Magnetoresistance of porous polycrystalline HTSC: Effect of the transport current on magnetic flux compression in intergranular medium

The hysteretic dependences of the magnetoresistance of porous (38% of the theoretical density) granular high-temperature superconductor (HTSC) Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x have been analyzed in the model of the effective intergranular field. This effective field has been defined by the superposition of the external field and the field induced by magnetic moments of superconducting grains. The magnetic flux compression in an intergranular medium, characterized by the effective field, controls the hysteretic behavior of the magnetoresistance. It has been found that the magnetoresistance hysteresis width for the studied porous HTSC depends on the transport current, in contrast to the superconductor of the same composition with high physical density (more than 90% of the theoretical value). For a porous superconductor, a significant current concentration occurs in the region of the grain boundaries, which is caused by features of its microstructure. A current-induced increase in the effective boundary length results in a decrease in the flux compression, a decrease in the effective field in the intergranular medium, and a magnetoresistance hysteresis narrowing with increasing current.     

Percolation effects in the conductivity and magnetoresistance of compacted chromium dioxide powder

The resistive and magnetoresistive properties of a submicron (120 nm) compacted CrO₂ powder with a Curie temperature of ∼385 K were investigated in the temperature range of 5–430 K for magnetic fields of up to 1.6 T. The specimen revealed a nonmetallic temperature dependence of resistance and high negative magnetoresistance (MR) (20%) in a low-temperature range. The MR magnitude rapidly decreased with an increase in temperature and was less than 0.3% for T > 200 K. Such MR behavior is shown to be characteristic for granular systems with spin-dependent intergranular tunneling. Some features of MR behavior in a low-temperature range (lower than 40 K) can be attributed to the percolation nature of the tunnel conductivity of such a granular system in the presence of a limited number of dominant conducting current paths.     

Magnetoresistive effect in RCu<Subscript>3</Subscript>Mn<Subscript>4</Subscript>O<Subscript>12</Subscript> perovskite-like oxides

The electrical properties of and the magnetoresistive effect in RCu₃Mn₄O₁₂ (R=rare-earth ion or Th) are studied. In all compounds of this series, the magnetoresistive effect amounts to 20% at liquid nitrogen temperature in the presence of a field of 0.9 T. An increase in the magnetoresistance with decreasing temperature and a high sensitivity to weak magnetic fields at low temperatures point to the intergranular nature of the effect. The magnetoresistance shows a peak in the vicinity of the Curie temperature T_C. Based on the dependences of the magnetoresistance on an external magnetic field, it is assumed that the magnetoresistance peak near T_C is related to the charge carrier scattering by magnetic inhomogeneities as in substituted orthomanganites. We believe that the magnetoresistance value near the magnetic ordering temperature depends on the synthesis conditions and the effect of the intergranular spacer on the transport properties of these compounds.     

Improved giant magnetoresistance in magnetic granular Co5 Cu95 alloys by direct-current joule heating

The dc joule heating technique has been used to produce giant magnetoresistance (GMR) Co₅Cu₉₅ granular alloys. At T=10 K, GMR as large as 28.5% has been observed in the as-quenched sample annealed with I=6A in a magnetic field up to 30 kOe. At room temperature, the joule-heated samples show higher GMR in comparison with that annealed by the conventional method.     

Improved giant magnetoresistance in magnetic granular Co5 Cu95 alloys by direct-current joule heating

The dc joule heating technique has been used to produce giant magnetoresistance (GMR) Co₅Cu₉₅ granular alloys. At T=10 K, GMR as large as 28.5% has been observed in the as-quenched sample annealed with I=6A in a magnetic field up to 30 kOe. At room temperature, the joule-heated samples show higher GMR in comparison with that annealed by the conventional method.     

Electric current-induced giant electroresistance in epitaxial La0.67Sr0.33MnO3 thin films

The electronic transport behavior of La_0.67Sr_0.33MnO₃ epitaxial thin films with different thicknesses has been investigated under various applied DC currents. The 20 and 70 nm thick films show a giant negative electroresistance (ER). In contrast, the films with 100 nm thickness show unusual giant positive ER, which can reach 30% with the current density of 1.8×10⁸ A/cm² at room temperature. It is interesting that the electric current can also change the magnetoresistance of the films. The results were explained by considering the spin polarized current induced increase of ferromagnetic metallic phase and current-induced lattice distortion via electron wind force under high current density.     

Mechanism of formation of a negative magnetoresistance region in granular high-temperature superconductors

The field dependences of the magnetoresistance of Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x samples with different densities, which have a foam microstructure and exhibit different diamagnetic responses, were studied at 77.4 K to identify the mechanism responsible for the formation of a negative magnetoresistance region in granular high-temperature superconductors. A region with negative magnetoresistance was found to exist in samples with magnetizations highest in absolute magnitude. This behavior finds a reasonable interpretation as due to the effect exerted by dipole moments of high-temperature superconductor grains on the effective intergranular field. The strength of this effective field has been estimated.     

