Mikhail P. Volkov

Magnetization reversal in YIG/GGG(111) nanoheterostructures grown by laser molecular beam epitaxy

Abstract Thin (4–20 nm) yttrium iron garnet (Y3Fe5O12, YIG) layers have been grown on gadolinium gallium garnet (Gd3Ga5O12, GGG) 111-oriented substrates by laser molecular beam epitaxy in 700–1000 °C growth temperature range. The layers were found to have atomically flat step-and-terrace surface morphology with step height of 1.8 Å characteristic for YIG(111) surface. As the growth temperature is increased from 700 to 1000 °C the terraces become wider and the growth gradually changes from layer by layer to step-flow regime. Crystal structure studied by electron and X-ray diffraction showed that YIG lattice is co-oriented and laterally pseudomorphic to GGG with small rhombohedral distortion present perpendicular to the surface. Measurements of magnetic moment, magneto-optical polar and longitudinal Kerr effect (MOKE), and X-ray magnetic circular dichroism (XMCD) were used for study of magnetization reversal for different orientations of magnetic field. These methods and ferromagnetic resonance studies have shown that in zero magnetic field magnetization lies in the film plane due to both shape and induced anisotropies. Vectorial MOKE studies have revealed the presence of an in-plane easy magnetization axis. In-plane magnetization reversal was shown to occur through combination of reversible rotation and abrupt irreversible magnetization jump, the latter caused by domain wall nucleation and propagation. The field at which the flip takes place depends on the angle between the applied magnetic field and the easy magnetization axis and can be described by the modified Stoner–Wohlfarth model taking into account magnetic field dependence of the domain wall energy. Magnetization curves of individual tetrahedral and octahedral magnetic Fe3+ sublattices were studied by XMCD.

Obtaining of crystals of polyelemental solid solutions of rare earth hexaborides

Single crystals of polyelemental rare earth hexaborides with the preset formula La0.5(Ce0.1Pr0.1Nd0.1Sm0.1Eu0.1)B6 were obtained for the first time. Synthesis and crystallization were performed by the solution–melt method in an immiscible Al/Pb system. Step-by-step chemical analysis was made with the aid of a CAMEBAX microprobe. The inclusion of all rare earth metals (REMs) in the hexaboride lattice was proven, and differences in the composition of obtained crystals caused by nonstationarity of the bulk crystallization process were found. The lattice periods of the polyelemental REM hexaborides were found to be smaller than that of hexaboride of lanthanum, the main element of the metal sublattice. The measured microhardness of the new material lies within the range of the microhardness values of hexaborides of all its constituent REMs. Speculations are provided on the peculiarities of the growth mechanism, crystallization, and composition of the obtained crystals.

High-power laser based on amplifiers with Yb:YAG elements of advanced geometries

High average and high peak power picosecond laser composed of ytterbium fiber oscillator, Yb:YAG thin-rod preamplifiers and Yb:YAG thin-disk multipass amplifier is under development. 82 W average power at 11.5 kHz repetition rate is achieved.

Prospects of using rare-earth hexaborides in thermoelectric single-photon detectors

The results of the numerical simulation of heat propagation processes occurring after the absorption of single photons with energies of 1 eV–1 keV in a three-layer sensor of a thermoelectric detector are analyzed. Different configurations of the sensor with a tungsten absorber, a thermoelectric layer of cerium hexaboride, and a tungsten heat sink are considered. It is shown that sensors for detecting photons of a specific spectral range with the required energy resolution and counting rate can be developed by varying the geometric sizes of the sensor layers. It is concluded that a three-layer sensor has a number of advantages in comparison with a single-layer sensor and has characteristics allowing consideration of the thermoelectric detector as a realistic alternative to superconducting single-photon detectors.

Nanostructured magnetic films of iron oxides fabricated by laser electrodispersion

Nanostructured FeO films with an average nanoparticle size of the order of 6–10 nm were fabricated by laser electrodispersion. Annealing at T = 300°C in vacuum resulted in the disproportionation of FeO particles into Fe3O4 and α-Fe, while the films exhibited a marked crystal orientation (texture with the [111] axis). The coercive force and the saturation magnetization of the synthesized nanostructured Fe3O4/α-Fe films were as large as ~660 Oe and ~520 emu/cm3, respectively. These values are considerably higher than the corresponding parameters of polycrystalline Fe3O4 films.

