Magnetic Field Dependent Microwave Absorption Research Articles

Ferromagnetic resonance studies of nanocrystalline La 0.6Pb 0.4MnO 3 thin films

Systematic magnetic field dependent microwave absorption measurements have been performed on nanocrystalline La 0.6Pb 0.4MnO 3 (LPMO) thin films over a temperature range of 77–300 K. In addition to a regular ferromagnetic resonance (FMR) signal below the Curie temperature of 230 K, we have observed a second FMR line at temperatures ≤170 K, which is accompanied by a nonresonant microwave absorption signal centered at zero magnetic field. The two components of FMR lines are identified to originate from bulk and surface magnetic orderings of the LPMO nanocrystallites. The temperature dependence of the linewidth ( Γ) revealed that the bulk of the crystallites contain chemical inhomogeneities, while the surface has a magnetic spin glass character. The hysteresis of the nonresonant absorption signal in LPMO films is found to differ from those of superconductors and, to originate due to magneto-induced microwave conductivity of surface spin glass.

Magnetic field dependent microwave absorption studies on a MgB2superconductor

Polycrystalline MgB2, synthesized using a conventionalsolid-state reaction route, has been investigated for its magnetic fielddependent microwave absorption using a standard X-band EPR spectrometer. Thechanges in microwave absorption as a function of temperature and/or magneticfield were recorded by following the microwave reflectivity of a sample-loadedcavity. The modulated low-field microwave absorption signal was found to besimilar to that observed in high-temperature superconducting materials. Thefield dependent direct microwave absorption in the 0-10 kG range was used todetermine Hc1 values at different temperatures, and the value ofHc1(0) was found to be 250 G. The absorbed microwave power has beenfound to obey a (H)1/2 dependence with a change of slope indicating atransition from a strongly pinned flux lattice to the flux flow regime.

Properties of YBa2Cu3O7—δ–Pb(Sc0.5Ta0.5O3 Superconducting Composites

Microwave absorption of the superconductor YBa2Cu3O7—δ–ferroelectric relaxor Pb(Sc0.5Ta0.5)O3 composite of (0.5/0.5) composition in weight was studied. The composite exhibits a critical temperature enhancement up to Tc-onset = 95 K clearly visible both in the transport and microwave absorption measurements. The structural investigations indicate the unchanged structure of YBa2Cu3O7—δ. The studies of magnetic field-dependent microwave absorption show a presence of intergrain weak links. The first intergrain penetration critical field Hc1J = 4 Oe. Both the inter- and intragrain penetration regimes can be observed.

Magnetic field dependent microwave absorption in pure superconducting tin near Tc

The microwave absorption in a weak magnetic field H ⪡ H c( T) is reported in pure polycrystalline tin in the superconducting state near T c. A novel experimental technique is used which employs an ESR apparatus with the perpendicular configuration of the static and microwave magnetic fields and the radio-frequency modulation of the static field. This measurement is related to the true superconducting state of the bulk material. The characteristic decrease of the microwave absorption in a static magnetic field is detected at 0.8 < T/ T c < 0.98 and ω/ T c = 0.12 which is predicted by the microscopic theory of the surface impedance of type-I superconductors.

Surface influence on magnetic field dependent microwave absorption in pure superconducting tin nearT c

It is shown that the magnetic field dependent microwave absorption in the superconducting tin changes its form when the tin surface is oxidized.

Magnetic field dependent microwave absorption evidence for a mixed liquid-solid vortex phase in Bi 2Sr 2CaCu 2O 8 + x

The depinning of vortices can be observed as a change in the slope of the dependence of the surface resistance on B 1 2 . However, close to the transition magnetic field it is observed that there is a departure from the B 1 2 dependence. It is shown that this deviation is inconsistent with second order behavior associated with a vortex glass. The deviation, however, can be accounted by the existence of a mixture of solid and vortex liquid phases resulting from a magnetic field dependent distribution of pinning barriers.

