03 novembre 2011

How much do spins in a complex and disordered paramagnetic material contribute to macroscopic magnetization? Through the study of doped polyaniline samples, the SCIB/RM laboratory jointly with the SPrAM/LEMOH and the Polytechnic University of Warsaw have shown that pulsed Electron Spin Resonance (ESR) is an efficient method to “sort” and identify the different spin states.


How to quantify the magnetic properties of a sample? With a magnetometer, one sums without distinction all the contributions. ESR spectroscopy with continuous wave is better suited for paramagnetic systems, but does not permit to easily identify the spin states present in a complex material. In pulsed ESR, it is possible to measure the contributions of the different spin states – the famous S of quantum mechanics – in a univocal way.


We applied this method to new materials of interest for molecular magnets: the doped polyanilines. These polymers, mostly studied for their electrical properties, have recently been found to be interesting for their magnetic properties also. Our colleagues from Warsaw have synthesized specific polymers for several purposes:

i) to reach a high level of doping. More than 70% of the monomers carry an unpaired electron and thus an electronic spin

ii) to induce ferromagnetic coupling between the spins created by the doping.


The figure shows the spectrum corresponding to a branched polymer exhibiting several different spin states: non-coupled spins (S=1/2), but also higher spin states showing that a noticeable fraction of electrons are coupled by pairs (S=1), by 3 (S=3/2) and by 4 (S=2). The second spectrum, corresponding to a linear polymer is very different: all of the electrons are coupled but only two by two (S=1). It is thus possible to better understand the magnetic interactions that occur in the polymer and to propose novel structures in order to visualize higher spin states.


Further reading: Gosk J et al., Journal of Applied Physics 109 (2011) 074911


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