Ac Inductive Measurements Research Articles

Spin to charge conversion in Si/Cu/ferromagnet systems investigated by ac inductive measurements

Semiconductor/ferromagnet hybrid systems are attractive platforms for investigation of spin conversion physics, such as the (inverse) spin Hall effect. However, the superimposed rectification currents originating from anisotropic magnetoresistance have been a serious problem preventing unambiguous detection of dc spin Hall electric signals in semiconductors. In this study, we applied a microwave frequency inductive technique immune to such rectification effects to investigate the spin to charge conversion in heterostructures based on Si, one of the primitive semiconductors. The Si doping dependence of the spin-orbit torque conductivity was obtained for the Si/Cu/NiFe trilayer system. A monotonous modulation of the spin-orbit torque conductivity by doping and relative sign change of spin to charge conversion between the degenerate $n$- and $p$-type Si samples were observed. These results unveil spin to charge conversion mechanisms in semiconductor/metal heterostructures and show a pathway for further exploration of spin-conversion physics in metal/semiconductor heterostructures.

Critical Current Characteristics and Flux Pinning in Fe-Based Pnictide Superconductor

The superconducting Ba1-xKxFe2As2(x = 0.4) single crystals were prepared by the so-called FeAs self-flux method. The critical temperature by ac susceptibility measurement was estimated to be about 36 K. The magnetic field and temperature dependences of critical current densities were investigated by an ac inductive measurement (Campbell’s method). Unlike the phenomenon of co-existence of the global and local critical current densities observed in many polycrystalline Fe-based superconducting pnictides, it was found that only a uniform critical current density (Jc) flows through the whole sample. The value of Jcat 20 K and 1 T was about 5×108A/m2, which is much smaller than the local critical current density observed in polycrystalline samples. This result implies that a dissimilarity of flux pinning mechanism exists between these two kinds of materials. The force-displacement characteristic of fluxoids in sample was investigated. The Labusch parameter was found to increase monotonously with increasing magnetic field, while the interaction distance was proportional to the fluxoid spacing. These results are consistent with the prediction based on a simple flux pinning mechanism.

Critical Current Density and Flux Pinning Properties in Superconducting MgB<sub>2</sub> with and without SiC Doping

It is well known that SiC doping in superconducting MgB2 improves the upper critical magnetic field (Bc2) and the critical current density (Jc) under high magnetic field. However, the relationship between SiC doping and the flux pinning mechanism has not been clarified. In this study, several MgB2 samples with and without SiC doping were prepared by the conventional in situ powder-in-tube method. The critical current densities and the force-displacement characteristics of fluxoids in samples were investigated by an ac inductive measurement (Campbell’s method). The Labusch parameter (αL) and the interaction distance (di) were estimated from the obtained force-displacement profile. It was found that SiC doping enhances the values of αL, but does not change the characteristics of the magnetic field dependence of αL apparently. Namely, αL vs. B3/2 characteristics in the pure samples and SiC doped samples are almost the same. Such a result of αL properties implies that the pinning mechanism in the SiC doped samples could be consistent with the conventional pinning theory. On the other hand, di, which is considered to be proportional to the size of pinning potential, decreases rapidly with increasing magnetic field, especially in the pure samples. For high magnetic field region, the variations of di were deduced to be caused by flux creep. The depth of pinning potential, U0, was estimated by using the values of αL and di. The values of U0 give evidence of that SiC doping can prevent the flux bundles moving to another pinning center under high magnetic field.

Critical Current Densities and Force-displacement Characteristics of Fluxoids in Ba1-xKxFe2As2 Single Crystal

The superconducting Ba1-xKxFe2As2 (x = 0.4) single crystals were prepared by the so-called FeAs self-flux method. The critical temperature by ac susceptibility measurement was estimated to be about 36 K. The magnetic field and temperature dependences of critical current densities were investigated by an ac inductive measurement (Campbell's method). Unlike the phenomenon of co-existence of the global and local critical current densities observed in many polycrystalline Fe-based superconducting pnictides, it was found that only a uniform critical current density (Jc) flows through the whole sample. The value of Jc at 20 K and 1 T was about 5×108 A/m2, which is much smaller than the local critical current density observed in polycrystalline samples. The force-displacement characteristic of fluxoids in sample was investigated. The Labusch parameter was found to increase monotonously with increasing magnetic field, while the interaction distance was proportional to the fluxoid spacing. These results are consistent with the prediction based on a simple flux pinning mechanism.