Interface scattering and spin-dependent transport in granular magnetite

We have prepared nearly monodisperse Fe₃O₄ of ∼50 nm by a chemical route and investigated the electrical and magnetic transports of the composite granular system. A Verwey transition is observed in the vicinity of 113 K. Above and below the Verwey transition, the electrical transport is dominated by electron hopping behavior showing a good linear relation between resistance and T^−1/2. The magnetoresistance (MR) increases with the applied field and does not follow the magnetization to reach the saturation at 10 KOe field. This indicates that the MR is mainly arising from the spin-dependent scattering of electrons through the grain boundaries. The temperature dependence of MR shows it has the highest MR value near the Verwey transition.     

Dominant influence of the compression effect of a magnetic flux in the intergranular medium of a granular high-temperature superconductor on dissipation processes in an external magnetic field

Experiments have been presented that demonstrate the effect of the compression of a magnetic flux in grain boundaries of a granular high-temperature superconductor in an external magnetic field on the dissipation processes. The compression of the magnetic flux is associated with the diamagnetic behavior of superconducting grains and the existence of a Josephson medium in grain boundaries. Under these conditions, grain boundaries are in an effective magnetic field that depends on the magnetic state (magnetization) of the superconducting grains. Based on the analysis of experimental data (dependences of the electrical resistance R and magnetization on the magnetic field H and temperature T, as well as current-voltage characteristics), the conclusion has been drawn that it is the temperature evolution of the effective magnetic field in the intergranular medium which primarily determines the behavior of the dependences R(T) in weak external magnetic fields of no more than ～10³ Oe. This should be taken into account in the interpretation of experiments on the magnetoresistance effect in granular high-temperature superconductors in terms of different theories. The conclusion drawn here also implies a significant correction of the previously obtained results.     

Domain observation and transition from giant magnetoresistance to anisotropy magnetoresistance in Ag1−x(Ni0.8Co0.2)x granular films

A series of Ag_1−x(Ni_0.8Co_0.2)_x granular film samples were prepared using an ion-beam cosputtering technique. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were performed to investigate the microstructure of these samples. The results measured using a vibrating sample magnetometer (VSM) show a gradual change from superparamagnetism to ferromagnetism as x increases in these samples. Magnetoresistance was measured using a conventional four terminal method at room temperature. As x increases, a transition from giant magnetoresistance (GMR) to anisotropic magnetoresistance (AMR) has been observed. The stripe-type domains have been observed using magnetic force microscopy (MFM) in the high x samples, and the domains gradually disappear as x decreases. It suggests that the transition from GMR to AMR may result from intergranular interaction (not only dipolar) in the samples as x increases.     

Magnetic and transport properties of granular and Heusler-type glass-coated microwires

We studied magnetic and structural properties of granular Co_xCu_100−x (5<x<40 at%), Cu₆₃Fe₃₇ and Heusler-type Ni₂MnGa glass-coated microwires. We found that the structure of Co–Cu microwires consists of two phases: fcc Cu for all the samples and fcc α-Co present for higher Co content. In the case of low Co content, Co atoms are distributed within the Cu matrix. The quantity and the size of grains strongly depend on the geometry of the microwire. Co–Cu and Fe–Cu microwires exhibited considerable magnetoresistance (MR). For Co_xCu_100−x microwires at x≥30 the anisotropic contribution to MR has been observed. Temperature dependences of magnetization measured without an external magnetic field (ZFC) and in the presence of a field (FC) show considerable difference below 20 K, indicating the presence of small α-Fe or Co grains embedded in the Cu matrix. Annealed Ni₂MnGa microwires showed ferromagnetic behavior with Curie temperature about 330 K and polycrystalline structure with space group I4/mmm and lattice parameters a=3.75 Å and c=6.78 Å.     

Optical and magnetooptical properties of granular alloys with giant magnetoresistance in the IR region of the spectrum

The features of the optical and magnetooptical properties of granular alloys with giant magnetoresistance in the IR region are examined in reference to the magnetorefractive effect and the equatorial Kerr effect. Calculations are performed within the semiclassical approximation with consideration of spin-dependent scattering in the bulk of the granules and on their surfaces (interfaces). The expressions obtained for σ _xx(ω) and σ _xy(ω) are found to be sensitive to scattering on the surfaces and in the bulk of the granules, as well as to granule size, the type of impurities trapped on the interfaces, the frequency of the incident light, and the external magnetic field. For granular thin films exhibiting giant magnetoresistance, the theory predicts significant relative changes in the optical reflection and transmission coefficients when the sample is magnetized to saturation (0.02% and 20%, respectively, for giant magnetoresistance of the order of 20%), as well as Kerr and Faraday effects that are nonlinear with respect to magnetization. Zh. éksp. Teor. Fiz. 116, 1762–1769 (November 1999)     

Magnetoresistance of a Two-Dimensional TbTe<Subscript>3</Subscript> Conductor in the Sliding Charge-Density Wave Regime

The magnetoresistance of a TbTe₃ two-dimensional conductor with a charge-density wave (CDW) has been measured in a wide temperature range and in magnetic fields of up to 17 T. At temperatures well below the Peierls transition temperature and in high magnetic fields, the magnetoresistance exhibits a linear dependence on the magnetic field caused by the scattering of normal charge carriers by “hot” spots of the Fermi surface. In the sliding CDW regime in low magnetic fields, a qualitative change in the magnetoresistance has been observed associated with the strong scattering of carriers by the sliding CDW.     

