Granular Films Research Articles

Ejection of boron from L10 phase for sputtered FePt-BN granular films by post-annealing

In this study, post-annealing under NH3 gas atmosphere was conducted in order to promote the two-phase separation of FePt-BN granular films. Magnetic properties and structural analysis for the Fe50Pt50–30 vol% BN granular films revealed that (1) saturation magnetization (M s) of the granular film increased from 500 to 650 emu cm−3 with increasing post-annealing time from 0 to 10 min, (2) the orientation of FePt grains changes from (002) to (111) and fully ordered with increasing post-annealing time up to 30 min, (3) the unit cell volume of FePt grains in the granular film decreased from 55.91 to 55.55 Å3 with increasing post-annealing time from 0 to 10 min, which suggests dissolve and eject of solid solution element. From the above, M s reduction for the FePt-BN granular films is considered to be caused by the solid solution of B in the FePt grains.

Magnetic properties and nanostructure of FePt-TiN granular films deposited with N2 gas addition for heat assisted magnetic recording media

Magnetic properties and nanostructure for FePt-30vol%TiN deposited while flowing N2 gas are investigated. When N2 gas addition of 3%–5% are introduced, compared with that of without N2 gas, reduction in the lattice constant ratio and the unit cell volume, the increase in the degree of order of the granular film are observed. These indicate the segregation of Ti into grain boundaries as the result of the N2 gas addition through nitridization of Ti dissolved in the FePt grains which lead to the enhancement of saturation magnetization, perpendicular magnetic anisotropy, and magnetic anisotropy field of the granular film. Therefore, magnetic decoupling in the FePt-TiN similar to the FePt-BN granular films is promoted by the segregation of nitrides formed by nitridization of metal element solidly soluble in FePt magnetic grains due to nitrogen deficiency.

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).

A Miniaturized Electrochemical Nitrate Sensor and the Design for Its Automatic Operation Based on Distributed Model

As one of the most common inorganic ions in daily food and ecological environment, nitrate (NO3−) is potentially harmful to both environment and humans. Therefore, the detection of nitrate concentration is quite important. Chemical sensors, as analytical measurement tools, usually demonstrate high selectivity, fast analysis speed, simple structure, convenient operation, and low cost and are considered to be a promising method for ion identification and detection. In this paper, a miniaturized electrochemical electrode is fabricated based on the microfabrication technology, and an electrochemical sensor for the detection of nitrate concentration in water is developed by combining the deposition technology of nanometal particles with electrochemical analysis technique. The developed electrochemical sensor utilizes electrochemical deposition to prepare a copper (Cu) granular sensitive film with a branch-cluster structure on the surface of a circular working electrode (Pt). Using copper’s characteristics of electrocatalytic reduction of NO3− in the acid solution environment (pH = 2), the concentration of NO3− in the water sample was calculated by measuring the reduction current on the working electrode when the electrocatalytic reduction reaction of NO3− occurred on the surface of the copper-sensitive membrane. The sensitive film is characterized and detected both with the scanning electron microscope (SEM) and X-ray diffraction analysis (XRD) technique; and the microsensor is used to detect nitrate standard samples; a high detection sensitivity (24.9 μA/mmol·L−1) is achieved in the concentration range of 0.0 to 3.0 mmol/L. At the same time, a MEMS three-electrode sensor without gold wire bonding and insulating glue packaging is designed, and an automatic detection system is constructed with a STM32f103 microcontroller as the controller to realize the automated detecting process for nitrate ions. At the same time, a distributed system is constructed, which can describe the nitrate concentration in every position of the water area and realize the monitoring of the whole water area. The detection system can meet the working environment under most conditions and has practical significance.

Ferromagnetic resonance in FePt thin films at elevated temperatures

Understanding of damping processes in ferromagnetic thin films at elevated temperatures has significant implications for heat-assisted magnetic recording, spin-transfer torque memory, and magnetic sensors operating at high temperatures. Through cavity-based high-temperature ferromagnetic resonance (FMR) measurements, this work examined the FMR linewidth and damping properties of continuous cubic FePt thin films at elevated temperatures. The data show that the FMR linewidth and the Gilbert damping constant both increase monotonically when temperature is increased from room temperature toward the Curie temperature. This temperature dependence is opposite to that observed previously in FePt thin films that are granular, rather than continuous, and have L10 structure, rather than cubic structure; in those films, the FMR linewidth decreases monotonically with an increase in temperature [PR Applied 10, 054046 (2018)]. These opposite results originate from the difference in the crystalline structure and microstructure of the films. In the previous work, the granular L10-order FePt films hold dense material imperfection and thereby may host strong two-magnon scattering (TMS); the TMS-produced damping decreases with an increase in temperature, giving rise to reduced FMR linewidths at high temperatures. In the current work, the continuous cubic FePt films have much less imperfection and thereby host weak TMS, and the dominant damping mechanism is spin-flip magnon-electron scattering (SF-MES). The SF-MES process becomes stronger with an increase in temperature, giving rise to larger linewidth and higher damping at high temperatures. This work and the previous work together demonstrate that for a given thin-film material, the temperature dependence of the FMR linewidth critically relies on the structural properties of the film. They also indicate that one can engineer damping in magnetic thin films through the control of the structural properties of the films.

