Magnetic crystallographic anisotropy of Fe/GaAs(100) epitaxial films

An investigation was made of the anisotropic properties of Fe films obtained by molecular-beam epitaxy on GaAs(100) substrates. It is shown that at thicknesses t=40–50 Å the Fe films begin to exhibit cubic magnetic anisotropy. At thicknesses t>100 Å the first constant of cubic anisotropy K₁ has values similar to those for “bulk” Fe single crystals. Films of intermediate thickness 50<t<100 Å possess both isotropic and anisotropic phases. Pis’ma Zh. Tekh. Fiz. 25, 88–95 (January 26, 1999)     

The “negative trapping” effect in the magnetic-field dependence of absorption in high-temperature superconductors

This paper reports the first study of the absorption of an rf electromagnetic field in high-temperature superconducting samples of YBa₂Cu₃O_7−x in the range of magnetic fields corresponding to the onset of the “negative trapping” effect. It is shown how additional annealing and degradation of the sample influence the onset of the effect. A modification of the model of dynamic interaction of superconducting loops is used to explain the observed phenomena. Pis’ma Zh. Tekh. Fiz. 23, 20–25 (March 26, 1997)     

Dispersion of magnetostatic waves in a tangentially magnetized ferrite wafer with normal uniaxial anisotropy

A dispersion law is obtained and analyzed for the first time for magnetostatic waves in a tangentially magnetized ferrite wafer with uniaxial anisotropy where the axis is perpendicular to the plane of the wafer and the applied static magnetic field is weaker than the anisotropy field. This model qualitatively describes the dispersion of magnetostatic waves in hexa-and orthoferrite wafers and also in an unsaturated ferrite wafer. Pis’ma Zh. Tekh. Fiz. 25, 73–78 (November 26, 1999)     

New Insights on the Intrinsic Anisotropy of YBCO Films

Careful investigation of the angular dependence of resistivity ρ(θ) (θ is the angle between the magnetic field and the ab-planes) and the temperature dependence of resistivity ρ(T) within the superconducting transition in an applied magnetic field B up to 1 T for a series of YBa₂Cu₃O_7−δ (YBCO) thin films revealed a large variation of intrinsic anisotropy factor γ. The series of films studied included both optimally doped and underdoped samples of different T _c , critical current density J _c , film thickness, and preparation techniques. The variation in the shape and depth of the minimum measured for ρ(θ) near θ=0° could be directly correlated to the intrinsic anisotropy of the YBCO films. The results of fitting of ρ(θ) using Bardeen–Stephen theory allowed a quantitative determination of the value of γ which varies between 7 and 230, and is independent of T _c , film thickness, or J _c . The sharper the minimum in ρ(θ) around θ=0° the larger is the anisotropy. For highly anisotropic film, ρ(θ) showed an identical behavior for B ∥ J and B ⊥ J (i.e., ρ(θ) is independent of the angle θ between B and J for this film). The large variation in γ could be attributed to the “buckling” of the CuO₂ planes.     

Ferromagnetic Resonance Study of Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As Fabricated on (311) GaAs Wafers by Mn Ion Implantation and Pulsed-Laser Melting

We present a ferromagnetic resonance (FMR) study of Ga_1−x Mn _x As fabricated by Mn ion implantation (II) into (311) GaAs wafers followed by pulsed-laser melting (PLM). We measured the angular dependence of FMR at 4 K, and the data were then fitted by Stoner–Wohlfarth model to obtain the cubic and uniaxial anisotropy parameters. The observed angular behavior of FMR can be understood in terms of two contributions: a cubic anisotropy field parallel to the 〈001〉 axes, and a uniaxial anisotropy field parallel to the [311] direction. Our results show that the magnetic anisotropy fields in II-PLM (Ga,Mn)As are fundamentally similar to those in Ga_1−x Mn _x As samples grown by molecular beam epitaxy (MBE), indicating that the two different growth methods lead to materials with very similar magnetic properties.     

Effect of electric fields on the amorphous state⇒Crystal transition in BiPbSrCaCuO (2223) films

Results of studying the influence of electric fields on the structure and electrical properties of BiPbSrCaCuO (2223) films undergoing the transition from the amorphous to a crystalline state are discussed. It is shown that passing a current through the samples during high-temperature annealing substantially increases the growth anisotropy, improves the quality of the texture, and increases the electrical conductivity of the films by between one and five orders of magnitude compared with control samples. It is concluded that the mechanism is attributable to carrier transport in (001) type planes, which facilitates changes in the atomic order. Unlike in the control samples, this family of planes plays a leading role in structural transformations. Pis’ma Zh. Tekh. Fiz. 25, 71–77 (January 26, 1999)     

Quantitative analysis of endohedral metallofullerenes by Rutherford backscattering of protons

