Electrical resistivity and magnetoresistance of nonuniformly mechanically strained films (30 nm) of the La0.67Ca0.33MnO3 manganite

The structure, electrical resistivity, and magnetoresistance of predominantly oriented La_0.67Ca_0.33MnO₃(30 nm)/LaAlO₃ films are investigated after partial relaxation of biaxial mechanical stresses. The negative magnetoresistance MR of the films reaches a maximum at T = 235–240 K. The full width at half-maximum of the peak in the curve MR(T) for a film is five to six times greater than that for a manganite layer grown on a substrate with a small lattice mismatch. For T < 150 K, the temperature dependence of the electrical resistivity ρ of the films is fitted well by the relationship ρ = ρ₀ + ρ₁ (H)T ^4.5, where ρ₀ ≡ ρ(T = 4.2 K) and ρ₁(H) is a parameter that is independent of temperature but dependent on the magnetic field H. The parameter ρ₁(H = 0) for the La_0.67Ca_0.33MnO₃(30 nm)/LaAlO₃ films is several times larger than that for thin manganite layers only weakly strained by the substrate. The electrical resistivity ρ₁ decreases almost linear as the quantity μ₀ H increases in the field range 1–5 T.     

Anomalous Hall effect in (Co41Fe39B20) x (Al-O)100 − x nanocomposites

The concentration dependence of the coefficient R _s characterizing the anomalous Hall effect (AHE) has been studied by measuring the electrical resistivity ρ, magnetoresistance, and the magnetic field dependence of magnetization and Hall resistivity of (Co₄₁Fe₃₉B₂₀) _x (Al-O)_{100 ? x} nanocomposite thin films. It has been demonstrated that the AHE coefficient increases by more than an order of magnitude with a decrease in the percentage x of the amorphous ferromagnetic metal from 60 to 30 and its behavior is described by the relation R _s ～ ρ ^m , where m = 0.46 ± 0.1. At the same time, the coefficient characterizing the normal Hall effect grows by a factor of less than 10. The mechanisms underlying the giant Hall effect in nanocomposites have been discussed.     

Low-field peak effect in type II superconductors

The combined effect of a surface (edge) barrier and volume pinning on the dependence of critical current I _c on the magnetic field (I⊥H ₀) in bulk type II superconductors is investigated. In low magnetic fields, there is a portion of the curve I _c(H ₀) where I _c grows with H ₀, causing a nontrivial peak effect in this field range. Such behavior is explained by the combined effect of a surface (edge) barrier and volume pinning, the latter being rather sensitive to the transport current density distribution in a superconductor.     

Modification of the CVD-graphene resistivity by post-processing sample annealing

In this paper, we present the results of an additional annealing effect on the temperature dependences of the resistivity for CVD-graphene samples of a large area. We found that an annealing in a Ar/H₂ mixture at different temperatures modifies both the value of the resistivity and the slope of its temperature dependence. The annealing effect on the resultant sample quality depends on the type of the ρ(T) dependence for the initial sample. For samples with a metallic-like ρ(T) dependence, a low-temperature annealing (at T = 250 °C) results in a slight decrease in the resistivity value and an increase of the ρ(T) curve slope. Increasing the annealing temperature up to T = 400 °C leads to a stronger increase in the ρ(T) curve slope but to an increase in the resistivity value. For samples with a semiconductor-like ρ(T) dependence, increasing the annealing temperature up to T = 750 °C results in a gradual suppression of the activation character of the resistivity behavior at low temperatures. The additional annealing is concluded to be accompanied by two processes: a cleaning of the graphene surface from adsorbed contaminations and an additional defect formation in the graphene structure. A relative role of these processes in dependence on the annealing temperature and the type of the ρ(T) dependence for the initial sample is discussed.     

Magnetoresistance of La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films grown on a substrate with low lattice mismatch

The structure, electrical resistivity, and magnetoresistance of La_0.67Sr_0.33MnO₃ heteroepitaxial films (120-nm thick) practically unstrained by lattice mismatch with the substrate were studied. A strong maximum of negative magnetoresistance of ≈27% (for μ₀H = 4 T) was observed at T ≈360 K. While the magnetoresistance decreased monotonically in magnitude with decreasing temperature, it was still in excess of 2% at 150 K. For T < 250 K, the temperature dependence of the electrical resistivity ρ of La_0.67Sr_0.33MnO₃ films is fitted well by the relation ρ = ρ₀ + ρ ₁(H)T^2.3, where ρ₀ = 1.1×10^?4 Ω cm, ρ₁(H = 0) = 1.8×10^?9 Ω cm/K^2.3, and ρ₁(μ₀H = 4 T)/ρ₁(H = 0) ≈0.96. The temperature dependence of a parameter γ characterizing the extent to which the electrical resistivity of the ferromagnetic phase of La_0.67Sr_0.33MnO₃ films is suppressed by a magnetic field (μ ₀H = 5 T) was determined.     

