Phase transition in the vortex structure of granular YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> HTSCs in weak magnetic fields

This study aims at establishing the interrelation between the current-carrying capacity and peculiarities of magnetoresistance of granular YBa₂Cu₃O_{7 ? δ} HTSCs (T _c = 92.5 K). The transverse magnetoresistance of several batches of YBa₂Cu₃O_{7 ? δ} HTSC samples with noticeably different values of critical supercurrent density j _c is measured in magnetic fields H _ext up to H _ext ^max ≈ 500 Oe in a wide range of transport currents (5 mA ≤ I ≤ 1600 mA) at T = 77.4 K. Samples with relatively high values of j _c (H _ext = 0) ≥ 100 A/cm² do not exhibit any anomalies in their field dependences. Magnetoresistance jumps δρ_BG-VG/ρ_273K are observed for samples with low values of j _c ≥ 20 A/cm² in fields H _BG-VG ≈ 200–260 Oe. The width ΔH _BG-VG of the anomalous resistance region increases upon an increase in I. The magnetoresistance jumps decrease with increasing I in increasing field H _ext(0 → H _ext ^max ) and increase in decreasing field H _ext(H _ext ^max → 0). It is found that these peculiarities of the field dependences of magnetoresistance are associated with a first-order phase transition (in magnetic field) in the vortex structure of YBa₂Cu₃O_{7 ? δ} HTSCs of the Bragg glass-vortex glass type.     

Magnetoresistance of superconducting grains in ceramic YBa2Cu3O7 − δ high-temperature superconductors in weak magnetic fields

The current-voltage characteristics $E(j)_{H_{treat} } = const$ of ceramic (granular) YBa₂Cu₃O_6.95 samples preliminarily magnetized in different transverse magnetic fields H _treat have been measured in a zero field (H _ext = 0) at T = 77.3 K for elucidating specific features of dissipation in superconducting grains of high-temperature superconductors (HTSCs). The current-voltage curves measured in the range 0 ≤ H _trap ≤ H _c2J (where H _trap is the magnetic field trapped as a result of the pretreatment in H _treat and H _c2J is the upper critical field of the Josephson weak links) have been used to construct the field dependences of the magnetoresistance ρ_A(H _treat) _{j = const} of superconducting grains. It has been established that the magnetoresistance ρ_A of the superconducting grains is significantly lower than the magnetoresistance ρ_J for the Josephson medium. The dependence of ρ_A on H _treat and on the transport current density j has been investigated. It has been shown that the dependences ρ_A(H _treat) _{j = const} exhibit a clearly pronounced tendency to saturation, ρ_satur, and the value of ρ_satur increases with increasing j. It has been found that the lower critical field H _c1A of the superconducting grains strongly depends on the transport current density.     

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 .     

Enhancement of low-field magnetoresistance in Fe3O4 particles induced by ball milling

A series of Fe₃O₄ particles with different size have been obtained by mechanical ball milling from t=0-450 h. Crystal structure and microstructure of the samples are analyzed by XRD and SEM. An emphasis has been placed on magnetic and transport properties. The experimental results indicate that the sample t=350 exhibits an enhancement of magnetoresistance (MR) comparing with initial powder compress sample (t=0). The low-field magnetoresistance reaches MR=−6.04% at Verwey temperature 120 K and MR=−2.54% at 290 K. Thermal behavior TGA analysis and investigation of magnetic properties have revealed that there is an oxide layer on surface of Fe₃O₄ particles. It is considered that the enhanced magnetoresistance can be taken into account in terms of spin-dependent tunneling effect between Fe₃O₄ particles.Temperature dependence of magnetization and resistivity are measured in order to study electrical and magnetic behavior near Verwey transition. In addition, we also discuss ball milling time dependence of coercivity H_c and specific magnetization M_s of these samples.     

Tunneling magnetoresistance in exchange-biased CoFeB/AlOx/Co/IrMn junctions

A series of exchange-biased magnetic tunneling junctions (MTJs) were made in an in-plane deposition field (h) = 500 Oe. The deposition sequence was Si(1 0 0)/Ta(30 Å)/CoFeB(75 Å)/AlO_x(d Å)/Co(75 Å)/IrMn(90 Å)/Ta(100 Å), where d was varied from 12 Å to 30 Å. The MTJ was formed by the cross-strip method with a junction area of 0.0225 mm². The tunneling magnetoresistance (ΔR/R) of each MTJ was measured. The high-resolution cross-sectional transmission electron microscopic (HR X-TEM) image shows the very smooth interface and clear microstructure. X-ray diffraction (XRD) demonstrates that the IrMn layer of the MTJ exhibits a (1 1 1) texture. From the results (ΔR/R) increases from 17% to 50%, as d increases from 12 Å to 30 Å. The tunneling resistance (R_o) of these junctions ranges from 150 Ω to 250 Ω. The exchange-biasing field (H_ex) of the MTJ is 50-95 Oe. Finally, the saturation resistance (R_s) was measured as a function of the angle (α) of rotation, where α is the angle between h and the in-plane saturation field (H_s) = 1.1 kOe. The following figure presents the dependence of R_s on α, instead of originally expected independence, the curve actually varies with a period of π.     

