Magnetization and compensation temperature of transition-metal–rare-earth multilayers in a model with long-range interactions

A model is proposed to describe the behavior of magnetization and compensation temperature in transition-metal–rare-earth multilayers. Long-range exponentially decreasing ferromagnetic (antiferromagnetic) interactions are considered between the same (different) kind of atoms. The magnetization and compensation temperature are shown to decrease with increasing single layer thickness. Good agreement is obtained with experimental data on Tb/Co system.     

X-ray and neutron studies of nanoscale atomic diffusion in thin films and multilayers

In the present work we review some X-ray and neutron based techniques capable of measuring diffusion lengths in thin films with an accuracy of a fraction of a nanometer. The techniques have been used for studying both self-diffusion of the constituent species in a thin film, as well as interdiffusion at the interfaces in multilayers. The high accuracy of the techniques in depth profiling of an element or a specific isotope makes very low diffusivities ∼10⁻²³ m²/s, amenable to measurements, and allows one to study the subtle effects of factors like internal stresses or structural relaxation on self-diffusion in compositionally homogeneous films. Depth selectivity of X-ray standing wave technique in multilayers makes it possible to distinguish between diffusion at the two types of the interfaces, namely A-on-B and B-on-A, in a single multilayer structure.     

Model of magnetization in thin magnetic films with one domain and two-domain basic structures

The magnetization processes in thin magnetic films are described by a model analysing the behaviour of one domain and two-domain basic structures (BS) in the applied magnetic field. These structures include the film areas with nearly constant crystal and magnetic parameters. The minimum of BS free energy including the energy in the internal magnetic field, the energy of the induced anisotropy and the domain-wall energy are taken into account. The initial and hysteresis curves of the sample depend on the function of distribution for the BSs are calculated. A good qualitative agreement with the results of other authors is observed if the films consists of one-domain or two-domain BS only. Our experimental data give also some support of the model.     

Magnetization in permalloy thin films

Thin films of permalloy (Ni₈₀Fe₂₀) were prepared using an Ar+N₂ mixture with magnetron sputtering technique at ambient temperature. The film prepared with only Ar gas shows reflections corresponding to the permalloy phase in X-ray diffraction (XRD) pattern. The addition of nitrogen during sputtering results in broadening of the peaks in XRD pattern, which finally leads to an amorphous phase. The M-H loop for the sample prepared with only Ar gas is matching well with the values obtained for the permalloy. For the samples prepared with increased nitrogen partial pressure the magnetic moment decreased rapidly and the values of coercivity increased. The polarized neutron reflectivity measurements (PNR) were performed in the sample prepared with only Ar gas and with nitrogen partial pressure of 5 and 10%. It was found that the spin-up and spin-down reflectivities show exactly similar reflectivity for the sample prepared with Ar gas alone, while PNR measurements on 5 and 10% sample show splitting in the spin-up and spin-down reflectivity.      

Magnetization reversal asymmetry in Fe/MgO(0 0 1) thin films

We study the in-plane magnetization process in 200 Å Fe(0 0 1) thin films grown by sputtering at normal incidence. In spite of this growth geometry, a small uniaxial in plane magnetic anisotropy, whose origin is not totally understood, is found superimposed to the expected cubic biaxial one. This has a dramatic effect both on the reversal process and the domain structure. A combined longitudinal and transversal Kerr study shows the different switching processes (180° walls along the main easy axis versus 90° along the secondary easy axis) depending on the relative orientation of the magnetic field with respect to the Fe crystallographic axes. Remarkably, this two- and sometimes three-step switching process appears only when the field is applied along certain crystallographic directions. These findings are corroborated by domain observations.     

Heat-induced effective exchange coupling in magnetic multilayers with semiconductors

Two ferromagnetic films separated by an amorphous semiconducting spacer layer are exchange coupled across the spacer. The coupling is reversibly temperature dependent with a positive temperature coefficient making such layered systems a 2-D realization of the concept of heat-induced magnetism. By studying ferromagentic Fe layers separated by amorphous Si, Ge, or ZnSe layers we explore the possibilities to generate such an effective exchange coupling and address the question of the mechanism responsible for it.     

