Recently, there has been a growing interest in developing and using magneto-optical polymer nanocomposites (MO-PNCs) as efficient Faraday rotators 1,2. A common strategy to improve the Verdet constant, V, of MO-PNCs is by increasing the nanoparticle (NP) loading 3. However, since the absorption coefficient, α, also increases with the loading, the figure-of-merit, FOM = V/α, remains unchanged. In this work, we first showed that the FOM in Fe3O4-based MO-PNCs is independent of NP loading in the absence of aggregation and decreases when NPs aggregate. Secondly, we demonstrated that the FOM is strongly sensitive to the NP size and composition via doping with terbium ions (Tb3+). In the loading study, we used a polymerizable ligand to fabricate poly (methyl methacrylate) films containing 8 nm Fe3O4 NPs with high optical quality 4. When the NP loading was increased from 3.5 to 12.8 wt%, the Verdet constant of the films improved from 4500 to 13500 °/Tm whereas the FOM remained constant at 2.9 °/T. This result confirms our expectation that increasing the loading does not enhance the overall MO performance. When the NP diameter was increased from 8 to 17 nm, we observed a 7-fold enhancement in the FOM from 2.9 to 23.1 °/T (Fig. 1) which can be attributed to the NPs reaching the critical size (~20 nm) at which their magnetic domains become stable against thermal fluctuations 5. For larger sizes, however, the Verdet constants saturated while FOMs declined due to increased aggregation. The FOM was further improved to ~70 °/T by doping with the highly MO-active Tb3+ ions (Fig. 2) 6. The drop in FOM at high Tb3+ doping is likely due to the increased lattice distortions and failure to incorporate the dopants inside the NPs 7. At the optimal doping and size, the Verdet constant of 4wt% loaded MO-PNC film is 5.6×105 °/Tm which is among the highest reported in the literature, especially for moderately loaded films 1. The MO-PNCs reported in this work are promising candidates for compact room-temperature optical magnetometers that can be used for sensing and medical imaging.

References:

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