Granular Thin Films Research Articles

Influence of the external magnetic field on structural characteristics of granular Co-Cu thin film alloys

We present the results of experimental studies of the influence of an external magnetic field on the structural characteristics (surface roughness and structural entropy) of nanogranular thin-film system based on Co and Cu. The samples CoxCu100-x in a wide range of compositions (17 at. % ≤ x ≤ 69 at. %) with the thickness of d = 35 nm were obtained by electron-beam co-evaporation using two independent electron guns. After obtaining, the films were annealed in a vacuum at a temperature of 800 K for 30 min. The studies of magnetoresistive properties have shown that the maximum giant magnetoresistance (GMR) amplitude (1.7 % in the transverse and 1.6 % in the longitudinal measurement geometries) in a magnetic field of H = 4.5 kOe at room temperature was observed in the Co21Cu79 film alloy. Transmission electron microscopy showed that this sample contains hcp-Co granules with a size of L = 5 ÷ 12 nm in a matrix of metastable fcc-Cu(Co) solid solution. The influence of the external magnetic field on the structural characteristics and surface morphology of the Co21Cu79 thin-film alloy was investigated using atomic force microscopy (AFM). It was found that a first impact of application of external magnetic field with H = 0.5 kOe causes a decrease in the values of the arithmetic mean Ra, and quadratic mean Rq, of the film roughness and structural entropy S. A decreases in Ra by 8.5 %, Rq by 6.5 %, and structural entropy S by 6.5 % were observed. After application of a larger magnetic field with H = 1.0 kOe and during the subsequent relaxation of the structure within 15 h after the field is turned off, the values of the structural parameters of the film surface did not change significantly.

Stratification and film ripping induced by structural forces in granular micellar thin films

HypothesisInteractions across incredibly thin layers of fluids, known as thin films, underpin many important processes involving colloids, such as wetting-dewetting phenomena. Often in these systems, thin films are composed of complex fluids that contain dispersed components, such as spherical micelles, giving rise to oscillatory structural forces due to preferential layering under confinement. Modelling of thin film dynamics involving Derjaguin-Landau-Verwey-Overbeek (DLVO) type forces has been widely reported using the Stokes-Reynolds-Young-Laplace (SRYL) model, and we hypothesize that this theory can be extended to a concentrated micellar system by including an oscillatory structural force term in the disjoining pressure. ExperimentsWe study the drainage behaviour of thin films comprising sodium dodecyl sulfate (SDS) micelles across a range of concentrations and interaction conditions between an air bubble and a mica disk using a custom-built dual-wave interferometry apparatus. FindingsEarly-stage film behaviour is dominated by hydrodynamics, which can be well reproduced by the SRYL model. However, experimental profiles drain significantly faster than predicted, transitioning into a structural force dominated phase characterised by four types of film ripping instabilities that we term ‘waving’, ‘ridging’, ‘webbing’, and ‘hole-sheeting’. These instabilities were mapped according to SDS concentration and approach velocity, providing insight into the interplay between structural forces and hydrodynamic conditions.

Percolation and the Phase Slip in Granular SmFeAsO1-xFx Thin Films

A set of granular SmFeAsO1-xFx films were synthesized on LaAlO3 substrates with different percentage of the coverage of the substrate surface, and then their electrical properties were studied. At high temperatures, as the substrate coverage is increased, the films go from an insulating to a metallic behavior; and then, at low temperatures, the films that present a resistance RN of few kOhms in the normal state show a transition to the superconducting state with a width in the R(T) curves that changes with the coverage of the substrate surface. The line shape of these curves can be explained by the Ambegaokar-Halperin (AH) model of the resistance produced by the thermal activated phase slippage at the intergranular junctions if the dimensionality for the films is presumed appropriately. For the thicker films, that can be considered as a 3D network of the Josephson junctions, a temperature dependence of the AH parameter gamma given by simA(1 − t)3/2, fits the R(T) curve satisfactorily. However, for the films that cover the surface of the substrates only partially, the AH model fits the R(T) curves only if gamma ~ A´(1 − t2)2, which is suitable for a 2D network of the Josephson junctions.

