  We present MuMax, a general-purpose micromagnetic simulation tool running on
Graphical Processing Units (GPUs). MuMax is designed for high performance
computations and specifically targets large simulations. In that case speedups
of over a factor 100x can easily be obtained compared to the CPU-based OOMMF
program developed at NIST. MuMax aims to be general and broadly applicable. It
solves the classical Landau-Lifshitz equation taking into account the
magnetostatic, exchange and anisotropy interactions, thermal effects and
spin-transfer torque. Periodic boundary conditions can optionally be imposed. A
spatial discretization using finite differences in 2 or 3 dimensions can be
employed. MuMax is publicly available as open source software. It can thus be
freely used and extended by community. Due to its high computational
performance, MuMax should open up the possibility of running extensive
simulations that would be nearly inaccessible with typical CPU-based
simulators.
