Effect of constant-energy surface anisotropy on the thermoelectric figure-of-merit of the n-Bi2 (Te,Se,S)3 solid solutions

A study is reported of the thermoelectric and galvanomagnetic properties of n-Bi₂Te_3−x−y Se_xSe_y solid solutions for 0.12⩽x⩽0.36 and 0.12⩽y⩽0.21 within the 80–300 K temperature region. The thermoelectric figure-of-merit Z has been found to correlate with the parameters of the many-valley energy-band model including anisotropic carrier scattering. It is shown that a decrease in the constant-energy surface anisotropy and scattering anisotropy results in a growth of Z for optimum carrier concentrations in the solid solution. Fiz. Tverd. Tela (St. Petersburg) 41, 187–192 (February 1999)     

Frozen magnetic states in quasi-two-dimensional Cr1/3−x NixTaS2 compounds with competing exchange interactions

A study of the dynamic and static magnetic susceptibility of quasi-two-dimensional, doubly intercalated compounds Cr_1/3−x Ni_xTaS₂ (0.08⩽x⩽0.23) is reported. It is shown that cooling from paramagnetic phase in this concentration range produces a spin-glass-type frozen magnetic state. In contrast to the classical 3D spin glasses, however, this state sets in within a broad temperature range rather than at a phase transition. A magnetic phase diagram of the studied compounds has been constructed. Fiz. Tverd. Tela (St. Petersburg) 39, 1801–1805 (October 1997)     

Magnetic, electrical, and crystallographic properties of thin La1−x SrxMnO3 films

A study is reported of the magnetic, electrical, and crystallographic properties of La_1−x Sr_xMnO₃ (0.15⩽x⩽0.23) epitaxial films grown on single-crystal substrates of (001)ZrO₂(Y₂O₃) having the fluorite structure and (001)LaAlO₃ having the perovskite structure. It was found that films with close compositions for x=0.15 and 0.16, grown on different substrates, have different properties, namely, the film on a fluorite substrate is semiconducting and has a coercive strength 30 times that of the film on a perovskite substrate; the temperature dependence of electrical resistance of the latter film has a maximum around the Curie point T _C and follows metallic behavior for T<T _C. These differences are explained as due to different structures of the films. The x=0.23 film on the perovskite substrate has been found to exhibit a combination of giant magnetoresistance at room temperature with a resistance of ≈300 Ω which is useful for applications. The maxima in resistance and absolute value of negative magnetoresistance are accounted for by the existence of two-phase magnetic states in these films. Fiz. Tverd. Tela (St. Petersburg) 40, 290–294 (February 1998)     

Infrared lattice-reflection spectroscopy of Zn1−x CdxSe epitaxial layers grown on a GaAs substrate by molecular-beam epitaxy

Results are presented of the first measurements of infrared reflection spectra of Zn_1−x Cd_xSe films (x=0–0.55; 1) grown on a GaAs substrate by molecular-beam epitaxy. It is shown by a mathematical analysis of the experimental spectra that the investigated Zn_1−x Cd_xSe alloy system manifests a unimodal rearrangement of its vibrational spectrum as the composition is varied. Fiz. Tverd. Tela (St. Petersburg) 41, 982–985 (June 1999)     

Direct observation of phase separation in La1−x CaxMnO3

In a study of the infrared optical absorption spectra of polycrystalline La_1−x Ca_xMnO₃ (0.1⩽x⩽0.8), a contribution from free charge carriers is observed below the Curie temperature for compositions with x⩽0.4. The existence of this contribution for samples with a high resistivity is a direct proof of the existence of “metallic” drops in an insulating matrix. The relative volume of the “metallic” phase is estimated. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 89–92 (10 July 1998)     

High-frequency excitations of stripe-domain lattices in magnetic garnet films

Magnetic garnet films of composition (Y,Bi)₃(Fe,Al)₅O₁₂ have been grown by liquid phase epitaxy on [111] and [110] oriented substrates of gadolinium gallium garnet. The domain wall resonance and the two branches of the domain resonance of periodic stripe domains are measured as function of the bias induction applied in the film plane parallel to the stripes. Resonance frequencies up to 7.5 GHz are observed. An improved version of the hybridization model is developed to describe these resonances. It turns out that hybridization of the domain resonance branches is determined by the cubic anisotropy for [111] oriented films, while for [110] oriented films coupling of the domain resonances is mainly caused by the orthorhombic anisotropy. The theoretical model is in excellent agreement with experiments, no fitting parameters are used. It is also used to derive the phase relation between the precessing magnetizations of neighbouring domains.     

