The dependence of the critical temperature T c (l,⌬) on film thickness l and ratio of surface (J s ) to bulk (J b ) exchange interaction strengths ⌬ϵJ s /J b Ϫ1 in quantum and classical Heisenberg models are studied by using the effective-field theory within the framework of the differential operator technique. It is found that for ⌬ Ͻ⌬ c ͑where ⌬ c depends on the specific model͒, the critical temperature T c (l,⌬) of the film is smaller than the corresponding bulk critical temperature T c b of the infinite system and as l is increased, T c also increases approaching T c b for large values of l. However, in the case of ⌬Ͼ⌬ c , T c is larger than both the bulk T c b and the surface T c s critical temperatures of the corresponding semi-infinite systems and, as the film thickness l further increases, T c decreases and approaches, for large value of l, the surface magnetic transition T c s .

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