Novel magneto-optic behaviour from a polysquaraine

Abstract

We report that poly((2,5-bis(1-methylpyrrol-2-yl)thiophene)squaraine) can be synthesized as a dark green insoluble powder which when subjected to shear force and pressed as a disk exhibits a gold-green near optical quality surface with semi-metallic behaviour. Reflectivity measurements at a wavelength of 819 nm reveal a high (72°) pseudo-Brewster angle and non-zero p-reflectivity whilst electrical measurements using a four-point probe return a conductivity of 1 × 10−5 S cm−1. Unexpectedly the disks also exhibit magneto-optic (MO) activity which it appears must arise from a weak magnetic component intrinsic to the samples. In both the longitudinal and transverse Kerr configurations large fractional changes in reflectivity (ΔI/I ≈ 2.5 × 10−2) are observed across a wide range of angles of incidence for wavelengths between 400 nm and 1064 nm on application and reversal of a magnetic field. Anomalously for these configurations all the MO effects observed are quadratic in the applied field and no first-order effects linear in applied field are observed for any state of incident polarisation. Examined using conventional magnetometry, disk samples return saturation magnetization values of 4.13 × 10−3 emu g−1 on a vibrating sample magnetometer and smaller samples similarly processed and prepared for examination by Squid magnetometer confirmatory values of 4.9 × 10−3 emu g−1. Magnetization curves from both instruments have a similar form, saturating at about 1.14 kOe, and are also in close correspondence with curves derived by plotting the magneto-optic signal as a function of field after allowance for the quadratic nature of the observed MO response. Similarly, both the magnetic and magneto-optic behaviour of all samples is isotropic in plane.

Taken together, all experimental observations on this fully organic polymer appear, surprisingly, to be commensurate with the development at room temperature of some form of magnetic state throughout very limited regions of the material. A very tentative model able to reconcile the magnetic and MO observations on the supposition that the magnetic state may be of a reduced dimensional nature is presented. We are very aware that the appearance of an intrinsic room temperature magnetism in the material studied is highly unlikely and that this interpretation of the results presented is strongly dependent on using MO evidence to support chemical analysis, which included multi-elements scans on an ICP and ESR, in precluding contamination.