Four-circle X-ray Diffraction Research Articles

Effects of a strong gravitational field on Mn-trimers and magnetic properties of hexagonal YMnO3 single crystal

Under strong gravitational fields, the unique uniaxially distorted crystalline state induces structural changes in compounds other than those caused by high pressure and high temperature. We subjected the hexagonal YMnO3 single crystal to a strong-gravitational field (0.78 × 106 G, 400 °C) and determined the changes in the crystalline structure and physical properties of the recovered metastable sample. According to four-circle single-crystal X-ray diffraction data, the structural analyses indicated changes in the inequivalent nearest-neighbor Mn-Mn and Mn-O bond distances, thereby demonstrating that trimerization distortion occurred in the ab plane under the strong gravitational field. Magnetic measurements indicated that there were changes in the magnetic anisotropy, which were explained by changes in the spin–orbit interaction due to lattice distortion.

Change in crystal structure and physical properties of the Multiferroics YMnO3 single crystals by Strong gravitational field

Many researchers have studied the multiferroicity of the hexagonal RMnO3 (R: rare-earth element) for both applications and fundamental studies. To investigate the relationship between the structure and physical properties of materials, some people apply the chemical pressure effect. The procedure of chemical pressure effect involves substituting rare-earth elements for ones which have a different ionic radius. Mashimo et al. have developed a high-temperature ultracentrifuge apparatus that can generate extended duration strong gravitational field in excess of 106 G under a wide range of temperatures (up to 500°C). Strong gravitational fields directly act on each atom as a different body force. This can cause the change in crystal structure. Thus, we subjected YMnO3 single crystal to strong gravity experiments (0.78×106 G, 400°C, 2 h) and investigated the resulting changes in the crystal structure and physical properties of the gravity sample. The single crystal four-circle X-ray diffraction measurements revealed the change in the nearest neighboring Mn-Mn and M-O bond distances. The temperature dependence of magnetic susceptibility by SQUID showed the change in the magnetic anisotropy of gravity sample.

Preparation of TaO2 thin films using NbO2 template layers by a pulsed laser deposition technique

TaO2 thin films were prepared using rutile-type NbO2 template layers on Al2O3(0001) substrates by a pulsed laser deposition technique. The structural and electronic properties of the films were characterized by performing four-circle X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) measurements. Four-circle XRD measurements revealed the formation and epitaxial growth of (100)-oriented tetragonal TaO2 on the NbO2 template layers. On the other hand, XPS measurements showed two kinds of valence states, namely Ta4+ and Ta5+ in the surface region of the film. The results from the XRD and XPS measurements suggest that the film is composed of epitaxially grown TaO2 with an amorphous Ta2O5 surface layer. It is found from control experiments that the NbO2 template layers play an important role in inducing the crystallization of the TaO2 films and promoting the formation of Ta4+ in the films deposited thereon. The results indicate the effectiveness of the NbO2 template layers for the formation of TaO2 thin films. Our work presents a promising method for the preparation of TaO2 thin films and provides a starting point for further research on TaO2.

Identification of Crystal Structure and Crystallographic Features of NiMnGa Thin Films by Combination of X-Ray Diffraction (XRD) and Electron Backscatter Diffraction (EBSD)

In this work, NiMnGa thin film composed of non-modulated martensite (NM) and seven-layered modulated martensite (7M) was produced. The crystal structure and lattice constants were determined by X-ray diffractometer (XRD). The preferred crystallographic orientation of martensite was determined using the four-circle XRD. SEM/EBSD was employed to verify the crystal structure of the martensite and to reveal its crystallographic features correlated with the microstructure. According to the XRD patterns, the crystal structure of NM and 7M was determined as tetragonal and monoclinic crystal structure, respectively. Pole figures measured by four-circle diffractometer revealed that the NM martensite possesses (004)NM and (220)NM preferred plane texture close to the substrate surface, whereas the 7M martensite has (2 0 20)7M, (2 0 )7M and (040)7M preferred plane texture close to the substrate surface. SEM/EBSD analysis shows that the surface layer of the film is mainly composed of NM martensite that is organized in variant groups. In each variant group, all the martensite plates consist of paired lamellar (112)NM compound twins and there are eight orientation variants in each variant group.