New line of thermodynamic phase transitions on the x-T diagram of a dilute Heisenberg system with short-range interactions Li0.5Fe2.5−x GaxO4

It is established on the basis of an investigation of the temperature dependences of the saturation magnetization σ_S(T) and the magnetic contribution to the specific heat C _m(T) that in the reentrant region of the x-T diagram (0.9⩽x⩽1.5) of dilute iron spinels Li_0.5Fe_{2.5− x} Ga_xO₄ there exists between the line of Curie points T _C(x) and the line of freezing temperatures T _f(x) another line of first-order phase transitions T ₁(x) to a noncollinear ferrimagnetic phase: T _C(x)⩽T ₁(x)⩽T _f(x). Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 329–333 (10 March 1998)     

Transport properties of antiferromagnetic fcc-iron alloys

A study is reported of the electrical resistance and thermopower of Fe_xNi_80−x Cr₂₀ fcc alloys within the 44⩽x⩽70 at. % range. It is shown that, at low temperatures, they typically exhibit minima in the temperature dependences of electrical resistance. The appearance of these anomalies is attributed to the formation of a gap in the conduction electron spectrum due to the onset of long-or short-range antiferromagnetic order in the alloys. The effect of magnetic field on the magnetic states appearing in frustrated antiferromagnetic alloys has been studied, and an H-T magnetic phase diagram constructed. Fiz. Tverd. Tela (St. Petersburg) 40, 101–105 (January 1998)     

Characteristics of magnetic order in perovskite manganites La1−x CaxMnO3

Solid solutions of the system La_1−x Ca_xMnO₃ are synthesized, and their magnetic and electrical properties are investigated. As x is increased, the crystal lattice changes symmetrically from an orthorhombic (x=0 and x=0.8) to a cubic structure (0<x⩽0.6 and x=1). Nonstoichiometric LaMnO₃ and compositions with a low Ca content (0<x<0.4) are collinear ferromagnets. A metal-insulator transition is observed in them near the Curie temperature. Compositions with 0.6⩽x⩽0.9 exhibit a semiconductor-type conductivity and an unusual temperature behavior of the magnetization and the susceptibility with very distinct Curie and Néel temperatures. The magnetic properties of the solid solutions are explained on the basis of the model of ferro-antiferromagnetic phase separation. Zh. éksp. Teor. Fiz. 116, 1664–1675 (November 1999)     

Phase transitions and magnetic-transport phenomena in the system La2/3Ba1/3(Mn1−x Cox)O3

A study is performed of the crystalline structure, magnetization, and magnetotransport properties of the system La_2/3Ba_1/3(Mn_1−x Co_x)O₃ with perovskite structure. It is shown that cubic solid solutions exist over the entire range of cobalt concentrations 0⩽x⩽1. Compositions with x⩽0.2 are ferromagnets with maximum resistance near T _C . Compositions with 0.2<x<0.4 manifest properties of inhomogeneous ferromagnets. Measurements of magnetic properties indicate the absence of long-range magnetic order in compositions with 0.5⩽x⩽0.9, which are probably spin glasses. The spontaneous magnetization of cobaltate (2μ _B per formula unit) corresponds to ferromagnetic ordering of the moments of the Co³⁺ and Co⁴⁺ ions found in the intermediate spin state. It is conjectured that the magnetoresistance consists of an extrinsic and an intrinsic contribution. The first arises as a result of intergrain transport of spin-polarized charge carriers, and the second, as a result of magnetic ordering near T _C . The magnetoresistance is essentially independent of the spontaneous magnetization and decreases abruptly as the cobalt concentration is increased with a corresponding transition from long-range to short-range magnetic order. Zh. éksp. Teor. Fiz. 116, 604–610 (August 1999)     

Temperature dependence of the exchange interaction constant in iron garnet films

The spin-wave resonance method is used to investigate the temperature dependence of the exchange interaction constant A in iron garnet films. The experimental results are compared with theoretical results based on the theory of second-order phase transitions. It is shown that the temperature dependence A(T) for films of pure yttrium iron garnet is different from A(T) for films exhibiting a point of compensation of the magnetic moment. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 528–532 (10 April 1996)     

Influence of microwave treatment on the electrophysical characteristics of technically important semiconductors and surface-barrier structures

An investigation is made of changes in the electrophysical parameters of narrow-gap (Cd_xHg_1−x Te x=0.22–0.24) and wide-gap (gallium arsenide, indium and gallium phosphides) semiconductor materials and Schottky-barrier diode structures based on these materials, stimulated by microwave electromagnetic radiation. It is shown that the parameters of materials and device structures may be improved by defect gettering. An analysis is made of possible mechanisms for the interaction between microwave radiation and the objects being studied. Zh. Tekh. Fiz. 68, 49–53 (December 1998)     

Phase transitions in RbxK1−x LiSO4 mixed crystals

Structural phase transitions in Rb_xK_1−x LiSO₄ mixed crystals (with x varying from 0 to 1) have been studied from the melting point to liquid-nitrogen temperature. Calorimetric (DTA and DSM) data, birefringence and optical polarization measurements were used to construct the full phase (T-x) diagram. It has been established that crystals of most compositions (x⩽0.95) grow in the hexagonal-trigonal KLiSO₄ structure. Replacement of K by the larger Rb atom results in a considerable increase of the region of existence of the P31c phase and expulsion of the high-temperature hexagonal phase. Fiz. Tverd. Tela (St. Petersburg) 40, 1341–1344 (July 1998)     