Growth of Y3Fe5O12/GaN layers by laser molecular-beam epitaxy and characterization of their structural and magnetic properties

Laser molecular-beam epitaxy has been employed to obtain layers of yttrium-iron garnet (YIG) Y3Fe5O12 on gallium nitride substrates. It was found that there exists a polycrystalline YIG phase without admixtures of other structural phases. A magnetic anisotropy of films of the “easy-magnetic plane” type was found. The gyromagnetic ratio and the demagnetizing field 4πMS were calculated.

Nanometer Structured Epitaxial Films and Foliated Layers Based on Bismuth and Antimony Chalcogenides with Topological Surface States

The thermoelectric and galvanomagnetic properties of nanometer structured epitaxial films and foliated layers based on bismuth and antimony chalcogenides were investi‐ gated, and an increase in the figure of merit Z up to 3.85 × 10‐3 K‐1 was observed in the Bi0.5Sb1.5Te3 films over the temperature range of 180–200 K. It is shown that an increase in the Seebeck coefficient and the change in the slope on temperature, associated with changes in the effective scattering parameter of charge carriers and strong anisotropy of scattering in the films, lead to enhance power factor due to the growth of the effective mass of the density of states. These features are consistent with the results of research of oscillation effects in strong magnetic fields at low temperatures and research of Raman scattering at normal and high pressures in the foliated layers of solid solutions (Bi, Sb)2(Te, Se)3, in which the topological Dirac surface states were observed. The unique properties of topological surface states in the investigated films and layers make topological insulators promising material for innovation nanostructured thermoelec‐ trics.

Inhomogeneous Microstructure and Electrical Transport Properties at the LaAlO3/SrTiO3 Interface

Medium-energy ion spectroscopy (MEIS), scanning transmission electron microscopy (STEM) and X-ray photoemission spectroscopy (XPS) were used to investigate the composition and microstructure of LaAlO3/SrTiO3 (LAO/STO) interfaces grown by pulsed laser deposition of LAO on TiO2-terminated STO substrates under different oxidizing conditions. MEIS and XPS indicated Sr/La and Al/Ti intermixing within several atomic layers at all studied interfaces. XPS and STEM revealed that La diffuses deeper than Al. Analysis of the MEIS data suggests inhomogeneous lateral distribution of the diffused elements. This is further supported by the observation of a large positive magneto-resistance at low temperatures. We discuss the role of lateral inhomogeneities on the formation of the electron gas at the LAO/STO interface.

Features of growth and galvanomagnetic properties of the Bi2Te3-based epitaxial films

Galvanomagnetic properties of the thermoelectric Bi2-xSbxTe3 (x=1.5) films grown by hat wall technique were studied in the wide (5-14 T) range of the magnetic field through the temperature interval 77-300 κ The results were analyzed in the terms of the many-valley model of energy spectrum with isotropic carrier scattering. An anisotropy of the constant-energy surface becomes weaker along the binary and bisector axes (m1/m2) in a view of turn on the tilt angle (Θ) between the principal axes of the constant-energy ellipsoids and the crystallographic axes for the film in comparison with bulk Bi2-xSbxTe3. Features of temperature dependence of the degeneracy parameter (βd) for the films can be explained by additional charge carrier scattering at block boundaries. In the model with anisotropic scattering mechanism, carrier scattering along bisector directions are primary in the films. Obtained data on anisotropy of the constant-energy surface and scattering mechanism looks important for formation of the thermoelectric Bi2-xSbxTe3 film heterostructures with enhanced figure of merit.Galvanomagnetic properties of the thermoelectric Bi2-xSbxTe3 (x=1.5) films grown by hat wall technique were studied in the wide (5-14 T) range of the magnetic field through the temperature interval 77-300 κ The results were analyzed in the terms of the many-valley model of energy spectrum with isotropic carrier scattering. An anisotropy of the constant-energy surface becomes weaker along the binary and bisector axes (m1/m2) in a view of turn on the tilt angle (Θ) between the principal axes of the constant-energy ellipsoids and the crystallographic axes for the film in comparison with bulk Bi2-xSbxTe3. Features of temperature dependence of the degeneracy parameter (βd) for the films can be explained by additional charge carrier scattering at block boundaries. In the model with anisotropic scattering mechanism, carrier scattering along bisector directions are primary in the films. Obtained data on anisotropy of the constant-energy surface and scattering mechanism looks important for formation of the thermoe...