Polarized neutron grazing incidence diffraction on magnetic epitaxial thin films

Surfaces and interfaces characterization by neutron reflectometry

J. Kent Howard, some years ago, published an extensive review of the phenomenon of what he referred to as “grain-to-grain” or “polycrystalline” epitaxy in thin films. Of the six or so different applications of this phenomenon, from superconductors to magnetics, none has greater technological significance than applications to magnetic thin films. This derives from the magnetic anisotropy of these thin films which has such a profound influence on their magnetic properties. In particular, magnetocrystalline anisotropy is often used to engineer thin films with the desired orientation of the easy axis of magnetization for magnetic record ing media applications. Magnetocrystalline anisotropy is intimately connected to the the crystallographic preferred orientation of the grains composing the magnetic layer which can be controlled through the skillful use of epitaxy with a suitable sublayer. The techniques of AEM, especially elongated probe microdiffraction (EPMD), have proven themselves invaluable for characterizing the micro-structure of such thin films.

The crystal structure of magnetic amorphous oxide thin films in the Ni-Fe-P system made by the two source evaporation method was studied. When the amorphous thin films are heated, the magnetization disappears at the Curie temperature (Tc1), but, at almost exactly the same time, crystallization begins and causes the magnetization to once again increase. Finally the magnetization of the crystal line phase disappears at (Tc2). The amorphous oxide films are presumed to possess a magnetic coupling similar to that in nickel ferrite.

ABSTRACTWe report the direct measurement of elastic strain effect on the electrical and magnetic properties of single domain epitaxial SrRuO3 thin films, using a lift-off technique. The as-grown films on vicinal (001) SrTiO3 substrates are subjected to elastic biaxial compressive strain within the plane and tensile strain normal to the plane. In contrast, the lift-off films prepared by chemical etching of SrTiO3 substrates, are completely strain free with bulk like lattice. Our measurements indicate that the elastic strain can significantly affect the electrical and magnetic properties of epitaxial ferromagnetic SrRuO3 thin films. For the strained films, the Curie temperature (Tc) was suppressed to 150K and the saturation magnetic moment (Ms) was decreased to 1.15μB/Ru atom as compared to a Tc of 160K and Ms of 1.45μB/Ru atom for the strain free films. These property changes are attributed to the structural distortion due to the elastic strain in the as-grown epitaxial thin films. Our results provide direct evidence of the crucial role of lattice strain in determining the properties of the perovskite epitaxial thin films.

Magnetic materials either in bulk or thin films are ubiquitous in our daily life. Technology based on magnetic materials range from chunk of bulk ferromagnet to thin film spintronics. In solid state bulk materials, information about its magnetic structure together with crystal structure is absolutely necessary to manipulate them in applications. Neutron diffraction is an important tool to determine atomic magnetic moments and its directions at the lattice site in the magnetic unit cell. It also investigates the type of magnetic ordering in conventional as well as new exotic materials. Magnetic thin films are engineered materials in which nanometer or sub-nanometer thickness scale films are grown. At such thicknesses nanoscale magnetic properties are fundamentally different than its bulk counterpart. Neutron reflectometry is a unique tool to investigate nano-magnetism in thin films. Moreover, in multilayer thin films generally used for spintronics, polarized neutron reflectometry is indispensable characterizing tool which investigates the magnetic properties in different layers and at the interfaces. In this chapter, we will introduce how neutron diffraction and reflectometry techniques play unique role in the investigation of magnetic structure and magnetic properties of functional bulk and nano-scale thin films.

Media for use in perpendicular magnetic recording must have a magnetic anisotropy directed normal to the film surface; as recording densities continue to rise, this method is fast becoming a center of attention. In a previous report, we focused on magnetic thin films with a perpendicular anisotropy caused by the shape anisotropy due to nonmagnetic impurities in films, the orientation of the film crystal and a columnar microstructure, and reported on the magnetic properties of (FeMn)Bi system thin films. In connection with this, we studied the magnetic properties and structure of FeBi thin films, and found that slight changes in the Bi concentration had a profound effect on the growth of crystal grains, with corresponding changes in the magnetization and coercivity of the evaporated ffilms. When columnar grains form in the Fe (110) direction, normal to the substrate, as occurs in 3000 A thick films with a Bi concentration of 1.6 at% and grown at a substrate temperature of approx. 100°C, perpendicular magnetization film with a coercivity of 500 Oe and a remanent magnetization of 630 G is obtained.

