H. Ferrari

Nonconventional short-time dc magnetometer for superconducting films

A nonconventional technique for the measurement of magnetic relaxation in superconducting films at short times (∼10−5 s) is described. This technique combines the application of a pulsed magnetic field and a synchronized high-energy pulsed laser. Remanent magnetic relaxation in (Y;Gd)Ba2Cu3O7−δ thin films is reported over five decades time at reduced temperatures above 0.8.

Magneto-Optical Imaging in an Undergraduate Laboratory Course

Magneto-optical imaging has become a powerful technique for the measurement of local magnetic fields. The technique consists in measuring the rotation in the light polarization plane when light travels through a sensitive garnet (Yttrium Iron Garnet, YIG). The plane rotation is a function of the local magnetic field at which the garnet is exposed. We describe an experiment for an undergraduate laboratory course in optics and electromagnetism that consists of two applications of the technique: (i) The characterization of the properties of the YIG as a function of the magnetic field generated by a Helmholtz pair of coils, and (ii) The measurement of the magnetization of previously recorded audio tapes. In the first case, the magnetic field is calculated by modeling the Helmholtz coils as two parallel circular coils of radius R separated at a distance 2R. The measured change in the light polarization plane is described as the corresponding to magneto-optical layers with in plane anisotropy. In the second application, computer generated functions (e.g. square and sawtooth) of different frequencies were directly recorded with a tape recorder. The magnetized tape patterns are observed using the same characterized YIG and a commercial polarized light microscope.

Magnetization in high-temperature superconducting strips during transverse magnetic field application

Abstract We have investigated both experimentally and numerically the magnetization, M(t), in high-temperature superconducting strips during the application of a transverse magnetic field, Ha(t). Experimental results in GBCO strips were obtained with a short time non-conventional technique for Ha(t)∼(1−exp(t/τ)), with τ∼1 μs . Calculations take into account the effect of flux creep and/or ohmic overcritical damping and satisfactorily describe the experimental results.

Flux and current distributions in type-II superconducting strips

Abstract Current density and penetrated magnetic flux distribution are calculated for a strip of constant thickness d that carries a time-dependent transport current I. Calculations assume a logarithmic activation energy U( j)∼ ln ( j c /j) to account for thermal activated motion of vortices, and a critical sheet current Jc(B,r)=jc(B,r)d that may be magnetic-field and position dependent ( j and jc are the current and critical current densities).

Inhomogenous current density transport properties in isotropic HTS

Isotropic HTS I–V characteristis at different sample locations have been measured, where geometry imposed an inhomogeneous current density. Above and below the intergranular glass transition, local current density scales with applied current with a local scaling factor independent of temperature.