Precise Determination of the Threshold Current for Third-Harmonic Voltage Generation in the ac Inductive Measurement of Critical Current Densities of Superconducting Thin Films

In an ac inductive method to measure critical current densities J/sub c/ of superconducting films by detecting third-harmonic voltages V/sub 3/ cos 3/spl omega/t, ac magnetic fields are generated with ac currents I/sub 0/ cos /spl omega/t in a small coil mounted just above the film. J/sub c/ is calculated from the threshold coil current I/sub th/, at which full penetration of magnetic field is achieved and V/sub 3/ starts to emerge. For convenience, I/sub th/ has been usually determined by a constant-voltage criterion, e.g., V/sub 3///spl radic/2 = 50 /spl mu/V. However, V/sub 3/ is proportional to I/sub th/, and the determination of I/sub th/ by the constant-voltage criterion inherently causes a systematic error. We confirmed this by comparing the obtained J/sub c/ with those obtained from dc magnetization and by a transport method. We propose to use a constant-resistance criterion, such as V/sub 3//I/sub 0/ = 2 m/spl Omega/, as an alternative method to determine I/sub th/ accurately.

Effect of flux flow on microwave losses inYBa2Cu3O7−xsuperconducting films

Microwave losses in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ (YBCO) films carrying overcritical currents were measured at several temperatures between 74 and 85 K, and found to be in good agreement with the predictions of the vortex dynamics model for the magnetic flux flow. For the measurements a 2.3-GHz microstrip resonator made of an epitaxial double-sided YBCO film on ${\mathrm{CeO}}_{2}$ buffered sapphire was equipped with a Cu coil, which is usually used for ac (1.1 kHz) inductive measurements of the critical current density ${J}_{C}.$ The ac current density induced in the film, ${J}_{\mathrm{ind}}$ controlled the vortex flow, while the microwave surface impedance ${(Z}_{S}{=R}_{S}{+iX}_{S})$ of the film was measured as a function of ${J}_{\mathrm{ind}}.$ Both ${R}_{S}$ and ${X}_{S}$ remained fairly constant for ${J}_{\mathrm{ind}}<{J}_{C},$ indicating a weak contribution of the thermally activated flux flow to the microwave loss. The loss revealed a sharp increase at ${J}_{\mathrm{ind}}\ensuremath{\approx}{J}_{C}.$ In the flux flow regime, ${J}_{\mathrm{ind}}>~{2J}_{C},$ both $\ensuremath{\Delta}{R}_{S}$ and $\ensuremath{\Delta}{X}_{S}$ were linear with ${J}_{\mathrm{ind}}.$ This behavior of the microwave losses agrees well with the dependence $\ensuremath{\Delta}{Z}_{S}={\ensuremath{\rho}}_{f}/2{\ensuremath{\lambda}}_{L}$ predicted by the vortex dynamics model $({\ensuremath{\lambda}}_{L}$ is the London penetration depth and ${\ensuremath{\rho}}_{f}$ is the complex flux flow resistivity of type-II superconductors). The ratio $\ensuremath{\Delta}{X}_{S}/\ensuremath{\Delta}{R}_{S}$ in the flux flow regime was independent of ${J}_{\mathrm{ind}}$ and increased with decreasing temperature from 2.7 at 85 K to 7.9 at 74 K, in agreement with the model.