Anisotropic magnetic and magnetotransport properties in morphologically distinct Nd0.6Sr0.4MnO3 thin films

We investigate the magnetic and magnetotransport properties of nanostructured Nd0.6Sr0.4MnO3 (NSMO) thin films grown on (100) oriented SrTiO3 (STO) substrates. The thin films of 100 nm thickness fabricated using the pulsed laser deposition technique possess two distinct surface morphologies—granular and nano-rod type. The morphological change present in the system significantly affects the magnetic and magnetotransport properties of the thin films. Magnetization measurements revealed that the films with rod-type morphology exhibit improved in-plane magnetic anisotropy. The colossal magnetoresistance (∆R/R(H = 0)) of the granular sample is ∼91 %, and the rod morphology sample is ∼97 % at 3 T magnetic field. Additionally, magnetotransport studies revealed that the granular thin films display a characteristic butterfly-shaped low-field magneto-resistive (LFMR) behavior with the value of LFMR of up to ∼10 %. Furthermore, it is observed that the thin film’s morphology has a significant effect on the anisotropic magnetoresistance ratio (AMR). Thin films with rod-type morphology show an enhanced AMR of ∼30 % around its metal-insulator transition temperature. Such morphology-dependent tunability in magnetoresistance properties over a wide temperature range is potentially interesting for developing oxide-based sensors and devices.

Microstructure evolution in FePt-Cr2O3 granular thin films

Increase in the FePt grain density is necessary to achieve an ultra-high recording density of the heat-assisted magnetic recording. Herein, the highest grain density in FePt-Cr2O3 granular thin films was obtained by utilizing a densified FePt nucleation layer. The reduced magnetic properties were attributed to the grain shape and effect of elemental diffusion at the FePt/Cr2O3 interface. The equilibrium shape of the FePt grains indicated that the solid–liquid interfacial energy in the FePt granular system is a key factor in controlling the microstructures.

Study of epsilon-near-zero response in Al substituted titanium oxynitride thin films using computational and experimental investigations

Titanium oxide (TiO2) is considered as an important semiconductor material due to having promising tuneable optoelectronic, thermoelectric, and photovoltaic applications. First-principles calculations based on the density functional theory were used to elucidate the effect of dopant on various properties. Thin films have been fabricated using a reactive magnetron sputtering. X-ray diffraction analysis displayed the formation of the rutile phase of Titania in thin films. Atomic force and scanning electron microscopy revealed the growth of uniform, smooth, and well distributed granular thin films. Photoluminescence intensity was observed to decrease with an increase in dopant concentration. Experimentally obtained energy bandgap of the un-doped and doped TiO2 thin film samples was found to decrease and observed as 2.90 eV for pure Titania and 0.85 eV for maximum dopant contents. The present study exhibited that prepared oxynitrides thin films with epsilon near zero response are suitable candidate for enhanced optoelectronic and thermoelectric applications.

Role of oxygen vacancies in the structural phase transformations of granular TiO2 thin films

Tailoring the structural phases of titanium dioxide (TiO2) is nowadays highly attracting for diverse applications, ranging from optical coatings, where rutile outperforms anatase, to photocatalysis, where a mix of the two is preferred. In this framework, the very high temperature transformation of anatase to rutile constitutes a big drawback. Here, we investigate the structural transformations of granular TiO2 thin films subjected to thermal treatments in different gaseous environments, such as air, oxygen, and vacuum, with the latter expected to enhance the formation of oxygen vacancies. To this aim, we used a combined approach of X-ray diffraction (XRD), Raman Spectroscopy (RS), Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FESEM), and we demonstrate that the achievement of a crystallization temperature as low as 150 °C for the anatase and 250 °C for the rutile is possible. By using a Gibbs free energy minimization approach, the combination of granular morphology and oxygen vacancies is proposed to tune both the amorphous to anatase transition and the anatase to rutile transformation. Finally, we explore the possibility of a low-dimensional growth of anatase-TiO2 by studying phonon confinement-like effect and crystallization kinetics thanks to time-dependent RS experiments.

