Thermomagnetic avalanches are commonly observed in hard type-II superconductors. From the viewpoint of practical applications of type-II superconductors thermomagnetic avalanches are problematic, because they may drive the superconducting sample into a normal or, at least, into a resistive state. The dynamics of the thermomagnetic avalanche depends on the processes of thermal and magnetic diffusion. The equations describing dynamics of the thermomagnetic avalanche are nonlinear, because many of the parameters influencing diffusion processes changes nonlinearly with temperature and with the magnetic field. At certain conditions, this may lead to unusual behavior of the thermomagnetic avalanche dynamics, e.g. to the development of dendritic flux structures in superconducting films [1].

In present work we have investigated dynamics of thermomagnetic avalanches in melt-textured YBaCuO samples. Our experiments were performed by using a miniature Hall probe placed on the surface of the investigated sample. Samples of different shapes were investigated. We have found that in melt-textured YBaCuO superconductor the thermomagnetic avalanche is accompanied by giant convergent oscillations of the surface induction. Similar experiments we have performed on other type-II superconductors. Convergent oscillations of the surface induction were also observed in the case of a slab of conventional NbTi superconductor. In the case of the slab of Bi₂Sr₂CaCu₂O_8+δ high temperature superconductor the dynamics of the thermomagnetic avalanche is also complex, consisting of two processes of different time scales. However, in this case no convergent oscillations were found.

[1] T.J. Johansen, et al., Europhys. Lett. 59 (2002) 599.

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1. The impedance of the melt-textured YBaCuO superconducting slab in high dc magnetic field