Ba–Ca–Cu oxycarbonate thin films, prepared by pulsed laser deposition: structure, growth mechanism and superconducting properties

Ba–Ca–Cu oxycarbonate superconducting thin films on LaAlO 3 (100) substrates were grown by pulsed laser deposition in off axis geometry. X-ray diffraction, selected area electron diffraction, and high-resolution transmission electron microscopy indicated that the films consisted of different phases of the (C x Cu 1− x )Ba 2Ca n−1 Cu n O 2 n+3 compound. The main phase was the n=4 phase (C x Cu 1− x )Ba 2Ca 3Cu 4O 11, besides other phases as the n=1,2,3,5… phases and precipitates of CaO. The films were grown predominantly a-axis oriented and contained, in a small amount, c-axis oriented domains of the n=4 phase near the film surface. An onset of superconductivity at 115 K and critical temperatures up to 78 K were found by AC resistive and inductive measurements.

Determination of critical current density in high temperature superconducting thin films in case of inductive measurements

In the frame of Bean critical state model an analysis of ac inductive measurements response of high temperature superconducting thin films has been made. The full magnetic moment and ac losses during the magnetic field cycle has been determined. A connection is found between the critical current density, J c and the field of maximum losses, H*. Estimation of J c at different areas of the film gives information about the sample's homogeneity.

Intergrain and Intragrain Critical Current Densities of Ag-Doped YBa2Cu3O7-y Superconductors

The effects of Ag doping on the intergrain and intragrain critical current density properties of YBa2Cu3O7-y superconductors were investigated. As a result of AC inductive measurement and four-probe DC measurement, it becomes clear that both intergrain and intragrain critical current densities are enhanced below 10 wt% Ag. This improvement suggests that Ag particles which precipitated in YBa2Cu3O7-y grains act as pinning centers. At high Ag concentration of more than 15 wt%, intergrain critical current density decreases though the intragrain critical current density is enhanced. The influence of pinning force and grain boundaries is discussed at a macroscopic level.

Preparation of epitaxial TlBa2Ca2Cu3O9 high T c thin films on LaAlO3 (100) substrates

Epitaxial TlBa2Ca2Cu3O9 high Tc thin films were prepared on LaAlO3 (100) substrates by a combination of laser ablation and thermal evaporation of thallium oxide. X-ray diffraction patterns of θ-2θ scans showed that the films consisted of highly c axis oriented TlBa2Ca2Cu3O9. φ scan measurements revealed an epitaxial growth of the TlBa2Ca2Cu3O9 thin films on the LaAlO3 (100) substrates. Ac inductive measurements indicated the onset of superconductivity at 110 K. At 6 K, the critical current density was 4×106 A/cm2 in zero magnetic field and 6×105 A/cm2 at a magnetic field of 3 T parallel to the c axis.

Preparation of TlBa2Ca2Cu3O9±δ high T c thin films by laser ablation in combination with thermal evaporation of thallium oxide

TlBa2Ca2Cu3O9±δ high Tc thin films were prepared on MgO 〈100〉 surfaces by a combination of laser ablation from a stoichiometric Ba2Ca2Cu3Ox target and the thermal evaporation of thallium oxide. X-ray diffraction measurements showed that the films consisted of predominantly c axis oriented TlBa2Ca2Cu3O9±δ, and scanning electron microscopy revealed that the surfaces had a flat, platelike morphology. The ac inductive measurements indicated that the onset of superconductivity occurred at 117 K with a transition width (10%–90%) of ∼3 K. Zero resistivity was reached at 120 K. The critical current density was ∼3×104 A/cm2 at 110 K.

The effect of oxygen on intergrain critical currents of YBa2Cu4O8 and Y(Ba1.6Sr0.4)Cu4O8 ceramic samples

The authors have investigated the effect of quenching on the intergrain critical current, Jc(inter), of YBa2Cu4O8 and Y(Ba1.6Sr0.4)Cu4O8 bulk samples. The quenching process was performed by quenching the samples from different annealing temperatures into liquid nitrogen. The critical current was deduced by AC inductive measurements. It was found that Jc(inter) was progressively lowered with increasing quenching temperature. For Sr-doped samples the suppression of the Jc(inter) by oxygen removal was more appreciable than for the undoped samples. The results suggest that weak links at grain boundaries are sensitive to quenching treatment although the grains themselves are thermally stable.

