A Systematic Approach to Multiphysics Extensions of Finite-Element-Based Micromagnetic Simulations: Nmag

Extensions of the basic micromagnetic model that include effects such as spin-current interaction, diffusion of thermal energy or anisotropic magnetoresistance are often studied by performing simulations that use case-specific ad-hoc extensions of widely used software packages such as OOMMF or Magpar. We present the novel software framework "Nmag" that handles specifications of micromagnetic systems at a sufficiently abstract level to enable users with little programming experience to automatically translate a description of a large class of dynamical multifield equations plus a description of the system\´s geometry into a working simulation. Conceptually, this is a step towards a higher-level abstract notation for classical multifield multiphysics simulations, similar to the change from assembly language to a higher level human-and-machine-readable formula notation for mathematical terms (FORTRAN) half a century ago. We demonstrate the capability of this approach through two examples, showing 1) a reduced dimensionality model coupling two copies of the micromagnetic sector and 2) the computation of a spatial current density distribution for anisotropic magnetoresistance (AMR). For cross-wise validation purposes, we also show how Nmag compares to the OOMMF and Magpar packages on a selected micromagnetic toy system. We, furthermore, briefly discuss the limitations of our framework and related conceptual questions