Preferential Orientation Response Switching of Optically Pumped Multiferroic Nickel Iodide

Nickel Iodide (NiI₂) has attracted attention in the two dimensional magnetism community due to its Multiferroicity below 60K, observed down to the two layer limit. Here we demonstrate the changing response of NiI₂ to optical pumping of magnetic states, utilizing pump-probe measurements of the Magneto Optical Kerr Effect (MOKE) and Dichroism responses. The MOKE response of bulk NiI₂ crystal demonstrates a dependence on the electric field orientation of the pump pulse, with a C2 symmetry at all temperatures measured, from 4K to 80K. The crystal orientation in which the pump produces the maximum MOKE signal changes abruptly when the sample is cooled below the multiferroic transition, and again, unexpectedly, at a lower temperature. At this lower temperature, there is no reported transition in the magnetic ordering of NiI₂. Additionally, when measuring the MOKE response as a function of pump-probe delay, at both transitions we see additional frequencies appearing in the Fourier transform of the temporal response. Future MOKE and pumped THz measurements could yield insight into the coupling between the coexisting electric dipole and magnetic states of NiI2, providing more insight into the exact mechanisms making NiI₂ a low dimensional Multiferroric.