Magnetic optical rotary dispersion and magnetic circular dichroism in methylammonium lead halide perovskites

Abstract Large magnetic optical rotary dispersion (Faraday rotation) has been demonstrated recently in methylammonium lead bromide. Here, we investigate the prospect of extending the active spectral range by altering the halogen. We also investigate the origins of large Faraday rotation in these diamagnetic materials using magnetic circular dichroism (MCD) spectroscopy and the Kramers–Kronig relations. We find that, while MAPbCl 3 (MA = methylammonium) single crystals exhibit a large Verdet constant in the blue, no appreciable Faraday rotation is observed in the red/near infra‐red for MAPbI 3 single crystals. However, in all film samples, we find clear evidence of large MCD resulting from the Zeeman splitting of the highly resonant 1s exciton state. Our Kramers–Kronig calculations of Faraday rotation based on MCD data matches well with the dispersion of our experimental data for MAPbCl 3 and MAPbBr 3 , with some deviation in magnitude—demonstrating the excitonic nature of Faraday rotation in these materials. However, our calculations predict significant Faraday rotation in MAPbI 3 , contrary to our experimental results, indicating a potential discrepancy between the properties of the thin film and single crystal.