Structural, electronic and optical study of Zr:CeO2 thin films by computational and experimental approach

Transition metal oxide shows some exceptional optical and electronic properties explore in recent investigations. In this context, the current study presents first principal and experimental investigations of pure and Zr doped CeO2 thin films. The simulations were performed using density functional theory based Wien2k-code with PBE-GGA approximation. The uniform and well-distributed granular thin films were grown for experimental investigations. The simulations and experimental results show a good resemblance in outcomes. The density of states predicts the p-d hybridization between Ce and O atoms while band structure reduce with Zr incorporation in CeO2 structure. Crystallographic analysis reveals the presence of crystalline cubic phase with space-group 225-F-m3m in thin films. Experimentally observed bandgap follows the reducing trend with Zr incorporation as predicted by simulations data and found as 2.38 eV for pure CeO2 and 1.51 eV for 6.25% Zr doping content. Optical parameters were recorded as a function of photon energy which strongly influenced by Zr concentration. The sharp increase in optical absorption and real part of dielectric constant in Zr containing composition makes these materials more efficient and favorable for photovoltaic and optoelectronic applications.

Characterization of Electrodeposited Cobalt/Copper (Co/Cu) Granular Multilayered Thin Films onto the Anodic Aluminum Oxide (AAO) Substrates

Herein, we report on the characterization of a series of cobalt/copper granular thin films alloys electrodeposited onto the previously pre-treated aluminum substrates, at different deposition time. And a systematic study of their structural and magneto-optical properties were investigated. The proper deposition potentials of Co, and Cu were obtained from the cyclic voltammetry method. It was observed that the easy axis direction of magnetization was parallel to the film plane for all films. The X-ray diffraction (XRD) results revealed that all films had a mixture of hexagonal close-packed (hcp) and face centered cubic (fcc) phases. The Scanning electron microscope images (SEM) and atomic force microscopy (AFM) showed some parallelepiped and ellipsoid shaped rods with an averaged dimension about of 600 nm length, 600 nm width and 200 nm thickness for a sample electrodeposited at room temperature. These rods are irregularly arranged. Their irregular arrangement could explain the large d-spacing in the sample at this temperature. The magnetic analysis of the granular thin films alloys showed the saturation magnetization increased and the coercivity decreased as the deposition time increased. These results showed that the changes in magnetic and structural properties of granular cobalt/copper alloys deposits were substantially depending on the variation of deposition time. Therefore, the change of the deposition time was seen to control the properties of the alloys deposits and hence their properties could be modified for desired purpose.

A comparative computational and experimental study of Al–ZrO2 thin films for optoelectronic applications

Modern optoelectronic devices are highly dependent upon the electronic and optical characteristics of semiconducting materials. In this work, pure and Al-doped ZrO2 compositions were studied comparatively studied using simulations and experimental investigations. The uniform and well-distributed granular thin films were experimentally deposited. X-ray diffraction analysis reveals the tetragonal crystal structure with the space group 137 P4_2/nmc. The density of states predicts the p-d hybridization in ZrO2 and presents overlapping of Al-states at the Fermi level in Al-containing compositions. Band structure was deduced using generalized gradient approximations that were found to reduce with the increment of Al content in the structure. The absorption coefficient increases with an increase in Al content in the low energy region and redshift is noticed attributed to the inter-band absorption in semiconductors. The experimentally calculated energy band gap value decreases from 2.76 eV for ZrO2 to 1.80 eV for the maximum content of Al composition.

