer scale, is fundamentally important for developing high-performance flexible photonics and superconducting electronics, but the study is rare thus far. This work reports the high-quality epitaxy of 2-in. titanium nitride (TiN) films on flexible fluorophlogopite-mica (F-mica) substrates via reactive magnetron sputtering. Combined measurements of spectroscopic ellipsometry and electrical transport reveal the superior plasmonic and superconducting performance of TiN/F-mica films owing to the high single crystallinity. More interestingly, the superconductivity of these flexible TiN films can be manipulated by the bending states, and enhanced superconducting critical temperature TC is observed in convex TiN films with in-plane tensile strain. Density functional theory calculations reveal that the strain can tune the electron-phonon interaction strength and the resultant superconductivity of TiN films. This study provides a promising route toward integrating scalable single-crystalline transition-metal nitride films with flexible electronics for high-performance plasmonics and superconducting electronics.

MXene-decorated nanofiber film based on layer-by-layer assembly strategy for high-performance electromagnetic interference shielding

With the increase of electromagnetic radiation pollution, there is an urgent need to develop lightweight, ultra-thin, and high electromagnetic shielding composite film to eliminate unwanted electromagnetic interference. Herein, based on the layer-by-layer assembly strategy, a simple solution dip coating method was used to achieve the combination of two-dimensional transition metal (MXene) and nanofiber film, thereby constructing a conductive composite film with a unique undulating structure. The silicon-containing hydrophobic layer imparted excellent hydrophobic properties to the composite film while maintained the stability of the sandwich structure. The contact angle (CA) of the composite film with ultra-thin thickness (29um) was 125.49° and the conductivity was as high as 71.91 S/cm. The hydrophobic treatment would not damage the flexibility and mechanical properties of the composite film. In particular, the average shielding effectiveness (SE) and SSE/t could as high as 28.82 dB and 12422.41 dB cm2g−1, due to the layered structure formed by the connection of countless MXene nanosheets is beneficial to promote multiple absorption and reflection. This layer-by-layer assembly strategy provides a facile route in fabricating light-weight, flexible and ultrathin composite film for high-performance electromagnetic shielding, which is of great significance for broadening the practical application of MXene-based conductive materials.

Resonant Soft X-ray Reflectivity in the Study of Magnetic Properties of Low-Dimensional Systems

In this review, the technique of resonant soft X-ray reflectivity in the study of magnetic low-dimensional systems is discussed. This technique is particularly appealing in the study of magnetization at buried interfaces and to discriminate single elemental contributions to magnetism, even when this is ascribed to few atoms. The major fields of application are described, including magnetic proximity effects, thin films of transition metals and related oxides, and exchange-bias systems. The fundamental theoretical background leading to dichroism effects in reflectivity is also briefly outlined.

Electrochemical Visualization of Gas Bubbles on Superaerophobic Electrodes Using Scanning Electrochemical Cell Microscopy.

Electrocatalytic gas evolution reactions, where gaseous molecules are electrogenerated by reduction or oxidation of a species, play a central role in many energy conversion systems. Superaerophobic electrodes, usually constructed by their surface microstructures, have demonstrated excellent performance for electrochemical gas evolution reactions due to their bubble-repellent properties. Understanding and quantification of the gas bubble behavior including nucleation and dynamics on such microstructured electrodes is an important but underexplored issue. In this study, we reported a scanning electrochemical cell microscopy (SECCM) investigation of individual gas bubble nucleation and dynamics on nanoscale electrodes. A classic Pt film and a nonconventional transition-metal dichalcogenide MoS2 film with different surface topologies were employed as model substrates for both H2 and N2 bubble electrochemical studies. Interestingly, the nanostructured catalyst surface exhibit significantly less supersaturation for gas bubble nucleation and a notable increase of bubble detachment compared to its flat counterpart. Electrochemical mapping results reveal that there is no clear correlation between bubble nucleation and hydrogen evolution reaction (HER) activity, regardless of local electrode surface microstructures. Our results also indicate that while the hydrophobicity of the nanostructured MoS2 surface promotes bubble nucleation, it has little effect on bubble dynamics. This work introduces a new method for nanobubble electrochemistry on broadly interesting catalysts and suggests that the deliberate microstructure on a catalyst surface is a promising strategy for improving electrocatalytic gas evolution both in terms of bubble nucleation and elimination.

STUDY OF FILMS OF TRANSITION METALS AS MATERIALS FOR LASER NANOSTRUCTURING

This paper presents the results of a study of direct laser writing on thin films of transition metals (Hf, Ti, Zr, Ta, V). The films were deposited on fused silica substrates. A comparison of laser writing on the indicated films is carried out from the point of view of the presence of contour writing. As it was proved earlier, when writing on zirconium films, contour writing leads to formation of periodic nanostructures with a period equal to the writing step (250-500 nm). Materials were identified that are promising from the point of view of writing oxide nanostructures for the further formation of the diffraction phase microrelief of DOEs.

In Situ Tensile Testing of Nanometer-Thick Two-Dimensional Transition-Metal Carbide Films: Implications for MXenes Acting as Nanoscale Reinforcement Agents

<i>In Situ</i> Tensile Testing of Nanometer-Thick Two-Dimensional Transition-Metal Carbide Films: Implications for MXenes Acting as Nanoscale Reinforcement Agents