Samples containing endohedral metallofullerenes of gadolinium (Gd@C₈₂) and dysprosium (Dy@C₈₂) are investigated by the Rutherford backscattering of protons. It is shown that the concentration of endohedral metallofullerenes can be determined to within a few percent. The measured concentration of endohedral metallofullerenes Gd@C₈₂ and Dy@C₈₂ in samples prepared by selective multistep extraction is ∼60%. Pis’ma Zh. Tekh. Fiz. 25, 24–30 (May 26, 1999)     

Properties of micromagnetic structures in amorphous FeCuNbSiB alloys

Results are presented of a magnetooptic investigation of the surface micromagnetic structure of FeCuNbSiB ribbons in the initial state and after annealing at 550 °C for 1 h. In the initial state the samples were amorphous, whereas after annealing they exhibited a nanocrystalline structure with typical grain sizes of 10–12 nm. Dispersion of the magnetic anisotropy was observed in the samples studied; this was responsible for the nonuniformity of their local magnetic properties. It was found that the linear dimension of the magnetic inhomogeneities in the initial and annealed samples was 120–150 and 50–70 μm, respectively. Pis’ma Zh. Tekh. Fiz. 25, 62–67 (January 12, 1999)     

Deformation stability of a flat domain wall in magnetic films

The forces determining the orientation of domain walls in films of magnetooptic materials with a figure of merit of order unity are studied. The behavior of small perturbations of the position of a flat domain wall in the presence of an in-plane component of the anisotropy vector is analyzed. The forces arising when the orientation of the domain wall deviates from the easy-magnetization axis and striving to return the wall to its initial state are conventionally represented as a gradient “effective magnetic field.” The forces exerted by the “effective field, ” due to the in-plane component of the anisotropy vector, on the perturbed domain wall are calculated. The orientational stability conditions for a planar domain wall are found. An explanation is given for the experimentally observed predominant orientation of striped domains. Pis’ma Zh. Tekh. Fiz. 25, 49–56 (September 12, 1999)     

Angular dependence of the current-voltage characteristics and voltage fluctuation spectrum in ceramic superconductors

Measurements were made of the current-voltage characteristics and voltage fluctuation spectrum as functions of the angle between the magnetic field applied in the plane of the sample and the direction of the transport current in a YBa₂Cu₃O_7−δ ceramic superconductor. The angular dependence of the fluctuation spectrum was measured first. It is shown that the current-voltage characteristics are not described using the vortex line cutting model and the voltage fluctuations are not caused by independent motion of vortices in the bulk of the sample. The results are consistent with the model of a self-organized critical state. Pis’ma Zh. Tekh. Fiz. 25, 8–13 (February 12, 1999)     

Anisotropic Carbon Phases Prepared from Fullerite C70 under High Pressure

Abstract

We present the structural and ultrasonic study of carbon phases prepared by quenching after heating of fullerite C₇₀ in the temperature range 300–1100°C at pressures 4 and 7.5 GPa. The main aspect of the work concerns the structural and elastic anisotropy of samples resulted as consequence of quasi hydrostatic pressure with an additional pressure component along the axis of pressure load. Structural anisotropy correlates with anisotropy of the tensor of effective elastic constants taken for the medium with single axis of anisotropy. Comparison of the data obtained with anisotropy effects, earlier observed in the phases synthesized from C₆₀ in the similar quasi hydrostatic conditions, clarifies the role of non‐spherical symmetry of C₇₀ molecule.     

Electrical and photoelectric properties of GeS layered crystals grown by different techniques

Thermally stimulated currents and anisotropic electrical conductivity are studied in GeS layered crystals prepared by the Bridgman-Stockbarger, Pizzarello, and sublimation techniques. All the crystals arep-type, regardless of the growth technique, owing to the presence of Ge vacancies. The conductivity anisotropy in the melt-grown crystals is high compared to the vapor-grown GeS. The anisotropy rises exponentially with temperature. The concentrations and ionization energies of traps in GeS crystals are determined from thermally stimulated current curves. The spectral response of the photocurrent through the crystals prepared by sublimation, whose structural perfection is higher than that of the melt-grown crystals, is governed by the spectral dependence of the absorption coefficient forad ≪ 1 (near-edge region) and by the spectral dependence of reflectivity for αd > 1 (high-α region). Regardless of the growth technique, the 293-K photocurrent spectra of GeS crystals show strongly polarized peaks at 1.65 (E ∥a) and 1.78 eV (E ∥b), which are due to the Λ ₁ ^v → Λ ₁ ^c and Δ ₂ ^v → Δ ₂ ^c optical transitions. The low-temperature photoresponse athv < 1.7 eV is due to absorption by Si impurity.     

