Three-dimensional ising model in the fixed-magnetization ensemble: A MonteCarlo study

We study the three-dimensional Ising model at the critical point in the fix ed-magnetization ensemble, by means of the recently developed geometric clu ster Monte Carlo algorithm. We define a magnetic-field-like quantity in ter ms of microscopic spin-up and spin-down probabilities in a given configurat ion of neighbors. In the thermodynamic limit, the relation between this fie ld and the magnetization reduces to the canonical relation M(h). However, f or finite systems, the relation is different. We establish a close connecti on between this relation and the probability distribution of the magnetizat ion of a finite-size system in the canonical ensemble.