Stress effect on the magnetoresistance of amorphous ferromagnets

To investigate the magnetization in amorphous ferromagnetic alloys we measured the change in the magnetoresistance (H⊥j and H 6j) simultaneously applying a tensile stress along the direction of the ribbons. With increasing stress we observed a complete alignment of the magnetic domain vectors M_s parallel andantiparallel (for λ_s > 0) or perpendicular (for λ_s < 0) to the stress axis. It is found that on average a value of σ_s = 10 kg/mm² is sufficient in all measured samples to produce this effect. The change in the magnetoresistance by stress annealing indicates that a preferred domain orientation in stress direction is induced.     

On the theory of surface impedance of metals placed in a magnetic field

The smooth non-monotonous dependence of the metal surface impedance upon the magnetic field H is investigated theoretically for the cases of diffuse and specular reflection of electrons from the specimen boundary. The type of the electron-surface interaction has been found to have very little effect on the magnitude of the impedance Z_α(H) in the range of weak magnetic fields [equation (1)]. In a strong field [equation (2)] the surface impedance behaves differently for diffuse and specular reflection. The form of the Z_α(H) function depends essentially on the ratio of the electromagnetic wave frequency ω and the collision frequency of electrons ν. This provides a possibility of establishing experimentally the frequency of electron collisions with volume scatterers.     

Electrical conduction and critical magnetic field for superconductivity of a Nb3Se4 single crystal

With respect to the quasi-one dimensionality of single crystals of Nb₃Se₄, the electrical resistivity from 1.3 to 320 K and the critical magnetic field for superconductivity are measured. The resistivity along the Nb-chain direction is represented as a sum of a temperature independent and an intrinsic temperature dependent term. The temperature dependence of the intrinsic resistivity subjects to T³ form between 10 and 80 K above which it tends to a T linear form. The critical magnetic field is proportional to the temperature difference from the transition temperature. Its dependence is well fitted by the elliptical fluxoid model of Ginzburg-Landau theory. The ratio of the parallel and the perpendicular to the c-axis is 5.7.     

Angular dependence of the upper critical field in Bi2Sr2CuO6+δ

The angular dependence of the upper critical magnetic field was investigated in a wide range of temperatures in very high-quality Bi₂Sr₂CuO_6+δ single crystals with critical temperature T _c (midpoint) ? 9 K in magnetic fields up to 28 T. Although the typical value of the normal state resistivity ratio ρ_c/ρ_ab≈10⁴, the anisotropy ratio H _c2∥ab/H _c2⊥ab of the upper critical fields is much smaller and shows an unexpected temperature dependence. A model based on strong anisotropy and small transparency between superconducting layers is proposed.     

Electrical and magnetic properties of ε-Fe3N nanoparticles synthesized by chemical vapor condensation process

ε-Fe₃N nanoparticles synthesized by chemical vapor condensation (CVC) are covered with shells of disordered Fe₃O₄ phase, as observed by a transmission electron microscopy. The zero-field cooling and field cooling temperature dependence of magnetization, ac susceptibility as a function of frequency, magnetic hysteresis loops, and the temperature dependence of resistivity of the ε-Fe₃N nanoparticles are systematically studied. The results indicate the existence of complex magnetic properties, such as superparamagnetic behavior, exchange bias, magnetic dipole interaction, and the possible coexistence of ferromagnetic and spin-glass-like states and/or disordered surface spins of the shells at low temperatures. The temperature dependence of resistivity ρ(T) for compacted ε-Fe₃N nanoparticles in a temperature range of 110 K< T< 300 K can be well described by the mechanism of fluctuation-induced tunneling conduction, while that below 110 K can be ascribed to conducting electrons scattered by localized magnetic moments and impurity as well as the influence of freezing of spin-glass-like moments and/or disordered surface spins of the shells.     

