Fe Layers Research Articles

Air and O2-Assisted Catalytic VACNT Growth Optimization for Uniformity and Throughput

The development of an optimized air or O2-assisted multi-wall vertically aligned carbon nanotubes (VACNT) process that adjusts the vertical height profile of a standard H2O vapor-assisted VACNT process is reported. The effect of the air or O2 chemical vapor deposition (CVD) precursor flow rate, the catalytic Fe layer thickness, the process growth temperature, and the H2/C2H4 ratio on VACNT length was first investigated to find the optimum growth conditions. Spatial distribution height mapping of VACNT structures on six patterned 4′′ catalyst Si wafers prepared with a 70–90 min long O2-assisted growth step shows an average growth height of 1.8–2.2 mm, with a standard deviation of less than 10%. Characterization techniques included Raman spectroscopy, scanning electron microscopy (SEM), and spatial height mapping analysis for a range of Fluid channel Array Brick (FAB) components with a length of 30 mm, a width range of 2.5–15 mm, a fluid channel diameter range of d = 5–100 mm, and a fluid channel closest gap range of g = 5–50 mm. A significant finding is that the O2-assisted VACNT growth process optimization efforts enable 2 mm parts processing with square edges, flat top surfaces, uniform height tolerances, and maximum catalyst wafer utilization for application in engineering devices.

Theory of superdiffusive spin transport in noncollinear magnetic multilayers

Ultrafast demagnetization induced by femtosecond laser pulses in thin metallic layers is caused by the outflow of spin-polarized hot-electron currents describable by the superdiffusive transport model. These laser-generated spin currents can cross the interface into another magnetic layer and give rise to magnetization dynamics in magnetic spin valves with noncollinear magnetizations. To describe ultrafast transport and spin dynamics in such nanostructures, we develop here the superdiffusive theory for general noncollinear magnetic multilayers. Specifically, we introduce an Al/Ni/Ru/Fe/Ru multilayer system with noncollinear Ni and Fe magnetic moments and analyze how the ultrafast demagnetization and spin-transfer torque depend on the noncollinearity. We employ ab initio calculations to compute the spin- and energy-dependent transmissions of hot electrons at the interfaces of the multilayer. Taking into account multiple electron scattering at interfaces and spin mixing in the spacer layer, we find that the laser-induced demagnetization of the Ni layer and the magnetization change of the Fe layer strongly depend on the angle between their magnetizations. Similarly, the spin-transfer torques on the Ni and Fe layers and the total spin momentum absorbed in the Ni and Fe layer are found to vary markedly with the amount of noncollinearity. These results suggest that by changing the amount of noncollinearity in magnetic multilayers, one can efficiently control the hot-electron spin transport, which may open a way toward achieving fast, laser-driven spintronic devices.

Controlling electron beam induced deposition of iron from Fe(CO)5: Inhibition of autocatalytic growth by NH3 and reactivation by electron irradiation

Focused electron beam induced deposition (FEBID) is a versatile direct-write approach to produce nanostructures from organometallic precursor molecules. Ideally, the material is deposited only when precursors interact with and are dissociated by the impinging electrons so that the process is spatially defined by the electron beam. In reality, however, thermal surface reactions as known from chemical vapor deposition can also contribute to the dissociation of the precursors. They often produce material with higher purity but can also impair the spatial selectivity of the electron-induced deposit growth. This work aims at an approach to suppress such thermal chemistry and to re-enable it within an area defined by the electron beam. We have, thus, used a surface science approach to study the inhibition of autocatalytic growth (AG) of Fe from Fe(CO)5 by NH3 and the reactivation of AG on the surface by electron irradiation. The experiments were performed under ultrahigh vacuum conditions using thermal desorption spectrometry to characterize adsorption and reactivity of Fe(CO)5 on Fe seed layers that were prepared by dosing Fe(CO)5 during electron irradiation of the entire sample surface (referred to as EBID herein). Auger electron spectroscopy was used to monitor deposit growth and to reveal the potential inhibition of AG by NH3 as well as the reactivation of the surface by electron irradiation. The results show that adsorption of NH3 slows down AG on deposits prepared by EBID but not on Fe layers produced by AG. Electron irradiation after adsorption of NH3 reactivates the surface and thus re-establishes AG. We propose that co-injection of NH3 during FEBID from Fe(CO)5 could be a viable strategy to suppress unwanted AG contributions and, therefore, enhance the spatial control of the deposition process.

