Weak Ferromagnetism and Tricriticality in PureLa2CuO4

New data for the field dependent magnetization $M\left(H\right)\mathrm{}$ of pure antiferromagnetic ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$ are fitted by a theory which treats the interlayer coupling using mean-field theory (MFT), and includes the Dzyaloshinskii-Moriya coupling. The fits yield a novel measure of the two-dimensional (2D) staggered susceptibility ${\chi }_{2\mathrm{D}}^{†}$, which deviates below 360 K from the 2D Heisenberg model. The fits also imply that the weak-ferromagnetic transition below the Néel 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 $\chi =\frac{\mathrm{dM}}{\mathrm{dH}}$.