  We studied the critical state stability in a large cubic sample of a single
crystalline La(1.85)Sr(0.15)CuO(4) for different sample orientations with
respect to the external magnetic field as well as the dynamics of the flux
jumps. It is shown that thermomagnetic avalanches develop in dynamic conditions
characterized by significantly lower magnetic diffusivity than the thermal one.
In this case, critical state stability depends strongly on cooling conditions.
We compared predictions of the isothermal model and of the model for the weakly
cooled sample with experimental results. In both models, the field of the first
flux jump decreases with an increase of sweep rate of the external magnetic
field. We also investigated the influence of external magnetic field on the
dynamics of the following stages of the thermomagnetic avalanche. It is shown
that the dynamics of the flux jumps is correlated with the magnetic diffusivity
proportional to the flux flow resistivity.
