MgO Single Crystals Research Articles

Depth profiles of damage creation and hardening in MgO irradiated with GeV heavy ions

The effect of irradiation with GeV heavy ions (U, Au, Bi) on the structure and mechanical properties of MgO single crystals was studied. The methods of nanoindentation, dislocation mobility, optical absorption and photoluminescence (PL) spectroscopy, X-ray diffraction and atomic force microscopy were used for damage characterization. The ion-induced increase of hardness and reduction of dislocation mobility was observed. The depth profiles of hardness, dislocation mobility and PL were investigated, and the contribution of electronic and nuclear loss mechanisms was confirmed. The efficiency of damage vs. average absorbed energy for heavy and light ions was compared. The change in the mechanism of plastic deformation at indentation was observed after severe irradiation due to the immobilization of dislocations by ion-induced extended defects. The results show that MgO single crystals maintain integrity and micro-plasticity at indentation, and exhibit improved hardness after irradiation with swift heavy ions at fluences up to 7 × 1013 ions/cm2.

Strain relaxation in epitaxial γ′-Fe4N ultrathin films

To investigate substrate-induced strain effects, we grew epitaxial γ’-Fe4N ultrathin films on three different single crystal substrates, MgO, SrTiO3 (STO), and LaSrAlTa oxide. The lattice mismatch was −9.7%, −2.6%, and −1.5% between the Fe4N layer and the MgO, STO, and LSAT substrates, respectively. Therefore, the strain was varied depending on the substrates. Cross-sectional transmission electron microscopy revealed that the lattice strain of Fe4N layer on the STO and LSAT maintained from the interfaces up to ∼2 nm, note here that we achieved to induce strain for the entire Fe4N film thickness. On the other hand, rapid relaxation appeared through many misfit dislocations in the case on MgO. The strain effects in the ultrathin Fe4N films were confirmed as the different behaviors for both ferromagnetic resonance and anisotropic magnetoresistance. These results broaden the amount of modulation in Fe4N based device properties by strain when we reduce the thickness down to several nanometer, which is a dimension encountered in recent spintronic switching devices.

Built-in electric field induces polarization rotation in bilayer BiFeO3/(Ba,Sr)TiO3 thin films

The crystal structure of BiFeO3/BaxSr1-xTiO3 (BFO/BST) heterostructures with x = 0.2, 0.6 and 0.8, grown on single-crystal MgO (001) substrate was investigated by x-ray diffraction and Raman spectroscopy in order to determine the influence of mismatch-induced strains and spontaneous polarization in BST buffer layers on BFO layers. The lattice parameter of the BFO layers was shown to decrease with increasing concentration of Ba ions, despite the increasing in-plain lattice parameters of tetragonal unit cells of BST layers. The rhombohedral angle of the crystal structure of BFO layers demonstrates an increase towards the ideal cubic perovskite structure with the appearance of the built-in electric field, induced by the spontaneous polarization in buffer layers. This result provides a remarkable tool for the control of polarization in BFO layers and other ferroelectric films in general, by changing the built-in electric field from ferroelectric buffer layer without changing a single crystal substrate.

Realization of Tunable Localized Surface Plasmon Resonance of Cu@Cu2O Core-Shell Nanoparticles by the Pulse Laser Deposition Method.

Cu@Cu2O core–shell nanoparticles (NPs) not only possess a stabilized structure but also exhibit better photocatalytic performance as compared to pure Cu2O. Therefore, preparation of Cu@Cu2O core–shell NPs is key toward efficient photocatalysis applications. In this paper, the fabrication of Cu@Cu2O core–shell NPs on single-crystal MgO(100) substrates has been studied systematically by pulse laser deposition. Scanning electron microscopy (SEM) images show that the average diameter of NPs is enlarged from 89.9 to 150.3 nm with the increasing of oxygen pressure. Transmission electron microscopy (TEM) images indicate that the NPs have elongated hexagons and a core–shell structure with a shell thickness of about 10 nm. UV–vis absorption spectra show that the position of the localized surface plasmon resonance (LSPR) peaks shifts from 648 to 858 nm and the full width at half-maximum (fwhm) of the LSPR peaks broadens from 226.7 to 436.5 nm with increasing average diameter of NPs. According to the analysis, the red shift of the LSPR peaks is caused by enlargement of the core diameter; higher fwhm is a result of broadened particle size distribution and the elongated morphology of NPs. Therefore, the width and range of LSPR peaks of the absorption spectrum can be tuned using this method, which is beneficial for enhancing the light absorption and improving the photocatalytic efficiency of Cu@Cu2O core–shell NPs.

