Granular Films Research Articles

Effect of an agglomeration-suppressed metal island buffer layer on nanostructure of FePt-oxide granular films for heat assisted magnetic recording media

Effect of an agglomeration-suppressed metal island buffer layer on nanostructure of FePt-oxide granular films for heat assisted magnetic recording media

Revealing the pinning landscape and related vortex pattern evolution in granular superconducting films

Most superconducting electronics based on films exhibit granular structures. It has been suggested that grain boundaries form a network with relatively weak superconductivity, potentially acting as pinning centers. Yet, so far, detailed microscopic studies of the pinning landscape and its relation to vortex behavior remain scarce. Here, we imaged the vortex lattices (VL) in granular Nb films using magnetic force microscopy over large scanning areas at various magnetic fields. A non-monotonic evolution in the degree of vortex lattice ordering was observed with increasing vortex density, driven by a combination of vortex-vortex interactions and pinning effects. The spatial distribution of pinning potential within the film was directly mapped using a recently developed scanning quantum vortex microscope (SQVM). Instead of the network formed by grain boundaries, the pinning landscape presents a network-like structure, yet with domains significantly larger than the individual grains. The results of numerical simulations based on pinning landscape revealed by SQVM well reproduce our experiments. The pinning force per unit length at low magnetic fields was calculated. The critical current density, estimated from the relative positions of vortices, aligns well with the critical state model. Our work illustrates the relationship between the evolution of the vortex lattice with magnetic field and the structural features of granular Nb film, providing new insights into the design of high-performance superconducting electronic devices.

Study of structural, morphological and optical properties of flash evaporated cadmium sulphide thin films

Cadmium sulphide thin films were successfully synthesized by the flash evaporation method under vacuum, then annealed at various temperatures. X-Ray Diffraction patterns showed the formation of the hexagonal wurtzite phase. Structural properties of this most stable phase remain approximately the same at all investigated temperatures. Scanning Electronic Microscopy showed the formation of granular films. The Energy Dispersive X-ray spectra revealed the possible formation of stoichiometric compounds. The obtained UV–Vis spectra revealed that the prepared films exhibit a moderate direct band gap with high transmittance in the visible range. Detailed interpretation of the structural, optical, and optoelectronic findings revealed that annealed CdS thin films hold promising potential for the fabrication of the next generation of the modern optoelectronic devices. Cadmium sulphide thin films were successfully synthesized by the flash evaporation method under vacuum, then annealed at various temperatures. X-Ray Diffraction patterns showed the formation of the hexagonal wurtzite phase for all temperatures with almost the same structural properties. UV–vis spectra revealed that the prepared films exhibit a moderate direct band gap with high transmittance in the visible range, which reinforces the usefulness of these films for photovoltaic and photocatalytique applications. Scanning Electronic Microscopy shows the formation of granular films, while the Energy Dispersive X-ray spectra revealed the possible formation of stoichiometric compounds.

Measurements of fluctuation-induced in-plane magneto-conductivity of granular aluminum film

The phenomenon of Berezinskii–Kosterlitz–Thouless (BKT) phase fluctuations and the superconducting fluctuations is investigated in a 40 nm thick granular aluminum film using magneto-transport measurements. The transport measurements suggest the possibility of strong electron–phonon (el–ph) interactions in contrast to a Bardeen–Cooper–Schrieffer superconductor. It shows a BKT transition of 2.304 K and a superconducting mean-field transition at 2.32 K. The presence of the resistive tail even before the BKT transition reflects the abundance of thermally activated free vortices. By analyzing the excess conductivity, Gaussian–Ginzburg–Landau superconducting fluctuations are observed above the superconducting transition, which causes rounding of the transition region even before the superconducting transition. The temperature dependence of the fluctuation conductivity in zero magnetic field exhibits distinct signatures of the two-dimensional direct Aslamazov–Larkin theory, with a significant contribution from the Maki–Thompson (MT) model. Furthermore, the anomalous behavior of the fluctuation conductivity at higher temperatures and perpendicular magnetic fields (up to 700 mT) is explained in terms of the total-energy cutoff (=0.72) in the low-wavelength region of the superconducting fluctuations and a pair-breaking parameter (∼0.031). Further studies on the pair-breaking parameter indicate the presence of the el–ph scattering, which diminishes the MT contribution. Our study carries important bearings on how the BKT phase fluctuations and superconducting amplitude fluctuations control the conductivity of granular superconductor near and above the transition region as non-equilibrium properties of weakly disordered granular superconductors. This research is of significance, offering insights into the fundamental properties of granular superconductivity and aiding in the comprehension of nano-structured thin film devices.

