Anisotropic Films Research Articles

Chiral metasurfaces in anisotropic thin film lithium niobate and its nonlinear effect

Different from conventional two-dimensional photonic metasurfaces, chiral photonic metasurfaces, which are composed of a chiral pattern with subwavelength dimensions, exhibit different chiroptical physical phenomena, such as spin-preserving, nonlinear chiroptical effects, and optical vortex generation. However, the effect of symmetry in the vertical direction on the linear and nonlinear response of the chiral quasi-two-dimensional metasurface has been seldom reported. In this study, we design and simulate a chiral metasurface using topological patterns in lithium niobate (LN) thin film to obtain the optimal circular dichroism and its corresponding nonlinear effect. Triangular holes with a certain rotation constitute a quasi-two-dimensional metasurface, which possesses outstanding chirality for the spin state of light, thus allowing it to control linear and nonlinear chiralities. The nonlinear conversion efficiency ηSHG reaches 2×10−4 and 4×10−3 on the LN structure with substrate and without substrate, respectively. Nonlinear circular dichroism CDSHG is 5.8 dB at 783 nm on a SiO2 substrate. This tunable chiral metasurface is expected to be applicable to nonlinear chiroptical responses and chiral light modulation.

Out-of-plane Ferromagnetic Resonance and Surface Spin Wave Modes in Ni80Fe20 Film on Ripple-Patterned Sapphire Substrate

The study of surface magnetic anisotropy of magnetic films is beneficial to understanding the behaviors of films at high frequencies and further promoting their high-frequency magnetic applications. We investigated the out-of-plane ferromagnetic resonance (FMR) and surface spin wave modes in 40-nm Ni80Fe20 (NiFe) films deposited on ripple-patterned sapphire substrate with periodicity and a flat sapphire substrate by broad-band FMR technique. When measuring the FMR of the film on ripple-patterned substrate, the in-plane angle (\U0001d711 H of the external applied magnetic field was along the direction of the ripples (\U0001d711 H = 90°) and perpendicular to the ripple direction (\U0001d711 H = 0°) respectively. The spin-wave resonance spectra consisting of up to two surface spin wave (SSW) modes were observed as the external magnetic field polar angle θH varied from “in-plane configuration” (θH = 90° ) to “out-of-plane configuration” (θH = 0°). As the decrease of θH , the two surface spin excitations disappeared in sequence and further only FMR mode was observed in the region θH which was smaller than the “critical angle” θc . The θc of \U0001d711 H = 90° is found to be different from that for \U0001d711 H = 0°. The dependences of field shift vs. out-of-plane angles θH for each SSW mode were analyzed by the surface inhomogeneity model. From the model, the surface anisotropy constants Ks of films were obtained. The results show that the values of surface anisotropy constants for the two SSW modes are different. Compared with the film sputtered on flat sapphire substrate, after introducing ripple-patterned substrate, the variation of Ks is only on the order of 0.01 erg/cm2 for the film of this thickness.

Thermally induced all-optical ferromagnetic resonance in thin YIG films

All-optical ferromagnetic resonance (AO-FMR) is a powerful tool for the local detection of micromagnetic parameters, such as magnetic anisotropy, Gilbert damping or spin stiffness. In this work we demonstrate that the AO-FMR method can be used in thin films of yttrium iron garnet (YIG) if a metallic capping layer (Au, Pt) is deposited on top of the film. Magnetization precession is triggered by heating of the metallic layer with femtosecond laser pulses. The heat pulse modifies the magneto-crystalline anisotropy of the YIG film and shifts the quasi-equilibrium orientation of the magnetization, which results in precessional magnetization dynamics. The laser-induced magnetization precession corresponds to a uniform (Kittel) magnon mode, with the precession frequency determined by the magnetic anisotropy of the material as well as the external magnetic field, and the damping time set by a Gilbert damping parameter. The AO-FMR method thus enables measuring local magnetic properties, with a resolution given by the laser spot size.

Effect of the Crystallographic Anisotropy of a Magnetic Film on the Detection of a Frequency-Modulated Signal

Effect of the Crystallographic Anisotropy of a Magnetic Film on the Detection of a Frequency-Modulated Signal

Aging effects in critical behavior of Heisenberg anisotropic ultrathin films

We present the results of Monte-Carlo studies of the non-equilibrium properties of ferromagnetic Heisenberg films. Aging effects were observed in non-equilibrium critical behavior. The calculations were carried out for both high-temperature and low-temperature initial states. The characteristic correlation time, which diverges at the transition temperature in the thermodynamic limit, was obtained as a function of system size and waiting time.