Epitaxial thin films can offer unprecedented definition of an electrocatalyst surface, defining the crystallographic orientation, interaction with a current collector, and providing an atomically flat surface for quantitative spectroscopy yielding mechanistic insights. This review introduces experts in thin films to the important parameters of electrochemical setups, techniques, and methods for quantifying electrocatalytic activity and comparing it across the literature. Examples are drawn from the literature measuring oxygen electrocatalysis on epitaxial oxide thin films. We share best practices in the robust measurement of intrinsic activity of thin films, including methods of electrical contact and assessment of charge transport in situ. We conclude by highlighting recent insights from epitaxial films in understanding oxygen electrocatalysis.

By lifting an epitaxial thin film off its growth substrate, we directly and quantitatively demonstrate how elastic strain can alter the magnetic and electrical properties of single-domain epitaxial SrRuO3 thin films (1000 Å thick) on vicinal (001) SrTiO3 substrates. Free-standing films were then obtained by selective chemical etching of the SrTiO3. X-ray diffraction analysis shows that the free-standing films are strain free, whereas the original as-grown films on SrTiO3 substrates are strained due to the lattice mismatch at the growth interface. Relaxation of the lattice strain resulted in a 10 K increase in the Curie temperature to 160 K, and a 20% increase in the saturation magnetic moment to 1.45 μB/Ru atom. Both values for the free-standing films are the same as that of the bulk single crystals. Our results provide direct evidence of the crucial role of the strain effect in determining the properties of the technologically important perovskite epitaxial thin films.

Developing high frequency soft magnetic thin films is still a hot issue due to the requirement of the technology in high frequency field. Traditionally, tuning the high frequency performance of magnetic thin films is realized by tuning the in-plane uniaxial anisotropy field of magnetic thin films with amorphous or nanocrystalline structure. There are several methods to obtain the in-plane uniaxial magnetic anisotropy such as applying an in-plane magnetic field during thin film deposition, magnetic field annealing, addition of elements or compounds, and patterned thin films to create shape anisotropy method [1]. Moreover, with the increase of the grain size, the high frequency performance becomes worse due to the decrease of the in-plane uniaxial anisotropy field and associated the cut-off frequency according to the random exchange anisotropy model of Herzer [2]. However, there are few works are concerned the high frequency properties of epitaxial single crystal film. In this work, Fe0.84Si0.16 (FeSi)/Ta (5 nm) thin films with thickness of 70 nm have been deposited on (001) MgO substrates by rf magnetron sputtering at 400 °C with a base pressure below 3'10 −5 Pa and a processing Ar pressure of 0.2 Pa, and then heat-treated for 1 h at 800 °C with the heating rate of 50 °C/min. MgO/FeSi/Ta shows (002) and (004) peaks, which indicated that FeSi(001)[110]//MgO(001) [100] film is epitaxial growth. The mainly chemical order phase is B2 with a significant A2 disorder phase according to the X-rays diffraction peak intensity ratio [3]. Angular dependence of remanence ratio (ARR) and hysteresis loops with the magnetic field applied parallel to in-plane film were measured by vibrating sample magnetometer (VSM) at room temperature. A four-fold symmetry of remanence ratio from the ARR graph was observed, indicating the cubic magnetic crystalline anisotropy of FeSi films. However two adjacent easy axis direction remanence ratios are not completely equivalent, which may be caused by a superstition of an extra in-plane uniaxial anisotropy with easy axis along the [100] direction as shown in Fig. 1. Thus, we may conclude the presence of a superstition of a strong four-fold magneto-crystalline anisotropy and a small in-plane uniaxial magnetic anisotropy with easy magnetization axis along the [100] and [010] crystallographic directions of FeSi and hard magnetization axis along the [110] and [110] crystallographic directions. The saturation field along [110] direction reaches 550 Oe and the saturation magnetization is about 13 kGs. The dynamic magnetic properties are investigated by the one-terminal micro-strip transmission-line perturbation method in the frequency range 0.1–9 GHz. The permeability spectra are shown in Fig. 2 with an extra magnetic field applied along the [100] easy axis direction. One can see the natural ferromagnetic resonance frequency is around 8.0 GHz for epitaxial FeSi thin films on (001) MgO substrate. The quite large resonance frequency is ascribed to the large magneto crystalline anisotropy field of FeSi films with cubic anisotropy. The obtained Gibert damping constant (a) through fitting the complex permeability (μ) based on Lan-dau-Lifshitz-Gibert equation with the variation of applied magnetic field is around 0.016, which is larger than that of pure Fe film. It maybe understood by the chemical disorder of Fe and Si atoms with A2 structure.