Flash co-evaporation as a vapor quenching method for the deposition of ferromagnetic–non-magnetic granular metal-matrix nanocomposite films

An FM-NM film (FM - ferromagnetic metals: Fe, Ni, Co; NM - conductive metals: Al, Cu, Ag, Au, etc.) is a type of magnetic nanogranular material with properties of metal-matrix nanocomposite or metastable crystalline alloys material systems. It is believed possible to deposit these films using vapor quenching techniques, for which flash evaporations are included. This study aims to find whether approaches and practices of the flash co-evaporation (FcoE) technique for creating magnetic nanogranular thin films are possible and realistic. This work introduces some technical basis, the thermodynamics of evaporation of related materials for the establishment of the FcoE technique, and some structural and magnetic properties of the FcoE deposited Co-NM films (NM = Ag, Al, Cu). The FcoE technique seems less applicable to such thin films so far. So, this technique is expected to contribute to the enrichment of deposition techniques for nanostructured material systems. Since then, a nanogranular film technology based on the FcoE technique applied to the FM-NM-type immiscible metal alloys, whose constituents are not thermodynamically identical, has been preliminarily set up. This technique is expected to provide another simple and efficient way to generate magnetic nanogranular or metal-matrix nanocomposite materials for research in nanoelectronics and spintronics.

Metallic nanofilms on Si(100) and SiO2 grown with a ruthenium precursor

Ruthenium (Ru) nanofilms (<3 nm) were prepared using tricarbonyl(trimethylenemethane)ruthenium, Ru(TMM)(CO)3 at 230 °C. We show that the surface morphology and electrical conductance of Ru nanofilms are substantially different on H:Si(100) and SiO2/Si(100) substrates. Two-dimensional (2D) Ru nanofilms (∼1 nm) were formed on H:Si(100), while thick (∼3 nm) granular Ru films were formed on SiO2 substrate under the same growth conditions, as confirmed by cross-sectional transmission electron microscopy and X-ray photoelectron spectroscopy. Using scanning probe microscopy, the metallic conductance of Ru grains on H:Si(100) substrates was recognized. On ultrathin (1 nm) SiO2/Si(100) substrates, the spatial separation of Ru grains facilitates the single electron tunneling (SET) phenomenon in the double barrier tunnel junction structure. The results emphasized the difference in carrier transport in Ru nanofilms on Si and SiO2 substrates.

Enhanced orbital magnetic moment in an FeCo-BaF2 granular film revealed by x-ray magnetic circular dichroism

FeCo-insulator fluoride nanogranular films belong to an excellent material group that exhibits a huge Faraday effect, which is several times larger than those of conventional materials, in the optical communication wavelength band (1310 ∼ 1550 nm). In this study, FeCo-BaF2 granular films were prepared on glass substrates by a tandem sputtering method. The Faraday effect was measured and a huge Faraday rotation angle of −2.3 deg./μm was observed for the wavelength of 1550 nm. The ratio of orbital magnetic moment to effective spin magnetic moment (morb/mspineff) was evaluated by x-ray magnetic circular dichroism (XMCD) measurements, and was found to be 0.43 for Fe and 0.24 for Co. The obtained values are several times larger than those of Fe-Co bulk samples. The elemental selectivity of the XMCD measurements reveals that Fe has a larger morb/mspineff value than that of Co. Such an increase in morb is attributed to the reduction of the symmetry at the interface between FeCo and BaF2 in the granular film, and is thought to be the origin of the giant Faraday effect in FeCo-fluoride granular films.

