MgO Substrates Research Articles

Strong correlation between uniaxial magnetic anisotropic constant and in-plane tensile strain in Mn4N epitaxial films

Ferrimagnetic Mn4N is a promising candidate for current-induced domain wall motion assisted by spin-transfer and spin–orbit torques. Mn4N can be doped to have perpendicular magnetic anisotropy (PMA) and a small spontaneous magnetization. However, the origin of the PMA of Mn4N has yet to be fully understood. Here, we investigated the relationship between the ratios of the perpendicular lattice constant c to the in-plane lattice constant a of Mn4N epitaxial thin films (c/a) and the uniaxial magnetic anisotropic constant (Ku) in Mn4N thin films grown on MgO(001), SrTiO3(001), and LaAlO3(001) substrates. The lattice mismatches between Mn4N and these substrates are approximately −6%, −0.1%, and +2%, respectively. All the Mn4N thin films had PMA and in-plane tensile distortion (c/a < 1) regardless of the Mn4N thickness and substrate. Although the magnitude of c/a depended on several factors, such as the Mn4N layer thickness and substrate, we found a strong correlation between c/a and Ku; Ku increased markedly when c/a deviated from 1. This result indicates that the origin of PMA is tensile distortion in Mn4N films; hence, it might be possible to control the magnitude of Ku by tuning c/a through the Mn4N layer thickness and the substrate.

Epitaxial Fe3O4 Films Grown on R-Plane Sapphire by Pulsed Laser Deposition

We have studied the influence of growth temperature and molecular oxygen pressure on the properties of thin (≤180 nm) epitaxial magnetite (Fe3O4) (001) films grown by pulsed laser deposition on R-plane single-crystal sapphire (Al2O3 ($$\bar {1}$$012)) with and without MgO seed layer. The electrical, morphological, and structural characteristics of the films have been investigated as functions of growth conditions. Fe3O4 has been shown to have a stable growth plateau at pressures in the range (4–9) × 10–5 Torr. The properties of the epitaxial Fe3O4 films grown on a MgO seed layer (5 nm thick) approach those of magnetite films grown on single-crystal MgO substrates and are superior to those of films grown on pure R-plane sapphire. The best electrical characteristic of the films and the corresponding crystal structure can be obtained at elevated growth temperatures, whereas reduced growth temperatures minimize the roughness of their surface and maximize its homogeneity. These conditions can be reconciled by high-temperature high-vacuum annealing of magnetite films grown at reduced temperatures.

Strained lateral structure and its relaxation in a Cu2O thin crystal epitaxially grown on MgO surface

We measured in-plane X-ray diffraction and investigated the strained lateral structure and its relaxation in a Cu2O thin crystal epitaxially grown in a small gap between paired MgO substrates. Although one side MgO was peeled off to irradiate Cu2O with X-ray, a thin MgO layer remained on Cu2O, and diffraction peaks of (400)-planes in both MgO and Cu2O were detected. The peak has four-fold symmetry on sample rotation, and the lattice constant estimated from the diffraction angle is 4.254 Å. The lattice constant is larger than that of MgO and smaller than that of strain-free Cu2O. Furthermore, it was found that the lattice strain in Cu2O relaxes at a depth of ≈0.15 μm from MgO on the basis of the estimated X-ray penetration depth under consideration of low incident angle, absorption and refraction of X-ray.

Spin-phonon interaction increased by compressive strain in antiferromagnetic MnO thin films

MnO thin films with various thicknesses and strains were grown on MgO substrates by pulsed laser deposition technique, then characterized using x-ray diffraction and infrared reflectance spectroscopy. Films grown on (0 0 1)-oriented MgO substrates exhibit homogenous biaxial compressive strain which increases as the film thickness is reduced. For that reason, in paramagnetic phase, the frequency of doubly-degenerate phonon increases with the strain, and splits below Néel temperature TN due to the magnetic-exchange interaction. The phonon splitting in the MnO (0 0 1) films is 20% larger than that of the bulk MnO. Films grown on (1 1 0)-oriented MgO substrates exhibit a huge phonon splitting already at room temperature due to the anisotropic in-plane compressive strain. Below TN, the lower-frequency phonon splits in the IR spectra and the higher-frequency phonon strongly hardens in AFM phase; these features are evidences for a spin-order-induced structural phase transition from tetragonal to a lower symmetry phase. Total phonon splitting is 55 cm−1 in (1 1 0)-oriented MnO film, which is more than twice the value in bulk MnO, but since the splitting is present already in paramagnetic phase, we cannot clearly correlate it with the value of exchange coupling constant. Nevertheless, at least observation of enhanced phonon splitting in strained MnO (0 0 1) films show that the exchange coupling could be enhanced by the compressive strain which supports recent theoretical predictions published by Wan et al (2016 Sci. Rep. 6 22743) and Fischer et al (2009 Phys. Rev. B 80 014408).

