Field Modulated Microwave Absorption Research Articles

Strengthening of pinning by magnetic particles in high temperature superconductors

Strengthening of flux-line pinning in high- T c compounds by ferromagnetic particles created by implantation of iron ions has been observed. Measurements were carried out at single crystals of Bi 2Sr 2CaCu 2O 8 and thin films of YBa 2Cu 3O 7 by means of field-modulated microwave absorption technique. In case of YBa 2Cu 3O 7, experiments were performed on composite structure consisting of the superconducting film and non-conducting SiO 2 film containing magnetic particles.

Dynamic response of microwave absorption triggered by magnetic modulation in high anisotropy Bi 2Sr 2CaCu 2O x crystals

We study magnetic field modulated microwave absorption in Bi 2Sr 2CaCu 2O x single crystals. In addition to the well-known signal corresponding to the field derivative of the thermally-activated flux-flow resistivity, which appears close to T c and depends linearly on the modulation level, we observe a new signal with a nonlinear (NL) dependence on the magnitude of the modulation field. This new signal (i) extends to low temperatures, (ii) appears only above some threshold modulation level, (iii) strongly depends on the magnitude and direction of the d.c. magnetic field, and (iv) appears only in highly anisotropic superconductors. It cannot be simulated from the microwave dissipation measurements of Enriquez et al. where an almost identical experimental set up was used but without magnetic field modulation. Hence, the NL signal is due to dynamic response triggered by the magnetic modulation. We suggest that the NL signal arises from the removal of vortices out of their shallow pinning potential well by the modulation magnetic field.

Field-modulated microwave absorption in thin YBaCuO films

Hysteretic behaviour of field-modulated microwave absorption is investigated in thin YBaCuO films with thickness d≈50 or 400 nm, by scanning the external magnetic field in opposite directions, and in constant magnetic fields when the specimen is rotated. A negative hysteresis loop of field-modulated microwave absorption is detected in the films with d≈50 nm . An addition signal corresponding to decrease of the absorption when the field is parallel to the film surface is found.

Microwave absorption in micron- and nano-size YBaCuO powders

Size-dependent effects of microwave absorption are investigated in micron and submicron size YBaCuO powders. It is shown that the hysteresis and step-like change of field-modulated microwave absorption (FMMA) decreases when the particle size decreases. Two contributions to the relaxation of the FMMA signals were found.

Nonlinear microwave response to magnetic modulation in BSCCO

We study magnetic field modulated microwave absorption in BiSrCaCuO single crystals. In addition to the well-known signal corresponding to the field-derivative of the thermally activated flux-flow resistivity, which appears close to T c and linearly depends on the modulation level, we observe a new signal with a nonlinear dependence on the modulation field. This new signal (i) extends to low temperatures, (ii) appears only above some threshold modulation level, (iii) strongly depends on the magnitude and direction of the dc magnetic field, (iv) appears only in highly anisotropic superconductors.

Induced microwave absorption by magnetic modulation in untwinned and twinned YBaCuO crystals and its comparison with resistivity measurements

We studied the magnetic field modulated microwave absorption across T c for an untwinned and for a twinned YBa 2Cu 3O 7− δ crystal ( T c≅93 K) which were cleaved from the same high quality single crystal. The absorption signals were analyzed using the Tinkham model. A good fit to the model was obtained for the untwinned crystal for B‖ c and for B>0.2 T, while for the twinned crystal we observed pronounced deviations from the model which we attribute to the presence of twin boundary pinning. The angular dependence of the activation energy of vortex motion in the untwinned crystal shows a sharp increase when the direction of magnetic field is almost parallel to the a– b plane, which is suggestive of a lock-in transition. Although field-dependent resistivity measurements on untwinned crystals clearly demonstrate a vortex melting transition, microwave studies do not reveal any signature of it. This agrees with results of other groups showing that high frequency measurements in YbaCuO 7− δ single crystals, unlike Bi 2Sr 2CaCu 2O x single crystals, are rather insensitive to the vortex melting transition.

New memory effects detected in field-modulated microwave absorption of magnetized YBa 2Cu 3O 7−δ powders

We describe two new memory effects detected as additional changes in signals of field-modulated microwave absorption of YBa 2Cu 3O 7−δ powders. They appear when the scanning direction of the field is reversed or when the sample is cooled. The observed effects can be attributed to vortex distribution in superconducting YBa 2Cu 3O 7−δ particles.