Memory of frozen and rotatable antiferromagnetic spins in epitaxial CoO(1 1 1)/Fe and NiO(1 1 1)/Fe bilayers

We combined X-ray magnetic linear and circular dichroism together with magnetooptic Kerr effect measurements in order to follow the magnetic properties of epitaxial CoO(111)/Fe(110) and NiO(111)/Fe(110) bilayers. We find that in both studied cases ferromagnetic sublayer plays a dominant role and determines the magnetic state of the neighboring antiferromagnet, however different interaction scenarios are observed. In CoO/Fe bilayers the antiferromagnetic spins are frozen and their orientation is imprinted by magnetization of Fe layer as the system passes the Néel temperature of CoO. For NiO/Fe bilayers, the antiferromagnetic spins are rotatable and always follow the reorientation of Fe magnetization that can be controlled by external magnetic field or via the temperature and thickness driven spin reorientation transition in Fe(110).

Inversion symmetry breaking in spin–orbit torque-induced magnetization switching to improve the recording density of multi-level magnetoresistive random-access memory

In this study, we prepared a multi-layer Tb–Fe/Pt/Tb–Fe wire to develop a multi-level magnetic memory. By applying current, magnetizations of the Tb–Fe layers were inversion symmetrically switched by spin– orbit torque (SOT) generated from the middle Pt layer. Measurements of SOT showed that its efficiency had opposite polarities in the top and bottom Tb–Fe layers. The switching current density of the top and bottom Tb–Fe layers shifted in opposite directions under a uniform perpendicular magnetic field. Because the perpendicular magnetic field broke the inversion symmetry of SOT generated from the middle Pt layer, it could be used to control the switching current. Our results prove that the additional uniform and perpendicular magnetic field can enhance the controllability of the magnetization state in case of multi-level SOT-induced magnetization switching.

Analysis and Mitigation of Negative Differential Resistance effects in Double-Gate Silicon-on-Insulator Negative Capacitance Field Effect Transistor with improved analog performance

Negative Capacitance Field Effect Transistors (NCFETs) offer the possibility of enhancing channel length scalability, with a key challenge being to ensure hysteresis and Negative Differential Resistance (NDR) free behavior. In this context, through TCAD-based simulations, for a symmetric Double-gate Silicon-on-Insulator (DG-SOI) baseline architecture with a Metal–Ferroelectric–Insulator–Semiconductor (MFIS) configuration, we first examine the root cause of NDR as being the FE layer with a small number of strongly coupled domains, which allows the drain to lower the surface potential (ΨSs) and internal gate voltage (Vint) on the source side. With this understanding of NDR, the inclusion of a paraelectric (PE) layer on the drain side of the gate stack, by splitting the stack into two halves laterally, while keeping the thickness of the stack unchanged, minimizes the drain’s influence on the source-sided electrostatics. This results in the mitigation of NDR effects along with superior analog performance parameters, thus making these FE–PE gate stack-based NCFET devices suitable for analog circuit applications.

Nonlocal orbital torques in magnetic multilayers

We investigate current-induced torques in Ni/Ti/Fe/Ti multilayers. In the multilayers, we find that the damping-like torque acting on the Ni magnetization increases with the thickness of the bottom Ti layer, despite the negligible spin Hall conductivity of Ti and the presence of the Fe interlayer that effectively absorbs the transverse spins. The nonlocal nature of the observed torque is consistent with the orbital torque arising from the orbital Hall effect in the Ti layer and orbital transport through the Fe layer. This observation highlights the unique features of the orbital currents, offering enhanced flexibility in the design of spintronic devices.

