Transition-metal Films Research Articles

Internal stress control of transition-metal thin films

The characteristics of sputtered thin films are affected by ion bombardment during sputter deposition. An ion bombardment parameter Pi, which is based on the magnitude of the ion momentum and the impingement ratio of ions to metal particles, has been previously proposed as a means of expressing the effects of ion bombardment. In this study, the effects of ion bombardment parameter Pi on the internal stress of thin films were evaluated. Consequently, the internal stress of Ni and Cu thin films were observed to increase linearly with the increase of Pi; however, a Mo thin film exhibited the converse result. The results presented herein indicate that the characteristics of thin films, irrespective of the target, can be controlled via manipulation of parameter Pi.

Thin Film Electrodes and Electrochemical Reactions at Three-Phase Boundary in Alkaline Media: A DFT Approach

With increased energy demands, fuel cells offer a viable solution due to their efficient conversion of energy from various fuels. Switching to a basic electrolyte, many metals that would be susceptible to corrosion in an acidic environment become stable in an alkaline one. This allows for the inclusion of non-precious metals in the electrodes. However, the kinetics in an alkaline environment tend to be sluggish; this must be overcome for alkaline fuel cells to become commercially realizable. While many model electrodes have been investigated for possibility to improve the kinetics in alkaline fuel cells, this work focuses on thin film electrodes and reactions happening at the interphase between the substrate and the film. The introduction of three-phase boundaries can allow the various reaction intermediates to preferentially bind to different locations, thus possibly allowing one to break away from scaling relationships. This could ideally enable the catalyst to reach higher catalytic activity [1]. To determine the effectiveness of these electrodes, density functional theory (DFT) is employed. Before work was done to model the interphases, extensive work was done examining various moiré patterns for various oxidation states of transition metal films, with a focus on FeOxHy films. In this work, the stability of a variety of transition metal (hydr)oxide thin films on Pt and Au are first examined for stability for their future prospects of viable catalysts for fuel cell applications. In addition, phase diagrams are constructed to determine the state of the film at a given voltage, something crucial for designing optimal catalysts. A DFT + U scheme is employed to model the thin films, where U values are fitted by correcting the formation energies. While the oxidation state and strain of the film are predicted from the full film phase diagrams, the nature of the interphase needed to be determined. The focus , hydrogen evolution and CO oxidation at these interphases with each reaction occurring at a different equilibrium potentials. This knowledge necessitated the creation of phase diagrams for the interfaces, to identify the most stable oxidation state the edge may take. With these phase diagrams, the thermodynamics and kinetics for the two reactions could be calculated with DFT along with nudged elastic band (NEB) calculations to calculate the activation barriers. These results are then compared with those on a clean Pt surface. [1] R. Subbaraman, D. Tripkovic, K. Chang, D. Strmcnik, A. P. Paulikas, P. Hirunsit, M. Chan, J. Greeley, V. Stamenkovic, N. M. Markovic. Nature Materials. 11, 550 (2012)

Giant perpendicular magnetocrystalline anisotropy of 3d transition-metal thin films on MgO

Magnetocrystalline anisotropy (MCA) of the Fe-based transition-metal thin films was investigated by means of first principles full-potential linearized augmented plane wave method. A giant perpendicular MCA (PMCA), up to 3 meV, was confirmed in a 7-layer Fe-Ni film/MgO(001), where an Fe2/Ni/Fe/Ni/Fe2 atomic-layer alignment with a bcc-like-layer stacking and the Fe/MgO interfaces play key roles for leading to the large PMCA. Importantly, we find that the PMCA overcomes enough over the magnetic dipole-dipole anisotropy that favors the in-plane magnetization even when the film thickness increases.

Electric-field-induced modification of the magnon energy, exchange interaction, and curie temperature of transition-metal thin films.

The electric-field-induced modification in the Curie temperature of prototypical transition-metal thin films with the perpendicular magnetic easy axis, a freestanding Fe(001) monolayer and a Co monolayer on Pt(111), is investigated by first-principles calculations of spin-spiral structures in an external electric field (E field). An applied E field is found to modify the magnon (spin-spiral formation) energy; the change arises from the E-field-induced screening charge density in the spin-spiral states due to p-d hybridizations. The Heisenberg exchange parameters obtained from the magnon energy suggest an E-field-induced modification of the Curie temperature, which is demonstrated via MonteCarlo simulations that take the magnetocrystalline anisotropy into account.