Crystalline Phases and Electronic Structures in Superconducting Bi – Sr – Ca – Cu Oxides

Two classes of samples, designated A and B, of layered Bi–Sr–Ca–Cu oxides having the same nominal composition 4 : 3 : 3 : 4, but different thermal histories, were investigated by using field modulated microwave absorption (ESR), powder x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and x-ray absorption near the edge structure (XANES). Previous electrical resistivity measurements showed that the B samples presented only two superconducting phases with midpoints of the transition temperatures at ∼80 K and ∼105 K. The microwave absorption technique indicated instead the presence of islands which became superconducting at the above-mentioned temperatures also in the A samples. The crystalline and electronic structures of the two types of samples are illustrated and discussed. A plausible theoretical interpretation of the experimental results, based on a quantum percolation model with Coulomb interaction, is also given.

Low field modulated microwave absorption and electrical properties in degraded and annealed granular YBa 2Cu 3O x

Modulated microwave absorption was examined on granular YBa 2Cu 3O x. The hysteric signal can be explain by three factors related to surface flux distributions, and by intergrain Meissner and critical current.

Field modulated microwave absorption and irreversibility line in [formula omitted] multilayers

We describe field modulated microwave absorption measurements in two series of YBa 2Cu 3O 7 PrBa 2Cu 3O 7 multilayers: those with PrBa 2Cu 3O 7 thickness in the range ∼2.5 to ∼10 nm and with a fixed YBa 2Cu 3O 7 thickness of ∼10 nm and those with different resistivities of the PrBa 2Cu 3O 7 barrier with the same thickness of ∼2.5 nm for YBa 2Cu 3O 7 and PrBa 2Cu 3O 7 layers. The Fourier spectrum of the reflected microwave power reveals one fundamental frequency which appears at T c and even harmonics of the modulation field frequency f at lower temperatures. The determination of the irreversibility line near T c inferred from the vanishing of the nonlinear response signal at 2 f in the presence of superimposed d.c. and modulation fields is reported. A comparison with results obtained on YBa 2Cu 3O 7 thin films is also given.

Search for new superconductors in the Y-Ni-B-C system

We have searched for superconductivity in a wide variety of stoichiometries in the Y-Ni-B-C system, using Yx(NiB)Cy phase spread alloy thin films and the magnetic field modulated microwave absorption technique. The superconducting critical temperature (Tc) varies with the stoichiometry, showing the highest Tc for a Y/Ni ratio of 1/2, attributed to the YNi2B2C phase. We found no other superconducting phases with a Tc higher than 10 K. Furthermore, a search in YxNiB(C1−zNz)y and YxNiBNy phase spread alloys showed no superconductivity.

Field modulated microwave absorption in YBa2Cu3O7/PrBa2Cu3O7 multilayers

Field modulated microwave absorption in YBa2Cu3O7/PrBa2Cu3O7 multilayers

Field modulated microwave absorption and irreversibility line in YBa 2Cu 3O 7 thin films

Measurements of the field modulated microwave absorption of YBa 2Cu 3O 7 thin films as a function of temperature and magnetic field are presented. The Fourier spectrum of the reflected microwave power below T c reveals a peak at the fundamental frequency of the modulation field and its even harmonics. The amplitude of the even harmonics decreases rapidly when the dc field superimposed on the modulation field is increased, then shows a maximum and vanishes above a characteristic field B ∗ . We show that this maximum is related to the full penetration of the modulation field and that the B ∗ (T) curve can be viewed as an irreversibility line which is consistent with that obtained from irreversible magnetization measurements.

Microwave studies of the superconducting state in Rb 3C 60

Methods of electron-spin resonance (ESR) and direct, non-linear field-modulated microwave absorption (FMMA) were applied for the measurements in low- and high-purity samples of rubidium-doped fullerene, Rb 3C 60. The coexistence of the normal strong ESR signals and regular series of weak absorption lines similar to those seen in systems of Josephson junctions was observed in the high-purity sample. The possible influence of the vortex lattice on the ESR signals was also studied. We determined from FMMA investigations using the Portis model the critical field μ 0H ∗ = 40 μ T , the depinning current density J c ∗(μ 0H 0 = 1 mT) ∼ 4 × 10 8 A/m 2 in low magnetic field and J c ∗(μ 0H 0 > 100 mT) ∼ 1.6 × 10 8 A/m 2 in higher fields. These values were generally one order of magnitude higher than the highest corresponding values previously observed in high-temperature superconductors (HTS's).