Continuous Hot‐Dip Galvanizing of Medium‐Manganese Third‐Generation Advanced High‐Strength Steels

The effects of process atmosphere oxygen partial pressure (pO2) on the preimmersion surface structures, interfacial reactive wetting products, and reactive wetting mechanisms of two prototype (0.15–0.20)C–(5.6–5.9)Mn–(0.4–1.9)Al–(1.1–1.5)Si–(0–0.6)Cr (wt%) third‐generation advanced high‐strength steels during continuous hot‐dip galvanizing are determined. In this study, the two‐stage thermal processing routes employed comprise an austenitization anneal followed by flash pickling and an intercritical anneal. All annealing treatments are conducted in a N2–5 vol% H2 process atmosphere with a controlled dew point. The substrates are austenitized at dew points of –30 or –10 °C, intercritically annealed at a dew point of –30, –10, or +5 °C, and then galvanized using a conventional 0.2 wt% Al (dissolved) bath. The preimmersion surfaces comprise a near‐pure Fe layer with dispersed nanoscaled oxide nodules. The primary reactive wetting mechanism is the direct wetting of the surface Fe, while oxide wetting, oxide cracking and liftoff, and oxide bridging by the liquid metal bath are secondary reactive wetting mechanisms. Zn ingress into the substrate via pores in the oxide network from selective dissolution during flash pickling is also noted. A well‐developed Fe2Al5–xZnx interfacial layer is observed for all metallic coating conditions. The resultant coatings show outstanding adherence after ASTM three‐point bend testing.

Catalytic Oxidation of Volatile Organic Compounds Using the Core–Shell Fe2O3-Cenospheric Catalyst in a Fluidised Bed Reactor

The results of selected volatile organic compounds (benzene, n-hexane, isopropanol, and formic acid) catalytic oxidation are presented on Fe2O3 cenospheres in the fluidised bed reactor. The core–shell Fe2O3-cenosphere catalyst was developed by applying an Fe layer on cenospheres by FB-MO-CVD (fluidised bed, metal–organic chemical vapor deposition) and following Fe layer oxidation. The efficiency of the decomposition of VOCs was tested in the range of 200 to 500 °C, using the method based on infrared spectroscopy (FTIR). The research was focused especially on the analysis of incomplete combustion products, such as CO and oxygen compounds. During the oxidation of isopropanol and n-hexane, in addition to carbon monoxide, species such as acetone, formaldehyde, and acetaldehyde were also detected. The oxidation of formic acid proceeded with only a slight emission of carbon monoxide, unexpectedly the oxidation of benzene proceeded in a similar way, and no other products of the incomplete oxidation were detected. In addition, the CO concentration was lower than in the case of conversion of isopropanol and n-hexane. For the presented solution, complete formic acid oxidation is possible at temperatures below 400 °C, and almost complete oxidation to CO2 of isopropyl alcohol, benzene, and n-hexane was achieved at 500 °C. Additionally, the possibility of conducting the autothermal process of oxidation of VOCs in a fluidised bed, provided that heat recuperation is used, was presented.

Magnetic anisotropy of Fe/MgO interfaces inserted with alkali halide layers

Perpendicular magnetic anisotropy (PMA) at the Fe/MgO interface has been widely studied for memory applications, and further improving PMA has been one of the central challenges. In this study we fabricate epitaxial Fe/alkali halide (LiF, NaCl, or CsI)/MgO multilayers using molecular beam epitaxy and investigate how the alkali halide layer insertion can enhance PMA at the Fe/MgO interface. We find that insertion of an ultrathin LiF layer (0.1--0.4 nm) enhances interfacial PMA, but thicker LiF layer insertion (0.6--1 nm) weakens it. For the CsI and NaCl cases, interfacial PMA energy decreases monotonically with CsI or NaCl thickness. The present results indicate that well-defined Fe-anion orbital hybridization achieved by good lattice matching between the Fe and insulator layers is essential to have strong PMA. It is also suggested that anion atoms with high electronegativities, such as F, are beneficial for interfacial PMA, probably because of the weaker Fe-F hybridization and stronger electron localization at the interface. Our study shall serve as a guiding principle for designing a new dielectric layer to achieve stronger PMA in ultrathin Fe films.