A simple digital system for ac magnetic measurements on superconductors

This article describes a simple apparatus to perform ac inductive measurements on superconductors. The ac susceptibility, the penetrated and trapped magnetic flux and the magnetization of a sample placed in an ac magnetic field are measured by processing the signal induced in a pick-up coil that surrounds the sample. The complex harmonic analysis of the signal is also performed. The setup consists of an amplifier, a three-channel recorder, and a standard desktop computer. The main advantages of the apparatus are its simple design and low cost. Its main drawback is the noise affecting the signals. Tests were performed on a high Tc superconductor.

Superconducting and magnetic properties of Y1Ba2Cu4O8±δ

Abstract We have carried out extensive characterization and measurements on a high purity Y 1 Ba 2 Cu 4 O 8±δ (124) sample. Metallographic investigations, X-ray diffraction and microprobe analysis have been performed and reveal a single phase with traces of CuO. Resistivity measurements give a resistivity ratio R (300 K)/ R (100 K) of ≈ 6.1, and a critical temperature T co of 82.5 K. The AC susceptibility shows a sharp transition at about same temperature. The Meissner effect, with a 55% fraction (at 4.2 K with H =20.3 Oe) confirms the bulk nature superconductivity. The specific heat difference C (0, T )− C ( H , T ) has been estimated from magnetization measurements in the vicinity of T c for μ 0 H =8 T and compared to the direct calorimetric measurement. Both methods indicate that the specific heat jump amounts to approximately 15 mJ/K 2 mol. Fluctuation contributions are found up to several degrees above T c . Inter- and intra-grain critical currents were determined at 4.2 K by AC inductive measurements and by measuring the irreversibility of the magnetization curve at a constant temperature (Bean's method). The results shows that there is no significant difference between the critical currents of the 124 phase and those of the 123 phase (Y 1 Ba 2 Cu 3 O 7 ).

Short-time relaxation of the critical current in oriented grained YBa2Cu3Ox and granular (Bi,Pb)2 Sr2Ca2Cu3Ox.

The time-dependent behavior of the critical current density {ital j}{sub {ital c}} is investigated by ac inductive measurements. The variation of {ital db}/{ital dt} of the ac field between 0.1 and 3 T/s reveals a short-time relaxation in the millisecond regime before {ital j}{sub {ital c}} exhibits the familiar logarithmic decay. At fields above the irreversibility line only this short-time relaxation is observed. Our experimental time scale allows us to obtain the unrelaxed critical current density {ital j}{sub {ital c}0} at certain fields and temperatures.

AC Inductive Measurement of Intergrain and Intragrain Currents in High-Tc Oxide Superconductors

Bulk intergrain current and closed intragrain current in sintered Y-Ba-Cu-O superconductors were measured at 77 K by using an ac inductive method. These currents can be separated because of the large difference in penetrating rates of the magnetic flux into specimens with respect to the ac field amplitude. The obtained intergrain current density agreed approximately with critical current density measured resistively. The closed intragrain current density amounted to 4.3×108 A/m2 at B=0.5 T and decreased gradually with increasing magnetic field. The present results show that the ac inductive measurement is one of the available nondestructive methods to characterize sintered oxide specimens.

Weak link problem and intragrain current density in polycrystalline Bi1Ca1Sr1Cu2Ox and Tl2Ca2Ba2Cu3O10

Polycrystalline sintered specimen were investigated by means of ac susceptibility and inductive critical current measurements. Main objects are whether the new highT c oxides have a weakly coupled intergranular structure and to get information about the intragrain critical current density. The Tl2Ca2Ba2Cu3O10 specimen shows a connective nature similar to YBa2Cu3O7 with low intergrain currents (100 A/cm2 at 100K,B=0 T) and high intragrain ones (4.5·105 A/cm2 at 100K,B=0 T). The investigation of the Bi1Ca1Sr1Cu2Ox specimen becomes complex due to the presence of two superconducting phases and the low intragrain critical current density of the lowerT c phase.