Rotation of the Fermi surface in the transition layer of bicrystals of an anisotropic metal

The Shubnikov-de Haas effect is investigated in bicrystals of p-type Bi_1−x Sb_x (0.063⩽x⩽0.088) alloys in static magnetic fields up to 15 T. The constant-energy surface of -holes in the transition layer of the bicrystals is found to be rotated ∼90° from its position in single domains (monoblocks). Fiz. Tverd. Tela (St. Petersburg) 39, 1713–1716 (October 1997)     

Microphase separation and magnetic Jahn-Teller polarons in LaSrAl1−xCuxO4−δ

An EPR study of the solid solutions LaSrAl_1−x Cu_xO₄, which are isostructural to La₂CuO₄, shows that microphase separation of the structure occurs already at small copper concentrations (x≈0.01). A phase enriched with Cu appears along with a phase of La₂AlO₄, which contains isolated CuO₆ centers. It is established that the nature of the states and the deformations of the CuO₆ centers is determined by internal Jahn-Teller factors. When x⩽0.1, EPR signals are detected for new dynamic centers, which are identified as CuO₆ Jahn-Teller centers with a hole delocalized among the four in-plane oxygen ions (the total spin S=1). When 0.1⩽x<1, local CuO ₆ ⁻ hole centers transform into magnetic Jahn-Teller polarons, which include five or more CuO₆ fragments, in the copper phase of the structure. Their transformations and the conditions for observing them are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 622–628 (April 1998)     

Properties and structure of GaAs films grown by molecular beam epitaxy on GaAs substrates with the (100), (111)A, and (111)B orientations

The structural perfection of GaAs epitaxial films grown by molecular beam epitaxy on substrates with the (100), (111)A, and (111)B orientations is investigated by double-crystal and triple-crystal x-ray diffractometry. It is found that the ratio γ of the molecular fluxes of arsenic and gallium has a strong influence on the structural quality of the epitaxial films. The optimum values of the parameter γ are found for each of the substrate orientations (100), (111)A, and (111)B. Zh. Tekh. Fiz. 69, 68–72 (July 1999)     

Mixed state and critical current in narrow semiconducting films

The mixed state of thin narrow superconducting films with an edge barrier placed in a transverse magnetic field is considered. The boundaries of the region for the existence of metastable mixed states with an assigned number of vortices N [H _min(N)⩽H⩽H _max(N)] are found. The magnetic-field dependence of the critical field is found for the films. The transition from the Meissner state to the static mixed state is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1773–1777 (October 1998)     

Two-dimensional weak localization effects in high temperature superconductor Nd2−x CexCuO4−δ

A systematic study of the resistivity and Hall effect in single-crystal films Nd_2−x Ce_xCuO_4−δ (0.12⩽x⩽0.20) is presented, with special emphasis on the low-temperature dependence of the normal state conductance. Two-dimensional weak localization effects are found both in a normally conducting underdoped sample (x=0.12) and in situ superconducting optimally doped (x=0.15) or overdoped (x=0.18) samples in a high magnetic field B>B _c2. The phase coherence time τ _ϒ (5.4 · 10⁻¹¹ s at 2 K) and the effective thickness of a CuO₂ conducting layer d (≃1.5Å) have been estimated by fitting 2D weak localization theory expressions to magnetoresistivity data for magnetic fields perpendicular to the ab plane and in plane. Estimates of the parameter d ensure strong carrier confinement and justify a model consisting of almost decoupled 2D metallic sheets for the Nd_2−x Ce_xCuO_4−δ single crystal. Zh. éksp. Teor. Fiz. 116, 1723–1734 (November 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Thermodynamic properties of the mixed elpasolites Rb2KGaxSc1−x F6 (x=0.6–1.0)

Measurements of the heat capacity of the elpasolite Rb₂KGaF₆ and the solid solutions Rb₂KGa_xSc_1−x F₆ with a gallium content in the range x=0.6–0.95 are performed. The nature of the influence of the chemical composition of the compounds and the hydrostatic pressure on the temperature and entropy of the ferroelastic phase transitions is established. Fiz. Tverd. Tela (St. Petersburg) 39, 1844–1849 (October 1997)     

Influence of structural deformations on the magnetic properties of Jahn-Teller complexes of divalent copper in CaxSr1−x F2 mixed crystals

Crystals of variable constituency Ca_xSr_1−x F₂:Cu (0⩽x⩽0.05, x≅0.5, 1.0⩾x⩾0.95) are investigated by EPR and x-ray structural analysis. The electron-Zeeman, intrinsic hyperfine, and ligand hyperfine interaction parameters are determined; they characterize the magnetic properties of the copper paramagnetic complexes formed in the crystals. Models of the molecular structures of these complexes are discussed, along with the influence of structural deformations and vibronic interactions on their magnetic properties. Fiz. Tverd. Tela (St. Petersburg) 40, 445–451 (March 1998)