  We investigate the phase transition in a three-dimensional classical
Heisenberg magnet with planar defects, i.e., disorder perfectly correlated in
two dimensions. By applying a strong-disorder renormalization group, we show
that the critical point has exotic infinite-randomness character. It is
accompanied by strong power-law Griffiths singularities. We compute various
thermodynamic observables paying particular attention to finite-size effects
relevant for an experimental verification of our theory. We also study the
critical dynamics within a Langevin equation approach and find it extremely
slow. At the critical point, the autocorrelation function decays only
logarithmically with time while it follows a nonuniversal power-law in the
Griffiths phase.
