Low-frequency spin dynamics in the orthorhombic phase of La2CuO4

An effective field theory is derived that describes the low-frequency spin dynamics in the low-temperature orthorhombic phase of La2CuO4. Restricted t o a single CuO2 layer the effective theory is a simple generalization of th e relativistic nonlinear sigma model to include all spin interactions allow ed by symmetry. Incorporating a weak interlayer interaction leads to two co upled nonlinear sigma models which provide an efficient description of the complete bilayer dynamics. Particular attention is paid to the weak-ferroma gnetic and spin-flop transitions induced by external magnetic fields. The m ain features of the observed (covert) weak ferromagnetism are thus accounte d for in a straightforward manner but some of the finer theoretical predict ions would require further experimental investigation. The derived framewor k is also suitable for the study of the structure and dynamics of magnetic domains in undoped La2CuO4.