Magnetostriction in the vicinity of spin-reorientation phase transitions in singlecrystal DyFe11Ti

The field, temperature, and angular dependences of longitudinal λ_‖ and transverse λ_⊥ magnetostriction in single-crystal DyFe₁₁Ti are investigated. Tensometric measurements were made in the temperature range from 78 to 300 K in magnetic fields up to 12 kOe. Measurements of the magnetostriction of single-crystal DyFe₁₁Ti, YFe₁₁Ti, and LuFe₁₁Ti imply that the sublattice of 3d transition metals makes only a small contribution to the magnetostriction in compounds RFe₁₁Ti, and that the primary contribution to the magnetostriction of these compounds comes from the rare-earth metal sublattice. The primary microscopic mechanism for magnetostriction is single-ion magnetostriction caused by the interaction of the anisotropic orbital electron cloud around the Dy³⁺ magnetic ion with the crystal field of the lattice. Fiz. Tverd. Tela (St. Petersburg) 41, 1647–1649 (September 1999)     

Characteristics of current flow in composites made from a high-temperature superconductor and the low-temperature superconducting metal oxide Ba(PbBi)O3

Composites are prepared from a high-temperature superconductor with the 1-2-3 structure and the low-temperature oxide BaPb_3/4Bi_1/4O₃ with various volume concentrations of the high-temperature and low-temperature superconductors. The temperature dependences of the electrical resistivities and critical current densities of the composites are measured. It is shown experimentally that these composites comprise a synthetically created network of Josephson weak links (with a specified degree of coupling). Fiz. Tverd. Tela (St. Petersburg) 39, 418–424 (March 1997) Deceased.     

Anomaly in the upper critical magnetic field common to YBa2Cu3O7−δ , HoBa2Cu3O7−δ , and Nd1.85Ce0.15CuO4−δ irradiated by helium ions

The high-temperature superconductors YBa₂Cu₃O_7−δ , HoBa₂Cu₃O_7−δ , and Nd_1.85Ce_0.15CuO_4−δ are found to possess a common anomaly in the variation of the upper critical magnetic field owing to irradiation by helium ions. While the resistivity ρ increases by many times, the increase in H _c2 from scattering by radiation defects typical of ordinary superconductors does not occur. In terms of the ordinary mechanism, the anomaly may be explained by a significant reduction in the conduction electron density resulting from a loss of oxygen, which causes a significant rise in ρ with a small change in the scattering. Fiz. Tverd. Tela (St. Petersburg) 41, 1372–1376 (August 1999)     

Anomalies of the electronic spin-lattice relaxation of Gd3+ in YBa2Cu4O8 and YBa2Cu3O6+x near 200 K

A sharp kink in the temperature dependence of the electronic spin-lattice relaxation rate near 200 K was found in the high-temperature superconductors YBa₂Cu₄O₈ and YBa₂Cu₃O_6+x . The effect is correlated with opening of a spin gap and microscopic phase separation. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 567–572 (25 April 1999)     

Metamagnetism and magnetostriction of the Ising antiferromagnet DyCrO3

The magnetostriction of the Ising antiferromagnet DyCrO₃ during metamagnetic transitions induced by a magnetic field applied along the a and b axes of the crystal is investigated experimentally and theoretically. The magnetostriction measurements performed in the temperature range 1.5–4.2 K in magnetic fields up to 3 T clearly reveal the two-step character of the transition from the original antiferromagnetic structure to the ferromagnetic structure with the intermediate formation of a low-symmetry structure (when H is parallel to the a axis). The unusual behavior of the magnetostriction discovered during the magnetizing process provides information on the nature of magnetostriction in rare-earth Ising metamagnets. Fiz. Tverd. Tela (St. Petersburg) 39, 668–670 (April 1997)     

