Quasi Two Dimensional Magnetic Properties of the Tri-Rutile Phases MTa₂O₆, M = V, Fe, Co, Ni

Abstract

<p>The structural and magnetic properties of the tri-rutile phases MTa₂O₆ (M = V, Fe, Co, Ni) were investigated. The nuclear and magnetic structures were refined from neutron powder diffraction data. The magnetic M²⁺ ions are located in planes perpendicular to the c axis. The distance between neighboring planes is approximately 4.5 Å. As a result, these compounds are quasi two dimensional magnetic systems, because the magnetic exchange interaction between ions in (001) planes is much larger than the interaction between ions in neighboring planes.</p> <p>Evidence for important two dimensional correlations has been obtained from magnetic susceptibility and heat capacity measurements. The magnetic susceptibility of these materials is characterized by a broad maximum at a temperature substantially above the ordering temperature. The experimentally observed magnetic susceptibility was compared to the predictions for 2-d quadratic layer models. The exchange interaction was antiferromagnetic in all cases. NiTa₂O₆ and VTa₂O₆ are Heisenberg systems with S = 1 and S = 3/2, respectively. The magnetic susceptibility of CoTa₂O₆ was consistent with an S = 1/2 Heisenberg model, while FeTa₂O₆ showed Ising-like behavior with S = 11.2.</p> <p>The magnetic structures of FeTa₂O₆ and VTa₂O₆ were determined. Mössbauer effect measurements were used to distinguish between two possible structural models for FeTa₂O₆. The magnetic moments of the Fe²⁺ ions points along <110> directions. The magnetic structure of VTa₂O₆ is similar, but the V²⁺ moments are oriented along <001>.</p>