Z. A. Samoilenko

Amorphous state and pulsed laser deposition of YBa2Cu3O7−δ thin films

Pulsed laser deposition of high Tc compounds onto unheated substrates, resulting in amorphous thin films, preserves to a great extent the composition of matter ejected from the target. This composition is of primary interest, both for understanding the dynamics of laser–target interaction and for practical (optimization) reasons. We have investigated the structure of amorphous and crystalline YBaCuO films obtained both in on-axis and off-axis deposition geometries, and correlated the results with optical and transport properties of these films. X-ray scattering reveals in amorphous films the existence of: (i) amorphous continuum of spatially disordered atoms, (ii) small (10–40 A) amorphous clusters which can be considered as mesoscopic order fluctuations in the amorphous continuum, and (iii) slightly larger (50–250 A) crystalline clusters exhibiting quasi-two dimensional (00l) or (11l) long range order. Crystalline films are predominantly (00l) oriented. Optical spectra of both crystalline and amorphous f...

Atomic order, electron structure, and critical parameters of epitaxial YBaCuO films

X-ray structural and optical methods were applied to make a detailed investigation of atomic and electronic structure of superconducting YBa2Cu3O7−δ films obtained by pulsed laser deposition. Optical transmission spectrum was found to depend not only on intraband (ℏω 1.95 eV) transitions, but also on contributions from charge transfer transitions O 2p⇒Cu 3d in the dielectric phase due to local order in a system of crystallographic (11l) planes. Superconducting properties of the films are determined by orientational structural transition (00l)+(11l)⇒(00l) and by Anderson type transition from a charge transfer O 2p⇒Cu 3d insulator to a strongly degenerate semiconductor with metallic conductivity. We show the effects of ordering on transition temperature Tc and demonstrate that at the concentration of (00l) clusters below the percolation threshold (20%) the behavior of Tc has catastrophic character.

Variety of LaSrMnO structures induced by growth conditions and laser irradiation

Crystallographic phase transitions in perovskite-like LaSrMnO metallic oxides are studied. The transitions are induced when internal stresses generated during film synthesis (at temperatures between 450 and 730°C) vary (decrease or increase) upon subsequent irradiation by a KrF laser emitting in the UV range. As the synthesis temperature Ts grows, the rhombohedral-to-orthorhombic phase transition occurs at 650–670°C. The resistivity is shown to be either temperature-independent, ρ(T)=const, at T<Tcrit, or varies and reaches a maximum, ρ(T)=ρmax, at the Curie temperature Tc. Optical transmission spectra taken at photon energies ℏω=0.5–2.5 eV exhibit both a high (0.8–0.9) and low (0.1–0.3) transmission coefficient t, depending on the synthesis temperature. As follows from X-ray diffraction data, the laser irradiation causes a phase transition only in LaSrMnO films grown at Ts<650°C. Phases of different size scales appear: the long-range-order orthorhombic matrix and mesoscopic-range-order rhombohedral clusters are observed in the films grown at Ts=450–550°C and the rhombohedral matrix with orthorhombic clusters, in the films grown at Ts=550–650°C.

Influence of a strontium-enriched intergranular cluster network on the electrical conductivity of In2O3-SrO ceramics

A metastable hexagonal R-phase is revealed in polycrystalline In2O3-SrO samples, which has the form of a network made up of mesoscopic clusters (60–180 Å in size). The clusters arise from strontium-enriched regions near grain boundaries in the main cubic structure of indium oxide. It is shown that annealing in oxygen at Ta ⩾ 300°C saturates dangling bonds between the R-phase and the matrix and makes the system metastable. This state shows up in the presence of (i) solitary diffuse maxima from the R-phase imposed on Debye lines from the main phase in the X-ray diffraction pattern and (ii) the electron cyclotron resonance (ECR) line with g = 1.875. In addition, the sample in this state acquires a high resistivity (ρ ∼ 106 Ω cm). Relaxation at T ⩽ 300°C after annealing at Ta > 300°C disrupts bonds between the strontium-enriched clusters of the R-phase and the indium oxide matrix. This causes spatial separation of the clusters, disruption of their coherent bonds with the matrix structure, and escape of excess oxygen from the sample along grain boundaries. As a result, a new stable state forms, which is characterized by (i) a series of diffuse maxima from the R-phase imposed on lines assigned to the main phase, (ii) the presence of the ECR line with g = 2 with the line with g = 1.875 retained, and (iii) the transition of the sample to a low-resistivity state (ρ ∼ 100 Ω cm).

Changes in the electronic, optical, and magnetic properties of LaSrMnO films upon transition from the rhombohedral phase to the orthorhombic phase

The properties of LaSrMnO films are investigated in the temperature range of the transition from the rhombohedral phase to the orthorhombic phase (600–650° C). It is shown that, with a variation in the growth temperature Ts, the change in the film properties is governed by the interaction of Mn-O metallic (ferromagnetic) clusters in the dielectric (antiferromagnetic) matrix. At Ts≤600°C, the low density of eg states and the dielectric gap (Eg=0.3–0.5 eV) are responsible for the following features: (i) the optical transparency in the range ℏω=0.5–2 eV, (ii) the difference between the FC and ZFC magnetizations M(T), (iii) the high resistance, and (iv) the appearance of the portions R(T) ≈ const in the dependence R(T) due to the transformation of clusters into a system of tunnel-coupled quantum dots. At Ts≥650°C, the local increase in the atomic and electronic densities leads to a decrease in the optical transmission and the resistance by three to nine orders of magnitude, the appearance of a maximum and a minimum in the dependences R(T) of the LaSrMnO films, and an increase in the magnetization M(10 K) by one order of magnitude. The inference is drawn that magnetic ordering of the system of tunnel-coupled clusters encourages an increase in the cluster size and in the content of the metallic (ferromagnetic) phase.

