Magnetically Modulated Microwave Absorption Research Articles

Microwave absorption by Co/Ce ferrite nanoparticles

Nanosized particles of CoFe2-xCexO4, with 0 ≤ x ≤ 0.29, were synthesized by the sol-gel autocombustion method and the microwave absorption of these ferrites was measured by the Magnetically Modulated Microwave Absorption (MAMMA) method. The results showed that the absorption is highest for a Ce concentration between 0.18 and 0.26, a result that could be useful for practical applications of this material.

Microwave absorption study of pinning regimes in Ba(Fe1−xCox)2As2 single crystals

Magnetic field dependent modulated microwave absorption (MMWA) measurements have been carried out to investigate vortex pinning effects in single crystals of the iron-based high-Tc superconductor Ba(Fe1−xCox)2As2 with three different cobalt doping levels of x = 0.07, 0.09 and 0.11. The dependence of the MMWA hysteresis loops on temperature, magnetic field and Co concentration have been measured and analyzed using a theoretical model of microwave absorption in superconductors. The analysis reveals that in the underdoped crystal (x = 0.07) the so called δTc-pinning due to magnetically ordered regions defines the temperature dependence of the critical current density, while in the optimally doped (x = 0.09) and overdoped (x = 0.11) samples the pinning is governed by structural imperfections due to the inhomogeneous distribution of the cobalt dopant and has the so called δl character.

Structure and superconductivity of VN–SiO 2 films obtained by thermal nitridation of sol–gel derived coatings

In this work studies of the structure and superconducting properties of xVN − (100 − x)SiO 2 films (where x = 90, 80, 70 and 60 mol%) are reported. The films were obtained through thermal nitridation (ammonolysis) of sol–gel derived V 2O 3–SiO 2 coatings (in a proper V 2O 3/SiO 2 ratio) at 1200 °C. This process led to the formation of a disordered structure with VN metallic grains dispersed in the insulating SiO 2 matrix. The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The superconducting properties of the samples were examined with magnetically modulated microwave absorption (MMMA). All the samples apart from x = 90 exhibited superconducting transition above 6 K.

Superconductivity of Mg–B layers prepared by a multi-energy implantation of boron into magnesium and magnesium into boron bulk substrates followed by the furnace and pulsed plasma annealing

B ions into Mg and Mg ions into B substrates were implanted in triple mode, i.e. each sample was sequentially implanted at three different energies starting from the highest one (80–150 keV range) to the lowest one (40–70 keV range). The energies and fluencies in each particular batch were simulated to yield a possibly large region in which the Mg:B ratio corresponds to stoichiometric MgB 2 compound. These structures were next annealed using high intensity hydrogen plasma pulses of energy densities between 1.5 and 4.0 J/cm 2, or furnace annealed at 350–600 °C in a stream of flowing Ar-4%H 2 mixture. The simulated profiles were in fair agreement with those derived from the RBS measurements. Magnetically modulated microwave absorption (MMMA), magnetization and resistance measurements showed that the superconducting transition onset temperature T c onset shifted from about 13 K in the best magnesium sample implanted with single-energy B ions, to 22–28 K for multi-energy implantation treatments. Respective shift in Mg-implanted boron samples was from about 33.3 K to 36.5 K. However, broadening of the transition to the superconducting state is observed for the multi-energy treatment in both cases. Possible reasons for these effects and proposed means to improve the method are discussed.

Superconducting properties of an NbN–SiO2 disordered system obtained by ammonolysis of NbN–SiO2 sol–gel derived coatings

Studies in superconducting properties of NbN–SiO2 films are reported. The films were obtained through nitridation of sol–gel derived Nb2O5–SiO2 coatings at 1200°C, a process leading to the formation of disordered structures with NbN metallic grains dispersed in the insulating SiO2 matrix. Electrical resistivity was measured with the conventional four-terminal method in the temperature range from 5 to 280K. The samples’ superconducting properties, examined with magnetically modulated microwave absorption (MMMA), depend on the NbN/SiO2 molar ratio and the film’s thickness.

Formation of superconducting regions of MgB 2 by implantation of magnesium ions into boron substrate followed by intense pulsed plasma treatment

Mg ion implantation into boron substrates followed by pulsed Ar plasma treatment was used to form MgB 2 compound. Rutherford Backscattering (RBS) analysis of the best samples revealed that the ratio of atomic concentration of Mg to B atoms could be close to the stoichiometry of MgB 2 in the range of about 100 nm beneath the surface. Results of magnetically modulated microwave absorption (MMMA), magnetic moment and electrical conductivity measurements indicate the presence of superconductive grains with critical temperature T c = 25 K. A gradual onset of microscopic percolation is inferred starting from 15 K although no full percolation has been reached.