Investigation of the deactivation and regeneration of an Fe2O3/Al2O3•SiO2 catalyst used in catalytic ozonation of coal chemical industry wastewater

Catalyst deactivation is an ongoing concern for industrial application of catalytic ozonation processes. In this study, we systematically investigated the performance of a catalytic ozonation process employing Fe2O3/Al2O3•SiO2 catalyst for the treatment of coal chemical industry (CCI) wastewater using pilot-scale and laboratory-scale systems. Our results show that the activity of the Fe2O3/Al2O3•SiO2 catalyst for organic contaminant removal deteriorated over time due to formation of a dense and thin carbonaceous layer on the Fe2O3 catalyst surface. EPR and fluorescence imaging analysis confirm that the passivation layer essentially inhibited the O3-catalyst interaction thereby minimizing formation of surficial •OH and associated oxidation of organic contaminants on the catalyst surface. Calcination was demonstrated to be effective in restoring the activity of the catalyst since the carbonaceous layer could be efficiently combusted during calcination to re-establish the surficial •OH-mediated oxidation process. The combustion of the carbonaceous layer and restoration of the Fe layer on the surface on calcination was confirmed based on SEM-EDX, FTIR and thermogravimetric analysis. Cost analysis indicates that regeneration using calcination is economically viable compared to catalyst replacement. The results of this study are expected to pave the way for developing appropriate regeneration techniques for deactivated catalysts and optimising the catalyst synthesis procedure.

Design and Fabrication of Segmented GeTe/(Bi,Sb)2Te3 Thermoelectric Module with Enhanced Conversion Efficiency

AbstractGeTe and (Bi,Sb)2Te3 are two representative thermoelectric (TE) materials showing maximum performance at middle and low temperature, respectively. In order to achieve higher performance over the whole temperature range, their segmented one‐leg TE modules are designed and fabricated by one‐step spark plasma sintering (SPS). To search for contact and connect layers, the diffusion behavior of Fe, Ni, Cu, and Ti metal layers in GeTe is studied systematically. The results show that Ti with a similar linear expansivity (10.80 × 10−6 K−1) to GeTe, has low contact resistance (3 µΩ cm2) and thin diffusion layer (0.4 µm), and thus is an effective metallization layer for GeTe. The geometric structure of the GeTe/(Bi,Sb)2Te3 segmented one‐leg TE module and the ratio of GeTe to (Bi,Sb)2Te3 are determined by finite element simulation method. When the GeTe height ratio is 0.66, its theoretical maximum conversion efficiency (ηmax) can reach 15.9% without considering the thermal radiation and thermal/electrical contact resistance. The fabricated GeTe/(Bi,Sb)2Te3 segmented one‐leg TE module showed a ηmax up to 9.5% with a power density ≈ 7.45 mW mm−2, which are relatively high but lower than theoretical predictions, indicating that developing segmented TE modules is an effective approach to enhance TE conversion efficiency.

Cluster-layered [Fe/Cr]30 structure exhibited Kondo-like effect studied by GISAXS and Mössbauer spectroscopy

Structure and morphology of epitaxial [Fe/Cr]30 multilayers with ultrathin Fe layers (nominally 0.12 nm and 0.08 nm) have been investigated by X-ray reflectivity, synchrotron Mössbauer spectroscopy at low temperature and grazing-incidence small angle X-ray scattering (GISAXS). The films demonstrate Kondo-like behavior of electrical resistivity. The GISAXS patterns reveal their cluster-layered structure. The observed side maxima give the information about the sizes and distances between lateral inhomogeneities. Mössbauer reflectivity spectra measured below the critical angle of the total reflection support the existence of the cluster-layered structure of the samples. Magnetic hyperfine field distributions show that largest number of 57Fe atoms is situated in interfaces of iron clusters and their number increases in the thinnest films.

Co:BaTiO3/Sn:BaTiO3 Heterostructure Thin‐Film Capacitors with Ultrahigh Energy Density and Breakdown Strength

