Periodic boundary conditions were applied in the x and y directions. Since the materials all had different lattice constants, it was not possible to create periodic surfaces with exactly the same dimensions. The α-Fe(110) surfaces had dimensions of x = 4.8, y = 4.8 and z = 1.1 nm. The Fe 3 C(010) surfaces had dimensions of x = 4.9, y = 4.7 and z = 1.05 nm. The Fe 3 O 4 (001) surfaces had dimensions of x = 5.0, y = 5.0 and z = 1.1 nm. At the start of the simulations, two of each type of surface were separated by a 3.0 nm in the z-direction.