Enhanced Microwave Absorption Properties of Intrinsically Core/shell Structured La0.6Sr0.4MnO3Nanoparticles

The intrinsically core/shell structured La0.6Sr0.4MnO3nanoparticles with amorphous shells and ferromagnetic cores have been prepared. The magnetic, dielectric and microwave absorption properties are investigated in the frequency range from 1 to 12 GHz. An optimal reflection loss of −41.1 dB is reached at 8.2 GHz with a matching thickness of 2.2 mm, the bandwidth with a reflection loss less than −10 dB is obtained in the 5.5–11.3 GHz range for absorber thicknesses of 1.5–2.5 mm. The excellent microwave absorption properties are a consequence of the better electromagnetic matching due to the existence of the protective amorphous shells, the ferromagnetic cores, as well as the particular core/shell microstructure. As a result, the La0.6Sr0.4MnO3nanoparticles with amorphous shells and ferromagnetic cores may become attractive candidates for the new types of electromagnetic wave absorption materials

Temperature-dependent phase reversal of nonresonant microwave and rf absorption in high-TcT_c superconductors

The phenomenon of temperature-dependent phase reversal of the nonresonant electromagnetic response of the high-$T_c$ superconductors is studied in the rf and the microwave ranges. Phase reversal is the changeover from a maximum at zero magnetic field and the decrease with the increasing field to a minimum at zero field and the increase with the increasing field. The results are analyzed using an effective-medium theory, treating the sample as a percolating network consisting of the superconducting regions separated by normal metallic regions. The experimental observations are shown to follow naturally from the dependence of the superconducting fraction on the temperature and the magnetic field

Anomalous d.c. field dependence of non-resonant r.f. absorption at high r.f. fields in YBa2Cu3O7δYBa_2Cu_3O_{7_\delta} powders

We have measured r.f. absorption at 14 MHz in powdered samples of sintered $YBa_2Cu_30_{7-\delta}$ at 77K $(T_c\approx 90 K)$ as function of low d.c. scan field $(H_{dc})$ for different amplitudes of the r.f. field $(H\omega)$. At low $H\omega\sim 4mG$, we obtained the well known non-resonant r.f. absorption that increased monotonically with $|H_{dc}|$ $(<{\sim}30G)$. At higher r.f. fields $(H\omega \geq20 mG)$, however, the phase of the derivative signal with respect to the d.c.-field $(H_{dc})$ scan reversed implying decreasing r.f. absorption with increasing $|H_{dc}|$. This anomalous $H_{dc}$ dependent r.f. absorption at higher $H\omega$ may be understood in terms of a non-linear response of weakly-connected superconducting rings distributed in the powdered sample. Here, the r.f. induced supercurrents drive the damped fluxons pinned in the Josephson junctions assumed to be non-hysteretic. The anomaly is absent in well sintered bulk samples

Certain novel features of the R.F. response of the YBa2Cu3O7−x superconductors

The r.f. absorption experiment performed on YBa2Cu3O7-x ceramic pellets using a CW NMR spectrometer shows some novel observed in the microwave range

Surface barrier effects in non-resonant microwave absorption by superconducting thin films of YBa2Cu3O7??

We present an unusual temperature dependence of hysteresis in the Lion resonant microwave absorption (NRMA) signals from superconducting thin films of YBa2Cu3O7-delta. We observe that the hysteresis increases with increase in temperature till T-c which we interpret as evidence for the presence of Bean-Livingston surface barriers (BLSB) in the single crystalline films

Line shapes of field-dependent nonresonant microwave and RF absorption in high-Tc superconductors

The field-dependent nonresonant absorption at microwave (9.1 GHz) and radio (19.5 MHz) frequencies has been studied in the superconducting state of $YBa_2Cu_3O_{7-x}$ pellets and epitaxially grown c-axis oriented thin films using conventional continuous-wave electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectrometers. The line shapes of the signals from the thin-film samples are observed to depend sensitively on the operating frequency; at 19.5 MHz derivative looking signals saturating at low magnetic fields (~10 Oe) are obtained while at 9.1 GHz, signals saturate only at fields of the order of a kOe. Two different possible origins of the losses, namely, diffraction reduction of junction currents in the weak links and the viscous motion of the fluxons are considered and compared. We find that the observed frequency dependence of the line shape can be explained only following the latter mode

Surface barrier effects in non-resonant microwave absorption by superconducting thin films of YBa2Cu3O7−δYBa_2Cu_3O_{7-\delta}

We present an unusual temperature dependence of hysteresis in the non resonant microwave absorption (NRMA) signals from superconducting thin films of $YBa_2Cu_3O_{7-\delta}$. We observe that the hysteresis increases with increase in temperature till $T_c$ which we interpret as evidence for the presence of Bean-Livingston surface barriers (BLSB) in the single crystalline films

Critical current densities of high pressure oxygen sputtered thin films of YBa2Cu3O7−xYBa_2Cu_3O_{7-x} by nonresonant RF absorption method

The critical current densities $(J_c)$ have been measured at 77 K in high pressure oxygen sputtered thin films of $YBa_2Cu_3O_{7-x}$ superconductor using the nonresonant $rf$ absorption technique. High values of $J_c(\sim10^5 A/cm^2)$ are observed in these relatively large area $(\sim1.2 cm^2)$ film

Critical current density measurements of YBa_2Cu_30_7_-_x thin films by non-resonant r f absorption method

A new, contact-less technique for the measurement of the critical current density $J_c$ using the non-resonant r.f. absorption phenomenon is proposed $J_c$ in thin film samples of YBa_2Cu_30_7_{-x} prepared by pulsed laser deposition are measured by this technique as well as the transport current method. Satisfactory agreement is obtained between the results of the two technique

Critical-current variation with Pr content in {Y_{1-x}Pr_xBa_2Cu_3O_7} epitaxial films

c-axis-oriented epitaxial {Y_{1-x}Pr_xBa_2Cu_3O_7} (x = 0, 0.05, 0.1, 0.2, 0.3, and 0.35) thin films have been grown with the pulsed-laser-deposition method. Superconducting transition temperatures of the films are within \pm 2 K of the corresponding polycrystalline bulk samples. The critical current density {J_c} of Pr-doped films for x = 0.1–0.3 is about two orders of magnitude lower than of the pure 1:2:3 films. This exponential fall in {J_c} with doping (x) can be understood semiquantitatively in terms of depairing due to a random-impurity (Pr-ion) potential causing localized suppression of the superconducting order parameter around the impurity to about the in-plane coherence length