No abstract

No abstract

Superparamagnetic iron oxide particles in commercial products Resovist and SH U555C, used as magnetic resonance imaging contrast agents, are polydispersed and covered with a thick organic coating so that the average particle sizes were reported between 3 and 62nm with different definitions. It is unclear which size determines the transverse relaxation rate 1∕T2 of water suspensions of such particles. Comparing the measured 1∕T2 and average sizes of different definitions determined from magnetization curve, transmission electron microscopy image, x-ray diffraction, and dynamic light scattering, it is found that the 1∕T2 behavior is basically determined by the diameter of bare single-crystal magnetic particles having the particle-volume-weighted average volume (about 14 or 9nm for Resovist or SH U555C) and is slightly influenced by their thick organic coating. This is explained by the low partial density of the coating substance and the overwhelming water occupation adjacent to magnetic particles.

No abstract

No abstract

We study the magnetic behavior of a finite superconducting ring in the presence of a uniform applied field directed along its axis by means of the critical-state model and the minimization of magnetic energy. We systematically study the dependence of the magnetization and the ac susceptibility upon the geometry of the ring and develop an approximate analytical expression for the case of narrow rings of any aspect ratio. Besides, we show how the critical-current density of the superconductor can be obtained from magnetization measurements and conclude that ring geometry is a very convenient one for such a purpose. In particular, we present an expression for the full penetration field in the case of finite rings, which allows us to find the value of the critical current from the value of the magnetic field at just one point on the axis of the ring.

No abstract

The field amplitude and frequency dependent complex ac susceptibility χ(Hm,f) of three Y–Ba–Cu–O disks made by a top-seeded melt growth technique has been measured at 77 K with the ac field applied along the c-axis of the samples (parallel to their thickness). A procedure based on the Bean model has been developed to calculate the critical-current density Jc near the surface of the sample from the measured χ(Hm) for the case where the maximum imaginary component χ′′ is not reached.