Low Field Non-resonant Microwave Absorption Research Articles

Low‐field non‐resonant microwave absorption in glass‐coated Co‐rich microwires

AbstractA study of low‐field non‐resonant microwave absorption (LFA) at 9.8 GHz, on as‐cast amorphous Co‐rich CoFeBSi microwires under different measuring geometries is presented. Results confirm that LFA is associated with the magnetization processes from the unmagnetized state (HDC = 0) to the saturated condition, in many aspects similar to Giant Magnetoimpedance (GMI), and clearly different from ferromagnetic resonance (FMR). LFA signal showed large variations in its maximum–minimum separation as a function of the measuring geometry, which is interpreted in terms of the total anisotropy in the process. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of magnetic materials by low-field microwave absorption techniques

A low-field non-resonant microwave absorption has recently been observed in a variety of magnetically ordered materials at low DC fields (−1000 Oe ⩽ H DC⩽+1000 Oe), which is known as low-field microwave absorption (LFA). It has been shown that LFA is essentially similar to giant magnetoimpedance (GMI), and clearly different from ferromagnetic resonance (FMR). LFA strongly depends on the anisotropy field of the sample. In contrast with FMR (which can be described as the homogeneous precession of spins in the saturated state), LFA can be thought as a spin rotation process occurring during the magnetic saturation. In this work, we present a detailed study of the basic features of LFA in several types of materials: ferrites and amorphous microwires and ribbons; in particular the effects sample shape, temperature up to the Curie transition, the influence of easy axis and the effects of annealings. These examples show that once LFA is fully understood, it can become a powerful characterization tool.

Characteristics of the magnetosensitive non-resonant power absorption of microwave by magnetic materials

By means of the modification of an electron paramagnetic resonance (EPR) spectrometer, two experimental techniques have been implemented to measure the magnetosensitive non-resonant absorption of microwaves (MANRAM): magnetically modulated microwave absorption spectroscopy (MAMMAS) and low field non-resonant microwave absorption spectroscopy (LFNORMAS). These techniques are proposed to be used in the characterization of magnetic materials at microwave frequencies. They are very sensitive, hence require very little amount of material for test in the form of crystals, glasses, powders, liquids; these wireless techniques are non-destructive. Magnetically modulated microwave absorption spectroscopy is carried out cooling or heating a sample under study in presence of a modulated magnetic field. The temperature of the material is taken and a graph of it versus the modulated absorption of microwaves is constructed. Low field non-resonant microwave absorption spectroscopy measures the magnetosensitive-modulated absorption of microwaves, by the material, with respect to cycled sweepings of an applied dc magnetic field. We present results for the ceramic superconductor YBa 2Cu 3O 7− x (YBCO), the antiferromagnetic (AF) MnO oxide and ferroelectromagnetic Pb(Fe 1/2Nb 1/2)O 3 (PFN). The absorption spectra are shown highlighting their main characteristics.

Superconducting State Volume in High Tc Superconductor and Non-resonant Microwave Absorption

The mechanism of low-field non-resonant microwave absorption is speculated, and it is exposed that the magnitude of the microwave signal indicates the superconducting state volume in the shell lying along inner surface of the sample.

Microwave absorption investigation on the formation of YBa2Cu3O7−σ by a binary metal oxide route involving interaction of BaCuO2 and Y2Cu2O5

Low-field microwave absorption has been used to discriminate between the YBa2Cu3O7−σ high temperature superconductor synthesized by reacting the binary metal oxides, BaCuO2 and Y2Cu2O5, versus synthesis with a single metal oxide route. This discrimination is more clearly seen by microwave absorption than by resistance measurements. Supplementary data was obtained by electron spin resonance in the g=2 region, scanning electron microscopy, thermal analysis and x-ray diffraction techniques. The scanning electron micrographs indicate a more separated granular structure for the material synthesized from the binary oxides. The low field non-resonant microwave absorption, which is characteristic of the superconducting phase, is twice as intense for the binary metal oxide preparation as compared to the single metal oxide preparation. Also, the superconducting transition temperature seems to be several degrees higher for the binary oxide preparation.