Magnetization Anisotropy in YBCO Superconductors Prepared MPMG Method

We have studied magnetization anisotropy in a YBa₂Cu₃O₇ sample synthesized by a modified melt powder melt growth (MPMG) method. Magnetization of the sample upon ZFC process was measured as a function of magnetic field oriented both parallel and perpendicular to the c-axis. Magnetization anisotropy parameter γ is determined from remanent magnetization as a function of temperature. Magnetic relaxation measurement has been carried out in selected magnetic fields in both field increasing and decreasing case. At 20 kOe and 25 K for the field increasing case, the activation energy was found to be 53.5 meV and 51.7 meV when H∥c and H⊥c, respectively. Calculations based on the critical-state model can reproduce quite well the experimental data for H∥c and H⊥c where we have chosen n=0.15 and n=0.3, respectively.     

Modified layered composite with high anisotropy of the piezoconstants e ij* and d ij*

A three-component composite combining features of structures with 2-2 and 3-1 connectivity and characterized by considerable piezoelectric anisotropy e ₃₃*/|e ₃₁|, d ₃₃/|d ₃₁*| is investigated for different volume concentrations of one of the components. An analysis is made of the physical factors influencing the piezoelectric anisotropy of the composite and causing the peculiar distribution of the internal stress and electric fields in this material. Pis’ma Zh. Tekh. Fiz. 24, 65–70 (June 12, 1998)     

Nonequilibrium effects accompanying condensation at a black sphere

A Monte Carlo simulation is used to make a detailed analysis of the nonequilibrium zone formed near the surface of a condensing sphere. Significant directional anisotropy of the temperature was observed. It was established that the nonequilibrium near an absolutely black sphere is responsible for the difference between the calculated values of the specific mass flow in comparison with the published data obtained assuming little difference between the vapor state at infinity and temperature saturation conditions at the surface of the sphere. Pis’ma Zh. Tekh. Fiz. 25, 43–48 (January 26, 1999)     

Dimensional Crossover in the Fluctuation Magnetization in YBa2Cu3O7- x and its Evolution with the Content of Oxygen

We have studied the field-induced dimensional crossover in the fluctuation magnetization of three single crystals of YBa₂Cu₃O_7-x, with superconducting transition temperatures, T_c = 62.5, 52, and 41 K. The dimensional crossover is observed by studying the diamagnetic vortex fluctuations of the lowest-Landau-level type which occur in isochamps magnetization curves, MvsT, for temperatures close to the transition temperature T_c(H). The study was accomplished by obtaining isochamps magnetization curves as a function of temperature for fields in the range of 0.1–5 T. Magnetization curves for each sample when plotted together show two distinct well resolved crossing points, one formed by low field curves and located at a higher temperature than the other formed by high field curves. A lowest-Landau-level scaling analysis is applied to the curves forming the crossing points and it is verified that lower field curves obey the three-dimensional form of this scaling while the higher field curves obey the two-dimensional form. The results allow to observe the evolution of the dimensional crossover field H_cross, 3D–2D, with the content of oxygen in YBa₂Cu₃O_7-x. It is observed that the evolution of the field H_cross with the content of oxygen in each sample qualitatively agrees with theoretical predictions and allow us to estimate the ratio of the anisotropy among the studied samples.     

Temperature dependence of transport losses in multicored high-temperature superconducting composites

Measurements were made of the ac transport losses at temperatures T≥77K in silver-sheathed (Bi, Pb)₂Sr₂Ca₂Cu₃O_x composites with N=19, 61, and 127 filaments. It was observed that an increase in temperature causes an increase in the transport losses which depend on the amplitude of the working transport current. It is shown that the increase in the transport losses results from a reduction in the critical current of the composites. Pis’ma Zh. Tekh. Fiz. 25, 83–87 (June 12, 1999)     

Surface light waves on the boundary of a photorefractive crystal with a diffusion-drift nonlinearity mechanism

All the possible types of surface light wave that can exist on the boundary of a photorefractive uniaxial crystal with a diffusion-drift nonlinearity mechanism are identified. It is shown that by varying the magnitude and direction of an external electric field E₀ one can control both the parameters and the type of the surface wave. In the particular case of zero field E₀ (the diffusion nonlinearity mechanism) the general solution agrees with the solution obtained in Ref. 5. Pis’ma Zh. Tekh. Fiz. 23, 31–36 (June 26, 1997)     

Influence of phase hardening on transition broadening in a (1−x)NaNbO3−xLiNbO3 system

Dielectric methods are used to study phase-transition broadening in a (1−x)NaNbO₃− xLiNbO₃ system. It is observed that the lowest values correspond to regions of transition between implanted solid solutions and substitutional solid solutions with 0.05≤x≤0.07. It is shown that the degree of broadening of the phase transition may be varied by phase hardening. Pis’ma Zh. Tekh. Fiz. 25, 1–4 (October 26, 1999)