Graphene–Silicon Layered Structures on Single‐Crystalline Ir(111) Thin Films

Single-crystalline transition metal films are ideal playing fields for the epitaxial growth of graphene and graphene-base materials. Graphene-silicon layered structures were successfully constructed on Ir(111) thin film on Si substrate with an yttria-stabilized zirconia buffer layer via intercalation approach. Such hetero-layered structures are compatible with current Si-based microelectronic technique, showing high promise for applications in future micro- and nano-electronic devices.

Dependence of all-optical magnetic switching on the sublattice magnetization orientation in Tb-Fe thin films

We demonstrate that the direction of all-optical switching (AOS) in rare-earth transition-metal (RE-TM) alloy Tb-Fe thin films depends on the orientation of the sublattice magnetization and not on the direction of the resulting net magnetization. For this purpose, we investigated the AOS ability for a sample dominated by the Fe sublattice magnetization (Tb24Fe76) and another dominated by the Tb sublattice (Tb30Fe70). This finding of the sublattice dependence of AOS contributes to the understanding of switching in RE-TM multilayered thin films and heterostructures.

Spin relaxation and spin Hall transport in5dtransition-metal ultrathin films

The spin relaxation induced by the Elliott-Yafet mechanism and the extrinsic spin Hall conductivity due to the skew scattering are investigated in $5d$ transition-metal ultrathin films with self-adatom impurities as scatterers. The values of the Elliott-Yafet parameter and of the spin-flip relaxation rate reveal a correlation with each other that is in agreement with the Elliott approximation. At 10-layer thickness, the spin-flip relaxation time in $5d$ transition-metal films is quantitatively reported about few hundred nanoseconds at atomic percent. This time scale is one and two orders of magnitude shorter than the values in Au and Cu thin films, respectively. The anisotropy effect of the Elliott-Yafet parameter and of the spin-flip relaxation rate with respect to the direction of the spin-quantization axis in relation to the crystallographic axes is also analyzed. We find that the anisotropy of the spin-flip relaxation rate is enhanced due to the Rashba surface states on the Fermi surface, reaching values as high as 97% in 10-layer Hf(0001) film or 71% in 10-layer W(110) film. Finally, the spin Hall conductivity as well as the spin Hall angle due to the skew scattering off self-adatom impurities are calculated using the Boltzmann approach. Our calculations employ a relativistic version of the first-principles full-potential Korringa-Kohn-Rostoker Green function method.

Manganese Precursor Selection and the Thermal Atomic Layer Deposition of Copper/Manganese Alloy Films

Our laboratory seeks to develop the growth of metallic first row transition metal thin films using atomic layer deposition (ALD). The microelectronics industry is calling for the growth of metallic first row transition metal films by the ALD method for a variety of applications, including copper metallization, seed layers for copper metallization, copper/manganese alloys for self-forming copper diffusion barriers, and magnetic alloys. The ALD growth of noble metal thin films has been explored extensively in the past ten years, due to the positive electrochemical potentials of these metal ions and relative ease of reduction to the metallic state. The low temperature ALD of high purity, low resistivity Cu films has been described, but ALD routes to the other metallic first row transition metal films remain poorly developed, largely because of the negative electrochemical potentials of most of the ions and a corresponding lack of powerful reducing co-reagents that can convert precursors in positive oxidation states to the metals. We will describe the synthesis, structure, and properties of a large series of new first row transition metal ALD precursors that combine high volatilities, high thermal stabilities, and high reactivities toward reducing agents. We will also report borane and other reducing agents that can react with the metal precursors to afford metallic films. Additionally, we will overview the thermal growth of metallic copper, nickel, cobalt, iron, manganese, and chromium thin films from these new precursors. Key advances include development of optimized ALD precursors with very similar ligands and chemistry, and identification of new reducing co-reagents that can rapidly reduce the positive oxidation state precursors to the metals.

Oxygen Vacancy Ordering: a Degree of Freedom that can Control the Structural, Electronic and Magnetic Properties of Transition-Metal Oxide Films

Oxygen Vacancy Ordering: a Degree of Freedom that can Control the Structural, Electronic and Magnetic Properties of Transition-Metal Oxide Films

Tailoring magnetic skyrmions in ultra-thin transition metal films.