Critical state of a thin superconducting plate in an inclined magnetic field

A solution is obtained for the Bean critical-state model in an inclined magnetic field which leads to localization of the irreversible magnetic moment induced by the shielding currents. An experimental investigation of YBa₂Cu₃O_y single crystals and epitaxial films, and also plates of classical superconductors using vector magnetometry yields results fully consistent with the model calculations. The established geometric effects have an important influence on measurements of the angular dependence of the critical current and its anisotropy in a high-temperature superconductor. Zh. éksp. Teor. Fiz. 114, 273–290 (July 1998)     

Investigation of the effect of uniaxial pressure on antiferromagnetic resonance in KFe11O17 Crystals

The deformation dependence of the resonance field in KFe₁₁O₁₇ single crystals was investigated by the AFMR method. The measurements were performed at T=77 K and ν=47.52 GHz for two orientations of the external pressure. The experimental data are discussed in terms of a model of a very simple easy-plane antiferromagnet taking account of the elastic and magnetoelastic contributions to the thermodynamic potential. The magnetostriction, magnetoelastic, and elastic contants are calculated and the results are λ=1.94×10⁻⁵, B ₁=2.75×10⁸ erg/cm³, and C ₁₁−C ₁₂=1.42×10¹³ erg/cm³, respectively. The alues of these constants imply that the origin of the initial gap in the AFMR spectrum is not of magnetoelastic origin. Fiz. Tverd. Tela (St. Petersburg) 40, 513–515 (March 1998)     

Magnetostriction of the spin-Peierls magnet CuGeO3

The temperature dependence of the longitudinal magnetostriction of a CuGeO₃ single crystal is measured within the temperature range 4.2–20 K in a magnetic field of 10 T. As the temperature is raised above 4.2 K, the magnetostriction at first increases from vanishingly small values, attains a maximum at a temperature of approximately 12 K, and then abruptly drops as the temperature approaches the spin-Peierls transition. The results are interpreted on the basis of a simple model utilizing the real pattern of magnetic excitations in the spin system. Fiz. Tverd. Tela (St. Petersburg) 40, 1671–1673 (September 1998)     

Magnetostriction constants of epitaxial La,Ga: YIG films measured by microwave resonance

The magnetostriction constants of Y_2.85La_0.15Fe_3.75Ga_1.25O₁₂ epilayers have been measured by observing the shift of the resonance line of a thin film which is stressed by three-point bending. The result is λ₁₁₁=−(0.75±0.15) × 10⁻⁶ and λ₁₀₀=−(0.4±0.1) × 10⁻⁶ which is in agreement with the measurements on bulk single crystals. This result indicates that there is no growth induced contribution to the magnetostriction in thin films of this garnet grown by liquid phase epitaxy.     

Muons in type-II superconductors: μ+ diffusion in ultra-pure niobiumdiffusion in ultra-pure niobium

The diffusivityD _μ of positive muons (μ⁺) in the mixed state of superconducting high-purity, high-perfection niobium single crystals is investigated by measurements of the relaxation of the transverse muon spin polarization (μ⁺SR). The method makes use of the strong magnetic field gradients existing in the mixed state of Type-II superconductors and monitorsD _μ through the variation of the magnetic field felt by the μ⁺ during their diffusion through the crystals. For μ⁺ near the centres of the flux lines inNb it givesD _μ(4.6 K)=(8±2)·10⁻¹¹m²S⁻¹. The positive temperature coefficient ofD _μ indicates that at liquid-helium temperatures the diffusivity of μ⁺ inNb is mainly due to phonon-assisted tunnelling processes.     

Magnetostriction of terbium molybdate in high magnetic field

Magnetostriction and magnetization M of Tb₂(MoO₄)₃ samples are measured at temperatures T = 4.2 and 20 K in fields H up to 14 T directed along the polar axis [001]. Results of the magnetostriction measurements obey the first law of thermodynamics: -MH = TΔS - W_E, where ΔS is the change of the entropy and W_E is the work of internal electric field done when ions move due to the magnetostriction.     

