Magnetic Properties of LnOFeAs Compounds from Anisotropic Heisenberg Model

In the present paper a ground state anisotropic Heisenberg Hamiltonian is presented to study the magnetic properties of LnOFeAs (also called ‘1111’) compounds. In absence of any general consensus on the ground state Hamiltonian of iron pnictides, the present Hamiltonian emphasize the role of Fe–As interaction and its role on the magnetic properties of the iron pnictides. Based on the model an expression is obtained for sub lattice magnetization and Neel temperature (TN) using Green’s function technique within random phase approximation. In the present model first nearest neighbours (J1), second nearest neighbours (J2), third nearest neighbours (J3) exchange coupling along with out-of-plane Fe–As exchange coupling (J⊥) is considered. Using the expression for Neel temperature, variation of Neel temperature (TN) with J⊥has been studied. The effect of J⊥on the sublattice magnetization is also studied. For realistic values of exchange coupling constants, Neel temperature of LnOFeAs system (Ln = La, Pr, Ce) has been evaluated with and without out-of-plane exchange coupling. The theoretical results with out-of plane exchange couplings are found in good agreement with experimental results.