Superconductivity Dependence on Epitaxial NbN Film Thickness

A series of NbN films (2.2–200 nm thick) were epitaxially grown on (100) oriented single crystal MgO substrates using reactive dc magnetron sputtering. As the film thickness decreased, the superconducting critical temperature ( T C) and the residual resistivity ratio both decreased, the normal state resistivity increased, while the zero temperature coherence length remained basically unchanged. In addition, a negative linear relationship existed between the T C and the normal state resistivity at 20 K. Hall Effect measurements showed that the carrier density of the 200-nm-thick NbN film that exhibited the highest T C (16.63 K) decreased from 1.12 × 1023 to 5.56 × 1022 e /cm−3 in going to the 2.2-nm-thick NbN film that exhibited the lowest T C (9.28 K). By fitting the data using McMillan theory for strong coupling superconductors, the T C for the NbN films was determined directly from the thickness-dependent carrier density.

Effect of Al2O3 Seed-Layer on the Dielectric and Electrical Properties of Ultrathin MgO Films Fabricated Using In Situ Atomic Layer Deposition.

Metal/insulator/metal (M/I/M) trilayers of Al/MgO/Al with ultrathin MgO in the thickness range of 2.20-4.40 nm were fabricated using in vacuo sputtering and atomic layer deposition (ALD). In order to achieve a high-quality metal/insulator (M/I) interface and hence high-quality dielectric ALD-MgO films, a 5 cycles (∼0.55 nm) thick ALD-Al2O3 seed layer (SL) was employed to demonstrate the dielectric constant (εr) is ∼8.82-9.38 in 3.30-4.95 nm thick ALD-MgO/SL films, which is close to that of single-crystal MgO εr ∼ 9.80. In contrast, a low εr of 3.55-4.66 for the ALD-MgO films of a similar thickness without a SL was observed. The effective oxide thickness (EOT) of ∼1.40 nm has therefore been achieved in the ultrathin ALD-MgO films, which are comparable to the EOTs of high-K dielectrics such as HfO2. In addition, the leakage current through the M/I/M structure is reduced by more than 1 order of magnitude with implementation of the SL. The high leakage current in the samples without a SL can be attributed to the nonuniform nucleation of the ALD-MgO on the Al surface with a significant portion of the Al surface remaining conductive as confirmed using in vacuo scanning tunneling spectroscopy (STS). With the SL, the STS study has confirmed a tunnel barrier height of 1.50 eV on 0.55 nm MgO with 0.55 nm Al2O3 SL with almost 100% coverage. In addition, molecular dynamics simulations point out the importance of deposition of ultrathin SL that has a significant effect on the initial nucleation of the Mg precursor. This result not only illustrates the critical importance of controlling the M/I interface to obtain high-quality dielectric properties of ultrathin ALD films but also provides an approach to engineering incompatible M/I interfaces using a SL for a high-quality dielectric required for applications in M/I/M tunnel junctions and complementary metal oxide semiconductors.

Magnetic in-plane components of FePt nanogranular film on polycrystalline MgO underlayer for heat-assisted magnetic recording media

A systematic study is conducted on the effect of MgO substrate/underlayer in terms of the surface roughness, grain boundary, and crystallographic texture on the magnetic in-plane components (medium noise) of the FePt-based heat-assisted magnetic recording (HAMR) media. Cross-sectional transmission electron microscope observations revealed that good (001) texture can be developed in FePt nanogranular films even when they are grow on a rough surface of a single crystal-MgO (001) substrate or across the grain boundaries of a (001)-textured polycrystalline-MgO underlayer. The misoriented FePt grains with magnetic easy axes (c-axes) tilted away from the film normal direction mainly originate from their epitaxial growth on the initially misoriented MgO underlayer grains. Micromagnetic simulation results revealed that the FePt grains with misorientation angle larger than ∼20° are attributed to the detected magnetic in-plane components. Based on these experimental results, we discuss how to improve the (001) texture of the polycrystalline underlayer as well as to develop FePt-based HAMR media with enhanced signal-to-noise ratio.