Large magnetothermoelectric power in Co/Cu, Fe/Cu and Fe/Cr multilayers

We report and discuss experimental data on the thermoelectric power of magnetic multilayers. Measurements of the thermoelectric power of Fe/Cr, Co/Cu and Fe/Cu multilayers have been carried out in the temperature range 4K < T < 150 K magnetic fields perpendicular to the layers. All specimens were found to exhibit pronounced magnetothermoelectric power (MTEP) effects correlating with their giant negative magnetoresistance. The main difference between the MTEP and the magnetoresistance is in their temperature dependence. Whereas the magnetoresistance is a decreasing function of temperature, the MTEP, at least in Co/Cu and Fe/Cu multilayers, is very small at low temperature and increases rapidly above 30–40 K. We ascribe this high temperature part of the MTEP to spin-dependent electron-magnon scattering and we propose a theoretical model.     

Magnetoresistive properties of cold-pressed CrO<Subscript>2</Subscript> powder,prepared by hydrothermal synthesis

The resistive, magnetoresistive and magnetic properties of cold-pressed CrO₂, powder prepared by hydrothermal synthesis from chromic anhydride have been studied. The powder particles (with a mean diameter of about 120 nm) were nearly spherical. The particles stabilized with a β-CrOOH surface layer. The powder compact (with a Curie temperature of about 385 K) revealed nonmetallic temperature behavior of the resistance (with an R(T) dependence close to exponential at T < 20 K). A giant negative magnetoresistance (MR) (∼20% at T ≈ 5K) is found. MR decreased rapidly with an increase in temperature (to 0.3% at T > 200 K). Such MR behavior is shown to be typical of a system of magnetic grains with magnetic (spin-dependent) tunneling.     

Structural, magnetic and transport properties of NiFeχAg(1−χ) heterogeneous alloys

We carried out a comprehensive study of structural, magnetic and electrotransport properties of as-deposited and annealed (Ni₈₀Fe₂₀)_χAg(_1−χ) heterogenous alloys prepared by sputtering. The NiFe atomic concentration was varied between 15% and 40%. These alloys consist of small magnetic particles (Ni₈₀Fe₂₀) embedded in a nonmagnetic matrix (Ag). The structures of these alloys were investigated by X-ray diffraction, scanning electron microscopy and high-resolution cross-section transmission electron microscopy. The magnetic measurements were made using SQUID magnetometry and ferromagnetic resonance. Magnetoresistance was measured with a conventional four-point probe between 1.5 K and room temperature in field range 0–6T. Three contributions to the magnetoresistance of these granular alloys have been clearly identified: the spin-valve (or giant) magnetoresistance as in multilayers, scattering on magnetic fluctuations (as in any ferromagnetic metal around its magnetic ordering temperature), and anisotropic magnetoresistance. These three contributions have their own dependences on the size of the magnetic particles, on the degree of intermixing between Ni₈₀Fe₂₀ and Ag, and on temperature. We discuss the different shapes and amplitudes of magnetoresistance versus Ni₈₀Fe₂₀ concentration or temperature and their evolution upon annealing in terms of the relative roles of these three contributions. The magnetoresistance in multilayers (current in-plane or perpendicular to the plane) and granular alloys are also compared.     

Giant negative magnetoresistance in a composite system based on Fe3O4 nanocrystals in a polymer matrix

Conducting polymer composites based on Fe₃O₄ nanocrystals in a polyvinyl alcohol matrix are synthesized. The current-voltage characteristics, the magnetization, and the magnetoresistance of the nanocomposites are investigated, and a giant negative magnetoresistance is observed. The decrease in the resistance at room temperature is found to reach 10% in a 10 kOe field. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 1, 37–40 (10 January 1998)     

SPIN-DEPENDENT INTERFACIAL TUNNELING AND MAGNETO RESISTANCE IN THE FAMILY OF GRANULAR PEROVSKITE La1-xSrxMnO*3

The magnetic transport properties in granular perovskite system La_1-xSr_xMnO^*₃ have been investigated. The spin-dependent inter facial tunneling and the corresponding giant magnetoresistance (GMR) effect have been observed in the whole temperature range below the Curie point T_c for the samp les with concentration x from 0.05 to 0.45. Theoretical analysis shows that the interfacial tunneling originates from the difference in magnetism between surfaces and cores, and the tunnel-type GMR stems from the field-induced change of interfacial magnetic order.