Investigation of the high temperature magnetic transition in CuTCNQ Phase II

Copper-7,7,8,8-tetracyanoquinodimethane (CuTCNQ) is a material that has attracted much interest for its electrical resistance switching. The material is a charge transfer salt consisting of a conducting, kinetic phase ("phase I") and an insulating, thermodynamic phase ("phase II"), both of which have been proposed to be involved with the resistive switching process. In addition to the interesting electrical properties, both phases exhibit quite different magnetic properties, with the phase 2 material showing a well-defined magnetic ordering below 11K, measurements suggesting antiferromagnetism with a small ferromagnetic component, possibly from a small deviation from the collinearity of the free radical electrons' magnetic moments. Earlier zero field (ZF) muon spin spectroscopy (MSS) measurements by us have revealed possible evidence of a second, weak, magnetically ordered component within the material, that may exist between 11K and at least 150K, but not at 300K. However, this magnetic state may in fact be nuclear in origin, though close inspection of magnetic susceptibility data suggest a subtle change in the electronic magnetic response between 200K and 300K. We would therefore like to measure ZF MSS at ten temperatures between 250K and 320K to confirm the existence of this apparent high temperature magnetically ordered phase, investigate the point at which is suppressed and determine whether it has a nuclear origin.