MuMax: A new high-performance micromagnetic simulation tool. 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 100 ×× can 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 two or three 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