Time-dependent low field microwave absorption in the high temperature superconductors

Abstract It is observed that the hysteresis in the applied magnetic field position and the intensity at the peak of the low field non-resonant microwave absorption (recorded in an EPR experiment with a modulation amplitude of ∼ 10 G) in the superconducting state of the cuprate superconductors, is time-dependent after the removal of a DC magnetic field sizably greater than the lower critical field. This intrinsic time-dependence, which we attribute to flux creep, is reported here for two copper oxide-based high temperature superconductors.

Stabilization of orthorhombic Ba2YCu3O7 − δ phase by the 5a group transition metal elements

The effect of partial replacement of copper atoms in the superconducting orthorhombic Ba 2 YCu 3 O 7 − δ phase with the 5a group transition metal elements has been studied. It was shown that in the Ba 2 YCu 3 M x O 7 − δ ( M ≡ V, Nb or Ta) system, the orthorhombic phase is stabilized in a wide range of dopant concentrations. A degradation of superconducting properties takes place gradually by increasing the dopant level, this being accompanied by formation of the other phases, i.e. BaVO 3, Ba 3V 2O 8, Ba 2YNbO 6, Ba 2YTaO 6 and CuO. X-ray powder diffraction, electron paramagnetic resonance spectroscopy, low field non-resonant microwave absorption, electrical resistance and magnetic susceptibility measurements were correlated.

Microwave absorption of single-crystal high Tc superconductors biased by dc current

The low field non-resonant microwave absorption of single-crystal YBCO was measured, as a function of the strength and direction of the applied magnetic field with respect to crystal axes, and by applying dc current parallel to the c axis. The transport critical current density may be obtained from these experiments.

Low-field microwave absorption in high-T c superconducting oxides

Low-field nonresonant microwave absorption in an electron paramagnetic resonance spectrometer has been observed in the superconducting state of different samples of the new high-Tc oxides, YBa2Cu3O7−x (powder pellets and single crystals), La1.8Sr0.2CuO4 , and Bi-Sr-Ca-Cu-O. It has been attributed to flux slippage occurring at the Josephson junctions existing in the granular composite materials. The dependence on particle size, temperature, microwave power, dc modulation amplitude, and magnetic field has been studied.

Paramagnetic resonance and non-resonant microwave absorption in the Bi-Ca-Sr-Cu-Oxide high temperature superconductors

Electron paramagnetic resonance and low field non-resonant microwave absorption are reported in the BiCaSrCuoxide high temperature superconductor. Two different Cu 2+ EPR resonances are observed in samples with semiconducting impurities phases, one having an isotropic g tensor and the other an anisotropic g tensor. An EPR signal is absent in samples in which no impurities phases are present. The low field microwave absorption attributed to inter-domain Josephson junctions formed within the grains of the material is observed and its temperature dependence in the superconducting phase has been measured.

Critical state and fluxon pinning in high-T c superconductors

The low-field nonresonant microwave absorption in high-T c superconductors results from the interaction of supercurrents with free fluxons and is a sensitive indicator of fluxon pinning. The existence of a critical state for both increasing and decreasing applied magnetic fields accounts for the change of sign of the signal with reversal of field sweep. When the sweep direction is reversed, a field interval 2H c1 = (8π/c) λJ c is required for the establishment of the reverse critical state, where J c is the critical current density and λ is a relaxation length. During this interval fluxons are first pinned and then depinned, causing a minimum in the absorption as has been observed directly in the scanned microwave signal.

Paramagnetic species and magnetic surface structures in high-TC superconductors as revealed by electron spin resonance and low-field nonresonant microwave absorption.

Paramagnetic species in the normal conducting state of various high Tc superconducting oxides such as the La-Ba (or Sr) -Cu-O, RE-Ba-Cu-O (RE=Y, Gd, Dy, Ho, Er and Tm) and Bi-Ca-Ba-Cu-O systems were characterized by ESR spectroscopy. Low-field nonresonant microwave absorption spectra were observed for the superconducting state of the above high Tc superconductors. The nonresonant absorption is caused to the reduction of the shielding effect by the supercurrent and intergrain Josephson current on the surface of above samples. Magnetic hysteresis and transient response observed in the nonresonant absorption arise from frozen-in magnetic fluxes, which are in a metastable state. The critical field obtained from the flux trapping allows one to estimate the average size of the superconducting clusters with a uniform phase in the spin-glass-like high Tc superconductors. It is suggested that the low-field nonresonant microwave absorption method is applicable to the characterization of thin-layer high Tc superconductors as well as novel composite types of superconductors.