Surface magnetism: phase transitions in quantum and classical models

We study the phase diagram and multicritical behavior of the semi-infinite quantum spin 1/2 anisotropic Heisenberg and classical O(n) models. We use correlated effective field theory. The multicritical point Δc and the crossover exponent φ are obtained as a function of the value n and anisotropy parameter. We have compared the results of systems with ferromagnetic and antiferromagnetic interactions.