Frustrated exchange interactions formation at low temperatures and high hydrostatic pressures in La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>Mn<Subscript>O2.85</Subscript>

The magnetic and thermal properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite are investigated in wide temperature (4–350 K) range, including under hydrostatic pressure (0–1.1 GPa). Throughout the pressure range investigated, the sample is spin glass with diffused phase transition into paramagnetic state. It is established, that spin glass state is a consequence of exchange interaction frustration of the ferromagnetic clusters embeded into antiferromagnetic clusters. The magnetic moment freezing temperature T _f of ferromagnetic clusters increases under pressure, freezing temperature dependence on pressure is characterized by derivative value ∼4.5 K/GPa, while the magnetic ordering T _MO temperature dependence is characterized by derivative value ∼13 K/GPa. The volume fraction of sample having ferromagnetic state is V _fer ∼ 13% and it increases under a pressure of 1.1 GPa by ΔV _fer ≈ 6%. Intensification of ferromagnetic properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite under hydrostatic pressure is a consequence of oxygen vacancies redistribution and unit cell parameters decrease. The most likely mechanism of frustrated exchange interactions formation is discussed.     

Magnetic phase separation in the (La<Subscript>0.3</Subscript>Sr<Subscript>0.7</Subscript>)<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>FeO<Subscript>3</Subscript> solid solution with a perovskite structure

The crystal structure and magnetic properties of the (La_0.3Sr_0.7)_0.5Ca_0.5FeO₃ solid solution with a perovskite structure have been investigated. The solid solution has been synthesized according to the high-pressure technique. The unit cell parameters have been refined using the Rietveld full-profile analysis under the assumption of the single-phase crystalline state and the two-phase model corresponding to the parent compositions. It follows from the calculations that the best agreement between the experimental data and the theoretical curve is observed for the two-phase model. The measurement of the magnetic properties also indicates the coexistence of two magnetic phases.     

Concentration-dependent structural transition in the La0.70Sr0.30MnO3−δ system

The crystal structure of anion-deficient La_0.70Sr_0.30MnO_3?δ manganites (δ = 0, 0.10, 0.15, and 0.20) has been studied by powder neutron diffraction at room temperature. The crystal structures of the stoichiometric La_0.70Sr_0.30MnO₃ and anion-deficient La_0.70Sr_0.30MnO_2.90 samples are satisfactorily described by the rhombohedral R $\bar 3$ c space group. The anion-deficient La_0.70Sr_0.30MnO_2.85 sample separates into two perovskite phases with the R $\bar 3$ c and I4/mcm space groups. The crystal structure of La_0.70Sr_0.30MnO_2.80 cannot be described by a single perovskite phase in the I4/mcm space group. It is found that the clustering of oxygen vacancies is the cause of structural phase separation at 293 K in anion-deficient La_0.70Sr_0.30MnO_3?δ manganites.     

Effect of long-range stresses on the structure of semiconductor heterosystems

The dependence between the misfit value of the interface layers f and the structural parameters of the misfit dislocations is obtained for arbitrary orientation of the interface of a semiconductor heterosystem subjected to the complete relaxation of misfit stresses. Such parameters are the distance D _i between neighboring dislocations of the ith family and projection of the edge component of the Burgers vector onto the interface (b _i ^e ). The number of families incorporated into the interface is determined by the orientation of the boundary and occurrence of the relaxation process. The role of specific cases of this expression for experimental and technological applications is discussed using the appearance of long-range shear and normal stresses in films with orientations of (001) and (111) as examples.     

Synthesis of Mg(Fe<Subscript>0.8</Subscript>Ga<Subscript>0.2</Subscript>)<Subscript>2</Subscript>O<Subscript>4</Subscript> by Gel Combustion Using Glycine and Starch

A powdery material Mg(Fe_0.8Ga_0.2)₂O₄ has been prepared by combusting a gel containing magnesium(II), iron(III), and gallium(III) nitrates and a glycine–starch mixture. The gel produced during the synthesis has been studied by thermal analysis (TGA/DSC) and IR spectroscopy. This mixture has been shown to be efficient to produce a homogeneous nanosized powderlike material Mg(Fe_0.8Ga_0.2)₂O₄. The morphology and properties of ceramic samples are characterized by scanning electron microscopy, X-ray powder diffraction, neutron diffraction, and vibrational magnetometry.     

Crystal structure and magnetic properties of nanosized Mg(Fe0.8Ga0.2)2O4-δ films on Si substrates

Film samples of nominal Mg(Fe_0.8Ga_0.2)₂O_4-δ composition were obtained on Si(100) substrates by oxygen-ion-beam sputtering of a Mg(Fe_0.8Ga_0.2)₂O_4-δ ceramic target. The film thicknesses were ～200 and ～400 nm. Field dependences of the specific magnetization of ～200-nm films annealed at different temperatures (800–1000°C) have been measured. The crystal structure, surface morphology, and magnetic characteristics of films of different thicknesses (～200 and ～400 nm) are investigated. The reasons for the discrepancy between the specific magnetizations of the films obtained and their ceramic analog are discussed.     

Initial stages of Ge epitaxy on Si(111) under quasi-equilibrium growth conditions

The results of the structural and morphological studies of Ge growth on a Si(111) surface at the initial stages of epitaxy by means of scanning tunneling microscopy and high-resolution transmission electron microscopy are presented. Epitaxy of Ge has been performed in the temperature range of 300 to 550°C under the quasi-equilibrium growth conditions and low deposition rates of 0.001–0.01 bilayers per minute. The stages of the formation and decay of the nanoclusters as a result of the redistribution of the Ge atoms into two-dimensional pseudomorphic Ge islands before the formation of the continuous wetting layer have been experimentally detected. The positions of the preferable nucleation of three-dimensional Ge islands on the wetting layer formed after the coalescence of the two-dimensional islands have been analyzed. The c2 × 8 → 7 × 7 → c2 × 8 phase transitions due to the lateral growth of the islands and the plastic relaxation of the misfit strains occur on the surface of the three-dimensional Ge islands when their strain state changes. The misfit dislocations gather at the interface and two types of steps lower than one bilayer are formed on the surface of the three-dimensional islands during the relaxation process.     

Effect of oxygen non-stoichiometry on the antiferromagnet-ferromagnet transition in Nd0.5Ca0.5MnO3−δ(δ≤0.12)

A decrease in the oxygen content in Nd_0.5Ca_0.5MnO_3?δ down to γ≤0.12 is shown to bring about a strong decrease in the magnetic field inducing a transition from the antiferromagnetic charge-ordered to the ferromagnetic charge-disordered state. The ferromagnetic phase in a Nd_0.5Ca_0.5MnO_2.92 sample is stable in the absence of an external magnetic field. A further increase in the content of oxygen vacancies stabilizes the antiferromagnetic charge-disordered state.