Ferromagnetism in the two dimensional t-t' Hubbard model at the VanHove density

Using an improved version of the projection quantum Monte Carlo technique, we study the square-lattice Hubbard model with nearest-neighbor hopping t and next-nearest-neighbor hopping t' by simulation of lattices with up to 20 x 20 sites. For a given R = 2t'/t, we consider that filling which leads to a singular density of states of the noninteracting problem. For repulsive interactions, we find an itinerant ferromagnet (antiferromagnet) for R = 0.94 (R = 0.2). This is consistent with the prediction of the T-matrix approximation, which sums the most singular set of diagrams.