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.

Microstructure evolution and highly sensitive surface enhanced Raman scattering properties of self-formation dual-scale Ag nanoparticles/ Ag-Zr alloy film

• Simple method was proposed to prepare monodisperse Ag particles/Ag-Zr films. • Ag nanoparticles (NPs) directly formed on the surface of as-deposited Ag-Zr films. • Size, number and nanogap of Ag NPs are sensitive to Ag-Zr film thickness. • Ag NP/Ag-Zr granular film displays effective Raman enhanced properties. Ag-22%Zr alloy films with different thicknesses were fabricated on flexible polyimide substrate by co-sputtering method. It is interested in finding that a lot of Ag nanoparticles are spontaneously formed on the as-deposited Ag-Zr films at room temperature. The phase, crystal structure, particle number and size, and surface enhanced Raman scattering (SERS) properties of as-deposited and annealed Ag-Zr alloy films were characterized by X-ray diffraction, high resolution transmission electron microscope, field emission scanning electron microscope and Raman spectrometer, respectively. Results show that Zr grains dispersed around Ag grains and had a coherent growth relation with Ag grains in the alloy film. Dual scale Ag nanoparticles were formed on the annealed Ag-Zr alloy films without template. An intense SERS effect was obtained by covering the Ag layer on the Ag-Zr alloy films. The SERS substrate showed good repeatability and revealed high sensitivity of enhancement factor with the detection limit concentration of Rhodamine 6G as low as 5 × 10-15 mol/L. It was demonstrated that the nanogaps between Ag nanoparticles behaved as hot spots, and thus enhanced local electromagnetic fields substantially. Furthermore, densely and uniformly distributed Ag nanoparticles enlarged the surface roughness and the surface area, resulting in further enhanced Raman signals.

Elucidation the effectiveness of acridine orange as light-harvesting layer for photosensing applications: Structural, spectroscopic and electrical investigations

In this research, acridine orange, AO, bio-photosensitive thin films are prepared using vacuum thermal evaporation technique for optoelectronic applications. First, the crystal structure of the deposited films compared to powder is investigated using the XRD technique. Next, the surface morphology of the deposited AO thin film is characterized using the FESEM technique giving rise to a uniform, homogeneous, and granular polycrystalline film. The amplitude and spatial roughness parameters of the film are estimated. Furthermore, the optical properties of the deposited AO thin film are spectrophotometrically characterized in the range from UV to NIR. The recorded absorbance, transmittance, and reflectance showed high UV and visible absorption characteristics with a direct energy gap of ∼2.238 eV. The optical constants, including refractive index and extinction coefficient, are found, and the dispersion behavior is analyzed using the one-oscillator model for estimating the oscillator and dispersion energies. The dielectric function and dielectric relaxation time of AO thin film are interpreted in detail. In addition, an organic/inorganic heterogeneous junction based on Ag/AO/p-Si/Al is fabricated and electrically evaluated using the current-voltage relation. The fabricated heterojunction shows a rectification behavior of ideality factor, barrier height, and rectification ratio ∼2.09, 0.707 eV, and 176, respectively. The charges dynamics mechanism in terms of band diagram and the density of interface states profile are analyzed. Eventually, the photoresponse of the engineered heterojunction is evaluated under the illumination of intensities that varies from 20 mW/cm2 to 100 mW/cm2. The fabricated photosensor showed a high stable performance with responsivity, specific detectivity, linear dynamic range, and ON/OFF ratio of about 68.4 mA/W, 1.11 × 1010 Jones, 69.3 dB, and 108.4, respectively. The photoresponse performance of the present device is considered more efficient and stable than many other organic/inorganic photosensors.

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.