AbstractFerroelectric (FE) capacitors exhibiting ultrahigh power densities are widely utilized as electrostatic energy storage devices in pulsed electronic devices. One approach to maximize the discharge energy density (Ud) of capacitors is to increase the breakdown strength (Eb) accompanied with high maximum polarization (Pm) while suppressing the energy loss. However, the inverse relationship between Eb and Pm challenges the simultaneous enhancement of Eb and Ud. To overcome this limitation, FE/relaxor FE (RFE) heterostructure capacitors composed of Co‐doped BaTiO3 (BTCO) and Sn‐doped BaTiO3 (BTS) epitaxial thin film layers to decouple the Eb and Pm values are fabricated and the simultaneous enhancement of the Eb and Ud up to 7.9 MV cm−1 and 117 J cm−3, respectively is achieved. The high Eb and Ud values can be attributed to the suppression of the leakage current at the BTCO/BTS interface, a narrower hysteresis loop contributed by the BTS, and high Pm and Eb from the BTCO layer. Additionally, the BTCO/BTS heterostructure capacitors exhibit excellent fatigue endurance of up to 108 cycles and are thermal stable even at 160 °C. Through properly designing the FE and RFE layers, thermally stable and reliable FE/RFE heterostructure capacitors exhibiting high Ud and Eb can be realized.

Ultrafast electron dynamics in Au/Fe/MgO(001) analyzed by Au- and Fe-selective pumping in time-resolved two-photon photoemission spectroscopy: Separation of excitations in adjacent metallic layers

The transport of optically excited, hot electrons in heterostructures is analyzed by femtosecond, time-resolved two-photon photoelectron emission spectroscopy (2PPE) for epitaxial Au/Fe/MgO(001). We compare the temporal evolution of the 2PPE intensity upon optically pumping Fe or Au, while the probing occurs on the Au surface. In the case of Fe-side pumping, assuming independent relaxation in the Fe and Au layers, we determine the hot electron relaxation times in these individual layers by an analysis of the Au layer thickness dependence of the observed, effective electron lifetimes in the heterostructure. We show in addition that such a systematic analysis fails for the case of Au-side pumping due to the spatially distributed optical excitation density, which varies with the Au layer thickness. This work extends a previous study [Beyazit et al., Phys. Rev. Lett. 125, 076803 (2020)] by new data leading to reduced error bars in the determined lifetimes and by a non-linear term in the Au-thickness dependent data analysis which contributes for similar Fe and Au film thicknesses.

Maximum energy product of exchange-coupled Sm(FeCo)12/α-Fe nanocomposite particle

The effects of the coating surface orientation of the α-Fe soft magnetic layer on the Sm(Fe0.8Co0.2)12 hard magnetic phase and the volume fraction of α-Fe, VFe, on the maximum energy product, (BH)max of exchange-coupled Sm(Fe0.8Co0.2)12/α-Fe nanocomposite magnet particles were micromagnetics OOMMF package was systematically investigated. The (BH)max of the reference model, Sm(Fe0.8Co0.2)12 particles without Fe layer, was 630 kJ/m3. In contrast, in the nanocomposite magnet particle model with soft magnetic layers on both sides of the hard magnetic phase, (BH)max reached a maximum value of 657 kJ/m3 at VFe = 12% (Fe layer thickness, tFe = 2 nm). In the model with α-Fe coating on the top and bottom surfaces of the hard magnetic phase, (BH)max = 636 kJ/m3 at VFe = 4% (tFe = 2 nm). Furthermore, the coating of the soft magnetic phase on both sides of the hard phase particles reduces the magnitude of the demagnetizing field, Hd of the nanocomposite magnet particles, indicating that the side coating of the soft magnetic phase is effective in increasing (BH)max. These findings allow for a greater degree of freedom in the design of nanocomposite magnets by adjusting not only the VFe volume fraction of the hard/soft phases but also their arrangement.

Brillouin Light Scattering from Magnetic Excitations.