Strong perpendicular magnetic anisotropy induced by broken symmetry of A1-CoPt

Strained CoPt films have been deposited on TiN intermediate layers with single crystal MgO as the substrate. At a thickness of 2.5 nm, CoPt films grow epitaxially on the TiN layer while being strained horizontally. On the other hand, the 10 nm sample is partially relaxed by misfit dislocations. Different strain conditions result in different c/a ratios of the respective lattices. With a c/a ratio of 0.974, the 2.5 nm CoPt film with a perpendicular coercivity of 11 kOe could be obtained at a temperature (400°C) much lower than the usual fabrication temperature (∼800°C). The broken symmetry induced by the lattice distortion is believed to be the main source of the perpendicular magnetization. This study may open new possibilities of fabricating CoPt based magnetic recording media at lower temperatures.

High-Pressure Third-Order Elastic Constants of MgO Single Crystal: First-Principles Investigation

Abstract High-pressure third-order elastic constants of materials have rarely been investigated experimentally and theoretically to date, so the predictive ability of the method of the volume-conserving, homogeneous deformations based on the first-principles total-energy calculations is tested for the first time in this work. Using this approach, the high-pressure third-order elastic constants C 111 − 3 C 112 + 2 C 123 ${C_{111}}-3{C_{112}}+2{C_{123}}$ , C 111 / 2 + 3 C 112 + C 123 ${C_{111}}/2+3{C_{112}}+{C_{123}}$ , C 144 − C 155 ${C_{144}}-{C_{155}}$ , and C 456 of the MgO single crystal are obtained successfully. Meanwhile, the reliability of this method is also verified by comparing the calculated structural properties and high-pressure second-order elastic constants of the MgO single crystal with the available experimental results and other theoretical predications. Results not only indicate the accuracy of our calculations but also reveal the feasibility of the present theoretical method. It is hoped that the present theoretical method and predictions on the high-pressure third-order elastic constants of the MgO single crystal would serve as a valuable guidance or reference for further related investigations.

Double epitaxy of tetragonal and hexagonal phases in the FeSe system

Epitaxial films which contain more than one crystallographic phase or orientation are of interest due to the possibility of altered magnetic, electrical, and optical properties. Thin films of FeSe have been grown on single-crystal MgO substrates under conditions that produce the simultaneous, epitaxial growth of tetragonal and hexagonal phases. We show that this double epitaxy is characterized by phase domains with a well-defined epitaxial relationship to each other and that the relative phase fraction can be controlled. For growth temperatures of 350–450 °C, the (0 0 1)-oriented tetragonal phase (β-FeSe) grows with its unit cell aligned with the cubic substrate, while a (1 0 1)-oriented hexagonal phase (Fe7Se8) shows domains with two different in-plane orientations separated by 45°. Additionally, the β-FeSe phase can be chosen to be (0 0 1)- or (1 0 1)-oriented with respect to the substrate, with the (1 0 1) orientation containing three rotated domains.

Channeling analysis in studying ion irradiation damage in materials containing various types of defects

Ion channeling is a powerful quantitative technique for studying ion-irradiation induced defect evolution in single crystalline materials. An iterative procedure to determine dechanneling yields has been developed for decades, serving as a major method for analyzing experimental channeling data. The applicability of such procedure is, however, generally limited to the crystalline damage with only point defects and local amorphous domains. For the other cases, such as irradiated metals, the assumption of direct-backscattering free has usually been made. In the present study, Ni, TiAl, MgO, and SrTiO3 single crystals are selected as four model materials, representing metals, intermetallic alloys, and ceramic compounds with different defect evolution processes under irradiation, to investigate the fidelity of applying dechanneling analysis on various types of defects. The pure dechanneling assumption is shown oversimplified in Ni irradiated with low fluence self-ions and may result in error on the derived damage profile. Moreover, the iterative procedure of dechanneling analysis is shown valid for more general situations than the randomly distributed atoms, including those not exhibiting a peak in channeling spectra. The disappearance of damage peak in channeling spectra is attributed to the combined effects of small (but non-zero) scattering factor, long-range damage effects, and non-ignorable damage level in pristine crystals. Furthermore, the ratio of direct backscattering to dechanneling areas provides information on defect configurations in the materials containing a well-defined damage peak in channeling spectra. The contribution from dechanneling sources increases from SrTiO3, TiAl, to MgO, according to the derived scattering and dechanneling factors.