Study on the Gilbert damping of polycrystalline YIG films with different capping layers

This paper describes the effect of 5-nm thick platinum (Pt), aluminum (Al) and silicon oxide (SiO_x) capping layers on the static and dynamic magnetic properties of 400-nm thick polycrystalline YIG films deposited on a Pt buffer layer. Both static and dynamic magnetic properties of Pt capped YIG film are totally different among all YIG films. Namely, the squareness of the magnetization curve for Pt capped YIG film increases, indicating that Pt capped YIG film is magnetically softer than other YIG films. Interestingly, the effective Gilbert damping parameter of Pt capped YIG films is about four times as large as those of other YIG films, and its value is approximately 9.52 × 10⁻⁴. However, the value of Gilbert damping is 2.55 × 10⁻⁴, 3.46 × 10⁻⁴ and 3.85 × 10⁻⁴ respectively for no capping, SiOx capping and Al capping samples respectively. This huge change in Gilbert damping parameter is mainly originating from the spin pumping effect, which arises at the interface of a material having strong spin orbit interaction such as Pt. Moreover, the enourmous increase in the value of effective anisotropic field and decrese in effective saturation magnetization indicates interface anisotropy is induced in Pt capped sample. These results suggest that the static and dynamic magnetic properties of YIG film can be controlled by selecting an appropriate capping layer.     

Structure and magnetic properties of wedged Co/Ti multilayers

Co/Ti multilayers with wedge-shaped Co or Ti sublayers were prepared using UHV (5×10⁻¹⁰ mbar) DC/RF magnetron sputtering. The planar growth of the Co and Ti layers was confirmed in situ by X-ray photoelectron spectroscopy. Cobalt sublayers grow on sufficiently thick titanium sublayers in the soft magnetic nanocrystalline phase up to a critical thickness d_crit3.0 nm. For a thickness greater than d_crit, the Co sublayers undergo a structural transition to the polycrystalline phase with much higher coercivity. Furthermore, for the Co/Ti multilayers with nanocrystalline Co sublayers with d_Co=2.7 nm we have observed a significant drop of the coercivity — typically from H_c3.5 kA/m to H_c0.2 kA/m — for Ti thickness d_Ti0.35 nm. The above effect could be explained by the existence of a minimum Ti sublayer thickness (d_min0.35 nm), which is required for the nanocrystalline growth of Co, and/or the formation of quasi-continuous non-magnetic layers for d_Tid_min giving rise to a decrease of the exchange energy between Co sublayers. Magnetic domains and walls studies revealed the structural transitions of the Co sublayers.     

Ferromagnetism in transition-metal-doped semiconducting oxide thin films

A rather complete work on transition-metal (TM)-doped TiO₂ thin films has been done and room ferromagnetism (FM) is found in the whole series of Sc/V/Cr/Mn/Fe/Co/Ni-doped TiO₂ films. Not only is it remarkable that for the first time, FM at high temperature was achieved in TM-doped TiO₂, but also a very big magnetic moment of 4.2μ_B/atom could be obtained, and direct evidences of real ferromagnets with big domains were shown as well. A similar chemical trend was achieved in TM-doped In₂O₃ films, however, the observed magnetic moment is rather modest, with the maximal value is of only 0.7μ_B/atom for Ni-doped In₂O₃ films. As regards TM-doped SnO₂ films, observed magnetic moments could be very large, with the maximum saturation of 6μ_B per impurity atom for Cr-doped SnO₂ thin films, but it could be influenced very much depending on substrate types. On the other hand, results on TM-doped ZnO films interestingly have revealed that in these systems, the magnetism more likely resulted from defects and/or oxygen vacancies.     

Electronic transport mechanisms in tetraphenyleprophyrin thin films

Thermally-evaporated thin films of tetraphenylporphyrin, TPP, with thickness range from (175 to 735) nm had been prepared. Annealing temperatures ranging from (273–473) K do not influence the amorphous structure of these films. The influence of environmental conditions: film thickness, temperature and frequency on the electrical properties of TPP thin films had been reported. It was found that dc conductivity increases with increasing temperature and film thickness. The extrinsic conduction mechanism is operating in temperature range of (293–380) K with activation energy of 0.13 eV. The intrinsic one is in temperatures >380 K via phonon assisted hopping of small polaron with activation energy of 0.855 eV. The ac electrical conductivity and dielectric relaxation in the temperature range (293–473) K and in frequency range (0.1–100) kHz had been also studied. It had been shown that theoretical curves generated from correlated barrier hopping (CBH) model gives the best fitting with experimental results. Analysis of these results proved that conduction occurs at low temperatures (300–370) K by phonon assisted hopping between localized states and it is performed by single polaron hopping process at higher temperatures. The temperature and frequency dependence of both the real and imaginary parts of dielectric constant had been reported.     

