Proton orbital coupling induced magnetic anisotropy in perovskite single crystals: Demonstration with Faraday rotation effect in deuterated crystals

In this report, eight halide-based perovskite single crystals high in crystallinity with cations of methylammonium, formamidinium, and cesium are grown at low temperature via a modified solvent evaporation method or an inverse temperature crystallization technique to investigate the Faraday rotation effect. The crystals are examined using various techniques to ensure the pure single crystal phase. Among the crystals investigated, only the MAPbCl₃ and MAPbBr₃ crystals display the Faraday rotation effect in a broad visible range. Superconducting quantum interference device measurements conducted at room temperature show diamagnetic behavior of all crystals. Contaminations from magnetic elements, which could possibly induce magnetic anisotropy, are excluded from the secondary-ion mass spectrometry results. The experimental results conclude that the spin-orbital coupling of the lead ion is unlikely to be responsible for the observed Faraday rotation effect. The combined effect of the proton orbital-orbital interactions in the CH₃NH₃ (MA) cation under optical excitation and confinement of the unit cell is proposed as the underlying mechanism for the observed magnetic anisotropy. Furthermore, conclusive results from Faraday rotation measurements conducted on the deuterated d3-CH₃ND₃PbX₃ (X = Cl, Br) and d3-CD₃NH₃PbCl₃ single crystals demonstrate the involvement of the protons in the MA cation in the Faraday rotation effect; however, not all protons contribute equally to magnetic anisotropy.