Low-frequency spin dynamics in the orthorhombic phase of La₂CuO₄

Abstract:

An effective field theory is derived that describes the low-frequency spin dynamics in the low-temperature orthorhombic phase of La₂CuO₄. Restricted to a single CuO₂ layer the effective theory is a simple generalization of the relativistic nonlinear σ model to include all spin interactions allowed by symmetry. Incorporating a weak interlayer interaction leads to two coupled nonlinear σ models which provide an efficient description of the complete bilayer dynamics. Particular attention is paid to the weak-ferromagnetic and spin-flop transitions induced by external magnetic fields. The main features of the observed (covert) weak ferromagnetism are thus accounted for in a straightforward manner but some of the finer theoretical predictions would require further experimental investigation. The derived framework is also suitable for the study of the structure and dynamics of magnetic domains in undoped La₂CuO₄.