Magnetic susceptibility anisotropies in a two-dimensional quantum Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interactions

The magnetic and thermodynamic properties of the two-dimensional quantum Heisenberg antiferromagnet (QHAF) that incorporates both a Dzyaloshinskii-Moriya and pseudodipolar interactions are studied within the framework of a generalized nonlinear sigma model. We calculate the static uniform susceptibility and sublattice magnetization as a function of temperature and we show that (i) the magnetic response is anisotropic and differs qualitatively from the expected behavior of a conventional easy-axis QHAF; (ii) the Neel second-order phase transition becomes a crossover, for a magnetic field B perpendicular to CuO2 layers. We provide a simple and clear explanation for all the recently reported unusual magnetic anisotropies in the low-field susceptibility of La2CuO4 [L. N. Lavrov , Phys. Rev. Lett. 87, 017007 (2001)], and we demonstrate explicitly why La2CuO4 cannot be classified as an ordinary easy-axis antiferromagnet.