Magnetic-field induced Bose-Einstein condensation of magnons and critical behavior in interacting spin dimer system TlCuCl3

Magnetization measurements were performed to investigate the critical behavior of the field-induced magnetic ordering in gapped spin system TlCuCl3. The critical density of the magnons was obtained as a function of temperature and the magnon-magnon interaction constant was evaluated. The experimental phase boundary for T < 5 K agrees almost perfectly with the magnon Bose-Einstein condensation (BEC) theory based on the Hartree-Fock approximation with realistic dispersion relations. The phase boundary can be described by the power law [H-N(T)-H-c] proportional to T-phi. With decreasing fitting temperature range, the critical exponent phi decreases and converges at phi(BEC) = 3/2 predicted by the magnon BEC theory.