Magnetic field-dependent microwave absorption in YBa2Cu4O8/YBa2Cu3O7mixes: a test of phase purity

Magnetic field-dependent microwave absorption in YBa2Cu4O8/YBa2Cu3O7mixes: a test of phase purity

Measurements of low magnetic field microwave absorption in 110-K superconducting Bi-Sr-Ca-Cu-O thin films

Studies on magnetic-field dependent microwave absorption in Bi(Pb)-Sr-Ca-Cu-O thin films superconducting above 100 K are reported. The films were DC magnetron sputtered on MgO and CaNdAlO/sub 4/ single crystals from Pb-rich targets and post-annealed at high temperatures in air for less than 1 h. The measurements were performed as a function of temperature, DC magnetic field, and the film magnetic history. The results show that the microwave absorption measurements are a very sensitive tool for high-T/sub c/ thin-film characterization, since even small variations in the film phase homogeneity, granularity, or crystalline orientation are clearly visible on the modulated absorption curves. It was found that the films characterized by the lowest absorption exhibited the highest critical current densities.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Observation of irreversible to reversible transition in Tl 2Ba 2Ca 2Cu 3O x by magnetic field dependent microwave absorption

The magnetic field dependence of the microwave absorption due to flux motion has been measured at several temperatures in the superconducting state of Tl 2Ba 2Ca 2Cu 3O x . At a given temperature there are two magnetic field regions in which the microwave absorption displays a different functional dependence on the magnetic field strength and there is a definite magnetic field, B′, separating the two regions. The lower field region is associated with the dynamics of flux motion in the irreversible, strong flux pinning regime, and the upper region with flux motion in the reversible fluid phase. The microwave absorption is shown to display a hysteresis in the field direction below B′ but not above it. A measurement of the temperature dependence of the transition magnetic field, B′, between the two regions allows a determination of the H- T irreversibility line for surface melting of the vortex solid.

Magnetic field dependent microwave absorption in high-T c superconducting cuprates

A strong microwave absorption at low magnetic fields is observed for a variety of metallic cuprates below their critical superconducting transition temperatures (Tc) using conventional electron paramagnetic resonance (EPR) instrumentation. The low-field microwave signal was investigated in the following high-temperature superconductors, YBa2Cu3O7–x(Tc= 95 K), GdBa2Cu3O7–x(Tc= 95 K), YBa2Cu4O8(Tc= 75 K), (Bi1.6Pb0.4)Sr2Ca2Cu3Oy(Tc= 115 K), Pb0.5Bi2Sr2Ca2.5Cu4.5Oy(Tc= 118 K)(Tl0.5Pb0.5)Sr2(Ca0.8Er0.2)Cu2Oy(Tc= 115 K), Tl2Ba2Ca2Cu3Oy(Tc= 118 K), TbTl2Ba2Cu3Oy(Tc= 84 K), Tl2Ba2Ca2Cu3O10(single-crystal, Tc= 118 K) and Bi2Sr2CaCu2O8(single-crystal, Tc= 80 K). We have investigated the usefulness of the technique as a practical non-intrusive method for screening potentially superconducting samples, with particular emphasis on sensitivity, the effect of microwave power and the inherent problems of studying signals at small magnetic fields. In the latter regard, we have also carried out studies using a radiofrequency EPR spectrometer operating at 300 MHz, which facilitated detailed calibration of the 0–200 G region, where the signal is most intense.

Microwave absorption from superconducting crystals of the Tl-Ca-Ba-Cu-O system

The authors present the results of non-resonant, magnetic field-dependent microwave absorption experiments carried out on five superconducting crystals from the Tl-Ca-Ba-Cu-O system. The absorption was found to be anisotropic with respect to the applied DC magnetic field. On cooling the sample, the onset of the absorption with the field parallel to the crystal c axis was detected at a higher temperature than with the field perpendicular to the c axis for all the crystals studied.