In situ soft x-ray absorption spectroscopic study of polycrystalline Fe/MgO interfaces

Interfacial interactions between a layer of iron and MgO hold the key to various phenomena like tunnel magnetoresistance, perpendicular magnetic anisotropy, interlayer exchange coupling, observed in the system. Interface structure has been studied in situ during deposition of iron on MgO surface, using soft x-ray absorption spectroscopy (SXAS). Sub-monolayer sensitivity of SXAS, combined with in situ measurements as a function of iron layer thickness, allowed one to study the evolution of interface with film thickness. Two different substrates namely MgO (0 0 1) single crystal, and a polycrystalline MgO film on Si substrate have been used in order to elucidate the role of the state of MgO surface in controlling the interface structure. It is found that at the interface of iron and MgO film, about two monolayers of Fe3O4 are formed. Fe3O4 being the oxide of iron with the highest heat of formation, the reaction appears to be controlled thermodynamically. On the other hand, on the interface with MgO (0 0 1) surface, FeO is formed, suggesting that the reaction is limited by the availability of oxygen atoms. Magnetic behavior of the FeO layer gets modified significantly due to proximity effect of the bulk ferromagnetic iron layer.

Ellipticity dependence transition induced by dynamical Bloch oscillations

The dependence of high-harmonic generation (HHG) on laser ellipticity is investigated using a modified ZnO model. In the driving of relatively weak field, we reproduce qualitatively the ellipticity dependence as observed in the HHG experiment of wurtzite ZnO. When increasing the field strength, the HHG shows an anomalous ellipticity dependence, similar to that observed experimentally in the single-crystal MgO. With the help of a semiclassical analysis, it is found that the key mechanism inducing the change of ellipticity dependence is the interplay between the dynamical Bloch oscillation and the anisotropic band structure. The dynamical Bloch oscillation contributes additional quantum paths, which are less sensitive to ellipticity. The anisotropic band-structure make the driving pulse with finite ellipticity be able to drive the pairs to the band positions with larger gap, which extends the harmonic cutoff. The combination of these two effects leads to the anomalous ellipticity dependence. The result reveals the importance of dynamical Bloch oscillations for the ellipticity dependence of HHG from bulk ZnO.

A waveguide-based flexible CO2-laser heating system for diamond-anvil cell applications

Fiber-based laser heating systems offer high flexibility and are increasingly used in high-pressure/high-temperature research. Previous fiber-based laser heating systems were based on lasers delivering radiation with 1-μm wavelength that show unfavorable heating characteristics for many sample materials. We report on the development and first experiments of a waveguide (fiber)-based CO2-laser heating system that can be used in a flexible manner for high-temperature research and for high-pressure/temperature experiments when combined with diamond-anvil cells. The waveguide-based CO2-laser heating system allows convenient heating of optically transparent samples without the need for adding an additional laser absorber. In a pilot experiment, the flexible heating system has been installed on a Brillouin scattering system to measure high-temperature sound-wave velocities in single-crystal MgO. The waveguide-based CO2-laser heating system offers perspectives for a variety of scientific applications, most importantly those that require the use of synchrotron facilities where space is limited and flexible solutions are required.

Josephson Tunneling Behaviors in NbN/AlN/NbN Junctions with an Ultrathin NbN Film

We fabricated epitaxial NbN/AlN/NbN tunnel junctions with an ultrathin NbN film as the base layer on single-crystal MgO (100) substrates. The measured thickness of the base layer was 4.7 nm. By measuring the electrical characteristics of the junctions, we showed that the junction had excellent Josephson tunneling properties, with a large gap voltage, a small gap voltage width, and a quality factor Rsg/RN of 42 at 4.2 K. The temperature dependence of the critical current for the junctions agreed with a theoretical calculation using the Ambegaokar-Baratoff relation over the temperature range of 3-12 K. An ideal Fraunhofer diffraction pattern appeared for the Josephson critical current as a function of magnetic field. Fiske steps were observed in current-voltage characteristics of the Josephson junctions at different applied magnetic field.