Although there is considerable interest in BiFeO3 owing to its versatile physical properties, which make it suitable for a wide range of applications, its high leakage current is a significant limitation. Among various methods for reducing the leakage current, substitution with transition-metal or rare-earth elements is widely recognized as the most effective approach. Herein, to enable in-depth studies of the physical properties of BiFeO3, high-quality epitaxial BiFeO3 thin films with a low leakage current must be formed. However, owing to the difficulty of controlling the element doping when pulsed laser deposition is used for epitaxial thin-film growth, studies on substitutional doping based on epitaxial BiFeO3 thin films have not been systematically carried out. In this regard, we establish an innovative approach for overcoming the high leakage current of BiFeO3 by fabricating artificially engineered superlattice-based epitaxial BiFeO3 thin films in which there is a significant reduction of the leakage current. The control of the element doping in epitaxial BiFeO3 thin films is easily regulated precisely at the atomic-scale level. The results of this study strongly suggest that superlattice-based epitaxial BiFeO3 thin films can be a cornerstone for exploring the reliable fundamental physical properties of substitutional doping in epitaxial BiFeO3 thin films.

We have investigated the intrinsic thermoelectric properties of epitaxial β-FeSi2 thin films and the impact of phosphorus (P) doping. Epitaxial β-FeSi2 thin films with single phase were grown on Si(111) substrates by two different techniques in an ultrahigh-vacuum molecular beam epitaxy (MBE) system: solid-phase epitaxy (SPE), where iron silicide films formed by codeposition of Fe and Si at room temperature were recrystallized by annealing at 530°C to form epitaxial β-FeSi2 thin films on Si(111) substrates, and MBE of β-FeSi2 thin films on epitaxial β-FeSi2 templates formed on Si(111) by reactive deposition epitaxy (RDE) at 530°C (RDE + MBE). Epitaxial SPE thin films based on codeposition had a flatter surface and more abrupt β-FeSi2/Si(111) interface than epitaxial RDE + MBE thin films. We investigated the intrinsic thermoelectric properties of the epitaxial β-FeSi2 thin films on Si(111), revealing lower thermal conductivity and higher electrical conductivity compared with bulk β-FeSi2. We also investigated the impact of doping on the Seebeck coefficient of bulk and thin-film β-FeSi2. A route to enhance the thermoelectric performance of β-FeSi2 is proposed, based on (1) fabrication of thin-film structures for high electrical conductivity and low thermal conductivity, and (2) proper choice of doping for high Seebeck coefficient.

The bulk photovoltaic effect (BPE) is a nonlinear light-matter interaction in which light is converted into electrical current. Unlike the junction based PV mechanism, which relies on an interface to induce band bending between two differently doped regions or materials, or the Dember effect involves uses non-uniform incident light intensity to generate asymmetrical populations of holes and electrons, the BPE occurs in materials which lack inversion symmetry or break time-reversal symmetry. The process in which light is converted into an electrical current by the BPE involves aspects of crystal structure, electronic structure, and carrier transport. The BPE is influenced by material synthesis not unlike the materials science and engineering paradigm of structure, properties, performance, and processing. This broad perspective is important to consider, as modern research in the BPE, e.g., light detection under a broad range of wavelengths, exploring the mechanisms of the BPE, and improving the power conversion efficiency, reveals other influences that can alter the observed response. To distinguish and elucidate the contribution from the BPE, BaTiO₃ epitaxial thin films grown by pulsed laser deposition were used a model system. The main goals of this thesis are (1) investigate the roles of biaxial strain and crystallographic orientation on the BPE response and (2) to probe the influence of electrode materials and their resistivity on BPE photocurrent in epitaxial thin films. These results deepen practical understanding of the BPE, which may help to improve the BPE response in future ferroelectric photovoltaic thin films.

Effect of ionizing radiations on the crystalline structure and electric resistivity of cadmium sulphide thin films

To achieve the epitaxial thin films deposition, it is necessary to use properly oriented substrates, with or without buffer layers, matching the lattice parameters of the epitaxial thin film we want to grow. In this work, the deposition of epitaxial Bi2SiO5(200) and BiFeO3(001) thin films on Si(001) substrates by pulsed electron deposition (PED) technique is reported without special substrate preparation. The deposition of epitaxial BSO(200) and T-BFO(001) directly onto Si(001) substrates during a single target deposition process is relevant and presents enormous potential to reduce costs and improve practicality, interface quality and BFO integration efficiency with Si(001) substrates.

The degree of orientation of mixed a‐ and c‐axis‐oriented epitaxial thin films was defined. This can be evaluated using several scanning methods in X‐ray diffraction. Reciprocal space mapping was chosen because it is adaptable to the modulations of lattice spacing, as well as the small modulations of orientation. The proposed method was demonstrated for superconducting YBa2Cu3O7–δ and magnetic FePt thin films. The increasing tendency of the degree of orientation with increasing deposition temperature agreed with those obtained from the pole figure measurements. The degree of orientation was used to evaluate the order parameter of the FePt film with overlapping sublattice and fundamental peaks. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.