Effects of Aging in the Critical Behavior of Anisotropic Ultrathin Heisenberg Films

Effects of Aging in the Critical Behavior of Anisotropic Ultrathin Heisenberg Films

The influence of magnetic film anisotropy on the frequency-modulated signal detection

We studied the possibility of using a magnetostriction converter for detecting a microwave signal with frequency modulation. The magnetization vector and elastic displacement oscillations in a normally magnetized ferrite film with crystallographic anisotropy are considered. We analyzed the effect of crystallographic cell orientation on the nature of the magnetization dynamics and elastic displacement oscillations, the establishment of various oscillation modes, and the detection process.

Free carrier plasma edge and plasmonic excitations in heavily doped surface grated n-type Si

Infrared spectroscopic ellipsometry (SE) in the MID IR region was applied at room temperature to heavily doped n++-Si to obtain information about the free carrier plasma edge and plasmonic excitations before and after the Si surface was grated with a period Λ of 294 nm to a nominal depth of 20 nm by using laser lithography. Besides, SE measurements were performed in the NIR-VIS-UV spectral range to validate the optical models for non-grated and grated Si. In the latter case a thin grated layer on silicon surface was alternatively modelled either within Bruggeman effective medium approximation (BEMA) or by treating this layer as an anisotropic thin film (ATF). A model that included SiO2 (2.1 nm), n++-Si substrate and a 24.5 nm-thick ATF for grated layer was found to be good enough to reproduce the obtained ellipsometric parameters in the accessed spectral range. Intraband optical transitions are shown to overwhelmingly contribute to the MID IR dielectric function (DF) below the free carrier plasma edge (FCPE) of non-grated n++-Si. The parameters obtained for conduction electrons from Drude fit to this function agree well with the relevant data reported for Si so far. FCPE of non-grated n++-Si is positioned at 1416 cm−1. For graded layer this value is preserved within determination accuracy. The life time of plasmonic excitations that appeared as a single peak with a perfect Lorentz line-shape in the dielectric energy loss function of non-grated Si and grated Si layer is about 70 fs. Shape induced optical anisotropy, along with presumably conical diffraction that identified itself through a distinct angle-dependent upturn in the reflection spectra of p-polarized light incident upon surface in the plane perpendicular to Λ is behind the whole set of ellipsometric data obtained for n++-Si with subwavelength grating on the surface.

Template-free self-assembly of mesoporous ZnO nanocluster/ polymethyl methacrylate based anisotropic nanocomposite thin films with enhanced interfacial interactions and tuneable optical properties

Nature inspired biomimetic growth of anisotropic, hierarchical nanostructures could offer insight into new and exciting crystalline properties for advanced multifunctional applications. Our study demonstrates diethanolamine-mediated mesoscopic self-assembly of semiconducting ZnO Quantum Dots (3–5 nm) into lattice-aligned, symmetrical superstructures (40–50 nm) via non-classical oriented attachment (OA) crystal growth observed in various biomineralization processes. Multifunctional nanocomposite thin films of self-assembled nanostructures and polymethyl methacrylate were spin-coated onto plasma-treated Si wafers and the surface and interfacial properties were rigorously studied. HR-TEM images depicted the OA growth process with neighboring nano-crystals having perfectly aligned lattices. Preferential orientation of the thin films along <100> direction was evident from the XRD data. Quantum confinement in ZnO QDs and surface defect originating sharp green PL emission were examined through UV–Vis absorption and Photoluminescence spectra respectively. DLS and Zeta potential studies of surface-engineered colloidal superstructures established excellent long-term physico–chemical stability with no agglomeration or transparency loss observed in ZnO mesocrystal suspension even after 6 months. Diethanolamine, due to its dual functionality radically enhanced the interaction between polar ZnO and non-polar PMMA matrix resulting in highly stable thin films (Class II hybrids) with enhanced surface and interfacial properties as evident from the extremely low surface roughness and homogenous nanofiller dispersion observed in AFM and FE-SEM studies. Chemical interactions at the interface were also established quantitatively by XPS binding energy measurements which suggested hydrogen bonds and covalent bonds between organic-inorganic phases promoted via diethanolamine surface engineering.

Impedance modeling for excluding contact resistance from cross-plane electronic conductivity measurement of anisotropic two-dimensional Ti3C2Tx MXenes

We propose impedance spectroscopy modeling as a convenient tool for estimating the cross-plane conductivity of anisotropic two-dimensional materials by excluding contact resistance with the underlying metal electrodes. We showcase this idea on two types of titanium carbide Ti3C2Tx MXenes, wherein Tx represents surface functional termination. These MXenes are synthesized from the same parent MAX phase material (Ti3AlC2) by different synthesis protocols and are studied in their plain as-synthesized states as well as after short annealing. The cross-plane conductivity of MXenes estimated by our approach appears to be up to an order of magnitude higher than is estimated by a conventional one. We validate our impedance modeling approach by comparing the specific contact resistance results derived from it to a direct estimation by transmission line measurement. We also deduce which functional group has a dominant footprint on the conductivity in each studied MXene by performing numerical simulations on cross-plane conductivity as well as on electronic band structures. The novel equivalent circuit developed herein may be extended to other anisotropic thin films to aid their application in various electronic devices.

