T. Dragon

Influence of nanocrystallization on the evolution of domain patterns and the magnetoimpedance effect in amorphous Fe73.5Cu1Nb3Si13.5B9 ribbons

The evolution of the magnetic domain patterns and the giant magnetoimpedance (MI) effect in the Fe73.5Cu1Nb3Si13.5B9 amorphous ribbons are studied as a function of the annealing time ta at 550 °C. It is shown that by annealing at 550 °C for ta from 1 to 150 h, very fine nanocrystalline bcc-FeSi grains are homogeneously formed in the amorphous matrix. Although with increasing ta the grain size remains very fine (⩽11 nm), the coercivity Hc increases rapidly from 0.0014 Oe for ta=3 h to 1.67 Oe for ta=150 h. For the nanocrystalline ribbons with ta=3 h, the domain structure is characterized by a few broad longitudinal together with some broad transverse domain patterns, connected to the minimum coercitive field. With increasing annealing time ta⩽10 h, only transverse domain patterns are observed and the transverse domain width gradually becomes narrow. Considering the different magnetic softness, domain structure, and magnetization process, various types of the MI effect are observed in these nanocryastalline...

Investigation of the mixed state of YBa2Cu3O7−δ superconductors

The magneto‐optical Faraday effect has been used to observe the penetration of magnetic flux into sintered YBa2 Cu3 O7−δ. In addition, measurements of the magnetic polarization are performed with a SQUID magnetometer. The magneto‐optical Faraday effect shows an inhomogeneous penetration of the magnetic flux into the sample. In a first step the flux penetrates into the intergranular regions and in a second step into single grains separately. A spatial gradient of the flux density within each grain is built up, indicating pinning forces. In the remanent state in each grain trapped flux is observed. The measured magnetization curves show good agreement with the magneto‐optical investigations of the flux penetration.The magneto-optical Faraday effect has been used to observe the penetration of magnetic flux into sintered YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. In addition, measurements of the magnetic polarization are performed with a SQUID magnetometer. The magneto-optical Faraday effect shows an inhomogeneous penetration of the magnetic flux into the sample. In a first step the flux penetrates into the intergranular regions and in a second step into single grains separately. A spatial gradient of the flux density within each grain is built up, indicating pinning forces. In the remanent state in each grain trapped flux is observed. The measured magnetization curves show good agreement with the magneto-optical investigations of the flux penetration.

Transverse domain structure related giant magnetoimpedance in nanocrystalline Fe73.5Cu1Nb3Si13.5B9 ribbons

A sensitive field- and frequency-dependent magnetoimpedance (MI) has been observed in nanocrystalline Fe73.5Cu1Nb3Si13.5B9 ribbons. A maximum value for the MI ration ΔZ/Z=[Z(H)−Z(Hmax)]/Z(Hmax), of more than 400% was obtained in these nanocrystalline ribbons (annealed for 3–5 h at 550 °C). A peak in the field dependence of the MI ratio ΔZ/Z was observed in nanocrystalline samples, but not in the as-quenched amorphous samples. The sensitivity attained a value larger than 60% Oe−1 in the field range 3–7 Oe at 800 kHz for the nanocrystalline ribbons. Domain observation experiments show that the transverse domain structure observed in nanocrystalline samples is responsible for their giant MI effects, and the domain wall movements in the transverse domain pattern region are dominant during the initial magnetization processes in both longitudinal and transverse directions of the applied field. The correlation between the magnetization processes and the giant MI effects is discussed briefly based on the view of ...

A new calibration technique for the determination of magnetic flux density with the magneto-optical Faraday effect

Abstract Using the high resolution magneto-optical Faraday effect the magnetic flux density distribution of circular shaped Y1Ba2Cu3O7−x thin films is investigated. A new calibration technique is developed to determine the z-component Bz(x,y) of the local magnetic flux density B (x,y) in the x,y-surface of the films with high accuracy. The observed flux density profiles agree well with calculations of B ( r ) using a simple model with homogeneous current distribution. The appearance of a negative magnetic flux and reversed flux lines at the edge of the films in the remanent state is caused by stray fields of the trapped flux. For a determination of the local critical current density jc, the local pinning forces pv and the magnetization of thin films from the observed magnetic flux density distribution B (x,y) the curvature of the magnetic field ∂Bz/∂r and ∂Br/∂z, respectively, has to be taken into account.

Investigation of flux distribution in YBaCuO thin films by means of the magneto-optical Faraday effect

Abstract The magneto-optical Faraday effect is used to observe the magnetic flux distribution in high T c superconducting Y 1 Ba 2 CU 3 O 7 thin films. In the Shubnikov phase three characteristic structures of flux distribution are observed. A homogeneous flux front penetrating from the edge of the specimen to the center, irregularly oriented linear or oval shaped regions acting like a drain where the flux penetrates easily, and a regular stripe structure. Measurements of the magnetization curves are comparable to the observed penetration behaviour of magnetic flux.