Structural and mechanical properties of amorphous AlMgB14 thin films deposited by DC magnetron sputtering on Si, Al2O3 and MgO substrates

AlMgB14 coatings have been deposited by DC magnetron sputtering from elemental targets on Si (001), Al2O3 (0001) and MgO (001) substrates at temperatures in the range of 25–350 °C. The structural and mechanical properties of AlMgB14 films were characterized by X-ray diffraction, scanning electron microscopy, nanoindentation, and analyzed as a function of deposition conditions and substrate materials. The results show that all films are X-ray amorphous, and the mechanical properties of the deposited films depend on the substrate and growth temperature. AlMgB14 thin films deposited at 350 °C are found to have smoother surfaces and containing more well-formed B12 icosahedra than the films deposited at lower temperature, which consequently increase the hardness of the deposited films. The maximum hardness and Young’s modulus of the as-deposited films are about 32.3 GPa and 310 GPa, respectively, for films deposited on Al2O3 substrate at 350 °C.

Effect of substrate material to the properties of screen-printed lead free (Bi0.5Na0.5)TiO3-based thick films

Lead-free piezoelectric 0.83(Bi0.5Na0.5)TiO3-0.17BaTiO3 (BNT-17BT) thick films were grown on MgO ceramic substrates via screen printing. Thick films with compressive stress formed a c-domain preferentially with a volume fraction of 0.88. High-temperature X-ray diffraction and piezoresponse force microscopy and the temperature dependence of the piezoelectric coefficient d33 data indicated that the thick films had a preferred c-domain during the transformation from the tetragonal phase to the cubic phase with temperature increasing. Based on polycrystalline MgO substrates with high thermal expansion coefficients, the thick films displayed a higher phase transition temperature and larger remanent polarization (Pr) values over 220 °C than those of thick films on Al2O3 substrates. The ferroelectric properties of BNT-17BT/MgO were better than those of soft PZT/YSZ and hard PZT/YSZ over 220 °C. The results showed the possibility of applying BNT-17BT thick films in a wide temperature range.

Synthesis, structure and thermal behavior of volatile mononuclear mixed ligand complexes of rare-earth dipivaloylmethanates with diethylenetriamine

Highly volatile and stable complexes of rare-earth elements with mononuclear structure are of great importance for gas phase deposition of functional thin film materials. Mixed ligand complexes with β-diketonate anions (e.g. thd− = 2,2,6,6-tetrametylheptane-3,5-dionate) and ancillary neutral donor ligands demonstrate mononuclear structure and sufficient volatility, however, they are unstable to neutral ligand elimination especially in case of light rare earth elements. Here diethylenetriamine (deta) has been applied as tridentate neutral ligand to improve the stability of mixed ligand complexes due to macrochelate effect and additional weak intramolecular interactions, e.g. hydrogen bonds. Synthesis of a series of mixed-ligand [Ln(thd)3(deta)], Ln = La (1L), Pr (2L), Nd (3L), Sm (4L) and Gd (5L) complexes, their X-ray single crystal structure characterization, DFT calculations, and thermal behavior study have been performed. Compounds 1L-5L demonstrate similar mononuclear molecular structure, but different molecular packing of three types, which may undergo mutual transformation. Compounds 1L-4L sublime intact in temperature range of 140–160 °C in vacuum without decomposition. 1L was successfully applied as volatile precursors for MOCVD preparation of epitaxial complex oxide thin films, (0 0 l) LaMnO3 and (0 0 l) LaAlO3, on (0 0 l) MgO and (0 0 l) SrTiO3 substrates.