Metal/insulator manganite multilayers

We have studied the transport and magnetic properties of strongly textured metal/insulator La_1−xSr_xMnO₃ (x=0.4,0.1) bilayers and trilayers, grown by DC magnetron sputtering over MgO and SrTiO₃ substrates. The multilayers present transport properties similar to those of the La_0.6Sr_0.4MnO₃ films, being very sensitive to deposition conditions. Magnetic multilayers show a metal–insulator transition around T_c (250 K) and colossal magnetoresistance which is maximum around T_c. No extrinsic magnetoresistance associated with the multilayered structure was observed, probably due to the presence of ferromagnetic coupling between the metallic layers, as suggested by magnetization measurements.     

Characteristics of ZrC/ZrN and ZrC/TiN multilayers grown by pulsed laser deposition

ZrC/ZrN and ZrC/TiN multilayers were grown on (1 0 0) Si substrates at 300 °C by the pulsed laser deposition (PLD) technique using a KrF excimer laser. X-ray diffraction investigations showed that films were crystalline, the strain and grain size depending on the nature and pressure of the gas used during deposition. The elemental composition, analyzed by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), showed that films contained a low level of oxygen contamination. Simulations of the X-ray reflectivity (XRR) curves acquired from films indicated a smooth surface morphology, with roughness below 1 nm (rms) and densities very close to bulk values.Nanoindentation results showed that the ZrC/ZrN and ZrC/TiN multilayer samples exhibited hardness values between 30 and 33 GPa, slightly higher than the values of 28-30 GPa measured for pure ZrC, TiN and ZrN films.     

Rapid initiation of reactions in Al/Ni multilayers with nanoscale layering

Research into nanoenergetic materials is enabling new capabilities for controlling exothermic reaction rates and energy output, as well as new methods for integrating these materials with conventional electronics fabrication techniques. Many reactions produce primarily heat, and in some cases it is desirable to increase the rate of heat release beyond what is typically observed. Here we investigate the Al-Ni intermetallic reaction, which normally propagates across films or foils at rates lower than 10 m/s. However, models and experiments indicate that local heating rates can be very high (10⁷ K/s), and uniform heating of such a multilayer film can lead to a rapid, thermally explosive type of reaction. With the hopes of using a device to transduce electrical energy to kinetic energy of a flyer plate in the timescale of 100's of nanoseconds, we have incorporated a Ni/Al nanolayer film that locally heats upon application of a large electrical current. We observed flyer plate velocities in the 2-6 km/s range, corresponding to 4-36 kJ/g in terms of specific kinetic energy. Several samples containing Ni/Al films with different bilayer thicknesses were tested, and many produced additional kinetic energy in the 1.1-2.3 kJ/g range, as would be expected from the Ni-Al intermetallic reaction. These results provide evidence that nanoscale Ni/Al layers reacted in the timescale necessary to contribute to device output.     

Morphology of alkali halide thin films

Two alkali halide adsorbate-substrate systems were investigated by atomic force microscopy (AFM) working in contact mode. Adsorbate film orientation relative to the substrate was determined from the arrangement of the atomic steps of the substrate and the edges of the forming islands. In this work we present experimental results obtained for systems: NaCl/LiF(0 0 1) and LiF/NaCl(0 0 1), which exhibit a strong tendency of the self-assembly into regular structures.     

Magnetization reversal in (0 0 1)Fe thin films studied by combining domain images and MOKE hysteresis loops

The magnetization reversal of epitaxial single-crystal Fe films has been studied by combining domain images and hysteresis loops. The reversal is quantitatively described by combining the coherent rotation model and the domain wall displacement model. The pinning energy exerted on the domain walls and the domain wall angle at the switching fields are obtained by fitting this model to experimental hysteresis loops. The field-dependent pinning energy and the domain wall angle in the reversal process, and the contributions of second-order magneto-optic effect to hysteresis loops, are revealed to be two important features of single-crystal Fe films.     