A microwave technique for characterization of SNS and grain-boundary junctions

We have utilized a magnetic field modulated microwave absorption technique for characterizing SNS and grain-boundary junctions. The technique is based on the measurements of the first derivative microwave absorption P, with respect to an applied magnetic field B at 9.5 GHz, as a function of dc magnetic B/sub dc/. A homodyne detection system is used. Due to flux quantization in a non-uniform junction a series of equally spaced microwave absorption lines can be observed for B/sub dc/ and, in strongly coupled junction cases, the dependence of I/sub c/ on magnetic field and temperature can be extracted. Both the intra-grain boundary in a melt-textured YBa/sub 2/Cu/sub 3/O/sub x/ bulk material and thin film single grain boundary junction (GBJ) YBa/sub 2/Cu/sub 3/O/sub x/ were investigated. The result of the measurements agrees with a model in which the grain boundary consists of several sub-junctions in parallel. >

Microwave absorption and EPR studies of a new copper oxyfluoride superconductor synthesised through the ammonium bifluoride route

The recently reported superconductor Sr 2CuO 2F 2.4 is synthesised by a new route of fluorination by NH 4HF 2 and its superconducting phase is investigated by electron paramagnetic resonance, direct microwave absorption and non-resonant response employing magnetic field modulated microwave absorption. These studies have given clear evidence for the occurrence of a superconducting phase below 40 K. From the magnetic field dependence of the changes in microwave absorption, the lower critical field H c1 (perpendicular to the c-axis) is determined to be 100 G at 10 K. The EPR studies on this oxyfluoride have shown a Cu 2+ signal with 19F super hyperfine structure due to interaction with two equivalent F nuclei. In view of its low intensity it was assigned to a possible ‘defect site’ in the lattice. The line width of the EPR signal increased in the superconducting phase smearing out the 19F shfs below 35 K. This is consistent with what is normally expected in superconductors.

Remanent microwave absorptin in Y−Ba−Cu−O at low magnetic fields

Polycrystalline samples of YBa2Cu3O7 were studied by the microwave absorption technique at the low magnetic fields in the temperature range from 5 to 80 K. The direct microwave absorption and the field modulated microwave absorption were measured at 77 K. Analysis of the microwave absorption curves indicates that the field penetration into the sample starts from the lowest values of applied magnetic field within the whole temperature range.

Irreversibility line of YBa 2Cu 3O 7 thin films studied by field modulated microwave absorption

Field modulated microwave absorption has been studied in the superconducting state of epitaxial YBa 2Cu 3O 7 thin films prepared by MOCVD. Even harmonics of the modulation field frequency in the Fourier spectrum of the reflected microwave power have been observed. Their amplitude decreases with increasing the dc field superimposed on the ac field and vanishes above a characteristic field B ∗. We show that the B ∗(T) curve can be viewed as an irreversibility line which we compare to that obtained from irreversible magnetization measurements.

Direct field modulated microwave absorption in Ba 1− xK xBiO 3

Non linear, field modulated, direct microwave absorption (MA) was studied in the high- T c copper-free isotropic Ba 1− x K x BiO 3 (BaKBiO) powder samples. The microwave determined critical magnetic field gradient and the corresponding depinning current density were compared with the values obtained previously for YBaCuO (YBCO) and BiSrCaCuO (BSCCO).

Microwave absorption in non- and hydrogenated La 1.85Sr 0.15CuO 4 high-T c superconductor

The field-modulated microwave (MW) absorption in non- and hydrogenated La 1.85Sr 0.15CuO 4 high-temperature superconductors was measured. The relationship between the microwave hysteresis at high magnetic field ( H = 3 kG) and the irreversible magnetization has been investigated. It has been established that the temperature dependence of the MW hysteresis is not in a line with magnetic hysteresis data. The value of the pinning energy of 50 meV at 15 K in magnetic field of 3.0 kG for the non- and hydrogenated samples has been estimated.