High electrical conducting deep-ultraviolet-transparent oxide semiconductor La-doped SrSnO3 exceeding ∼3000 S cm−1

La-doped SrSnO3 (LSSO) is known as one of the deep-ultraviolet (DUV)-transparent conducting oxides with an energy bandgap of ∼4.6 eV. Since LSSO can be grown heteroepitaxially on more wide bandgap substrates such as MgO (Eg ∼ 7.8 eV), LSSO is considered to be a good candidate for a DUV-transparent electrode. However, the electrical conductivity of LSSO films is below 1000 S cm−1, most likely due to the low solubility of the La ion in the LSSO lattice. Here, we report that high electrically conducting (>3000 S cm−1) LSSO thin films with an energy bandgap of ∼4.6 eV can be fabricated by pulsed laser deposition on a MgO substrate followed by a simple annealing in vacuum. From the X-ray diffraction and the scanning transmission electron microscopy analyses, we found that lateral grain growth occurred during the annealing, which improved the activation rate of the La ion, leading to a significant improvement of the carrier concentration (3.26 × 1020 cm−3) and Hall mobility (55.8 cm2 V−1 s−1). The present DUV-transparent oxide semiconductor would be useful as a transparent electrode for developing optoelectronic devices, which transmit and/or emit DUV-light.

Mn3Ge-based tetragonal Heusler alloy thin films with addition of Ni, Pt, and Pd

We have investigated substitution effects of Ni, Pt, and Pd on phase formation and magnetic properties of D022-Mn3Ge thin films. We prepared (Mn1−xMx)3Ge thin films (M = Ni, Pt, Pd) at 650 °C by magnetron sputtering on MgO(0 0 1) substrates with x varying from 0.03 to 0.6. For improving the film quality, a Cr(0 0 1) seed layer was employed. The D022 structure formed only for the lowest concentrations of Ni and Pt. Nevertheless, the doped samples showed strong perpendicular magnetic anisotropy up to x = 0.1. For high Ni concentrations, we observed the formation of a soft ferromagnetic MnxNiy Ge phase with a Curie temperature of about 230 K, while in samples with high Pt content the antiferromagnet L10-MnPt phase is formed along with GePt.In contrast, for Pd substitution, the D022 structure is preserved up to x = 0.2, exhibiting strong perpendicular magnetic anisotropy and low saturation magnetization. Interestingly, the coexistence of the D022-Mn3Ge and a novel D022-(Mn1−xPdx)3Ge phase was revealed, which might have been facilitated by the low lattice mismatch to the Cr(0 0 1) seed layer. With further increase of the Pd concentration, the D022 structure vanishes and mainly the GePd and GePd2 phases are present.Overall within the investigated sample series, the saturation magnetization strongly decreases with increasing dopant concentration, offering the possibility to adjust the saturation magnetization in the range between 20 and 100 emu cm−3, while still preserving strong perpendicular magnetic anisotropy, which is important for spintronic applications.

Terahertz emission generated from Bi<sub>2</sub>Te<sub>3</sub>/CoFeB heterostructures grown by magnetron sputtering

High-performance terahertz emitters, which convert the femtosecond laser pulses into terahertz pulses, are essential for terahertz spectroscopy technology and terahertz wireless communication. Spintronic terahertz emitters based on ferromagnet/nonmagnet bilayers have attracted tremendous attention due to their high efficiency, ultra-broadband, low cost and high flexibility. Here, we systematically investigate the terahertz emission from polycrystalline topological insulator Bi<sub>2</sub>Te<sub>3</sub>/ferromagnetic CoFeB heterostructure grown by magnetron sputtering. The Bi<sub>2</sub>Te<sub>3</sub>/CoFeB heterostructure exhibits high efficiency of terahertz emission, and the polarization of terahertz waves can be controlled by the external magnetic field direction. The performance of Bi<sub>2</sub>Te<sub>3</sub>/CoFeB heterostructure is almost comparable to that of the Pt/CoFeB bilayer. In contrast, no terahertz emission is observed in the pure Bi<sub>2</sub>Te<sub>3</sub> or CoFeB film driven by femtosecond laser pulses, probably because the Bi<sub>2</sub>Te<sub>3</sub> prepared by sputtering is polycrystalline and the thickness of CoFeB is too thin. We also compare the performances of Bi<sub>2</sub>Te<sub>3</sub>/CoFeB grown on MgO, glass and high-resistivity silicon substrates, and find that the samples grown on MgO substrates exhibit the best emission performances. The glass substrate absorbs more terahertz waves than MgO substrate, resulting in a slightly weaker terahertz signal emitted from the Bi<sub>2</sub>Te<sub>3</sub>/CoFeB grown on the glass substrate. Although the absorption coefficient of high-resistivity silicon to terahertz waves is very small, the residual pump light excites the high-resistivity silicon to generate the photo-generated carriers, which change the conductivity of the high-resistivity silicon and reduce the transmittance of terahertz wave. We attribute the mechanism of the terahertz emission to the spin-charge conversion at the interface of Bi<sub>2</sub>Te<sub>3</sub>/CoFeB. The terahertz emission efficiency of our sample is expected to be able to be further improved by optimizing the samples. Moreover, with the sputtering method, it is possible to fabricate large area samples at low cost, which is critical for commercial applications.

