Transition Metal Thin Films Research Articles

Influence of spin polarization enhancement layer to rare earth–transition metal thin films for perpendicular tunneling magnetoresistance evaluated by ferromagnetic Hall effect

Magnetic properties of ion beam sputter-deposited TbFeCo and GdFeCo thin films with their compensation point were studied using ferromagnetic Hall effect measurement. Gdx(Fe90Co10)1−x thin films sputter deposited using Kr exhibited high squareness ratio and sharp magnetization reversal compared with those prepared using Ar as sputtering gas. Lower kinetic energy of sputtering ions was effective to attain higher squareness ratio. Ferromagnetic Hall effect clarifies the magnetization reversal process of a FeCo(xnm) layer contacted to a TbFeCo(50nm) layer. FeCo layer with thickness less than 2nm is magnetized together with TbFeCo layer. Ferromagnetic Hall measurement is a powerful tool to determine the magnetization process of very thin films and magnetically coupled layers.

Transport properties via surface localized states of Ru, Rh and Pd thin films on Ag(111)

We investigate the transport properties of some 4d transition metal (i.e. Ru, Rh and Pd) thin films fabricated on Ag(111) based on first principles calculation. We calculate the conductance observed by double tipped scanning tunnelling microscope and find that the conductances through Ru and Rh thin films are majority spin polarized. We discuss these results and compare them with our previous study for Fe thin films.

Ultrafast third-order optical nonlinearity of noble and transition metal thin films

We present transient ellipsometry measurements made upon Au, Cu, Ag, Ni, Pd, Ti, Zr, and Hf thin films under identical experimental conditions. Using an elliptically polarized pump beam, we have simultaneously observed the specular inverse Faraday effect (SIFE) and specular optical Kerr effect (SOKE) contributions to the optical polarization response, in order to extract the real and imaginary parts of the non-vanishing components of the third-order optical susceptibility tensor. The signal magnitudes and the extracted tensor components show a systematic variation that reflects the underlying band structure of the different metals. A time delay observed between the SIFE and SOKE signals is interpreted in terms of lifetimes for the transient polarization that may be of comparable magnitude to the pulse width. This suggests that formulae derived in the continuous wave limit may not be applicable in the description of ultrafast nonlinearities in metallic samples. The implications of our results for different areas of applied optics are discussed.

Anti-diffusion barriers for gold-based metallization to GaN

AbstractWe propose a new metallization scheme to p-GaN, where thin-film transition metal nitrides have been applied to improve thermal stability of gold-based metallization. In this metallization scheme the Pd/Au bilayer was used to form low-resistivity ohmic contact to p-GaN, while Ta-Si-N, Ti-Si-N, and Ti-W-N anti-diffusion barriers were used to protect contact metallization from interaction with Au overlayer. We present the details of optimization of process parameters of barrier layer fabrication and show that Ta0.34Si0.25Ni0.41 and Ti0.26Si0.0.17Ni0.57 thin films fabricated by reactive magnetron sputtering show excellent barrier properties under high temperature stress.

Self-assembly of novel nanowires by thermolysis of fullerene and transition metal thinfilms

A wide range of nanomaterials has been grown by thermal treatment ofpatterned condensed-phase precursors. We present a systematic study of thethermolysis of fullerene, amorphous carbon and transition metal thin films,trying to bridge previously reported results in the high temperature regime(>900 °C) and reporting novel structures for low temperature(<550 °C) processing. The synthesis of crystals of single-walled carbon nanotubes fromhigh temperature annealing of patterned, multilayered fullerene and nickelprecursor films, could not be reproduced. A thicker fullerene layer in thepresence of nickel was, however, transformed into a web-like carbon network.Low temperature processing of similar precursor patterns on sulfur-containingmolybdenum grids resulted in the self-assembly of nickel sulfide nanowires and filledMoS2 nanotubes. Cobalt was found to form cobalt sulfide structures. The strongly oxidizingbehaviour of iron resulted in an abundance of needle-like molybdenum oxidecrystals. None of the structural formations could be seen for amorphous carbon as asubstitutional thin film precursor. Based on the ease of changing precursor materials, thissimple, scaleable method addresses many nanomaterials, giving new insight intogrowth mechanisms as well as offering synthesis control for future applications.

