Magnetically coupled charge-transport crossover and giant negative magnetoresistance in iodine-incorporated Cr$_2$Se$_3$
We report the synthesis and comprehensive investigation of iodine-incorporated $Cr_2Se_3$, a non-van der Waals quasi-two-dimensional magnetic material, using structural, magnetic, transport, spectroscopic, and first-principles methods. Magnetization and electron spin resonance measurements reveal an antiferromagnetically ordered state below $T_N \approx 52$ K. At higher temperatures, a second, broader anomaly emerges near $T^* \approx 150$ K, coinciding with a shallow minimum in the temperature-dependent resistivity that resembles a metal-to-insulator-like crossover. Hall measurements indicate predominantly hole-like conduction at high temperatures ($\geq$ 150 K), while the nonlinear Hall response below T*, together with an anomaly in the third-harmonic electrical signal, suggests the emergence of mobility-dependent multichannel and spatially inhomogeneous transport. A large, non-saturating negative magnetoresistance reaching approximately -78\% at 25 K and 12 T further demonstrates strong coupling between charge transport and the magnetic state. Temperature-dependent Raman spectroscopy reveals no symmetry-changing structural transition near T*, whereas angle resolved photoemission spectroscopy (ARPES) measurements show no major reconstruction of the electronic structure occupied across the crossover. Taken together, these results identify the high-temperature anomaly as a broad magnetically coupled transport crossover arising from the interplay of short-range magnetic correlations, chemical disorder, and redistribution among competing conduction channels. These findings demonstrate that anion incorporation provides an effective route to tuning the coupled electronic and magnetic properties of non-layered Cr$_2$Se$_3$, establishing this system as a promising platform for investigating correlated transport and emergent spintronic functionalities in transition-metal chalcogenides.