Electrical resistivity and magnetotransport in La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films asymmetrically biaxially compressed by an NdGaO<Subscript>3</Subscript>(001) substrate

The La_0.67Ba_0.33MnO₃(40 nm) films are quasi-coherently grown on an NdGaO₃(001) substrate with an orthorhombic unit cell distortion of ～1.4%. The biaxial compressive stresses generated during nucleation and growth lead to a decrease in the unit cell volume of the grown layers. This, in turn, results in a decrease (by ～35 K) in the temperature of the maximum in the dependence of the electrical resistivity ρ of the layers on the temperature. For T < 150 K, the electrical resistivity ρ of the films increases in proportion to ρ₂ T ^4.5 and the coefficient ρ₂ decreases almost linearly with increasing magnetic field H. The negative magnetoresistance (≈?0.17 for μ₀ H = 1 T) reaches a maximum at temperatures close to room temperature. The response of the electrical resistivity ρ of the La_0.67Ba_0.33MnO₃(40 nm) films to the magnetic field depends on the crystallographic direction of the film orientation and the angle between H and I (where I is the electric current through the film).     

Magnetoresistance anisotropy of strontium ruthenate films grown coherently on a TiO2-terminated SrTiO3(001) substrate

The SrRuO₃ films with a thickness of 80 nm have been coherently grown on a TiO₂-terminated SrTiO₃(001) substrate. Biaxial mechanical stresses induce a considerable difference between the unit cell parameters of the SrRuO₃ layer in the substrate plane (??3.904 ?) and along the normal to the substrate surface (??3.952 ?). The electrical resistivity of the SrRuO3 film decreases practically linearly with increasing magnetic field strength H when the latter is parallel to the current I _b and the projection of the easy magnetization axis in the substrate plane. At T = 4.2 K, ??₀ H = 14 T, and the magnetic field oriented along the hard magnetization axis, the negative anisotropic magnetoresistance of the grown layers reaches 16% and exerts a notice-able effect on the response of electrical resistivity of the SrRuO₃ film to the magnetic field.     

Analysis of the spin splitting of maxima of quantum oscillations of the resistivity of n-Bi-Sb semiconductor alloys in a magnetic field parallel to the bisector axis

In samples of semiconductor alloys n-Bi_0.93Sb_0.07 with different electron concentrations (n ₁ = 8 × 10¹⁵ cm^?3, n ₂ = 1.2 × 10¹⁷ cm^?3, and n ₃ = 1.9 × 10¹⁸ cm^?3), dependences of the electrical resistivity on magnetic fields up to 45 T parallel to the current and the bisector axis (H ‖ C ₁ ‖ j) have been measured at temperatures of 1.5, 4.5, and 10 K. The obtained dependences ρ₂₂(H) demonstrate quantum oscillations of the resistivity (Shubnikov-de Haas effect), and, in high magnetic fields, there is a resistivity maximum far away from other maxima. On assumption that this maximum is related to the spin-split Landau level N = 0^? for electrons of the main ellipsoid, the spin-splitting parameters are calculated for electrons of the main ellipsoid: γ₁ = 0.87, γ₂ = 0.8, and γ₃ = 0.73. Using these values, the oscillation maxima can be reliably related to the numbers of split Landau levels for electrons of the main and secondary ellipsoids. The dependences of the resistivity ρ₁₁ and the Hall coefficient R _31.2 on magnetic field have been measured in a transverse magnetic field at H ‖ C ₁ and j ‖ C ₂ on the sample with the electron concentration n ₄ = 1.4 × 10¹⁷ cm^?3. Using similar analysis, the spin-splitting parameter is found to be γ₄ = 0.85, which is close to the value of γ₂ = 0.8 obtained for the sample with close electron concentration (n ₂ = 1.2 × 10¹⁷ cm^?3) during the measurements in a longitudinal magnetic field. The quantum oscillation maxima of Hall coefficient R _31.2 are shifted to the range of high magnetic fields as compared to the quantum oscillation maxima of resistivity ρ₁₁.     

Effect of lateral tensile stresses on the low-temperature electrical resistivity and magnetoresistance of La0.67Ca0.33MnO3 nanofilms

The thickness of the buffer layer of strontium titanate introduced between an La_0.67Ca_0.33MnO₃ manganite film and a (001)La_0.29Sr_0.71Al_0.65Ta_0.35O₃ substrate is varied (d ₁ = 7–70 nm) to influence effective misfit m in their lattice parameters. As m increases, electrical resistivity ρ of the film increases sharply and the maximum in the ρ(T) dependence shifts toward low temperatures. At T < 150 K, the temperature dependence of ρ of the manganite film obeys the relationship ρ = ρ₁ + ρ₂ T ^4.5, where parameter ρ₁ is independent of the temperature and magnetic field. Coefficient ρ₂ decreases with increasing magnetic field and increases with the misfit between the lattice parameters of the film and substrate, i.e., when the effective hole concentration in the manganite layer decreases.     