Tuning magnetic anisotropy in SrRuO3 thin film by Ru vacancies induced phase transition

Effective control of magnetic anisotropy, including perpendicular magnetic anisotropy (PMA) and lateral magnetic anisotropy, is important for the design of low-power and high-density spintronic devices. However, the rarity of oxide materials with PMA and stringent conditions required to control magnetic anisotropy have prevented its large-scale application. Here, we demonstrate that the magnetic anisotropy of SrRuO3 films can be specified on-demand by adjusting the content of Ru vacancies to control the structure of the films. With the increase in Ru vacancies, the structure of SrRuO3 changes from orthorhombic to tetragonal. The field angle dependence of the Hall resistance confirmed that the uniaxial magnetic easy axis of SrRuO3 thin films continuously rotates in the (100)pc crystallographic plane, which is identical to the continuous phase transition. Our results not only provide a way to continuously tune the physical properties of epitaxial oxide films by continuously changing the composition but also help to provide guidance for the on-demand design of spintronic devices.

Effect of laser power on structural and magnetic properties of anisotropic Nd-Fe-B film magnets prepared by PLD method

Anisotropic Nd-Fe-B film magnets are applied to miniaturized electronic devices and MEMS (Micro Electro Mechanical Systems) devices have been prepared by the sputtering method. However, the thickness of each film is mainly less than 20 μm. We have already fabricated anisotropic rare-earth film magnets by pulsed laser deposition (PLD) technique. In this study, an investigation has been carried out on increasing laser beam power for a high deposition rate. Although the thickness of the films increases as laser power increases, it is suggested that the residual magnetic polarization ratio and coercivity values of anisotropic Nd-Fe-B film magnets are not allowed to deteriorate with increasing laser power or deposition rate. These results suggest that we have the facility in obtaining perpendicularly anisotropic Nd-Fe-B thick-film magnets using the PLD method.

Preparation of Mechanically Anisotropic Polysaccharide Composite Films Using Roll-Press Techniques.

Natural polysaccharides are biocompatible and biodegradable; therefore, they can be used as feedstock for biodegradable structural materials and biomaterials. In this study, anisotropic polysaccharide composite films consisting of chondroitin sulfate C (CS) and chitosan (CHI) were fabricated from their polyion complex (PIC) gels by roll-press techniques. The obtained films (CS/CHI films) were thin and transparent, similar to the composite films prepared by hot-press techniques. The roll-press conditions were optimized, and it was observed that the molecular weight of CHI did not significantly affect the formability of the films, whereas the roll temperature and rolling speed were important. The tensile tests of the roll-pressed films revealed that the mechanical strength of the films in the mechanical direction (MD) was approximately 5 times higher than that in the transverse direction (TD), indicating that the roll-press techniques imparted mechanical anisotropy to the films. Additionally, the films shrank in the MD and expanded in the TD after immersion in aqueous solutions, followed by drying. Such anisotropic shrinking and expanding properties indicate that these films can be used as shape-memory materials.

An Oxidative Enzyme Boosting Mechanical and Optical Performance of Densified Wood Films.

Nature has evolved elegant ways to alter the wood cell wall structure through carbohydrate-active enzymes, offering environmentally friendly solutions to tailor the microstructure of wood for high-performance materials. In this work, the cell wall structure of delignified wood is modified under mild reaction conditions using an oxidative enzyme, lytic polysaccharide monooxygenase (LPMO). LPMO oxidation results in nanofibrillation of cellulose microfibril bundles inside the wood cell wall, allowing densification of delignified wood under ambient conditions and low pressure into transparentanisotropic films. The enzymatic nanofibrillation facilitates microfibril fusion and enhances the adhesion between the adjacent wood fiber cells during densification process, thereby significantly improving the mechanical performance of the films in both longitudinal and transverse directions. These results improve the understanding of LPMO-induced microstructural changes in wood and offer an environmentally friendly alternative for harsh chemical treatments and energy-intensive densification processes thusrepresenting a significant advance in sustainable production of high-performance wood-derived materials.