Brownian motion of massive skyrmions in magnetic thin films

We report on the thermal effects on the motion of current-driven massive magnetic skyrmions. The reduced equation for the motion of skyrmion has the form of a stochastic generalized Thiele’s equation. We propose an ansatz for the magnetization texture of a non-rigid single skyrmion that depends linearly with the velocity. By using this ansatz it is found that the skyrmion mass tensor is closely related to intrinsic skyrmion parameters, such as Gilbert damping, skyrmion-charge and dissipative force. We have found an exact expression for the average drift velocity as well as the mean-square velocity of the skyrmion. The longitudinal and transverse mobility of skyrmions for small spin-velocity of electrons is also determined and found to be independent of the skyrmion mass.     

Preparation of CuInS2 thin films by metal-organic decomposition

Metal-organic decomposition (MOD) technique has been developed as a low cost thin film CuInS₂ preparation method for solar cell application. XRD and Raman spectra measurement revealed that deposited films contain CuInS₂. Stoichiometric films with a bandgap of 1.53 eV and an FWHM of 0.45° were obtained from a solution with Cu/In=1.5.     

Instability and dynamics of thin viscoelastic liquid films

The instability, rupture, and subsequent growth of holes in a thin Jeffreys-type viscoelastic film under the influence of long-range van der Waals force are investigated using both linear stability analysis and nonlinear numerical solutions. The linear stability analysis of full governing equations valid for arbitrary wave numbers shows that although fluid rheology does not influence the dominant length scale of the instability, it significantly affects the growth rate. It is shown that neglect of inertia and solvent dynamics results in a nonphysical singularity in the growth rate beyond a critical value of relaxation time. We further carry out numerical simulations of a set of long-wave, nonlinear differential equations (also derived in Rauscher et al., Eur. Phys. J. E 17, 373 (2005)) governing the evolution of the free surface. The nonlinear simulations, in their domain of validity, confirm the results of the linear analysis. Interestingly, results from nonlinear simulations further show that both for Newtonian and viscoelastic liquids, the shape and the dewetting dynamics of a hole are identical when examined in terms of a rescaled time which depends on rheological parameters. Thus, viscoelasticity of Jeffreys type merely accelerates the growth rate, without however affecting the important morphological characteristics.     

Multiscale modeling of nanoindentation in copper thin films via the concurrent coupling of the meshless Hermite-Cloud method with molecular dynamics

This paper investigates the 2D nanoindentation of a copper thin film using a concurrent multiscale method. The method uses molecular dynamics (MD) simulation in the atomistic region, the strong-form meshless Hermite-Cloud method in the continuum region and a handshaking algorithm to concurrently couple them. A fully atomistic simulation is also carried out to validate the multiscale method. The results, namely the load versus indentation depth graph obtained from the multiscale method shows only slight quantitative variation from that of the full atomistic model. More importantly, the graphs from both simulations show a similar trend thus validating the 2D multiscale method. The displacement profile without discontinuities further supports the efficiency of the multiscale method in ensuring smooth exchange of information between the atomistic and continuum domains. The material properties extracted from the simulation include the force/unit length values obtained by dividing the maximum load on the indenter by its contact perimeter, instead of the hardness value obtained in 3D simulations. By restricting the atomic scale detail to the critical regions beneath the indenter, the multiscale method effectively saves computational resources to more than one order (close to 13 times less for this problem), thus making it feasible to simulate problems of larger dimensions that are not amenable to complete atomistic simulations.     

Investigations on the mechanism of carrier transport in plasma polymerized pyrrole thin films

Plasma polymerization is found to be an excellent technique for the preparation of good quality, pinhole-free, polymer thin films from different monomer precursors. The present work describes the preparation and characterization of polypyrrole (PPy) thin films by ac plasma polymerization technique in their pristine and in situ iodine doped forms. The electrical conductivity studies of the aluminium-polymer-aluminium (Al-polymer-Al) structures have been carried out and a space charge limited conduction (SCLC) mechanism is identified as the most probable mechanism of carrier transport in these polymer films. The electrical conductivity shows an enhanced value in the iodine doped sample. The reduction of optical band gap by iodine doping is correlated with the observed conductivity results.