Magnetoresistance hysteresis of granular YBa2Cu3O7−δ high-temperature superconductor in weak magnetic fields

The variation of the transverse magnetoresistance of YBa₂Cu₃O_～6.95 high-temperature superconducting ceramic with external magnetic field intensity H _ext first increasing from zero to ～500 Oe (Δρ⁺ / ρ_{273 K}) and then decreasing from about 500 Oe to zero (Δρ^? / ρ_{273 K}) is studied for transport current densities varying from j/j _c ≈ 0.01 to ≈0.99 (where j _c is the critical current density in the absence of the magnetic field) at 77.3 K. It is found that the field dependence of the magnetoresistance of YBa₂Cu₃O_～6.95 high-temperature superconductor is characterized by pronounced hysteresis, the difference Δρ⁺ / ρ_{273 K} ? Δρ^? / ρ_{273 K} increasing with j/j _c. As j/j _c grows, the effective critical fields of Josephson weak links, H _c2J , and the lower critical fields of superconducting grains, H _c1A , decline. When field H _ext rises, the critical fields are lower than when H _ext diminishes: H _c2J ⁺ < H _c2J ^? and H _c1A ⁺ < H _c1A ^? .     

Etude spectrale d'une famille d'opérateurs non-symétriques intervenant dans la théorie des champs de reggeons

In this paper, we study a few spectral properties of a non-symmetrical operator arising in the Gribov theory. The first and second section are devoted to Bargmann's representation and the study of general spectral properties of the operator: $$\begin{gathered} H_{\lambda ',\mu ,\lambda ,\alpha } = \lambda '\sum\limits_{j = 1}^N {A_j^{ * 2} A_j^2 + \mu \sum\limits_{j = 1}^N {A_j^ * A_j + i\lambda \sum\limits_{j = 1}^N {A_j^ * (A_j + A_j^ * )A_j } } } \hfill \\ + \alpha \sum\limits_{j = 1}^{N - 1} {(A_{j + 1}^ * A_j + A_j^ * A_{j + 1} ),} \hfill \\ \end{gathered}$$ whereA* _j andA _j ,j∈[1,N] are the creation and annihilation operators. In the third section, we restrict our study to the case of nul transverse dimension (N=1). Following the study done in [1], we consider the operator: $$H_{\lambda ',\mu ,\lambda } = \lambda 'A^{ * 2} A^2 + \mu A^ * A + i\lambda A^ * (A + A^ * )A,$$ whereA* andA are the creation and annihilation operators. For λ′>0 and λ′²≦μλ′+λ². We prove that the solutions of the equationu′(t)+H _{λ′, μ,λ} u(t)=0 are expandable in series of the eigenvectors ofH _λ′,μ,λ fort>0. In the last section, we show that the smallest eigenvalue σ(α) of the operatorH _{λ′,μ,λ,α} is analytic in α, and thus admits an expansion: σ(α)=σ₀+ασ₁+α²σ₂+..., where σ₀ is the smallest eigenvalue of the operatorH _{λ′,μ,λ,0}.     

Large negative magnetoresistance in thiospinel CuCrZrS4

We report on large negative magnetoresistance observed in ferromagnetic thiospinel compound CuCrZrS₄. The electrical resistivity increased with decreasing temperature according to the exp(T₀/T)^1/2, an expression derived from variable range hopping with strong electron-electron interaction. The resistivity under a magnetic field was expressed by the same form with the characteristic temperature T₀ decreasing with increasing magnetic field. Magnetoresistance ratio ρ(T,0)/ρ(T,H) is 1.5 for H=90 kOe at 100 K and increases divergently with decreasing temperature reaching 80 at 16 K. Results of magnetization measurements are also presented. A possible mechanism of the large magnetoresistance is discussed.     