Superconductivity of MgB 2 thin films prepared by ion implantation and pulsed plasma treatment

The experiments to synthesize thin MgB 2 inter-metallic compound with the use of ion implantation and plasma pulse treatment are presented. Mg was implanted with 3×10 18 cm −2 of 80 keV and 5×10 18 cm −2 of 100 keV B + ions and next treated with hydrogen and argon plasma pulses of duration of about 1 μs and fluence between 2 and 4 J/cm 2. Superconducting properties were examined by magnetically modulated microwave absorption (MMMA), magnetic moment and electrical conductivity measurements. The structural properties of the implanted and pulse-treated samples were examined by the X-ray diffraction (XRD) and Rutherford backscattering (RBS) methods. The main result consists in observation of MMMA hysteresis loop demonstrating the existence of superconducting regions with T c as high as 32 K. However, the zero-resistance effect has not been obtained due to incomplete global connectivity between the superconducting regions.

Application of magnetically modulated microwave absorption to study of giant magnetoresistance effect in the Ni−Fe/Cu multilayer system

Magnetically modulated microwave absorption (MMMA), magnetic hysteresisM(H), giant magnetoresistance (GMR) and ferromagnetic resonance (FMR) have been examined in the antiferro-magnetically coupled Py/Cu (Py=Ni83Fe17, permalloy) multilayer system. The correlation between results obtained by the MMMA technique, the standard GMR measurements, and magnetization reversal studies is shown. Microwave studies of GMR, magnetization reversal, and FMR for different orientations of the magnetic field with respect to the sample surface are presented.

The effect of the YBCO–PST composite composition on the superconducting carrier concentration determined by microwave studies under high pressure

The effect of hydrostatic pressure ( p<0.6 GPa) on the superconducting critical temperature T c in YBa 2Cu 3O 7− δ –Pb(Sc 0.5Ta 0.5)O 3 (YBCO–PST) composite is measured by the method of magnetically modulated microwave absorption (MMMA). The T c dependence on the PST fraction in weight x (0, 0.25, 0.5 and 0.75) is approximated by an inverted parabola function whereas the influence of pressure on T c is represented by the equation: d T c/d p=0.61(2)−1.72(6) x. The result may be explained assuming that PST phase in YBCO–PST composite influences the superconducting carrier concentration similar to the chemical substitution in YBa 2Cu 3O 7 [J.J. Neumeier, H.A. Zimmermann, Phys. Rev. B 47 (1993) 8385]. It is suggested that ions from PST diffuse to YBCO cell during the sintering of the composite.

Demagnetising effect in HTS in GHz frequency fields

In the present paper, we investigate the demagnetising effect in granular superconductors by applying the method of magnetically modulated microwave absorption (MMMA). The quantitative description is based on microwave measurements of demagnetising factors in cylindrical high-temperature superconductors (HTS). The results prove to be in good agreement with the calculations of Chen et al. [D.-X. Chen, J.A. Brug, R.B. Goldfarb, IEEE Trans. Magnetics 27 (1991) 3601] of demagnetising factors for cylindrical samples. The shielding effect caused by the sample as well as by individual grains, whose influence is especially noticeable for highly porous materials, is also discussed. The final results point to the necessity of allowing for the demagnetising effect in microwave absorption studies of superconductors.

Magnetically modulated microwave absorption studies on high- Tc powdered materials

Magnetically modulated microwave absorption (MMA) is performed on powdered high- T c Bi 2Sr 2Ca 2Cu 3O y , Bi 2Sr 2Ca 1Cu 2O y and Y 1Ba 2Cu 3O y materials. The temperature dependence of the zero field microwave absorption maximum and low field hysteresis is presented. It is observed that the zero field line and low field hysteresis depend on the particle size of powdered materials. We show that a model, which combines a model based on the existence of a critical state with pinning and depinning of fluxons during a modulation cycle and a weak link model, can account for the observed temperature and particle-size dependence of zero field line and hysteresis.

Electron paramagnetic resonance and magnetically modulated microwave absorption experiments on κ-(BEDT—TTF) 2Cu[N(CN) 2]Cl

Electron paramagnetic resonance (EPR) and magnetically modulated microwave absorption (MMMA) measurements were taken on differently prepared samples of the quasi two-dimensional organic conductor κ-(BEDT—TTF) 2Cu[N(CN) 2]Cl to study the interesting pressure dependent phase transition of this charge-transfer compound. The MMMA results undoubtedly confirm that the substance becomes a superconductor below 12.5 K, if a moderate pressure, e.g. just by embedding the substance in silicon grease during cooling, is applied. At an onset temperature of 25 K an additional microwave absorption is observed, which is about a factor of 100 weaker than the superconducting MMMA signal. We relate this microwave absorption to an antiferromagnetic ordering resulting in weak ferromagnetism by spin-canting occurring at this temperature. The result corresponds to the EPR measurements, where considerable changes in the main EPR parameters, i.e. linewidth, signal amplitude and susceptibility, are observed, giving evidence for an antiferromagnetic ordering below 50 K and a change to weak ferromagnetism below 25 K.

