Room temperature magnetic order in an organic magnet derived from polyaniline

We report on the synthesis and characterisation of a new type of polymer, PANiCNQ produced from polyaniline (PANi) and an acceptor molecule, tetracyanoquinodimethane (TCNQ). PANiCNQ combines a fully conjugated nitrogen containing backbone with molecular charge transfer side groups and this combination gives rise to a stable polymer with a high density of localised spins which are expected to give rise to coupling. Magnetic measurements suggest that the polymer is ferri- or ferro-magnetic with a Curie temperature of over 350 K, and a maximum saturation magnetization of 0.1 JT(-1) kg(-1). Magnetic force microscopy images support this picture of room temperature magnetic order by providing evidence for domain wall formation and motion. The magnetic measurements reveal that the magnetically ordered state develops with time, taking several months to reach completion; X-ray diffraction data demonstrate that there is a concomitant evolution of the polymer chains from an amorphous state to a partially ordered form. Estimates of spin density from integrated electron spin resonance lines are larger than values obtained from the saturation magnetisation by a factor of 7 which leads us to tentatively conclude that PANiCNQ exhibits ferrimagnetic order. (C) 2004 Elsevier Ltd. All rights reserved.