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 BEC theory based on the Hartree-Fock approximation with realistic dispersion relations. The phase boundary can be described by the power law [HN(T)-Hc] propto Tᵖʰⁱ. With decreasing fitting temperature range, the critical exponent phi decreases and converges at phiBEC =3/2 predicted by the magnon BEC theory.