  The dimerized S=1/2 spins of the Cu ions in TlCuCl3 are ordered
antiferromagnetically in the presence of a field larger than about 54 kOe in
the zero-temperature limit. Within the mean-field approximation all thermal
effects are frozen out below 6 K. Nevertheless, experiments show significant
changes of the critical field and the magnetization below this temperature,
which reflect the presence of low-energetic dimer-spin excitations. We
calculate the dimer-spin correlation functions within a self-consistent
random-phase approximation, using as input the effective exchange coupling
parameters obtained from the measured excitation spectra. The calculated
critical field and magnetization curves exhibit the main features of those
measured experimentally, but differ in important respects from the predictions
of simplified boson models.
