Room temperature magnetic order in an organic magnet derived from polyaniline Naveed A. Zaidi a, * , S.R. Giblin b , I. Terry b , A.P. Monkman a a Organic Electroactive Materials Research Group, Department of Physics, University of Durham, Rochester Building, South Road, Durham DH1 3LE, UK b Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK Received 21 November 2003; received in revised form 4 May 2004; accepted 2 June 2004 Abstract 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 21 kg 21 . 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. q 2004 Elsevier Ltd. All rights reserved. Keywords: Plastic magnet; Magnetic polyaniline; TCNQ 1. Introduction For many years chemists and physicists have striven to synthesize ever more intricate molecules which would support higher and higher densities of radical, spin bearing states. The molecules being so designed that the interactions between each spin gave rise to ferromagnetic ordering. In almost all cases, this interaction has proved to be weak so that the ferromagnetic phase was only observed at very low temperatures [1–4], also as the radicals had to be generated by oxidation, great difficulties arose when trying to maintain a high packing density and degree of order, whilst permitting access of the oxidising agents into the structure to form the radicals [5]. Thus, the magnetic order measured in all these systems is very small and only observed at very low temperatures. Two recent reports have given encour- agement though: a weakly ferromagnetic phase, at room temperature, of carbon was reported by Makarova et al. in C 60 [6], and Rajca et al. reported a ferromagnetic phase below 10 K in a polymeric form of a high spin state aromatic molecule [7]. Earlier, Torrance et al. synthesised poly(1,3,5-triaminobenzene) which when oxidised with iodine was reported to show a ferromagnetic phase up to 400 8C [8]. Unfortunately, this latter work has been difficult to reproduce. Crayston et al. [9] have recently reviewed other developments in the field of organic magnets. To overcome many of the problems associated with the generation of spins via oxidation in the molecule of interest, and to produce a high spin-bearing polymer that is stable in air, our magnetic polymer has been designed with emeraldine base polyaniline (PANi) as the starting material. PANi has already attracted interest as an organic magnet because of the potentially strong exchange interactions occurring through the p-conjugated back-bone. Ito et al. [10] have already shown that there is evidence for a weak ferromagnetic exchange interaction between the spins of oxidised meta-polyaniline at low temperatures. Moreover, Wienk et al. [11] have specifically identified PANi with its meta and para-aniline oligomers as building blocks for 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.06.002 Polymer 45 (2004) 5683–5689 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ44-191-334-3558; fax: þ 44-191-334- 3585. E-mail address: n.a.zaidi@durham.ac.uk (N.A. Zaidi).