Spherical cluster ensembles with fractal structure in LaSrMnO: New form of self-organization in solids

The growth of La0.7Sr0.3MnO3 films in magnetron plasma, in special conditions, leads to the appearance of ensembles of micron-sized spherical crystalline clusters with fractal structure, which we consider to be a new form of self-organization in solids. Each ensemble contains 105–106 elementary clusters, 100–250 A in diameter. Interaction of the clusters in the ensemble is realized through the interatomic chemical bonds, intrinsic to the manganites. Integration of peripheral areas of interacting clusters results in the formation of common intercluster medium in the ensemble. We argue that the ensembles with fractal structure built into paramagnetic disordered matrix have ferromagnetic properties. Absence of sharp borders between elementary clusters and the presence of common intercluster medium inside each ensemble permits to rearrange magnetic order and to change the volume of the ferromagnetic phase, providing automatically a high sensitivity of the material to the external field.

Structural transformations in Y-Ba-Cu-Ti-O metal oxides accompanying substitution of titanium for Y and Cu

Taking into account changes in the Cu charge states, general characteristics and distinctive features of structural transformations typical of high-temperature superconducting and ferroelectric phases in Y-Ba-Cu-Ti-O ceramic samples are studied. It is shown that with substitution of titanium for Y and Cu the concentration of Jahn-Teller ions Cu2+ detected by EPR technique does not rise higher than 10%. According to X-ray spectroscopy data, the Cu charge state lowers from Cu2.2+ to Cu0.5+ with increasing titanium content. It is found by X-ray diffraction studies that with increasing Ti content, the fraction of the layered structure containing (00l) planes of orthorhombic structure decreases and the growth of planes of the (11l) type pertaining to tetragonal structure is stimulated. The structural phase transition from the orthorhombic to tetragonal crystal structure mainly occurs in the range x=0.3–0.5.

Effect of clustering in LaCa(Sr)MnO epitaxial films on their magnetic, electron, and optical properties

This work is aimed at finding a correlation between the magnetic properties, conductivity, and crystal structure of La0.6Sr0.2Mn1.2O3 and La2/3Ca1/3MnO3 films prepared by pulsed laser (KrF) deposition in order to study the behavior of their M(T) and M(H) magnetizations. X-ray diffraction is used to investigate the cluster crystal structure of the manganites. It is shown that the variation of growth temperature and substrate type, along with laser irradiation, change not only the matrix structure (long-range order in ion arrangement) but also the cluster structure (mesoscopic order). The magnetic, electron, and optical properties revealed experimentally are explained by changes in the atomic and electron subsystems of the cluster structures, which provide magnetic interaction between clusters.

Self-organization of atomic order and electronic structure in LaSrMnO films

A linear relationship between the critical temperatures Tmax and Tmin in the temperature dependences of the resistance of La0.6Sr0.2Mn1.2O3 single-crystal films that have a mesoscopic irregularity (metallic clusters in an insulating matrix) is found. A correlation between the atomic order and electronic structure of the films is studied by taking X-ray diffraction patterns and optical absorption spectra. It is shown that a rise in Tmax and a simultaneous decrease in Tmin cause correlated local changes in cluster areas of the structure. Namely, the volume occupied by a family of Mn-O planes with large interplanar spacings (d=2.04–2.08 Å) shrinks, while the volume occupied by a family of closer spaced (d=1.90–1.99 Å) planes grows. In the electronic subsystem, the density of states at ℏω=1.5 and 2.4 eV, which are due to Mn3+ and Mn4+ ions, increases, and the contribution from Mn2+ states at ℏω=0.9 eV decreases. As the charge states associated with Mn3+ and Mn4+ ions become dominant, the Mn-O binding energy grows. As a result, the contribution of the structural states with smaller d increases, thereby raising the density of states in the electronic subsystem at energies between 0.5 and 2.7 eV. The effect of self-organization in the multicomponent LaSrMnO system shows up in the transition from the heavily distorted rhombohedral to the less distorted orthorhombic structure.

Evolution of Short-Range, Mesoscopic, and Long-Range Orders in Magnesium-Zinc Ferrites

The Mössbauer and X-ray diffraction techniques are used to study the evolution of structure and chemical inhomogeneities in magnesium-zinc ferrites. These are the octahedral short-range environment of a magnetic Fe3+ ion, mesoscopic quasi-planar regions of size 100–200 Å (clusters) with locally changed chemical composition and crystallographic order, and the general long-range order of the ferrite matrix. In each of these three inhomogeneities, the high-temperature long-term firing of the Mg-Zn ferrite (T=1280°C, τ=0–8 h) induces transitions (magnetic in the short-range order, concentration in the clusters, and structural in the matrix) within the time period τ=1–2 h. Subsequently, the structure stabilizes with the initial three-scale hierarchy retained.