Superposition of currents in hard superconductors placed into crossed ac and dc magnetic fields

The superposition of currents in YBCO melt-textured samples placed into crossed ac and de magnetic fields is predicted and observed. This superposition is a direct consequence of the critical state model. The de magnetic field distribution is shown to become uniform wherever the ac field has penetrated. Owing to this nonlinear process, the area of the dc magnetization loop diminishes and eventually disappears completely with an increase of the ac field magnitude. This means that under the action of the external ac field, the static magnetic properties of hard superconductors change and tend to the well known properties of soft ones.