High-frequency susceptibility and ferromagnetic resonance in multidomain thin films of the yttrium-iron garnet type

The field dependence of the high-frequency susceptibility and the ferromagnetic resonance were experimentally studied in a thin (d≈0.1 µm) (111)-oriented single-crystal film of substituted yttrium-iron garnet with the factor q?1. It was shown that the anomaly in the high-frequency susceptibility observed in a magnetic field H parallel to the normal to the film surface in the magnetization saturation region (H≈H_s) has a dual nature; more specifically, this anomaly is associated with an abrupt collapse of the stripe domain structure and a ferromagnetic resonance in the experimental configuration H ∥ [111] and h ⊥ H. In this case, the film transition from the inhomogeneous multidomain state to the homogeneous (single-domain) state at the point H≈H_s has no indications of a second-order phase transition. The experimental frequency-field dependence of ferromagnetic resonance (FMR) in the sample under study, having a characteristic minimum at the point ω₀=5 MHz and H_FMR=H_s, agrees qualitatively and quantitatively with calculations. The influence of the cubic magnetic anisotropy and the film thickness on the FMR spectrum and the orientation of the spontaneous magnetization in domains with respect to the film plane in the zero field H was theoretically studied.     

New insights in the T dependence of the electrical resistivity in size-effect dominated pure metal foils from measurements on high-purity Au at low T

Analysis of Soffer's size-effect theory for electrical resistivity shows, for measurements in such a T range for which the temperature dependent portion of the resistivity, ρ_i, is always much smaller than the residual bulk resistivity ρ_∞(0) of the metal studied, that while size-effects leave the essential T dependence of ρ_i unchanged, it may increase its absolute value and the observed residual resistivity ρ(0), thus explaining recent results of Caplin et al. This also corrects the general conclusion arrived at by the latter authors, i.e. that the T dependence of ρ of a metal foil of given residual resistivity is the same as that of a bulk sample of the same residual resistivity provided that the latter is governed by impurity scattering, as being true for a narrow T range only, i.e. for which ρ_i(T) ? ρ_∞(0). However, for this T range a procedure is outlined which allows one to extract values of the surface specularity parameter p_S and also ρ_∞ of the metal foils studied.     

Magnetic measurements of Fe-Ni-Nb-B and Fe-Co-Mo-Cu-B in the vicinity of the Curie temperature

The study of the transition between ferromagnetic and paramagnetic states has been investigated on selected metallic glass systems based on Fe-Ni-Nb-B and Fe-Co-Mo-Cu-B with T_C close to room temperature. Samples in the form of ribbons were prepared by planar flow casting and magnetostriction in parallel and perpendicular directions and saturation magnetostrictions have been determined on these samples in as-cast states together with hysteresis loops. In addition, a magneto-optic device for dynamic domain observation has been used for observation of domain structure. Magnetostriction measurements using direct method of measurement show the decrease of saturation magnetostriction towards zero upon approaching T_C. In paramagnetic state the field dependencies of magnetostriction in parallel and perpendicular configurations exhibit a linear dependence on the external magnetic field. In the transition region of temperatures the dependencies are a combination of ferromagnetic and paramagnetic field dependencies. The coercivity H_C in the materials investigated exhibits values below 20 A/m. The observed magnetic domains are typical for this class of amorphous alloys. The polarization in paramagnetic state increases gradually with increase in magnetizing field, reflecting the increasing amount of polarized regions.     

Kinetic effects in manganites La1 − x Ag y MnO3 (y ≤ x)

We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La_{1 ? x} Ag _y MnO₃ (y ≤ x) doped with silver as functions of temperature (4.2–350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression ρ_FM(T) = ρ₀ + AT ² + BT ^4.5 presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons.     

Giant magneto-impedance effect in CoMnSiB amorphous microwires

The giant magneto-impedance (GMI) ratio, ΔZ/Z=[(Z(H)−Z(H_max)]/Z(H_max), in a nearly zero magnetostrictive Co_68.5Mn_6.5Si₁₀B₁₅ amorphous microwire has been investigated for the frequency range 0.5–10 MHz, driving current amplitude of 0.5–2.5 mA, bias DC magnetic field up to 2400 A/m and under applied tensile stress up to 132 MPa. A maximum relative change in the GMI ratio up to around 130% is observed at a frequency of 10 MHz, magnetic DC field of about 180 A/m, driving current amplitude of 1 mA and under tension of 60 MPa. The tensile stress dependence of the magnetic field, H_m, corresponding to the maximum ΔZ/Z ratio allows to estimate the magnetostriction constant (λ_s≈−2×10⁻⁷) to be in good agreement with λ_s values estimated by different methods and in amorphous alloys with similar compositions.     

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.