Magnetic-field-induced crystal-hexatic-liquid-crystal transition in 2D binary crystals

We simulate two-dimensional (2D) triangular lattices composed of paramagnetic and non-paramagnetic spheres. By increasing the magnetic field, i.e., enhancing paramagnetic particles’ repulsions, randomly distributed paramagnetic particles on the initial lattice rearrange into a new triangular lattice. When the number ratio of the two types of particles cannot form a simple superlattice, i.e., the lattice spacing of paramagnetic spheres cannot well accommodate the close packed non-paramagnetic spheres, non-paramagnetic particles form a fluid instead of an amorphous solid. Thus, increasing the magnetic field, i.e., increasing the effective volume fraction, melts the initial lattice to a hexatic phase with hyperuniformity, then a liquid, and eventually re-solidifies to a crystalline skeleton of paramagnetic particles filled with fluidic non-paramagnetic particles. This novel order-disorder-order transition pathway casts light on the compositional order-disorder transitions in binary alloys and colloidal crystals.