Brillouin light scattering (BLS) has been established as a standard technique to study thermally excited sound waves with frequencies up to ~100 GHz in transparent materials. In BLS experiments, one usually uses a Fabry-Pérot interferometer (FPI) as a spectrometer. The drastic improvement of the FPI contrast factor over 1010 by the development of the multipass type and the tandem multipass type FPIs opened a gateway to investigate low energy excitations (ħω ≤ 1 meV) in various research fields of condensed matter physics, including surface acoustic waves and spin waves from opaque surfaces. Over the last four decades, the BLS technique has been successfully applied to study collective spin waves (SWs) in various types of magnetic structures including thin films, ultrathin films, multilayers, superlattices, and artificially arranged dots and wires using high-contrast FPIs. Now, the BLS technique has been fully established as a unique and powerful technique not only for determination of the basic magnetic constants, including the gyromagnetic ratio, the magnetic anisotropy constants, the magnetization, the SW stiffness constant, and other features of various magnetic materials and structures, but also for investigations into coupling phenomena and surface and interface phenomena in artificial magnetic structures. BLS investigations on the Fe/Cr multilayers, which exhibit ferromagnetic-antiferromagnetic arrangements of the adjacent Fe layer's magnetizations depending on the Cr layer's thickness, played an important role to open the new field known as "spintronics" through the discovery of the giant magnetoresistance (GMR) effect. In this review, I briefly surveyed the historical development of SW studies using the BLS technique and theoretical background, and I concentrated our BLS SW studies performed at Tohoku University and Ishinomaki Senshu University over the last thirty five years. In addition to the ferromagnetic SW studies, the BLS technique can be also applied to investigations of high-frequency magnetization dynamics in superparamagnetic (SPM) nanogranular films in the frequency domain above 10 GHz. One can excite dipole-coupled SPM excitations under external magnetic fields and observe them via the BLS technique. The external field strength determines the SPM excitations' frequencies. By performing a numerical analysis of the BLS spectrum as a function of the external magnetic field and temperature, one can investigate the high-frequency magnetization dynamics in the SPM state and determine the magnetization relaxation parameters.

First-principles study of anomalous Hall effect and anomalous Nernst effect in Fe2Si

We have performed density functional calculations of the electronic structure, anomalous Hall conductivity, and anomalous Nernst conductivity for the model crystal structure of a mineral Hapkeite Fe2Si contained in a meteorite Dhofar 280, which is believed to originate from the Moon. The calculated results showed that the model structure of Hapkeite Fe2Si exhibits a larger transverse thermoelectric effect than bcc-Fe and hexagonal Fe2Si. Based on the detailed analysis of the electronic structures and transport coefficients, we found that the origin of the large transverse thermoelectric effect is due to an anomaly in the density of states at the interface between the Fe layers and the Si–Fe alternative stacked layers. This finding is promising for the design of new transverse thermoelectric materials.

Insight into internal oxidation mechanism of hot rolled Si–Mn added high strength steel: Pure iron layer formation

In order to reveal special morphology formation mechanism of surface continuous Fe layer, through the vacuum sealing heat treatment experiment, the scale transformation process and the mechanism of oxygen pressure reduction in the coiling process of hot rolled steel are obtained. After discussion, it is considered that the surface pure Fe produced by heat treatment under the condition of vacuum sealing of C–Mn–Si steel sheet is due to the internal oxidation of Mn and Si in the substrate and decarburization reaction, and finally lead to the decomposition of oxide scale, resulting in the special structure of surface pure Fe, intermediate oxide scale and internal oxidation.

Stratification of the Fe/Si(001)2x1 interface by heat treatment of the wetting layer

The study was carried out by LEED, AES, EELS and AFM methods. Films of Fe/Si(001)2x1 were obtained at substrate temperatures of 30oC and source temperatures of 1250oC. Wetting layer (WL) Fe on Si(001)2x1 was formed by two-stage annealing at temperatures and thicknesses of 500oC and 250oC and 1 monolayer (ML) and 3 ML, respectively. Analysis and interpretation of the data obtained, taking into account possible reaction patterns, showed that after annealing at 1 ML thickness, the Fe composition corresponded to 2 ML Si/Fe. Further, at 2 ML, it changed to Fe/Si/Fe, at 3 ML, it changed to Fe-FeSi, and after annealing, to FeSi. At 4 ML, there was formation of FeSi/FeSi2 film. And, further, at 7 ML and 10 ML, the composition of the films became Fe3Si/FeSi2 and, respectively, Fe/Fe3Si/FeSi2. At the same time, the upper Fe3Si layers were coated with 0.6 ML and the Fe layers with 0.3 ML of segregated Si atoms, which number increased, after annealing at 250oC, to 0.6 ML in the latter case. In the obtained Fe film, the size and average grain height were 10-20 nm and, respectively, ~0.4 nm. Keywords: interface, wetting layer, multilayer film, layer composition, surface reaction patterns, Fe, Si(001)2x1.