The two-dimensional XY and Heisenberg models are studied with a quantum-mechanical generalization of
the Niemeijer and Van Leeuween renormalization-group method. In second-order cumulant expansion a fixed
point exists for the XY model, giving a critical temperature at J/kTc ~ 0.87 +- 0.05. For the Heisenberg model we find a fixed point at K* = 2.8. This result may not be reliable, since the truncation of the cumulant expansion is only valid for small K.