A new manganese-based single-molecule magnet with a record-high antiferromagnetic phase transition temperature

We perform both dc and ac magnetic measurements on the single crystal of Mn3O(Et-sao)3(ClO4)(MeOH)3 single-molecule magnet (SMM) when the sample is preserved in air for different durations. We find that, during the oxidation process, the sample develops into another SMM with a smaller anisotropy energy barrier and a stronger antiferromagnetic intermolecular exchange interaction. The antiferromagnetic transition temperature observed at 6.65 K in the new SMM is record-high for the antiferromagnetic phase transition in all the known SMMs. Compared to the original SMM, the only apparent change for the new SMM is that each molecule has lost three methyl groups as revealed by four-circle x-ray diffraction (XRD), which is thought to be the origin of the stronger antiferromagnetic intermolecular exchange interaction.

Adsorption-controlled growth of BiVO4 by molecular-beam epitaxy

Single-phase epitaxial films of the monoclinic polymorph of BiVO4 were synthesized by reactive molecular-beam epitaxy under adsorption-controlled conditions. The BiVO4 films were grown on (001) yttria-stabilized cubic zirconia (YSZ) substrates. Four-circle x-ray diffraction, scanning transmission electron microscopy (STEM), and Raman spectroscopy confirm the epitaxial growth of monoclinic BiVO4 with an atomically abrupt interface and orientation relationship (001)BiVO4 ∥ (001)YSZ with [100]BiVO4 ∥ [100]YSZ. Spectroscopic ellipsometry, STEM electron energy loss spectroscopy (STEM-EELS), and x-ray absorption spectroscopy indicate that the films have a direct band gap of 2.5 ± 0.1 eV.

Synthesis, structure and conductivity of molecular conductor β-(BEDT-TTF)(HSO 4)

A new BEDT-TTF-based radical salt, β-(BEDT-TTF)(HSO 4) [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene], has been synthesized by oxidative electrocrystallization. The crystal structure was determined by four-circle X-ray diffraction. The crystal belongs to monoclinic system, C2/c space group with the unit cell parameters of: a = 1.5996(2) nm, b = 1.06173(10) nm, c = 1.15083(11) nm, β = 120.534(8)°, V = 1.6834(3) nm 3, and R = 0.0611. In the title crystal, the BEDT-TTF radicals are stacked to form columns along the [1 0 1] axis and the molecular planes of BEDT-TTFs in adjacent columns are parallel to each other. The inter-stack side-by-side S…S short contacts along the b-axis help to form a kind of cationic conducting sheet parallel to (1 0 1) plane. The room temperature conductivity at certain direction on (1 0 1) plane of β-(BEDT-TTF)(HSO 4) single crystal was measured to be 11.7 S cm −1. From 100 K to 290 K, the resistivity–temperature curve demonstrates its typical semiconductivity with an excitation energy of 0.26 eV.

Substrate-controlled allotropic phases and growth orientation of TiO2epitaxial thin films

TiO2thin films were grown by pulsed laser deposition on a wide variety of oxide single-crystal substrates and characterized in detail by four-circle X-ray diffraction. Films grown at 873 K on (100)-oriented SrTiO3and LaAlO3were (001)-oriented anatase, while on (100) MgO they were (100)-oriented. On (110) SrTiO3and MgO, (102) anatase was observed. OnM-plane andR-plane sapphire, (001)- and (101)-oriented rutile films were obtained, respectively. OnC-plane sapphire, the coexistence of (001) anatase, (112) anatase and (100) rutile was found; increasing the deposition temperature tended to increase the rutile proportion. Similarly, films grown at 973 K on (100) and (110) MgO showed the emergence, besides anatase, of (110) rutile. All these films were epitaxically grown, as shown by φ scans and/or pole figures, and the various observed orientations were explained on the basis of misfit considerations and interface arrangement.

Applying uniform reversible strain to epitaxial oxide films

We demonstrate using four-circle x-ray diffraction that the piezoelectric substrate of Pb(Mg1/3Nb2/3)0.72Ti0.28O3(001) induces uniform reversible in-plane strain to epitaxially-grown oxide films and bilayers. The biaxial in-plane strain depends linearly on the applied electrical voltage. Utilizing the reversible strain, the strain-dependent lattice structure and the Poisson number characterizing the elastic response is determined for 200 nm thick SrTiO3, LaScO3, and BiFeO3 films. The uniformity and reversibility of the strain provides access to the direct quantitative measurement of strain-dependent properties of epitaxial oxide films.