Using MeV ion backscattering/channeling and MC simulations to characterize the composition and structure of buried metal–metal interfaces

The technique of MeV ion backscattering and channeling has been used to study the interface structure and stoichiometry for thin transition metal films deposited on Al single crystal surfaces. Analysis of the backscattering data is based on the concepts of shadowing and the well-known theory of Rutherford scattering. In some cases we have used X-ray photoemission spectroscopy (XPS), low-energy (keV) He + ion scattering spectroscopy, and Monte Carlo computer simulations of interface evolution to obtain additional information about the structure of buried interfaces. We first describe the growth of epitaxial fcc Ti films on Al single crystal surfaces, and alloy formation at the epitaxial Ag–Al interface. We next describe measurements for alloy formation at the Ni–Al(1 1 0) interface and present Monte Carlo simulations of the interface evolution. Finally, we describe an approach to stabilize metal–metal epitaxial interfaces using an extremely thin metallic interlayer. Specifically, we show that a single ML of Ti metal deposited at the Fe–Al interface, a system well known for considerable intermixing at room temperature, prevents interdiffusion and enables the epitaxial growth of Fe films on the Al(1 0 0) surface. The resulting structure is observed to be stable for temperatures up to about 200 °C.

Investigation of the interfacial interdiffusion and magnetic properties of [formula omitted

We study the structural stability and magnetic properties for 3d transition-metal thin films on Cu(100) by a detailed ab initio calculation. The diffusion of Ni atoms into the bulk is energetically more favorable than the formation of cluster at the surface. For the 1–3 monolayer (ML) thick CuNi surface alloy on Cu, we demonstrate that the subsurface formation of 1-ML, 2-ML, or 3-ML thick Ni films below a Cu cap layer is energetically a more favorable structure. Our result of the energetics of surface alloying is in good agreement with the recent LEED experiment and other theoretical results.

Diffusion barrier properties of transition metal thin films grown by plasma-enhanced atomic-layer deposition

Ta thin films were grown on Si(001) and polycrystalline Si substrates by plasma-enhanced atomic-layer deposition (PE-ALD) using TaCl5 and atomic hydrogen as precursors. The grown films have resistivity of 150–180 μm cm with a small Cl concentration between 0.5 and 2 at. %. The diffusion barrier properties were investigated using bilayer structures consisting of 200 nm Cu deposited by sputtering on ALD Ta films with various thicknesses. Three in situ analysis techniques consisting of x-ray diffraction, elastic light scattering, and resistance analysis were used to determine the diffusion barrier failure temperature of Ta films. The barriers were annealed at a temperature ramp rate of 3 °C/s from 100 to 1000 °C in forming gas. For this method using x-ray diffraction, the barrier failure temperatures were determined by monitoring the disappearance of the Cu(111) x-ray diffraction peak and appearance of Cu silicide diffraction peaks. At the diffusion barrier failure temperature, elastic light scattering indicated a rapid increase in surface roughness, and concurrent increases in sheet resistance of the blanket Cu/ALD Ta films on polycrystalline Si were monitored using a four-point-probe technique. The same analyses were performed on bilayer structures of Cu/physical-vapor-deposition Ta films with comparable thicknesses. The ALD Ta barrier films show significantly higher failure temperatures compared to PVD Ta; at least 70 °C at comparable thicknesses, which is thought to be due to the uniform, smooth surface and amorphous or nanocrystalline microstructure of ALD Ta.

Temperature dependence of the magnetization of transition-metal thin films: Fe- monolayers and bilayers

The functional integral theory of bulk itinerant magnetism is extended to be applied to low-dimensional transition-metal systems. Using a real-space recursive expansion of the local Green’s function we calculate the magnetization curves and the Curie temperature TC for the bulk, monolayer, and bilayer of Fe. Disorder is treated by using the virtual crystal approximation. The local-environment dependence of TC is analyzed by comparing the results for bulk and the (001) and (110) oriented films of Fe. Finally, the applicability of the theory is discussed.

Influence of the inert and active ion bombardment on structure of the transition metal thin films

The results of the experimental research of the inert (He, Ne, Ar, Kr, Xe) and active (O, N) ion impact on the transition metal structure are presented. Thin high-purity (99.999 at.%) films of nickel, chrome and iron were used in the experiment. The bombardment was realized under room temperature at high vacuum ( P<1×10 −7 Pa) by a separated ion beam of 10–10 3 keV. As a main result of the experiment, the full absence of crystal matrix changes was ascertained for all the transition metals irradiated by inert gas ions. The chemical nature of the crystal structure changes observed in transition metals being under active ion bombardment was found out too.

