Magnetic field tunability of spin polarized excitations in a high temperature magnet

Magnetic semiconductors are at the heart of modern device physics because they naturally provide a non-zero magnetic moment below the ordering temperature, spin-dependent band gap, and spin polarization that originates from exchange-coupled magnetization or an applied field creating a spin-split band structure. Strongly correlated spinel ferrites are amongst the most noteworthy contenders for semiconductor spintronics. NiFe$_2$O$_4$, in particular, displays spin-filtering, linear magnetoresistance, and wide application in the microwave regime. To unravel the spin-charge interaction in NiFe$_2$O$_4$, we bring together magnetic circular dichroism, photoconductivity, and prior optical absorption with complementary first principles calculations. Analysis uncovers a metamagnetic transition modifying electronic structure in the minority channel below the majority channel gap, exchange splittings emerging from spin-split bands, anisotropy of excitons surrounding the indirect gap, and magnetic-field dependent photoconductivity. These findings open the door for the creation and control of spin-polarized excitations from minority channel charge charge transfer in NiFe$_2$O$_4$ and other members of the spinel ferrite family.