Bose-Einstein condensation of dilute magnons in TlCuCl3

The recent observation [A. Oosawa et nl., J. Phys. Condens. Matter 11, 265 (1999)] of the field-induced Neel ordering in the spin-gap magnetic compound TICuCl3 is interpreted as a Bose-Einstein condensation of magnons. A Hartree-Fock-type calculation based on this picture is shown to describe the temperature dependence of the magnetization well.