Adsorption-Controlled Growth of BiFeO3 by MBE and Integration with Wide Band Gap Semiconductors

BiFeO3 thin films have been deposited on (001) SrTiO3, (101) DyScO3, (011) DyScO3, (0001) AlGaN/GaN, and (0001) 6H-SiC single crystal substrates by reactive molecular beam epitaxy in an adsorption-controlled growth regime. This is achieved by supplying a bismuth over-pressure and utilizing the differential vapor pressures between bismuth oxides and BiFeO3 to control stoichiometry in accordance with thermodynamic calculations. Four-circle x-ray diffraction and transmission electron microscopy reveal phase-pure, epitaxial films with rocking curve full width at half maximum values as narrow as 7.2 arc seconds (0.002 degrees). Epitaxial growth of (0001)-oriented BiFeO3 thin films on (0001) GaN, including AlGaN HEMT structures, and (0001) SiC has been realized using intervening epitaxial (111) SrTiO3 / (100) TiO2 buffer layers. The epitaxial BiFeO3 thin films have 2 in-plane orientations: [1120] BiFeO3 || [1120] GaN (SiC) plus a twin variant related by a 180 degrees in-plane rotation. This epitaxial integration of the ferroelectric with the highest known polarization, BiFeO3, with high bandgap semiconductors is an important step toward novel field-effect devices.

Magnetoresistance of Fe thin films on faceted Al2O3 substrates

Nanopatterned Fe thin films are prepared by e-beam evaporation on faceted Al2O3 substrates. In situ scanning tunneling microscopy investigations show that the Fe morphology consists of a grain structure which follows the shape of the substrate facets. By four-circle x-ray diffraction we demonstrate that the grains are textured with different but defined alignments. The facet morphology results in a pronounced magnetic anisotropy as shown by magnetization loops measured in a vibrating sample magnetometer. The morphology in conjunction with a conventional anisotropic magnetoresistance effect is the origin of a strongly anisotropic magnetoresistance of the samples.

Ni-Mn-Ga films in the austenite and the martensite structures at room temperature: Uniaxial texturation and epitaxial growth

AbstractNi-Mn-Ga films in the austenite and the martensite structures at room temperature have been obtained using the DC magnetron sputtering technique. Two elaboration processes were studied. A first batch of samples was deposited using a resist sacrificial layer in order to release the film from the substrate before vacuum annealing. This process leads to polycrystalline films with a strong (022) fiber texture. The martensitic phase transformation of such polycrystalline freestanding films has been studied by optical and scanning electron microscopy. A second batch of samples was grown epitaxially on (100)MgO substrates using different deposition temperatures. The texture has been analyzed with four-circle X-ray diffraction. Epitaxial films crystallized both in the austenite and the martensite structures at room temperature have been studied.

Growth and Characterization of Alternative Gate Dielectrics by Molecular-Beam Epitaxy

In this work, we have investigated conditions to grow epitaxial La2O3 and Sc2O3 thin films directly on (111) Si as well as epitaxial LaScO3 thin films on (100) Si using an alkaline earth oxide buffer layer. The films were structurally characterized by reflection high-energy electron diffraction (RHEED) during growth and four-circle x-ray diffraction (XRD) and high-resolution plan-view and cross-section TEM, including Z-contrast TEM after growth. The epitaxial La2O3 thin films grew with the following epitaxial relationship (0001) La2O3 // (111) Si La2O3 // [112] Si with 1 additional rotational twin variant. Sc2O3 thin films grew epitaxially with a cube-on-cube orientation relationship on (111) silicon. It was not possible to achieve epitaxial LaScO3 thin films without the use of an alkaline earth buffer layer. The epitaxial orientation relationship between the orthorhombic LaScO3 and the underlying Si was (101) LaScO3 // (001) Si and [010] LaScO3 // [110] Si.

Adsorption-controlled growth of EuO by molecular-beam epitaxy

Using molecular-beam epitaxy, we demonstrate the adsorption-controlled growth of epitaxial EuO films on single crystalline (110) YAlO3 substrates. Four-circle x-ray diffraction (XRD) reveals phase-pure, epitaxial, (001)-oriented films with rocking curve full width at half maxima as narrow as 34 arc sec (0.0097°). The critical thickness for the onset of relaxation of (001) EuO on (110) YAlO3 (∼2% lattice mismatch) was determined from XRD measurements to be 382±25 Å. A saturation magnetization of 6.96±0.07μB/Eu, a value close to the theoretical limit of 7μB/Eu, is observed.