Fabrication and optical properties of highly transparent MgO ceramics by spark plasma sintering

Highly transparent MgO ceramics have been prepared via Spark Plasma Sintering (SPS) at 1200°C for 10–60min, under a pressure of 100MPa. All of the prepared ceramics were visually transparent and exhibited relatively high in-line transmittance. The ceramic sintered for 60min demonstrated the highest in-line transmittance, of 60–80% in the 300nm to 9μm wavelength range; values that are close to that reported for MgO single crystals. Photoluminescence measurements revealed a peak located at ~360nm, which was determined to be due to oxygen vacancy color centers induced by the reducing atmosphere of the SPS process in vacuum atmosphere.

Microstructure and optical properties of ultra-thin NiO films grown by atomic layer deposition

High-quality ultrathin films of the p-type wide bandgap semiconductor NiO are potentially interesting candidates for a number of frontier applications. Here we have fabricated a series of NiO thin films with a precise thickness control by atomic layer deposition from Ni-bis(2,2,6,6-tetramethyl-3,5-heptanedionate) and O3 precursors. While randomly oriented polycrystalline films were obtained on glass and Si substrates, epitaxial growth was achieved on sapphire, SrTiO3 and MgO single crystals. Optical energy gaps were determined from UV–vis absorption spectra for the films grown on borosilicate glass, and found to decrease with decreasing film thickness. This observation is in contrast to the expected bandgap increase through quantum confinement, but can be explained by an increased Coulomb interaction. Furthermore, NiO films doped with Co, Cu and Mn were deposited. Electrical measurements showed that the doping has a much higher impact on the two-probe resistance than on the resistivity. A likely explanation is a change of the contact resistance by a change of hole density.

Vortex glass-liquid transition and activated flux motion in an epitaxial, superconducting NdFeAs(O,F) thin film

An epitaxial NdFeAs(O,F) thin film of 90 nm thickness grown by molecular beam epitaxy on MgO single crystal with Tc = 44.2 K has been investigated regarding a possible vortex glass-liquid tran-sition. The voltage-current characteristics show excellent scalability according to the vortex-glass model with a static critical exponent {\nu} of around 1.35 and a temperature-dependent dynamic exponent z increasing from 7.8 to 9.0 for the investigated temperature range. The large and non-constant z values are discussed in the frame of 3D vortex glass, thermally activated flux motion, and inhomogeneity broadening.

Surface preparation and the evolution of atomically flat step terrace morphology of MgO single crystals

We present a preparation method of vicinal stepped MgO (100) substrate surfaces, a prerequisite for high quality thin film growth. Through atomic force microscopy measurements, the surface morphology evolution of (100)-oriented MgO as a function of the annealing conditions is investigated. In particular, we discuss the effect of temperature, time and ambient atmosphere on the evolution of the MgO surface arrangement. At annealing temperature of 1000°C for 40 min, MgO surface morphology displays an atomically flat, single terminated surface with (100)-oriented, sharp terraces and equidistant steps equal to half of the MgO lattice parameter. Longer annealing leads to the formation of bunched terraces with large step heights of 1-5nm. The optimized recipe developed here efficiently removes surface adsorbates, namely hydroxyl and carbonate groups and restores the terraces on the surface.

Molecular beam epitaxy growth and optical properties of Mg3N2 films

Epitaxial Mg3N2 films with a (100) orientation have been grown by plasma-assisted molecular beam epitaxy on single crystal MgO substrates. The growth was monitored in situ by both reflection high-energy electron diffraction and optical reflectivity. The growth rate was determined from the optical reflectivity during growth. The index of refraction of Mg3N2 was measured by spectroscopic ellipsometry and found to be in good agreement with the in situ reflectivity. The optical bandgap was found to be ∼2.5 eV from transmission measurements.