  We study spin models on Euclidean black hole backgrounds. These resemble the
Ising model, but are inhomogeneous with two parameters, the black hole mass and
the cosmological constant. We use Monte-Carlo methods to study macroscopic
properties of these systems for Schwarzschild and anti-deSitter black holes in
four and five dimensions for spin-1/2 and spin-1. We find in every case that
increasing the black hole mass causes the spins to undergo a second order phase
transition from disorder to order and that the phase transition occurs at
sub-Planckian black hole mass.