Structural, optical, and electrical properties of e-beam deposited metamaterials of granular CdSe thin films on glass substrates with a thin buffer layer of HfO2 dielectric

Structural, optical, and electrical properties of granular thin films of CdSe deposited on HfO2 dielectric layer are reported. The thin films were deposited on fluorine-doped tin oxide (FTO) coated glass substrates by e-beam evaporation technique. Structural analysis revealed the amorphous nature of the HfO2, whereas wurtzite type hexagonal crystal structure was observed in the CdSe thin films. Microstructural analysis showed the granular nature of both the deposited layers. The optical and electrical properties of the thin films were studied in detail. The dielectric properties of the deposited layers were studied in correlation with their microstructure. The appearance of negative permittivity (NP) in the films indicates towards metamaterial type characteristics which has various potential applications viz. high permittivity capacitors, microwave filters, coil free resonators, and thin film transistors (TFTs).

Superconducting boron doped nanocrystalline diamond microwave coplanar resonator

A coplanar waveguide resonator (CPR) is presented for kinetic inductance (Lk) and penetration depth (λL) measurements of superconducting boron doped nanocrystalline diamond (B-NCD) at microwave frequencies of 0.4 to 1.2 GHz and at temperatures below 3 K. Using finite element modelling and experimental measurements, this work demonstrates that thin granular B-NCD films (thickness d≈ 500nm) on Si have a large penetration depth (λL≈3.8μm), and therefore an associated high kinetic inductance per square (Lk,□≈ 670 to 690 pH/□). These values are much larger than those typically obtained for films on single crystal diamond, which is likely due to the high granularity of the nanocrystalline films. Based on the measured Q factors of the structure, the calculated surface resistance is found to be around ≈ 1 to 6μΩ at T<2 K in the 0.4 to 1.2 GHz range, demonstrating the potential for granular B-NCD for high quality factor superconducting microwave resonators and highly sensitive kinetic inductance detectors.

Phonon heat capacity and self-heating normal domains in NbTiN nanostrips

Self-heating normal domains in thin superconducting NbTiN nanostrips with the granular structure were characterized via steady-state hysteretic current–voltage characteristics measured at different substrate temperatures. The temperature dependence and the magnitude of the current, which sustains a domain in equilibrium at different voltages, can only be explained with a phonon heat capacity noticeably less than expected for 3D Debye phonons. This reduced heat capacity coincides with the value obtained earlier from magnetoconductance and photoresponse studies of the same films. The rate of heat flow from electrons at a temperature to phonons in the substrate at a temperature is proportional to with the exponent p ≈ 3, which differs from the exponents for heat flows mediated by the electron–phonon interaction or by escaping of 3D Debye phonons via the film/substrate interface. We attribute both findings to the effect of grains on the phonon spectrum of thin NbTiN films. Our findings are significant for understanding the thermal transport in superconducting devices exploiting thin granular films.

Models of advanced recording systems: A multi-timescale micromagnetic code for granular thin film magnetic recording systems

