For 2D systems, the transition temperature depends on the spin dimensionality ( n ). In system with n = 1 , the planar spins can be oriented either in or out of plane. A spin dimensionality of two means that the spins are free to point in any direction parallel to the plane. A system with a spin dimensionality of three means there are no constraints on the direction of the spin. A system with n = 1 is described by the 2D Ising model. Onsager's solution to the model demonstrates that T C > 0 , thus allowing magnetism at obtainable temperatures. On the contrary, an infinite system where n = 3 , described by the isotropic Heisenberg model, does not display magnetism at any finite temperature. The long range ordering of the spins for an infinite system is prevented by the Mermin-Wagner theorem stating that spontaneous symmetry breaking required for magnetism is not possible in isotropic two dimensional magnetic systems. Spin waves in this case have finite density of states and are gapless and are therefore easy to excite, destroying magnetic order. Therefore, an external source of magnetocrystalline anisotropy, such as external magnetic field, or a finite-sized system is required for materials with n = 3 to demonstrate magnetism.