Key Parameters for Detectivity Improvement of Low Noise Anisotropic Magnetoresistive Sensors Made of La2/3Sr1/3MnO3 Single Layers on Vicinal Substrates

The current trend in magnetoresistive sensors development is to increase the sensitivity of single sensing elements by using multilayer structures and to design them into arrays. Such arrays are designed to compensate the excess low frequency noise of individual elements, which limits their magnetic resolution. Here, we report the modeling, design, and fabrication of single layer anisotropic magnetoresistive (AMR) sensors using low noise epitaxial La2/3Sr1/3MnO3 (LSMO) oxide thin films deposited on vicinal SrTiO3 substrates. The fabrication process is simple, and the operation of the sensor is based on a step-induced uniaxial magnetic anisotropy, described using the Stoner–Wohlfarth model. A coherent magnetization reversal process is observed by magneto-optical Kerr effect imaging. A good agreement between experimental data and the expected sensor response confirms the correct operation of the device. Three main fabrication parameters, namely the vicinal angle of the substrate, the deposition temperature, the thin film thickness, and their effects on film anisotropy field and device detectivity have been studied. Detectivity levels as low as 1.4 nT Hz–1/2 at 1 Hz and 240 pT Hz–1/2 in the white noise region are achieved with a single Wheatstone bridge element operating at 310 K. Compared to GMR and AMR sensors, these results are promising for further development and for their use as single layer LSMO low field AMR sensors, including applications as implantable biomedical devices.

Stretchable and biodegradable chitosan-polyurethane-cellulose nanofiber composites as anisotropic materials

Chitosan is a naturally derived biodegradable polymer with abundancy, sustainability, and ease of chemical modification. Polyurethanes are a family of elastic biocompatible polymers, and composites of polyurethanes have versatile properties and applications. Chitosan-polyurethane composites were recently developed but had insufficient strength and limited stretchability. In the current study, cellulose nanofibers (CNFs) were integrated in chitosan-polyurethane composites to prepare stretchable and anisotropic materials. A biodegradable polyurethane was first synthesized, end-capped with aldehyde to become dialdehyde polyurethane (DP) nanoparticles, and added with CNFs to prepare the DP-CNF composite crosslinker (DPF). The waterborne DPF crosslinker was then blended with chitosan solution to make polyurethane-CNF-chitosan (DPFC) composites. After blending, DPFC may form hydrogel in ~33 min at room temperature, which confirmed crosslinking. Composite films cast and dried from the blends showed good elongation (~420.2 %) at 60 °C. Anisotropic films were then generated by tension annealing with pre-strain. The annealed films with 200 % pre-strain exhibited large elastic anisotropy with ~4.9 anisotropic ratio. In situ SAXS/WAXS analyses unveiled that rearrangement and alignment of the microstructure during tension annealing accounted for the anisotropy. The anisotropic composite films had the ability to orient the growth of neural stem cells and showed the potential for biomimetic and tissue engineering applications.

Bct Structure and Uniaxial Magnetic Anisotropy of Fe-Co Films with V-N Added

bct Structure and Uniaxial Magnetic Anisotropy of Fe-Co Films with V-N Added

ANALYSIS OF STUDYING THE STRUCTURE OF EPITAXIAL IRON FILMS ON GALLIUM ARSENIDE Fe/GaAs

The anisotropy in thin films coincides with the crystallographic direction of an increase in the thickness of the iron layer. The value of the cubic anisotropy constant decreases. In this case, the value of the uniaxial anisotropy constant, on the contrary, increases. The dilution of the semiconductor matrix with iron atoms is also noted, and the formation of Fe-Ga or Fe-As compounds is assumed. An analysis of the surface sensitivity of the spectra of As and Ga shows that As is more easily mixed with Fe than with Ga.

Validity of DFT-based spin-orbit torque calculation for perpendicular magnetic anisotropy in iron thin films

In this study, the validity of our DFT-based self-developed JunPy+SOT calculation method has been rigorously confirmed by the perpendicular magnetic anisotropy (PMA) induced effective magnetic anisotropy field of 5.6 kOe in a 1.5 nm-thick iron thin film. The angular dependence of the magnetic anisotropy energy (MAE) and the total spin-orbit torque (SOT) indicate the uniaxial PMA corresponding to the out-of-plane rotations of the magnetization. Our results agree with the conventional MAE calculation but provide deeper insights into atomistic spin dynamics of local magnetic moments. The main advantage of this method is that the well-decomposed layer-resolved SOT and SOC-dominated spin current accumulation, which exhibit maximum magnitude but opposite sign at the surface, provide comprehensive physical understanding in angular momentum transfer between spin and orbital, competition between SOT and spin current accumulation, and precise spin torque acting on each local magnetic moment that is crucial for the atomistic spin dynamics especially in magnetic heterostructures.

In-plane magnetic anisotropy and domain structure of FeNi films deposited in the presence of a magnetic field of various configurations

FeNi films in circular shape with 3 mm in diameter obtained by magnetron sputtering in the presence of a constant magnetic field of various configurations are studied. The presence of a field created by a permanent magnet with a diameter of 1.2 mm, the magnetization of which is oriented perpendicular to the plane of the substrate, leads to the formation of a vortex-like magnetic structure in the film, characterized by diffuse domain walls. Keywords: thin magnetic films, induced magnetic anisotropy, magnetic domain structure, magnetron sputtering.