Study of the magnetic properties of ferrites in the CuGaxAlxFe2−2x O4 system having a frustrated magnetic structure

This paper reports a first preparation of samples of the CuGa_xAl_xFe_2−2x O₄ system (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7) and a study of temperature dependences of their spontaneous magnetization σ _s, coercive force H _c, and of the longitudinal, λ_∥, and transverse, λ_⊥, magnetostriction. Our experimental data have established that compositions with x>0.4 undergo two magnetic phase transitions, namely, from the paramagnetic to cluster spin-glass state at the Curie temperature T _C, and another transformation, at T _tr<T _C, from the cluster spin glass to a frustrated magnetic structure. It was found that the coercive force of ferrites with a frustrated magnetic structure is an order of magnitude lower than that of ferrimagnetically ordered ferrites. The behavior of the magnetostriction of frustrated ferrites was studied. Such ferrites were found to exhibit a considerable positive magnetostriction of the paraprocess λ _para. It was shown that the magnetostriction of spin-glass ferrites is lower by nearly an order of magnitude. Fiz. Tverd. Tela (St. Petersburg) 40, 1505–1509 (August 1998)     

Magnetostriction of Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>S (<Emphasis Type="Italic">x</Emphasis> = 0.27) crystals

The magnetostriction of Fe _x Mn_{1 − x} S (x = 0.27) single crystals in strong magnetic fields up to 120 kOe has been investigated. It has been found that the magnetostriction reaches colossal values (±3 × 10⁻⁴) atypical of compounds of 3d elements. It has been found that the magnetostriction changes sign when varying temperature and magnetic field; this behavior indicates an important role of the spin-phonon interactions in the formation of the magnetic order in solid solutions of iron-manganese sulfides.     

Specific heat of magnetic clusters with S=2 YBa2Cu3O7−δ

An analysis is made of published data from measurements of the temperature-and-field dependence of the specific heat in YBaCuO high-temperature superconductors at temperatures below 10 K. It is shown that the “excess” contribution to the specific heat can be explained by spin splittings in copper-oxygen clusters with S=2. The magnitudes of the necessary splittings agree with the available magnetic resonance data. Fiz. Tverd. Tela (St. Petersburg) 39, 1320–1322 (August 1997)     

Contribution of superconducting grains and grain boundaries to the magnetoresistance of ceramic YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> high-temperature superconductors in weak magnetic fields

The current-voltage characteristics of granular YBa₂Cu₃O_6.95 high-temperature superconductor samples have been measured at a temperature of 77.3 K in external transverse magnetic fields H _ext with a strength of up to H _ext ≈ 500 Oe for low transport current densities (0.1 A/cm² ≤ j ≤ 0.6 A/cm²). The current-voltage characteristics obtained have been used to construct dependences of the magnetoresistance ρ on the quantities j (ρ(j) _Hext=const) and H _ext(ρ(H _ext) _{j = const}). It has been revealed that the current and field dependences of the magnetoresistance exhibit anomalies at H _ext ≥ H _c1g , where H _c1g is the lower critical field of superconducting grains. A comparative analysis of the dependences ρ(j)H _{ext = const} and ρ(H _ext) _{j = const} has made it possible to develop concepts regarding the influence of the processes of redistribution of the magnetic field between grain boundaries and superconducting grains on the transport and galvanomagnetic properties of granular high-temperature superconductors. It has been established that the field dependences of the magnetoresistance exhibit specific features associated with the beginning of penetration of Josephson vortices into grain boundaries in the magnetic field H _c1J and with the breaking of a continuous chain of Josephson junctions in the magnetic field H _c2J .     

