  An exact four-dimensional black hole solution of gravity with a minimally
coupled self-interacting scalar field is reported. The event horizon is a
surface of negative constant curvature enclosing the curvature singularity at
the origin, and the scalar field is regular everywhere outside the origin. This
solution is an asymptotically locally AdS spacetime. The strong energy
condition is satisfied on and outside the event horizon. The thermodynamical
analysis shows the existence of a critical temperature, below which a black
hole in vacuum undergoes a spontaneous dressing up with a nontrivial scalar
field in a process reminiscent of ferromagnetism.
