The ferromagnetic behaviour of conducting polymers revisited

Abstract

The magnetic properties of doped pellets of poly(3-methylthiophene) showing room temperature ferromagnetic behaviour have been discussed in a previous article. The magnetic behaviour was attributed to a weak ferromagnetic phase, due to the superexchange interaction of polarons via the dopant anions. The Dzialoshinsky-Morya interaction among canted spins was proposed to explain the ferromagnetism. In this article the main conclusions of that work concerning the magnetic behaviour are revised. The basic assumption now is that the magnetic moments are spin 1/2 polarons that can interact antiferromagnetically and/or ferromagnetically. In the small crystalline regions of the polymer, which are identified with the polymer portion that remains ferromagnetic at room temperature, the interaction gives rise to S = 0 and 1 bipolarons and the S = 1 triplet state is lower in energy. In the disordered region, disorder will prevent the complete S = 1 and 0 coupling and bands of polarons ferromagnetically and antiferromagnetically coupled will appear. Using this approach, all the magnetization data can be qualitatively explained, as well as the electron spin resonance data.

Simultaneous observation of the magnetic and electric behavior in a correlated system near a metal-semiconductor transition: ESR in pellets of conducting polymers

“…For more ordered regions or crystalline regions of larger sizes, there is a competition between localized FM coupled polarons and delocalized ones, that is, between localization and delocalization. These conclusion have been previously obtained based on magnetization and ESR data in partially reduced poly͑3-methythiophene͒ pellets 3 and doped plasticized polyaniline films. 4 The interesting thing about our data is that the fitting of the ESR data for sample 2 with two Dysonian lines enables us to follow the behavior of the microwave conductivity as a function of temperature in each phase of the sample.…”

“…Thus, the doping that transforms a conjugated polymer in a conducting polymer is generally seen as a topological doping. In a recent article 3 we have discussed the connection between the formation of defects, polarons and bipolarons, in conjugated polymers with a nondegenerate ground state, with the observed magnetic susceptibility behavior. We have concluded that our magnetization and electron spin resonance ͑ESR͒ data in conducting polymer pellets could be explained assuming ferromagnetic ͑FM͒ and antiferromagnetic ͑AFM͒ interacting spin polarons.…”

Room-temperature organic ferromagnetism in the crystalline poly(3-hexylthiophene): Phenyl-C61-butyric acid methyl ester blend film

“…Organic ferromagnets as promising magnetic materials have caught the attention of the magnetism community due to potential applications such as organic spintronic devices, with logic, communications, and data storage. Their advantages are low cost, light weight, and large area production [1][2][3][4][5]. There have been tremendous efforts to search for transition metal-free ferromagnetic organic materials and developing the theory for the designing of such materials [2, 4, 5 -8].…”

“…For example, an organic π-conjugated polymer, synthesized with large net spin quantum numbers through designing cross link and alternating connectivity of radical molecules with unequal spin quantum numbers, can order ferromagnetically at very low temperatures, below 10 K [9]. After that, significant efforts from multiple groups have been devoted to increasing the Curie temperature of synthetic organic ferromagnets up to roomtemperature and beyond, which is important for practical applications [5,6,8,10]. Up to now, there are several reported transition metal-free RT ferromagnetic conducting or semiconducting polymers, such as perchlorate ion ClO 4 -doped poly(3-methylthiophene) [5] and polyaniline:tetracyanoquinodimeth ane (PANiCNQ) [11].…”

“…After that, significant efforts from multiple groups have been devoted to increasing the Curie temperature of synthetic organic ferromagnets up to roomtemperature and beyond, which is important for practical applications [5,6,8,10]. Up to now, there are several reported transition metal-free RT ferromagnetic conducting or semiconducting polymers, such as perchlorate ion ClO 4 -doped poly(3-methylthiophene) [5] and polyaniline:tetracyanoquinodimeth ane (PANiCNQ) [11]. However, the reproducibility of organic ferromagnetism is low and the intrinsic origin of ferromagnetism is still controversial.…”