UNIVERSALITY IN 2-DIMENSIONAL MAGNETIC SYSTEMS

Two-dimensional magnetic behavior in thin films and in layered magnets apparently falls into one of only two universality classes. The first is that of the 2D Ising model, characterized by a magnetization exponent beta=1/8. The second, with beta almost-equal-to 0.23, is observed in systems with planar spin symmetry. Recently it has been shown, using a modified renormalization group calculation, that beta=0.23 is a property of a finite size X Y model. Although there is no broken symmetry in this system, correlations below the Kosterlitz-Thouless transition temperature decay so slowly that a magnetization occurs in any realizable system, and beta=0.23 is a result of two-dimensional fluctuations. By considering a large number of experimental situations we demonstrate that this rule is generally obeyed, and can be used as a test to identify two-dimensional behavior.