Thermal stability and switching field of hard/soft-stacked perpendicular media

The time dependence of remanence coercivity and thermal stability were investigated for hard/soft-stacked media consisting of a magnetically hard granular layer underneath a very thin soft layer with a large saturation magnetization, M_s. The values of remanence coercivity at measurement times t′=10³ and 10⁻⁵ s (pulse field) were measured, and defined as H_r and H_r^P. The remanence coercivity on the recording time scale, H_r (1 ns), and the energy barrier, ΔE/kT, were evaluated by fitting H_r and H_r^P to Sharrock's equation taking into account the power law variation of the energy barrier, n. The value of H_r (1 ns) for a (Co–Pt)–SiO₂ (9 nm)/Co–SiO₂ (2 nm) stacked medium with an interfacial coupling control layer was about 9 kOe, which was less than half of that of a (Co–Pt)–SiO₂ (9 nm) conventional medium (=21.3 kOe). The value of ΔE/kT for the stacked medium was about 111 (n=0.7), and was not significantly different from the conventional medium. Moreover, no significant difference in the rate of decrease of H_r with increasing temperature was observed between media with and without interlayers. These results indicate that the use of a thin soft layer with high M_s was effective at significantly reducing H_r with no notable change in thermal stability.     

Electron entrainment by solitons in superlattices in an exteral magnetic field allowing for collisions

We found the current of electron entrainment by an electromagnetic soliton propagating along the axis of a 1-D semiconductor superlattice located in the external magnetic field (H₀). The direction of this field coincides with the direction of the soliton magnetic field (H_s). It is shown that entrainment current oscillations emerge and grow in number with H₀ for such a geometry if the condition H₀/H_s≥10² is fulfilled. Pedagogical University, Volgograd, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 5, pp. 662–666, May, 1997.     

Topological and thermodynamic investigations of ternary mixtures containing cyclic ether: Excess molar enthalpies

Excess molar enthalpies, H_ijk^E, of 1,3-dioxolane or 1,4-dioxane (i) + aniline (j) + benzene or toluene (k) ternary mixtures have been measured as a function of composition at 308.15 K. The H_ijk^E data have been fitted to Redlich-Kister equation to determine ternary adjustable parameters along with their standard deviations. It has been observed that H_ijk^E values predicted by Graph theory compare well with their corresponding experimental values. The observed data have been analyzed in terms of (i) Graph theory (which involves the topology of a molecule); (ii) Prigogine-Flory-Patterson; and (iii) Sanchez and Lacombe theories.     

Anisotropic contribution of magnetostriction to the elastic compliance constants in hexagonal materials: The Δs-effect

In this paper, we consider the single crystal analogue of the ΔE-effect in hexagonal materials with uniaxial magnetic anisotropy. We define Δs_ijld as the fractional change in the elastic compliance tensor s_ijkl in the demagnetized state relative to its value in the saturated state. The Δs-effect depends in general on the direction of the applied stress and the resulting magnetostrictive strain. We can show, however, that Δs_ijkl is always nonnegative when i=k and j=l. We then consider the measurements of the elastic constants in magnetic materials which have large magnetocrystalline anisotropy. The stresses applied in these measurements are not sufficient to rotate the magnetization away from the magnetically preferred direction, and hence cause no magnetostriction, and no Δs-effect. Therefore, it is not necessary to apply a saturating magnetic field along the easy axis in order to measure the true elastic constants in these materials.     

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.     

Molecular dynamics and phase transitions in paramagnetic [Mn(H2O)6][SiF6] investigated by H NMR

The temperature dependences of ²H NMR spectra and spin-lattice relaxation time T₁ have been measured for paramagnetic [Mn(H₂O)₆][SiF₆]. The obtained ²H NMR spectra were simulated by considering the quadrupole interaction and paramagnetic shift. The variation of the spectra measured in phase III was explained by the 180° flip of water molecules. The activation energy E_a and the jumping rate at infinite temperature k₀ for the 180° flip of H₂O were obtained as 35 kJ mol⁻¹ and 4×10¹⁴ s⁻¹, respectively. The spectral change in phases I and II was ascribed to the reorientation of [Mn(H₂O)₆]²⁺ around the C₃ axis where the E_a and k₀ values were estimated as 45 kJ mol⁻¹ and 1×10¹³ s⁻¹, respectively. From the almost temperature independent and short T₁ value, the correlation time for electron-spin flip-flops, τ_e, and the exchange coupling constant J were obtained as 3.0×10⁻¹⁰ s and 2.9×10⁻³ cm⁻¹, respectively. The II-III phase transition can be caused by the onset of the jumping motion of [Mn(H₂O)₆]²⁺ around the C₃ axis.     

