Hyperfine-Induced Asymmetric Overhauser Field in MAPbI ₃ Thin Films

Metal halide perovskites have rapidly transformed photovoltaic and optoelectronic research and are increasingly recognized as promising materials for spin-based optical functionalities. Despite this progress, the mechanisms governing spin relaxation in these systems remain insufficiently understood. Here, we demonstrate efficient dynamic nuclear polarization (DNP) in polycrystalline MAPbI₃ films, arising from spin flip-flop processes between localized electron and hole spins and the nuclear spin bath. This polarization generates Overhauser fields up to 35 mT for ²⁰⁷Pb nuclei and 15 mT for ¹²⁷I nuclei, indicating strong coupling with nuclear spin systems. We further show that the DNP building up strongly depends on the orientation of the applied magnetic field, revealing an intrinsic asymmetry of the hyperfine interaction. This behavior is quantitatively captured by a numerical model. These findings unveil a previously unexplored regime of asymmetric hyperfine coupling in halide perovskites and establish a basis for nuclear-assisted control of spin coherence in solution-processed opto-spintronic materials.