The effect of strain and strain symmetry on the charge-order transition in Bi0.4Ca0.6MnO3 films

The transition to a charge and orbital ordered (CO/OO) state in epitaxial manganite films is strongly influenced by lattice strain. Bi1− x Ca x MnO3 is a particularly interesting material due to its high transition temperature and its relation to other Bi-based materials. Here, we review its properties and show the effects of strain and strain symmetry on Bi0.4Ca0.6MnO3 films on SrTiO3 (STO) and LaAlO3 substrates with (pseudocubic) (001) and (011) orientations. Transport and magnetization data are compared to four-circle X-ray diffraction and high-resolution Z-contrast scanning transmission microscopy data. We observe the spontaneous formation of single-unit-cell thick, Bi-rich layers only on (001) STO substrates and different defect structures depending on the substrate type and orientation. This shows that the details of epitaxial strain play a role not only at phase transitions, but also during the growth of these materials. Results are compared to those published for other CO/OO manganite films.

Epitaxial (La, Sr)TiO3 on textured Ni–W as a conductive buffer architecture for high temperature superconducting coated conductor

The formation of epitaxial (La,Sr)TiO3 on biaxially-textured Ni–W metal alloy tape was examined as a conductive buffer layer for YBa2Cu3O7 film growth. A TiN epitaxial nucleation layer served as an effective template layer for subsequent (La,Sr)TiO3 film growth on the Ni–W tape. The TiN/(La,Sr)TiO3 multilayer was epitaxial on the Ni–W substrate as confirmed via four-circle X-ray diffraction. A high temperature superconducting YBa2Cu3O7 film was deposited epitaxially on the (La,Sr)TiO3/TiN/Ni–W tape. The YBa2Cu3O7 film exhibited a superconducting transition temperature of 86K and critical current density of 0.42×106A/cm2 at 77K.

Optical band gap of BiFeO3 grown by molecular-beam epitaxy

Bi Fe O 3 thin films have been deposited on (001) SrTiO3 substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase BiFeO3 films can be grown over a range of deposition temperatures in accordance with thermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial films with ω rocking curve full width at half maximum values as narrow as 29arcsec (0.008°). Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at 2.74eV for stoichiometric as well as 5% bismuth-deficient single-phase BiFeO3 films.

Adsorption-controlled molecular-beam epitaxial growth of BiFeO3

Bi Fe O 3 thin films have been deposited on (111) SrTiO3 single crystal substrates by reactive molecular-beam epitaxy in an adsorption-controlled growth regime. This is achieved by supplying a bismuth overpressure and utilizing the differential vapor pressures between bismuth oxides and BiFeO3 to control stoichiometry. Four-circle x-ray diffraction reveals phase-pure, untwinned, epitaxial, (0001)-oriented films with rocking curve full width at half maximum values as narrow as 25arcsec (0.007°). Second harmonic generation polar plots combined with diffraction establish the crystallographic point group of these untwinned epitaxial films to be 3m at room temperature.

Room-temperature growth of AlN/TiN epitaxial multi-layer by laser molecular beam epitaxy

We have fabricated epitaxial AlN thin films at room temperature on sapphire (0001) substrates with a TiN (111) epitaxial buffer layer by pulsed laser deposition in ultra-high vacuum (laser molecular beam epitaxy method). The TiN buffer layers were also fabricated at room temperature. Four-circle X-ray diffraction analysis and reflection high-energy electron diffraction results indicate the heteroepitaxial structure of AlN (0001)/TiN (111)/sapphire (0001) with the epitaxial relationship of AlN [10–10]||TiN [11–2]||sapphire [11–20]. The surface of the room-temperature grown AlN film was found to be atomically flat, reflecting the nano-stepped surface of ultrasmooth sapphire substrates. Then, we could achieve the room-temperature epitaxial growth of [AlN/TiN] multi-layer. The temperature dependence of resistivity of the AlN/TiN multi-layer film was also measured.

Synthesis of gold nanowires with controlled crystallographic characteristics

The controlled fabrication of poly- and single-crystalline Au nanowires is reported. In polycarbonate templates, prepared by heavy-ion irradiation and subsequent etching, Au nanowires with diameters down to 25 nm are electrochemically synthesized. Four-circle X-ray diffraction and transmission electron microscopy measurements demonstrate that wires deposited potentiostatically at a voltage of -1.2 V at 65 °C are single-crystalline and oriented along the [110] direction. By reverse-pulse electrodeposition, wires oriented along the [100] direction are grown. The wires are cylindrical over their whole length. The morphology of the caps growing on top of poly- and single-crystalline wires is a strong indication of the particular crystalline structure of the nanowires.