  It is shown that the observed anisotropy of magnetization at high magnetic
fields in RbMnBr3 , a quasi-one-dimensional antiferromagnet on a distorted
stacked triangular lattice, is due to quantum and thermal fluctuations. These
fluctuations are taken into account in the framework of linear spin-wave theory
in the region of strong magnetic fields. In this region the divergent
one-dimensional integrals are cut off by magnetic field and the bare easy-plane
anisotropy. Logarithmical dependence on the cutoff leads to the "enhancement"
of the anisotropy in magnetization. Comparison between magnetization data and
our theory with parameters obtained from neutron scattering experiments has
been done.

Condensed Matter