  A theory is put forward that the electronic phase transition at 0.2 K in
Ni-doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ is result of the formation of a spin
density wave in the system of Ni impurities. The driving force for the
transition is the exchange interaction between the impurity spins and the spins
of the conduction electrons. This creates a small gap at two of the four nodes
of the superconducting gap. The effect is to reduce the thermal conductivity by
a factor of two, as observed.
