Determining the coercivity of anisotropic Heisenberg magnets by Monte Carlo sampling: Application to the design of permanent magnets

•New Monte Carlo sampling-based algorithm for coercivity calculation proposed.•Design of AFM-FM exchange-coupled composite magnets.•Magnetic hardening of Fe16N2 for rare-earth-free permanent magnet applications.•AFM-Fe16N2 composite magnets have large coercivity and maximum energy product.