Skyrmions in magnetic materials offer attractive perspectives for future spintronic applications since they are topologically stabilized spin structures on the nanometre scale, which can be manipulated with electric current densities that are by orders of magnitude lower than those required for moving domain walls. So far, they were restricted to bulk magnets with a particular chiral crystal symmetry greatly limiting the number of available systems and the adjustability of their properties. Recently, it has been experimentally discovered that magnetic skyrmion phases can also occur in ultra-thin transition metal films at surfaces. Here we present an understanding of skyrmions in such systems based on first-principles electronic structure theory. We demonstrate that the properties of magnetic skyrmions at transition metal interfaces such as their diameter and their stability can be tuned by the structure and composition of the interface and that a description beyond a micromagnetic model is required in such systems.

Growth of polycrystalline nickel hydroxide films from aqueous solution. Solution chemistry, deposition methods, film morphology and texture

Deposition of functional ceramic films on different substrates by using soft aqueous solution processing can represent a cheap, low- energy and environmental friendly alternative to other film technologies. In this study β-Ni(OH)2 polycrystalline films were grown on polytetrafluoroethylene, NiO, glass, copper, and Y:ZrO2 substrates from Ni2+–NH4OH aqueous solutions. A careful control of solution supersaturation was essential to induce heterogeneous nucleation on the substrate and film growth. This was realized through the gradual decomposition of the nickel amino complexes Ni(NH3)n2+ using two different methods. Thermodynamic modeling of the Ni–NH4OH–H2O system was used to provide some general indications for defining the experimental conditions most suitable for film deposition and understanding the role of solution chemistry on film formation. Film microstructure and texture can be controlled by varying the substrate, the ammonia concentration, the ammonia evaporation rate, the soaking time and by adding a surfactant like sodium dodecylbenzensulphonate. As a result, films with different morphologies and orientations, as well as a uniform decoration of the substrate surface with Ni(OH)2 crystals, were obtained. The proposed method can be extended to the preparation of oxide and hydroxide films of other transition metals forming moderately stable amino complexes.

Surface-Enhanced Raman Scattering on Uniform Pd and Pt Films: From Ill-Defined to Structured Surfaces

We report borrowed surface-enhanced Raman scattering (SERS) at Pd and Pt films deposited on Au sphere segment void (SSV) SERS substrates and show that ultrathin, pinhole-free Pd and Pt films could be easily achieved by choosing constant-current deposition for the Pd deposition and under-potential deposited Cu-mediated deposition for the Pt deposition. The surface enhancement achieved from the Pd and Pt films was reproducible, owing to the ordered SSV structure of their surfaces, and borrowed additional enhancement from the long-range electromagnetic (EM) fields generated by the underlying Au SSV substrate. SERS experiments and theoretical simulations indicate that the distance over which the EM fields generated by a concave Au SSV surface can be transferred to the surface of the transition-metal films is approximately an order of magnitude larger than that for a convex sphere surface. This longer range of the EM fields allowed deposition of thicker, more robust Pd and Pt films while retaining the borrowed enhancement. These properties make such Pd and Pt films attractive as SERS surfaces to reveal surface species and reactive intermediates on these two typical electrocatalyst materials.

Hard X-ray photoemission spectroscopy of transition-metal oxide thin films and interfaces

Photoemission spectroscopy is a powerful experimental technique to study the electronic structures of solids, especially of transition-metal oxides. Recently, hard X-ray photoemission spectroscopy (HXPES) has emerged as a more relevant experimental technique to obtain clear information about bulk states. Here, we describe how HXPES can be conveniently applied to study the interesting subjects on oxide thin films such as the composition dependence and the film thickness dependence of the electronic structures and the interfacial electronic structure of multilayers.

Atomic-Layer Alignment Tuning for Giant Perpendicular Magnetocrystalline Anisotropy of3dTransition-Metal Thin Films

The magnetocrystalline anisotropy (MA) of Fe-based transition-metal thin films, consisting of only magnetic 3d elements, was systematically investigated from full-potential linearized augmented plane-wave calculations. The results predict that giant MA with a perpendicular magnetic easy axis (PMA) can be achieved by tuning the atomic-layer alignments in an Fe-Ni thin film. This giant PMA arises from the spin-orbit coupling interaction between occupied and unoccupied Ni dx2-y2,xy bands crossing the Fermi level. A promising 3d transition-metal thin film for the MgO-based magnetic tunnel junctions with the giant PMA was, thus, demonstrated.