Energetics of 3d thin film on Cu(0 0 1): CuNi surface alloy

We study the energetics and magnetism for 3d transition-metal (TM) thin film on Cu(0 0 1) by a detailed ab initio calculations. For 1-ML 3d TM on the Cu(0 0 1) substrate, we find the interaction between Ni and Cu atoms at the interface shows attraction, while the Fe–Cu interaction shows repulsion. At a coverage of 0.5 Ni ML, we find that an ordered ferromagnetic (2×2) surface alloy is not formed. For the 1–2-ML Ni films on Cu we demonstrate the energetically favorable structure in the case of several surface alloy including the coverage of 0.5 ML in (or below) the surface layer. Our ab initio results for the energetics of surface alloying are in agreement with LEED experiment.

Patterning ferromagnetism in Ni80Fe20 films via Ga+ ion irradiation

We demonstrate that focused Ga+ ion irradiation can comprehensively modify the ferromagnetic properties of Ni80Fe20 thin films. Magneto-optic Kerr effect measurements at room temperature and magnetoresistance measurements at temperatures between 1.5 and 270 K characterized the irradiation effects. Irradiation steadily reduced the films’ room temperature coercivity, and a dose of 1.0×1016 ions/cm2 at 30 keV was found sufficient to cause a loss of ferromagnetism at room temperature in films of thickness up to 15.5 nm. In situ end-point detection and postirradiation atomic force microscopy confirmed that the sputtering which accompanied doses up to 1.0×1016 ions/cm2 did not compromise the protective caps on these Ni80Fe20 films. We therefore conclude that the modification of ferromagnetic properties occurred primarily because of direct Ga+ ion implantation. From these results, we speculate that focused Ga+ ion irradiation could be a convenient tool for the nanoscale patterning of magnetic properties in 3d transition metal thin films.

Abnormal infrared effects and electrocatalytic properties of nanometer scale thin film of PtRu alloys for CO adsorption and oxidation

Nanometer scale thin films of PtRu alloy supported on glassy carbon (nm-PtRu/GC) were prepared using electrochemical codeposition under cyclic voltammetric conditions. The composition of the PtRu alloy was altered by varying the concentration of Pt4+ and Ru3+ ions in the deposition solution. STM results demonstrated that the nm-PtRu film was composed of crystallites appearing in a layered hexagonal form. The nm-PtRu/GC exhibited a high catalytic activity for CO oxidation, consisting mainly in a negative shift of potential for COad oxidation. In situ FTIR spectroscopic studies revealed that the nm-PtRu/GC alloy electrodes of different surface composition exhibit abnormal infrared effects (AIREs) as observed on electrodes of nanometer scale thin films of transition metals. The AIREs observed on nm-PtRu/GC electrodes consist in the inversion of COad bands, the significant enhancement of IR absorption and an increase in the FWHM. Following the increase in Ru component in the nm-PtRu thin film the FWHM was increased progressively from 20 cm−1 on a nm-Pt/GC to 55 cm−1 on a nm-Ru/GC electrode, and the enhancement factor of IR absorption by COL was between 10.5 to 13.1. The present study has provided new understanding of the structure and properties of nanometer scale thin film PtRu alloy material, and highlights its potential for fuel cell applications.

Study of the nonlinear magneto-optical Kerr rotations at the surfaces and interfaces in transition metal thin films

Based on the symmetric theory, the laws of Fresnel reflection and refraction and electronic theory at surface and buried interface, we have derived the dependence of NOMOKE angles Φ k, α (2) on the angle of incidence θ, the polarization angle ϕ, Φ and the photon energy ℏ ω in the magnetic films of bcc Fe, hcp Co and fcc Ni. The calculated results are given as follows: (i) comparing the values of Φ k, α (2) with the linear magneto-optical Kerr angles, we found that the values of Φ k, α (2) of three different transition metals have a great increase, especially, as θ→0°, Φ k, α (2)→90°; (ii) the values of Φ k, p (2) of three different transition metals all have an absolute maximum at about ℏ ω=1.5 eV and the values of Φ k, s (2) decrease with the increase of the photon energy on the whole in the range ℏ ω=0.64∼3.5 eV, but the curves of Φ k, α (2)∼ θ and ∼ℏ ω for three different transition metals are quite different; (iii) the values of Φ k, α (2) depend sensitively on the direction of magnetization at surfaces and interfaces of thin magnetic films.