Radio-frequency absorption study of the parameters of high-T c materials irradiated with fast reactor neutrons

The magnetic properties of YBa₂Cu₃O_7−x ceramics of various densities and Bi₂Sr₂CaCu₂O₈ quasicrystals irradiated with neutrons at fluences of 10¹⁶–10¹⁹ cm⁻² are investigated by a radio-frequency method (in the frequency range f=100–150 MHz). The electromagnetic absorption is used to estimate the parameters of a Josephson medium: the lower critical field, the critical current density, the grain size and intergranular distances, and the penetration depth of the magnetic field in the sample. The hysteresis of electromagnetic absorption in the indicated superconductors is investigated; the hysteresis effect can be used to obtain data on flux pinning processes and flux creep dynamics. It is shown that an increase in the critical current density is observed as a result of the injection of radiation-induced pinning centers. Fiz. Tverd. Tela (St. Petersburg) 39, 28–34 (January 1997)     

Workshop on fast structural changes

Copper-oxide (cuprate) high-temperature superconductors are doped Mott insulators. The undoped parent compounds are antiferromagnetic insulators, and superconductivity occurs only when an appropriate number of charge carriers (electrons or holes) are introduced by doping. All cuprate materials contain CuO₂ planes (Figure 1a) in their crystal structure; the doped carriers are believed to go into these CuO₂ planes, which are responsible for high-temperature superconductivity. High-temperature superconductors are characterized by their unusual physical properties, both in the superconducting state (below the superconducting transition temperature T_c) and in the normal state (above T_c). Since the discovery of high-temperature superconductivity in 1986 [1], these unusual physical properties and the mechanism of superconductivity have been prominent issues in condensed matter physics [2].     

Magnetic susceptibility and magnetic-field penetration-depth anisotropy in high-temperature oxide superconductors

A phenomenological theory of the magnetic-field penetration-depth (λ _iα ) anisotropy in high-temperature superconductors is constructed taking account of the magnetic susceptibility (χ _ik ) anisotropy of the surface layer. The factor of 1.9 change in the anisotropy λ _c /λ _ab with 4% substitution of Co for Cu ions in YBa₂Cu₃O₇ is virtually completely due to the increase in the magnetic susceptibility anisotropy. For Cu-Zn substitutions the change in λ _c /λ _ab is characterized primarily by a change in the anisotropy of the tensor of the reciprocal effective masses of the charge carriers (pairs). Fiz. Tverd. Tela (St. Petersburg) 41, 1552–1555 (September 1999)     

Electron spin resonance of copper pair centers in crystals with perovskite structure

Copper pair centers, which could be of interest for obtaining quantitative information about exchange interactions in superconductors based on cuprate perovskites, are observed in crystals with the perovskite structure by the ESR method. Such centers are investigated in KTaO₃:Cu and K_1−x Li_xTaO₃:Cu crystals. A model consisting of a chain of two equivalent Cu²⁺ ions and three oxygen vacancies, extending along the 〈 100〉 axis, is proposed for the centers. The exchange interaction in the pairs is ferromagnetic. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 12, 890–894 (25 June 1999)     

Peculiarities in galvanomagnetic properties of granular YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> high-temperature superconductors in very weak magnetic fields

The character of the evolution of a system of weak links in granular high-temperature superconductors under the action of an external magnetic field H _ext has been studied by measuring the current-voltage characteristics E(j)_{Hext = const}E{(j)_{{H_{ext}} = const}} of YBa₂Cu₃O_{7 − δ} (δ ≈ 0.05) ceramic samples. The measurements have been performed at T = 77.3 K in a range of very weak magnetic fields 0 < H _ext ≲ 0.5H _c2J, where H _c2J is the upper critical field of the Josephson weak links. The results have been used to construct the field dependences of the magnetoresistance Δρ(H _ext) of the superconducting ceramics. It has been established that the parameters of the power equation E = A(j − j _cJ)^ν and the magnetoresistance Δρ are nonmonotonic functions of the external magnetic field. The presence of extrema in the curves A(H _ext), j _cJ(H _ext), ν(H _ext), and Δρ(H _ext) indicates that different systems of weak links between grain boundaries, which are capable of forming extended Josephson contacts, undergo sequential transitions to a resistive state with an increase in H _ext.