Quasi-two-dimensional Bose-Einstein condensation of lattice bosons in the spin-1/2 XXZ ferromagnet K2CuF4

K2CuF4 is magnetically described as a spin-1/2, quasi-two-dimensional (2D), square-lattice XXZ ferromagnet with weak easy-plane anisotropy. The magnetic ordering for an applied magnetic field H parallel to the c axis is equivalent to the Bose-Einstein condensation (BEC) of lattice bosons, as discussed by Matsubara and Matsuda [T. Matsubara and H. Matsuda, Prog. Theor. Phys. 16, 569 (1956)]. Magnetization and specific-heat measurements were performed to obtain the temperature versus magnetic field phase diagram for H parallel to c. The phase boundary between polarized and ordered phases was found to be expressed by the power law H-c(T) - H-c(0) proportional to T-phi with exponent phi approximate to 1.0 in a wide temperature range, in agreement with the theory of quasi-2D BEC.