Manipulating the magnetic anisotropy of 3d transition-metal films on Cu(001) and their alloys on Rh(001) by electric field

The mechanism of electric field (EF) effects on the magnetocrystalline anisotropy (MCA) in metallic films is investigated by first-principles calculations. Start with a simple system of Fe, Co and Ni monolayer on Cu(001) substrate, we show that the key factor for a large EF-induced MCA modification is that the energy bands cross of d3z2−r2 and dyz (or dxz and dxy) is close to the Fermi level. In order to enhance the MCA modification by EF, 4d metal substrates (Rh, Pd) are also discussed. In particular, we find that the magnetization direction can be switched from out-of-plane to in-plane by a small EF for Fe1–xCox alloy films on Rh(001) substrate with x=0.5.

ChemInform Abstract: Reduction and Oxidation of Transition‐Metal Oxide Thin Films: Solid‐State Chemistry with Epitaxially Grown Thin Films

AbstractReview: 44 refs.

Reduction and Oxidation of Transition-Metal Oxide Thin Films: Solid-State Chemistry with Epitaxially Grown Thin Films

Abstract Reduction and oxidation of perovskite-structure transition-metal oxide thin films are highlighted. Oxygen ions are released from and incorporated into the perovskite-structure framework during the reduction and oxidation. Low-temperature topochemical reduction reaction with CaH2 enables us to prepare infinite-layer structure oxide thin films containing Fe and Ni. With the epitaxially grown thin film samples, anisotropic rearrangement and transfer of oxygen ions in the structures have been revealed. Selective reduction of layers in artificial superlattices was also found, and the observed behavior suggests that the oxygen-ion mobility in the artificial superlattices was confined within the two-dimensional brownmillerite layer. Specific structural features play important roles in the oxygen-ion transfer in solids at low temperatures. The obtained results would be a key to develop technologies related to energy and the environment.

Realization of the Switching Mechanism in Resistance Random Access Memory™ Devices: Structural and Electronic Properties Affecting Electron Conductivity in a Hafnium Oxide–Electrode System Through First-Principles Calculations

The resistance random access memory (RRAM™) device, with its electrically induced nanoscale resistive switching capacity, has attracted considerable attention as a future nonvolatile memory device. Here, we propose a mechanism of switching based on an oxygen vacancy migration-driven change in the electronic properties of the transition-metal oxide film stimulated by set pulse voltages. We used density functional theory-based calculations to account for the effect of oxygen vacancies and their migration on the electronic properties of HfO2 and Ta/HfO2 systems, thereby providing a complete explanation of the RRAM™ switching mechanism. Furthermore, computational results on the activation energy barrier for oxygen vacancy migration were found to be consistent with the set and reset pulse voltage obtained from experiments. Understanding this mechanism will be beneficial to effectively realizing the materials design in these devices.

Real‐Time Microscopy of Graphene Growth on Epitaxial Metal Films: Role of Template Thickness and Strain

Epitaxial transition metal films have recently been introduced as substrates for the scalable synthesis of transferable graphene. Here, real-time microscopy is used to study graphene growth on epitaxial Ru films on sapphire. At high temperatures, high-quality graphene grows in macroscopic (>100 μm) domains to full surface coverage. Graphene nucleation and growth characteristics on thin (100 nm) Ru films are consistent with a pure surface chemical vapor deposition process, without detectable contributions from C segregation. Experiments on thicker (1 μm) films show a systematic suppression of the C uptake into the metal to levels substantially below those expected from bulk C solubility data, consistent with a strain-induced reduction of the C solubility due to gas bubbles acting as stressors in the epitaxial Ru films. The results identify two powerful approaches--i) limiting the template thickness and ii) tuning the interstitial C solubility via strain--for controlling graphene growth on metals with high C solubility, such as Ru, Pt, Rh, Co, and Ni.

Electric Field-Induced Modification of Magnetocrystalline Anisotropy in Transition-metal Films and at Metal-Insulator Interfaces

We report results of first principles calculations for effects of an external electric field (E-field) on the magnetocrystalline anisotropy (MCA) in transition-metal (Fe, Co, and Ni) monolayers and at metal-insulator (Fe/MgO) interfaces by means of full-potential linearized augmented plane wave method. For the monolayers, the MCA in the Fe monolayer (but not in the Co and Ni) is modified by the E-field, and a giant modification is achieved in the Fe0.75Co0.25. For the Fe/MgO interfaces, the ideal Fe/MgO interface gives rise to a large out-ofplane MCA, and a MCA modification is induced when an E-field is introduced. However, the existence of an interfacial FeO layer between the Fe layer and the MgO substrate may play a key role in demonstrating an Efield-driven MCA switching, i.e., from out-of-plane MCA to in-plane MCA.