TEM observation of nitrogen-tunable bcc–bct–fcc transformation of iron-cobalt with added vanadium

We used transmission electron microscopy (TEM) to observe tunable bcc–bct–fcc transformation of FeCo films caused by the addition of auxiliary elements. The crystal structure of the FeCo with added V was bcc, and the transformation of the bcc structure through the bct to fcc was found to depend on the N content (x). TEM observation revealed that bct structure with c/a = 1.12 was obtained for x = 1.7 atomic%, while fcc structure with c/a = 2 was obtained for x = 9.6 atomic%. The c/a-values depended on x, so the bcc–bct–fcc transformation of the FeCoV was tunable by N content. This research was made possible by special preventative measures taken during preparation to avoid compressive stress, which could otherwise induce the bct FeCo lattice of axial ratio c/a > 1.00 to transform to bcc structure. A FeCo coating of thickness 20 nm was therefore deposited directly on a MgO substrate without any other metallic layers.

A Comparative Analysis of Asymmetric (BaTiO3) (1 − x)Λ/(BaZrO3) xΛ Superlattices via X‐Ray Diffraction and Raman Spectroscopy

This work is aimed at studying asymmetric ((BT) (1 − x)Λ/(BZ) xΛ) superlattices, grown by pulsed laser deposition onto (001) MgO substrates. The thicknesses of BT (ferroelectric) and BZ (paraelectric) layers were varied so that х ranged from 0 to 1 at a modulation period Λ of about 80 Å. The films were 400 nm thick. The out‐of‐plane lattice parameters of constituents were assessed using X‐ray diffraction. The lattice dynamic peculiarities of superlattices were probed via Raman spectroscopy; special attention is paid to the analysis of E(1TO) and A1(2TO) ferroelectric soft modes. A comparative analysis of data acquired via both experimental techniques reveals the enhancement of stress between BT and BZ layers with a decrease in symmetry from the tetragonal to a monoclinic phase due to strains induced by the lattice parameter mismatch between the constituents.

Polarization Switching along a Substrate in Thin Bi4Ti3O12 Films under Different Deformation Stresses

Single-crystal Bi4Ti3O12 films deposited on a 4-nm-thick Ba0.4Sr0.6TiO3 sublayer covering an MgO(001) substrate have been investigated. It has been found that the unit cells of Bi4Ti3O12 films in the resulting heterostructures are turned by 45° relative to the cells of the MgO substrate in the interface plane. In addition, the unit cells of the films are strained, the amount of strain depending on the Bi4Ti3O12 film thickness. When the films become about 40 nm thick, strain changes sign. It has been shown that reversible spontaneous polarization in Bi4Ti3O12 films with a 180° domain structure in the interface plane arises at a thickness of 10 nm and grows with thickness to 54 μC/cm2. The anisotropy of the film’s properties in the interface plane and the influence of unit cell distortion on the properties of the heterostructures have been confirmed by studying the dielectric performance of the films.

Fully transparent field-effect transistor with high drain current and on-off ratio

We report a fully transparent thin-film transistor utilizing a La-doped BaSnO3 channel layer that provides a drain current of 0.468 mA/μm and an on-off ratio of 1.5 × 108. The La-doped BaSnO3 channel is grown on a 100–150 nm thick unintentionally doped BaSnO3 buffer layer on a (001) MgO substrate by molecular-beam epitaxy. Unpatterned channel layers show mobilities of 127–184 cm2 V−1 s−1 at carrier concentrations in the low to mid 1019 cm−3 range. The BaSnO3 is patterned by reactive ion etching under conditions preserving the high mobility and conductivity. Using this patterning method, a sub-micron-scale thin film transistor exhibiting complete depletion at room temperature is achieved.

Wide range bandgap modulation in strained SrSnO3 epitaxial films

The bandgap of SrSnO3 films increases with IP strains on both LAO and MgO substrates, which are consistent with theory calculation results.

