Interplay of magnetic frustrations and lattice instability Original Research Article

We consider a situation when a lattice with antiferromagnetic (AF) interactions up to third near neighbors, is nearly frustrated between two AF phases: AFI and AFII. A quantitative theory describing momentum and temperature dependence of magnetic neutron scattering intensity is presented. The theoretical approach is based on the idea of Onsager reaction field and properly describes suppression of the Néel temperature in nearly frustrated state. We consider the interaction of the spin subsystem with lattice arising due to change in spin–spin coupling constants by lattice distortions and find that the system can undergo two subsequent magnetic transitions at TN1 and TN2. First, AFI order establishes at TN1 whereas at lower temperature, TN2 system undergoes structural phase transition and the second order parameter AFII appears: both order parameters, incompatible in the cubic structure, coexist at temperatures below TN2. We compare theoretical results with experimental properties of Mott Hubbard insulator NiS2 and show that this compound is a magnetic system with strong interplay of magnetic frustrations and lattice distortions.