The magnetic order of two-dimensional anisotropic antiferromagnets

We study the two-dimensional quantum Heisenberg antiferromagnet on the square lattice with easy-axis exchange anisotropy by means of Green’s function approach within random phase and Callen’s approximations. The Néel temperature T N , energy gap w 0 and staggered magnetization m are calculated. The theoretical predictions of T N and w 0 for K2NiF4, Rb2MnF4, K2MnF4, Rb2MnCl4 and (CH3NH3)2MnCl4 fit well to the measured values. The power law behavior of w ( T ) w ( 0 ) = β [ m ( T ) m ( 0 ) ] ν is also investigated. The exponents β and ν for K2NiF4 are in excellent agreement with the experimental results.