Studies of Abnormal Infrared Effects of Nanometer Thin Films of Transition Metals and Alloys in Acid Solutions

Studies of Abnormal Infrared Effects of Nanometer Thin Films of Transition Metals and Alloys in Acid Solutions

Geometric shape dependence of coercivity for patterned magnetic thin films

The micro-strips with patterned magnetic domains using electron beam lithography have been made to study the geometric shape dependence of coercivity. The size of the micro-strip is 10 μm×30 μm with 0.5 μm periods hole arrays pattern. Arrays with different types of geometry, such as square-, circle-, and ellipse-shapes have been made. Amorphous rear-earth transition-metal (RE-TM) thin films with magnetic perpendicular anisotropy were deposited on the micro-strips. The extraordinary Hall effect has been employed to measure the hysteresis loop and the coercivity of the sample. We have found that for the same deposited material, the magnitude of coercivity for various shapes are dissimilar. For example, the coercivity of the ellipse-shaped hole arrays is much larger than the coercivity of the square-shaped hole arrays. In addition, we observed that the coercivity of the patterned sites was larger than the sites without patterns for RE-dominated compounds and the coercivity of the patterned sites was smaller than the sites without patterns for TM-dominated compounds. The magnetic moments canting between RE and TM subnetworks will be used to explain the observed phenomena.

The Thermoelectric Power in Transition Metal Thin Films

The electrical conductivity and thermoelectric power of some transition metal films were investigated under ultrahigh vacuum conditions. The experimental results are interpreted in the framework of the modified classical size-effect theories. It was supposed there were two independent groups of charge carriers in the transition metal thin film. The surface and grain-boundary scattering have influence on charge transport of each group separately.

Permanent Magnetism in Exchange-Coupled Nanocomposites

ABSTRACTThe magnetic properties of permanent magnetic nanocomposites are investigated both experimentally and by model calculations. Rare-earth transition-metal films and nanocomposite thin films containing a L10hard phase (PtFe) have been prepared by plasma sputtering and heat treatment. By choosing suitable heat-treatment processes a nearly ideal nanostructure and high energy products of up to about 50 MGOe have been achieved for the Fe-Pt system. The behavior of the nanocomposites is discussed in terms of a model applicable to nearly ideal magnets, where there are no pronounced shoulders in the hysteresis loops. Analytic expressions for M(H) are obtained for a number of cases, and it is discussed how the loops depend on the micromagnetic parameters of the involved phases. A novel aspect of magnetic nanostructures is that very small magnetic grains exhibit very large energy products, even if they are intrinsically soft-magnetic.

Permanent Magnetism In Exchange-Coupled Nanocomposites

ABSTRACTThe magnetic properties of permanent magnetic nanocomposites are investigated both experimentally and by model calculations. Rare-earth transition-metal films and nanocomposite thin films containing a L10hard phase (PtFe) have been prepared by plasma sputtering and heat treatment. By choosing suitable heat-treatment processes a nearly ideal nanostructure and high energy products of up to about 50 MGOe have been achieved for the Fe-Pt system. The behavior of the nanocomposites is discussed in terms of a model applicable to nearly ideal magnets, where there are no pronounced shoulders in the hysteresis loops. Analytic expressions for M(H) are obtained for a number of cases, and it is discussed how the loops depend on the micromagnetic parameters of the involved phases. A novel aspect of magnetic nanostructures is that very small magnetic grains exhibit very large energy products, even if they are intrinsically soft-magnetic.

Magnetization reversal processes in perpendicular anisotropy thin films observed with magnetic force microscopy

We have carried out studies of the magnetic reversal process of a rare earth–transition metal thin film with perpendicular magnetic anisotropy using a magnetic force microscope (MFM) capable of applying in situ magnetic fields. The magnetization of the microscopic area shown in MFM images was determined for a number of field values comprising a complete hysteresis loop. This microscopic hysteresis loop was found to be nearly identical to a bulk hysteresis loop. Changes in the magnetization of the film as the hysteresis loop was traversed can be linked to individual microscopic domain changes evident in the MFM images. These studies show that the magnetization of this film was characterized by a two-stage process – fast and slow rates of change of magnetization with applied field. A second experiment in which the film was incompletely saturated and brought back to zero field showed that domain nucleation was not responsible for the rate of the fast process, but rather all magnetization changes were primarily limited by the low domain wall mobility. These observations are linked to previous work on magnetization processes in similar magnetic systems.