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.

Record Passive Radiative Cooling of over 15 °C by a Micro-Nanoporous Granular Composite Film

Record Passive Radiative Cooling of over 15 °C by a Micro-Nanoporous Granular Composite Film

High‐Throughput Optimization of Magnetoresistance Materials Based on Lock‐In Thermography

AbstractWith the giant magnetoresistance (GMR) effect serving as a vital component in modern spintronic technologies, researchers are dedicating significant efforts to improve the performance of GMR devices through material exploration and design optimization. However, traditional GMR measurement approaches are inefficient for comprehensive material and device optimization. This study proposes a high‐throughput current‐in‐plane GMR measurement technique based on thermal imaging of Joule heating utilizing lock‐in thermography (LIT). This LIT‐based technique is advantageous for efficiently evaluating films with varying compositions and thickness gradients, which is crucial for ongoing material exploration and design optimization to enhance the GMR ratio. First, it is demonstrated that using CoFe/Cu multilayers, the simple Joule heating measurement based on LIT enables quantitative estimation of the GMR ratio. Then, to confirm the usefulness of the proposed method in high‐throughput material screening, a case study is shown to investigate the GMR of CoCu‐based granular films with a composition gradient. These techniques allow to determine the optimum composition with maximum GMR ratio using the single composition‐gradient film and reveal Co22Cu78 as the optimal composition, yielding the largest GMR ratio among the reported polycrystalline CoCu‐based granular films. This demonstration accelerates the material and structural optimization of GMR devices.

Crossover from Cooper-pair hopping to single-electron hopping in Pb x (TiO2) 1−x granular films

The electrical transport properties of Pb x (TiO2) 1−x (x being the Pb volume fraction and ranging from ∼0.45 to ∼0.72) granular films are investigated experimentally. The charging energy of the Pb granules is reduced to less than the superconducting gap of Pb granules for the low temperature insulating films by using high-k dielectric TiO2 as the insulating matrix. For the insulating films in the vicinity of the superconductor-insulator transition, Cooper-pair hopping governs the low-temperature hopping transport. For these films, the low-temperature magnetoresistance is positive at low field and the resistivity vs temperature for Cooper-pair hopping obeys an Efros–Shklovsii-type variable-range-hopping law. A crossover from Cooper-pair-dominated hopping to single-electron-dominated hopping is observed with decreasing x. The emergence of single-electron-dominated hopping in the more insulating films is due to the causation that the intergrain Josephson coupling becomes too weak for Cooper pairs to hop between adjacent Pb granules.

Thermal measurements of boiling granular films.

This study focused on two familiar items in our daily lives: grains and boiling water. Accurately measuring boiling dense granular beds requires specialized instruments and novel techniques. An annular vertical riser was used to boil granular films. The riser was filled with glass beads and then immersed in liquid water. The water is heated by inducing eddy currents at the base of the riser, which is composed of carbon steel. Beyond the critical electric current, Ic, the Lyapunov exponent of the time series of the measured temperature distributions within the granular films became positive. This indicates the chaotic behavior of the films. Slightly above Ic, patterned instabilities with affine fractal-like structures were observed in the boiling granular films. Chaotic boiling granular films manifested percolation and transition to turbulence. Computer simulations of the above system under the guidance of generative AI (GenAI) successfully reproduced many critical features of the experimental observations, which can be considered a significant breakthrough in the modeling and simulation of complex systems. With this success in utilizing GenAI, more accurate instruments can be developed, resulting in the world of tomorrow rapidly becoming today's world.

Granular Aluminum Kinetic Inductance Nonlinearity

Granular Aluminum is a superconductor known for more than eighty years, which recently found its ap-plication in qubits, microwave detectors and compact resonators, due to its high kinetic inductance, critical magnetic field and critical current. Here we report on the nonlinear dependence of granular Aluminum inductance on current, which hints towards parametric amplification of the microwave signal in granular Aluminum films. The phase shift of the microwave signal reached 4 rad at a frequency of 7 GHz, which makes it possible to estimate the nonlinearity of the system as \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta \\phi {\ ext{/}}\\phi = 1.4{\\kern 1pt} \\% $$\\end{document} and the potential gain of the order of 17 dB.