Micromagnetic modelling provides the ability to simulate large magnetic systems reliably without the computational cost limitation imposed by atomistic modelling. Through micromagnetic modelling it is possible to simulate systems consisting of thousands of grains over a time range of nanoseconds to years, depending upon the solver used. Here we present the creation and release of an open-source multi-timescale micromagnetic code combining three key solvers: Landau-Lifshitz-Gilbert; Landau-Lifshitz-Bloch; Kinetic Monte Carlo. This code, called MARS (Models of Advanced Recording Systems), is capable of accurately simulating the magnetisation dynamics in large and structurally complex single- and multi-layered granular systems as is shown by comparison to established atomistic simulation results. The short timescale simulations are achieved for systems far from and close to the Curie point via the implemented Landau-Lifshitz-Gilbert and Landau-Lifshitz-Bloch solvers respectively. This enables read/write simulations for general perpendicular magnetic recording and also state of the art heat assisted magnetic recording (HAMR). The long timescale behaviour is simulated via the Kinetic Monte Carlo solver, enabling investigations into signal-to-noise ratio and data longevity. The combination of these solvers opens up the possibility of multi-timescale simulations within a single software package. For example the entire HAMR process from initial data writing and data read back to long term data storage is possible via a single simulation using MARS. The use of atomistic parameterisation for the material input of MARS enables highly accurate material descriptions which provide a bridge between atomistic simulation and real world experimentation. Thus MARS is capable of performing simulations for all aspects of recording media research and development. This ranges from material characterisation and optimisation to system design and implementation. The object orientated nature of MARS is structured to facilitate quick and simple development and easy implementation of user defined custom simulation types which can utilise either timescale or a combination of both timescales. Program summaryProgram title: MARSCPC Library link to program files:https://doi.org/10.17632/8mx7cndcdx.1Developer's repository link:https://bitbucket.org/EwanRannala/mars/Code Ocean capsule:https://codeocean.com/capsule/2549929Licensing provisions: MITProgramming language: C++Supplementary material: MARS testing methodology (PDF), HAMR simulation example video.Nature of problem: A combined model that enables the complete modelling of magnetic recording processes at elevated temperatures covering all time scales from writing (nanoseconds) up to long term data storage (years). The model must also accurately describe the granular nature of the recording media as grain sizes are reduced to a few nanometres.Solution method: Short timescale behaviours are captured via the Landau-Lifshitz-Gilbert and Landau-Lifshitz-Bloch solvers for low and high temperature systems respectively. The long time scale behaviours are captured via a kinetic Monte Carlo solver. To enable complex models which account for mixed timescale behaviours the solvers are implemented as a single class structure which allows for dynamic solver selection. The granular structure is generated via a Laguerre-Voronoi tessellation with a custom implemented packing algorithm to produce highly realistic grain size distributions. Complex thermal dependencies of materials can be incorporated via atomistic parameterisation forming a multi-timescale model of the material.

Microscopic quantum point contact formation as the electromigration mechanism in granular superconductor nanowires

Granular aluminium is a high kinetic inductance thin film superconductor which, when formed into nanowires can undergo an intrinsic electromigration process. We use a combination of experimental and computational approaches to investigate the role of grain morphology and distribution in granular aluminium thin films, when formed into nanowire constrictions. Treating the granular aluminium film as a network of randomly distributed resistors with parameters motivated by the film microstructure allows us to model the electrical characteristics of the nanowires. This model provides estimates of the dependence of sheet resistance on grain size and distribution, and the resulting device to device variation for superconducting nanowires. By fabricating a series of different length nanowires, we study the electromigration process as a function of applied current, and then compare directly to the results of our computational model. In doing so we show that the electromigration is driven by the formation of quantum point contacts between metallic aluminium grains.

Nanoarchitectonics for granular systems: in the case of disordered Mo–SiO x thin films

Granular systems composed of metallic granules embedded as artificial atoms in the insulating matrix, have been extensively studied over the last decade due to their importance for nanotechnological applications and fundamental research on disordered materials. However, fabrication of uniform granular systems with tunable functionalities is still challenging. Here, from a nanoarchitectonic perspective, we proposed a general fabrication approach which exploits the different oxygen affinity between involving chemical elements to realize granular systems. Such a routine was demonstrated in the prototypical Mo–SiO x granular systems when the Mo–Si alloy target was sputtered at room temperature under oxygen-poor conditions. This growth approach produces highly disordered Mo–SiO x granular thin films, which exhibit the tunable electronic behavior, and huge photo-response (I L/I D up to 107 at 100 K), over 100% external quantum efficiency (in the wavelength range of 500–750 nm) and a short response time (∼3 ms). Our work provides a new design principle for fabricating granular systems with tunable functionalities, which lays the foundation for understanding novel physical phenomena and rational design of multi-functional devices.

Understanding the growth of high-aspect-ratio grains in granular L1-FePt thin-film magnetic media

A systematic investigation has been performed to optimize the microstructure of L10-FePt–SiOx granular thin films as recording media for heat-assisted magnetic recording. The FePt–boron nitride (BN) nucleation layer, which is stable even at 700 °C, is used to control the grain sizes and microstructure during the high-temperature processing. The study finds that films of high-aspect-ratio FePt grains with well-formed silicon oxide (SiOx) grain boundaries require the grading of the deposition temperature during film growth as well as the grading of the silicon oxide concentration. Well-isolated columnar grains of L10-FePt with an average height greater than 11 nm and diameters less than 7 nm have been achieved. Transmission electron microscopy analysis of the microstructures of samples produced under a variety of non-optimal conditions is presented to show how the microstructure of the films depends on each of the sputtering parameters.

