A highly efficient Monte Carlo method for the calculation of the density ofstates of classical spin systems is presented. As an application, weinvestigate the density of states Omega_N(E,M) of two- and three-dimensionalIsing models with N spins as a function of energy E and magnetization M. For afixed energy lower than a critical value E_{c,N} the density of states exhibitstwo sharp maxima at $M = \pm M_{sp}(E)$ which define the microcanonicalspontaneous magnetization. An analysis of the form $M_{sp}(E) \propto(E_{c,\infty}-E)^{\beta_\epsilon}$ yields very good results for the criticalexponent $\beta_\epsilon$, thus demonstrating that critical exponents can bedetermined by analysing directly the density of states of finite systems.
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