Microwave absorption in BSCCO superconductor: Influence of different thermal treatments on oxygen stoichiometry

Modification induced by thermal treatments in different gaseous environments (nitrogen, helium, and oxygen), on the 2223 phase formatted in two BI(?B)SrCaCuO systems, were investigated by the microwave methods DMA (direct microwave absorption) and MAMMA (magnetically modulated microwave absorption). Si1.7Pb0.3Sr2Ca2Cu3Oy and Bi1.7Pb0.3Sr2 Ca2.5Cu3.5Oy systems were obtained by different preparation techniques, and as a result they exhibit small differences in the canon stoichiometry that affect the oxygen stoichiometry and their change under the same annealing conditions. The changes observed were attributed to the oxygen variation around the optimum concentration and. further the deviation of the oxygen content from this optimum value reduces the critical temperature of the 2223 phase.

Magnetically modulated microwave absorption (MMMA) measurements at low magnetic fields on the ferromagnetic state of [TDAE]C60

The ferromagnetic state and reported superconductivity of [TDAE]C60 (where TDAE is titrakis(dimethylamino)ethylene, see Figure) are investigated by magnetically modulated microwave absorption (MMMA). The results are compared with those‐reported using alternative physical measurement techniques, and a hypothesis proposed to explain the observed behavior. No evidence for superconductivity is found.magnified image

Variation of the microstructure and flux penetration in solid solutions prepared by melt processing

The substitution of for in solid solutions leads to a systematic decrease of the unit cell volume with increasing x value due to the smaller ionic radius of replacing in the structure. The growing strain, which is introduced into the structure by this substitution, distinctly influences the equilibrium in the process of crystal growth. This leads to a strong decrease of the texture and porosity of samples prepared by melting techniques, when the Sr concentration increases. The additional non-superconducting phases, which are formed in the process of the partial decomposition of the solid solution above the peritectic temperature, are characterized by optical microscopy in polarized light and electron microanalysis. Measurements of the magnetically modulated microwave absorption (MMMA) in (x = 0.4, 0.6, 0.8) solid solutions with different porosity and grain sizes prepared by the quench and melt technique allow one to analyse the inter- and intragrain flux penetration. The MMMA hysteresis width strongly depends on the sweep range of external magnetic field. The signal is reversible for the lowest sweep range, when the surface supercurrents allow Meissner shielding. When the sweep range increases, magnetic field begins to penetrate the sample through the intergrain weak links and irreversibility of the MMMA signal appears as a consequence of intergrain pinning of Josephson vortices. On further increase of the magnetic field sweep range, MMMA hysteresis width reflects the intragrain penetration. In contrast to porous samples, where all these stages can be clearly resolved, in solid solutions with low porosity flux pinning is enhanced and the role of intergrain Josephson coupling is significantly reduced.

Microwave measurements of the critical fields and currents in HTS

The technique of magnetically modulated microwave absorption (MMMA) has been used for measuring the critical fields and critical currents of granular high-temperature superconductors (HTS) and for investigating the Josephson/intergrain critical state.

Electrodynamics of high-Tc cuprates. Low field microwave absorption

Local electrodynamics of a high-Tc superconductor (HTS) is elaborated based on the concept of the complex conductivity of a hard type II superconductor containing weak links (WLs). The contributions of the two-fluid, the flux motion, and the weak link network to the total conductivity are taken into account. The expression for the surface impedance is derived and compared with the experimental results. Applicability of the model to the magnetically modulated microwave absorption (MMMA) experiments is discussed.

Particle-size dependence of modulated microwave absorption in powdered BiSrCaCuO superconductors

The effect of particle size on the magnetically modulated microwave absorption (MMA) signal in Bi 2Sr 2CaCu 2O y and PbBi 2Sr 2Ca 2Cu 3O y powdered samples is reported. It is found that for smaller particle sizes and for increasing temperature, the size of the irreversible hysteretic signal decreases. At the same time the modulation amplitude for which the hysteresis size is a maximum increases. In order to interpret the observed results, a macroscopic model is developed which takes into account the damped motion of fluxons as a loss mechanism, from which an indirect estimate of the penetration depth is obtained.

Thermogravimetric study of the reduction step in Nd 1.85Ce 0.15Cu 1.01O y as a function of the oxygen partial pressure

Thermodynamic measurements of the equilibrium oxygen partial pressure as a function of the oxygen content have been performed in Nd 1.85 Ce 0.15 Cu 1.01 O y at 900°C. These measurements suggest the presence of two kinds of defects: interstitial oxygen atoms for PO 2 higher than the one corresponding to the thermodynamic equilibrium CuO−Cu 2 O (1.4 × 10 −2 atm) and oxygen vacancies for oxygen partial pressure below this value. The superconducting transition of samples with controlled oxygen content was studied through Magnetically Modulated Microwave Absorption (MaMMA). The superconducting transition was observed only for samples quenched at PO 2 lower than 1.4 × 10 −2 atm. These results confirm that in order to obtain superconductivity in this material interstitial oxygen atoms must be removed from its structure.

Magnetically modulated microwave absorption in cuprate superconductors

Magnetically modulated microwave absorption (MMMA) is a highly sensitive microprobe, which is increasingly being used to extract a variety of information about the flux dynamics in the high-transition-temperature superconductors. The correspondence between the information obtained from the MMMA and the directly monitored (without modulation) field-induced microwave absorption techniques is discussed. The validity of some of the commonly held views about the features that characterize the MMMA signal are examined.