Dependence of magnetic properties in superparamagnetic Co-granular thin films on deposition rate and effectiveness of a new pulsed DC reactive sputtering method for film fabrication with high deposition rate.

The superparamagnetic thin films with a granular structure show a characteristic magnetization curve. In this study, Co-Al2O3 granular thin films were fabricated by the sputtering method with normal DC source for Co deposition, RF source for Al2O3 deposition, and pulsed DC reactive sputtering were used for fabrication of non-magnetic matrix of oxide or nitride of Al or Si. As a result, Co-Al2O3 superparamagnetic thin films with high magnetization could be obtained with high deposition rate. Very high deposition rate was obtained for fabrication of oxide or nitride films of Al or Si using the pulsed DC reactive sputtering method. This deposition rate was 5 times larger than RF sputtering method. It is expected that a sputtering method with DC source for Co deposition and pulsed DC source for matrix deposition will be effective to realize very high magnetization in the superparamagnetic granular thin films.

Transmission electron microscopy image based micromagnetic simulations for optimizing nanostructure of FePt-X heat-assisted magnetic recording media

The areal density of the heat-assisted magnetic recording (HAMR) technology has a potential to reach 4 Tb/in2. However, the bimodal grain size distribution and structural defects that are unavoidable in actual FePt-X nanogranular media must be minimized for further reduction of a jitter noise. Here we report transmission electron microscopy (TEM) image based micromagnetic simulations of the FePt-X nanogranular media in order to elucidate the role of structural defects such as [200] misalignment and {111} twins on the magnetic properties. Micromagnetic approximation of experimental out-of-plane hysteresis loops allows to evaluate micromagnetic parameters and volume fractions of the defects in MgO(6 nm)/FePt-BN(1 nm)/FePt-(BN,C,SiO2)(7 nm) granular thin films. Synchrotron XRD of the films validated that the structural defects can be accurately evaluated by the proposed approach. The developed TEM image based micromagnetic simulations can directly link nanostructure of the FePt-X media with magnetic properties, boosting not only the optimization of the HAMR media but also a design of magnetic nanomaterials in general.

Computation of magnetization, exchange stiffness, anisotropy, and susceptibilities in large-scale systems using GPU-accelerated atomistic parallel Monte Carlo algorithms

Monte Carlo algorithms are frequently used in atomistic simulations, including for computation of magnetic parameter temperature dependences in multiscale simulations. Even though parallelization strategies for Monte Carlo simulations of lattice spin models are known, its application to computation of magnetic parameter temperature dependences is lacking in the literature. Here we show how, not only the unconstrained algorithm, but also the constrained atomistic Monte Carlo algorithm, can be parallelized for any spin–lattice crystal structure. Compared to the serial algorithms, the parallel Monte Carlo algorithms are typically over 200 times faster, allowing computations in systems with over 10 million atomistic spins on a single graphical processing unit with relative ease. Implementation and testing of the algorithms was carried out in large-scale systems, where finite-size effects are reduced, by accurately computing temperature dependences of magnetization, uniaxial and cubic anisotropies, exchange stiffness, and susceptibilities. In particular for the exchange stiffness the Bloch domain wall method was used with a large cross-sectional area, which allows accurate computation of the domain wall width up to the Curie temperature. The exchange stiffness for a simple cubic lattice closely follows an mk scaling at low temperatures, with k < 2 dependent on the anisotropy strength. However, close to the Curie temperature the scaling exponent tends to k = 2. Furthermore, the implemented algorithms are applied to the computation of magnetization temperature dependence in granular thin films with over 15 million spins, as a function of average grain size and film thickness. We show the average Curie temperature in such systems may be obtained from a weighted Bloch series fit, which is useful for analysis of experimental results in granular thin films.