Bose-Einstein Condensation of Dilute Magnons in ${\mathrm{TlCuCl}}_3$

The recent observation [A. Oosawa et al., J. Phys. Condens. Matter 11, 265 (1999)] of the field-induced Néel ordering in the spin-gap magnetic compound ${\mathrm{TlCuCl}}_3$ 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.