Suppression of in-plane (001) texture component in FePt-oxide granular films by adding a carbon buffer layer

Investigation of magnetic properties and nanostructure of FePt-B2O3 granular film with carbon buffer layer (BL) of various thicknesses is reported. When the thickness of carbon BL is varied from 0 to 0.6 nm, saturation magnetization (Msfilm) is almost constant at around 750 emu/cm3 and perpendicular magnetic anisotropy (Ku⊥film) changes from around 1.0×107 to 2.0×107 erg/cm3. For the granular film with the carbon BL thicker than 0.6 nm, both Msfilm and Ku⊥film decrease. The reduction of Msfilm for the granular film by adding a carbon BL may be due to the alloying of carbon into the FePt magnetic grains. The enhancement of Ku⊥film for the film with a 0.6 nm carbon BL is considered due to the reduction of the in-plane texture component which is supported by the in-plane XRD. The reduction of Ku⊥film for the film with a carbon BL thicker than 0.6 nm is considered due to random growth of magnetic grains on a continuous carbon BL which is supported by the TEM cross-section images. According to these results, the employment of an un-continuous thin carbon BL is a promising method to enhance c-axis texture orientation of the FePt-oxide granular films.

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.

Electron-phonon relaxation in a model of a granular film

Electron-phonon relaxation in a model of a granular film

Effect of C Replacement on Magnetic Properties and Nanostructure of FePt-Nitride Granular Film for HAMR Media

Effect of C Replacement on Magnetic Properties and Nanostructure of FePt-Nitride Granular Film for HAMR Media

Crystalline and transport characteristics of ferrimagnetic and antiferromagnetic phases in Mn3Ga films

The Mn3Ga material is a promising candidate for memory and computing devices owing to its rich crystalline structures of tunable ferrimagnetic and collinear and non-collinear antiferromagnetic phases. In particular, Mn3Ga with non-collinear antiferromagnetic order exhibits giant anomalous and topological Hall conductivities and is a potential material platform for hosting spin-related quantum phenomena. In this study, we demonstrate Mn3Ga films grown on thermally oxidized Si substrates, with and without the Ta buffer, under different deposition temperatures (Ts). With increasing Ts, the dominant crystalline structure across all Mn3Ga films evolves from a cubic to hybrid tetragonal and hexagonal texture, wherein the crystalline orientation of spins endows the films with in-plane magnetic anisotropy. For Ta/Mn3Ga and Mn3Ga films grown under high Ts, the inhomogeneity in surface energy of the buffer layer results in a non-uniform granular film in the former. Notably, the Mn3Ga films of hexagonal texture exhibit topological Hall signatures. The density functional theory calculations on the hexagonal Mn3Ga phase corroborated with the experimental magnetic, structural, and transport properties. These findings establish an important platform for tailoring Mn3Ga films toward multifunctional applications.

Control of grain density by varying lattice mismatch in FePt-C film for heat assisted magnetic recording

Controlling the number density of grains in magnetic recording media is critical for improving the areal density of the recording. Herein, we report the dependence of grain density and microstructure of FePt-based granular films for heat-assisted magnetic recording media on the lattice mismatch between the FePt films and substrates SrTiO3, MgAl2O4, MgO and TiO2. The grain density and grain size of the FePt films increased and decreased, respectively, with the increasing lattice mismatch between the films and substrates. The optimum lattice mismatch for the FePt-based granular films was found to be 8%, which corresponds to the MgO substrate. FePt films grown on TiO2 substrate, which had the largest lattice mismatch of 16%, showed poor morphology and low grain density. The results imply that an underlayer with which a film develops a suitable mismatch plays a key role in controlling the microstructure and magnetic properties of the film.