Magnetic properties of (Nd,Pr)–Fe–B nanocomposite over quenched and annealed ribbons

The base alloys of nominal composition (Nd_0.75Pr_0.25)_yFe_balanceB_x (y=10−9.2 and x=6−19.2) were chosen to study the influence of RE/B ratio, smaller than stochiometric composition on magnetic properties of over quenched and annealed ribbons. From X-ray diffraction analysis of these ribbons, the α-Fe and Fe₃B phases were observed along with (Nd,Pr)₂Fe₁₄B major phase. The average grain size was calculated using these patterns as: 35 nm for α-Fe, 45 nm for (Nd,Pr)₂Fe₁₄B and 22 nm for Fe₃B particles. TEM analysis also supported the nano distribution of the above phases. These X-ray graphs support the idea of exchange coupling between hard and soft phases responsible for the observed magnetic properties. In these ribbons the saturation magnetization J_s and remnant magnetization J_r increases from 1.19 T to 1.66 T and from 0.65 T to 0.91 T, respectively as RE/B ratio increases. The increase in J_s and J_r may be attributed to the presence of exchange coupling between these phases. The corresponding coercivity _jH_c decreases from 673.33 to 271.33 k Am⁻¹. The maximum energy product (BH)_max initially increases from 72.42 kJ m⁻³to 109.85 kJ m⁻³ up to RE/B≈1 and then decreases to 58.5 kJ m⁻³, depending on the shape of second quadrant B–H loop. The coercivity mechanism observed from initial hysteresis curve was considered to be nucleation of domain wall.     

Co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature in nanocrystalline La0.7Te0.3MnO3

Sol-gel prepared nanocrystalline La_0.7Te_0.3MnO₃ has rhombohedral crystal structure (space group R3¯C) at room temperature and orders ferromagnetically at ∼280 K (T_C). A large magnetic entropy change of ∼12.5 J kg⁻¹ K⁻¹ is obtained near T_C for a field change of 50 kOe. This magnetocaloric effect could be explained in terms of Landau theory. The temperature dependence of electrical resistivity shows metal-insulator transition at T_C and a giant magnetoresistance of ∼52% in 50 kOe. The co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature makes nanocrystalline La_0.7Te_0.3MnO₃ a promising material for magnetic refrigeration and spintronic device applications.     

Magnetoresistance of single Co nanowires

Polycrystalline single Co nanowires are prepared by electron beam lithography on GaAs substrates at room temperature. The width of the Co nanowires is varied between 150 and 4000 nm. Magnetoresistance measurements are carried out in a temperature range between 1.5 and 45 K applying magnetic fields μ₀H up to 4.5 T parallel and perpendicular to the current direction. The in plane (longitudinal) magnetoresistance (MR) shows pronounced features at magnetic fields H_c (coercive fields) indicating the magnetization reversal process. From the MR-curves we determined H_c as a function of the angle α between current and field direction (from in plane to out of plane) and of the width w of the Co nanowires. The H_c=H_c(α,w) behavior allows to discuss the reversal process in more detail.     

Phonon mediated interaction for triplet pairing in Pd

The pairing interaction due to phonon exchange is calculated for the triplet p-state of Pd, using the atomic-site representation for the heavy d electrons. It is found that this interaction is repulsive, λ^Ph₁ < 0, and that its magnitude is small compared with the singlet s-state interaction of BCS, λ^Ph₀. The net interaction due to both phonons and spin-fluctuations is attractive but it is so small that the observation of a triplet p-state in Pd appears unlikely at the lowest accessible temperatures.     

Low temperature magnetoresistance in La1.32Sr1.68Mn2O7 layered manganite under hydrostatic pressure

The La_1.32Sr_1.68Mn₂O₇ layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature (T_C) and a metal-insulator transition (T_MI) at 118 K in the ambient pressure. The applied pressure shifts the T_MI to higher temperature values and induces a second metal-insulator transition (T²_MI) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T_C. When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.     

Relaxation phenomena in amorphous Co75−x Mn x B25 induced by stress annealing

The change of the magnetization direction in amorphous ferromagnets by the application of a magnetic field and a tensile stress is investigated by magnetoresistivity measurements. Attention is focussed on permanently remaining influences on the domain structure induced by stress annealing treatments. To observe any dependence on the sign of the magnetostrictionλ _s, we investigated the amorphous Co_75?x Mn _x B₂₅ system where the sign ofλ _s changes from negative to positive values with increasing Mn content. Surprisingly the stress-annealed samples showed a similar behavior independent of the sign of the magnetostriction.