Algorithm for determining the structural characteristics of amorphous nanogranulated composites from the microwave waves reflection or conductivity

We present an algorithm for calculating structural characteristics of amorphous nanogranular composites (mean free path of electron, average sizes of granules and gaps between them) by the measured reflection coefficients of microwave waves or conductivity. The algorithm was tested with amorphous granular composite films with thickness of 2.2–5.8 μm, deposited on a sitall substrate. We showed that the mean free path of electrons fully corresponded to the average size of the granules at certain contents of the metal phase and reflection coefficients of microwave waves. The results were obtained with the usage of atomic force and scanning electron microscopy.

Effect of N₂ Addition on Nanostructure and Magnetic Properties of Sputtered FePt-BN Granular Films During Deposition for Heat Assisted Magnetic Recording

Magnetic properties and nanostructure for FePt-30 vol% BN granular films deposited while flowing N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas with ratio between 0% and 5% with respect to the total flow of N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> + Ar are investigated. Here is a list of the results: 1) A maximum coercivity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {c}}$ </tex-math></inline-formula> ) of 18.9 kOe, a saturation magnetization ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M_{\mathrm {s}}$ </tex-math></inline-formula> ) of 600 emu/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , a perpendicular magnetic anisotropy ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K_{\mathrm {u\bot }}$ </tex-math></inline-formula> ) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.7\times 10^{\mathrm {7}}$ </tex-math></inline-formula> erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , and a magnetic anisotropy field ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {k}}$ </tex-math></inline-formula> ) of 58 kOe, with smaller grain size and higher degree of grain separation are obtained when N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas with a ratio of around 2% is added; 2) higher fraction of N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas ratio leads to an increase of the amount of magnetic grains with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$c$ </tex-math></inline-formula> -axis parallel to the film plane; 3) lattice constant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$a$ </tex-math></inline-formula> remains constant, a minimum value of lattice constant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$c$ </tex-math></inline-formula> and the ratio <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$c/a$ </tex-math></inline-formula> are also observed for a N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ratio of around 2%; 4) there are two phenomena observed in nanostructure and magnetic properties defined by the N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ratio. For N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ratios under 2%, changes in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$c/a$ </tex-math></inline-formula> ratio, the degree of order, and the agglomeration suppression of grains enhance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M_{\mathrm {s}}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K_{\mathrm {u\bot }}$ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {k}}$ </tex-math></inline-formula> . This indicates that the segregation of B into grain boundaries as the result of the N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> addition. For N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ratios over 2%, the change of tetragonal into cubic FePt magnetic grains, the reduction of the degree of order, and the decrease of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M_{\mathrm {s}}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K_{\mathrm {u\bot }}$ </tex-math></inline-formula> suggest that excess nitrogen dissolves into the FePt magnetic grains; and 5) another consequence of the 2% N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ratio is the enhancement of the magnetic grain decoupling along with the increase of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {k}}$ </tex-math></inline-formula> , this results in the maximization of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {c}}$ </tex-math></inline-formula> .

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.

Facile synthesis of PdCu nanocluster-assembled granular films as highly efficient electrocatalysts for formic acid oxidation

Facile synthesis of PdCu nanocluster-assembled granular films as highly efficient electrocatalysts for formic acid oxidation

Machine Learning Approach for Evaluation of Nanodefects and Magnetic Anisotropy in FePt Granular Films

This paper reports a machine learning approach for evaluating micromagnetic and microstructural parameters from demagnetization curves of FePt granular films for heat-assisted magnetic recording (HAMR) media. We developed a neural network to predict parameters of magnetic anisotropy and volume fractions of defects such as [200] misoriented grains, {111} twined variants, and disordered grains. The neural network was trained on a synthetic dataset of out-of-plane demagnetization curves that were simulated using the micromagnetic model constructed from actual nanostructure of a FePt-X HAMR medium. Predicted nanodefects agreed well with those estimated by synchrotron X-ray diffraction, and the demagnetization curve simulated with the predicted parameters accurately reproduced the experimental one. This work paves the way for a high-throughput magnetometry-based characterization of FePt granular media for its structural optimization toward higher areal density of HAMR.

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.

Tomographic imaging using conductive atomic force microscopy

Conductive atomic force microscopy was used to image permalloy nanostructures embedded in Al2O3. Two layers of permalloy nanostructures embedded in Al2O3 were fabricated in a stacked manner using nanosphere lithography and electron beam evaporation. Bias dependence conduction of the bottom and top layers of the permalloy nanostructures was demonstrated. A simple phenomenological model based on thermionic emission, direct tunnelling and Fowler-Nordheim tunnelling was used to explain the experimental findings. When applied to Co-HfO2 granular films, the technique enabled 3D reconstructed images of the films to obtain grain size information in a non-destructive manner. This relatively simple and cost-effective technique has the potential to be developed to image nanostructures and devices tomographically.

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.