The effect of microstructure on magnetic phase transitions in an Ising model

Magnetic phase transition of a model nanocrystalline phase is investigated using the approaches combining several computational methods: (1) the nanocrystalline microstructure is constructed using a stochastic optimization approach and Voronoi construction. Nanosized grains with high-angle grain boundaries and random crystallographic orientations are represented by Voronoi cells whose sizes have a log-normal distribution. (2) The equilibrium atomic positions are obtained by molecular dynamics simulation. (3) Critical behavior of the phase transitions in the nanostructured material is investigated using a two-dimensional Ising model. Monte Carlo simulation is performed to analyze the critical temperature and critical exponents using equilibrium cluster algorithm and nonequilibrium critical short-time dynamics. We find that the critical phenomenon does not depend on the ratio between the number of atoms inside the grains and that on the grain boundaries. However, the magnetic properties such as Curie temperature, coercivity and hysteresis loop are significantly affected by the grain boundaries.