Effect of substrate temperature on structure, magnetic and electrical transport properties of Mn1.60Ga films

L10-MnGa alloys have great application potential in the fields of permanent magnet and spintronics. L10-Mn1.60Ga thin films were prepared on MgO (100) substrates by magnetron sputtering under different substrate temperatures ranging from 200 to 600 °C, and the effect of the substrate temperature on the structure, magnetic and electrical transport properties of the films were observed. The results show that the films deposited at 300–600 °C substrate have good (001) orientation. The coercivity and effective anisotropy of film are enhanced at 400 °C substrate temperature, because the large tensile stress/strain induces a large coercivity and high effective anisotropy. When the substrate temperature increases to 500 °C, and the abnormal Hall resistivity of the film reaches the maximum value. The magnetic and electrical transport properties of the L10-Mn1.60Ga film are improved effectively with an appropriate substrate temperature.

Dynamic magnetic properties of amorphous Fe80B20 thin films and their relation to interfaces

We present a ferromagnetic resonance study of the dynamic properties of a set of amorphous Fe-B films deposited on Corning Glass® and MgO (001) substrates, either with or without capping. We show that the in plane anisotropy of the MgO grown films contains both uniaxial and biaxial components whereas it is just uniaxial for those grown on glass. The angular dependence of the linewidth strongly differs in terms of symmetry and magnitude depending on the substrate and capping. We discuss the role of the interfaces on the magnetization damping and the generation of the anisotropy. We obtained values of the intrinsic damping parameters comparable to the lowest ones reported for amorphous films of similar compositions.

Structure and Ferroelectric Properties of Thin Heteroepitaxial NaNbO3 Films Obtained by RF Cathode Sputtering

Thin films of sodium niobate (NaNbO3) on a MgO(001) substrate with a predeposited SrRuO3 layer were obtained for the first time by the method of RF cathode sputtering in an oxygen atmosphere. X‑ray diffraction data showed the obtained films to be single-phase and single-crystalline. The parameters of the unit cells of NaNbO3 and SrRuO3 layers in the tetragonal approximation were found to be $${{c}_{{{\text{NaNb}}{{{\text{O}}}_{3}}}}}$$ = 0.3940(1) nm, $${{a}_{{{\text{NaNb}}{{{\text{O}}}_{3}}}}}$$ = 0.389(1) nm, $${{c}_{{{\text{SrRu}}{{{\text{O}}}_{3}}}}}$$ = 0.4004(1) nm, and $${{a}_{{{\text{SrRu}}{{{\text{O}}}_{3}}}}}$$ = 0.392(3) nm. Misfit strain of the unit cell of NaNbO3 amounted to e33 = 0.007 and e11 = 0.002. The results of dielectric and piezoelectric measurements showed that the films occurred in a ferroelectric state.

Large-area ultrathin Te films with substrate-tunable orientation.

Anisotropy in a crystal structure can lead to large orientation-dependent variations of mechanical, optical, and electronic properties. Material orientation control can thus provide a handle to manipulate properties. Here, a novel sputtering approach for 2D materials enables growth of ultrathin (2.5-10 nm) tellurium films with rational control of the crystalline orientation templated by the substrate. The anisotropic Te 〈0001〉 helical chains align in the plane of the substrate on highly oriented pyrolytic graphite (HOPG) and orthogonally to MgO(100) substrates, as shown by polarized Raman spectroscopy and high-resolution electron microscopy. Furthermore, the films are shown to grow in a textured fashion on HOPG, in contrast with previous reports. These ultrathin Te films cover exceptionally large areas (>1 cm2) and are grown at low temperature (25 °C) affording the ability to accommodate a variety of substrates including flexible electronics. They are robust toward oxidation over a period of days and exhibit the non-centrosymmetric P3121 Te structure. Raman signals are acutely dependent on film thickness, suggesting that optical anisotropy persists and is even enhanced at the ultrathin limit. Hall effect measurements indicate orientation-dependent carrier mobility up to 19 cm2 V-1 s-1. These large-area, ultrathin Te films grown by a truly scalable, physical vapor deposition technique with rational control of orientation/thickness open avenues for controlled orientation-dependent properties in semiconducting thin films for applications in electronic and optoelectronic devices.

Fabrication of soft-magnetic FeAlSi thin films with nm-order thickness for the free layer of magnetic tunnel junction based sensors

FeAlSi epitaxial films were fabricated on single crystalline MgO(100) substrates and their structural and magnetic properties were investigated to apply them to the free layer in magnetic tunnel junction (MTJ) based sensors. We found that the film composition must be precisely controlled, and the post-annealing temperature was varied to obtain a D03-ordered structure and soft magnetic property. By adjustment of the film composition and optimizing annealing temperature, we succeeded in obtaining D03-ordered FeAlSi thin films with a small surface roughness and low coercivity. The fabricated FeAlSi film is greatly useful as a free layer material in MTJ based sensor devices.