WEAK FERROMAGNETISM AND TRICRITICALITY IN PURE LA2CUO4

New data for the field dependent magnetization M(H) of pure antiferrom agnetic La2CuO4 are fitted by a theory which treats the interlayer cou pling using mean-field theory (MFT), and includes the Dzyaloshinskii-M oriya coupling. The fits yield a novel measure of the two-dimensional (2D) staggered susceptibility chi2D(dagger), which deviates below 360 K from the 2D Heisenberg model. The fits also imply that the weak-ferr omagnetic transition below the Neel point T(N) is weakly first order, with a tricritical point close to T(N). The MFT nicely reproduces the order parameter and the cusp in chi = dM/dH.