Antiferromagnetic states in CopMg1−pO (abstract)

Abstract

CopMg1−pO offers an excellent prototypical example for the study of antiferromagnetism in diluted fcc lattices. The system retains the same crystallographic structure (NaCl type) over the entire 0≤p≤1 range. The Co2+ ions form a randomly diluted fcc spin lattice with predominantly next-nearest-neighbor antiferromagnetic (AF) exchange coupling. No significant changes in J are expected with magnetic dilution, since the lattice parameter changes only slightly (∼1%) between p=1 and p=0. The χ(p,T) data1 for the system reveal a distinct crossover effect at p≊0.45. A similar behavior of the χ(p,T) curve was observed in an isostructural system EupSr1−pTe, and was interpreted as an AF→SG (SG=spin glass) transition.2 Recently, we have reported low-T neutron-diffraction data3 from CopMg1−pO samples with various p, showing a breakdown of the type-II AF long-range order (LRO) in the system at p≊0.47. In this paper we present the results of measurements of the temperature dependence of magnetic peak intensities and widths which provide a further evidence for a sharp LRO-SRO (short-range order) phase boundary in the system. However, neutron-diffraction data alone cannot answer the question of whether the SRO phase is a SG state. In order to obtain a closer understanding of the LRO-SRO transition in type-II antiferromagnets, we have carried out Monte Carlo simulations on diluted fcc spin arrays. Since CopMg1−pO is an Ising-like system (due to significant crystal-field anisotropy), and some other materials of current interest (e.g., EupSr1−pTe) are Heisenberg systems, we discuss this question in context of simulation data obtained from both Heisenberg and Ising spin arrays.