The pyrite-structure transition-metal disulfide $\mathrm{Ni}{\mathrm{S}}_{2}$ is in principle a model cubic antiferromagnetic Mott insulator that can be doped through insulator-metal transitions with both electrons and holes (in ${\mathrm{Ni}}_{1--x}{\mathrm{Cu}}_{x}{\mathrm{S}}_{2}$ and ${\mathrm{Ni}}_{1--x}{\mathrm{Co}}_{x}{\mathrm{S}}_{2}$), eventually inducing superconductivity and ferromagnetism, respectively. Magnetism and transport have proven challenging to understand in $\mathrm{Ni}{\mathrm{S}}_{2}$, however. The antiferromagnetic spin structure below $\ensuremath{\sim}39\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is complex due to frustration, while unexplained weak ferromagnetism emerges below $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Surface conduction is also now understood to dominate in $\mathrm{Ni}{\mathrm{S}}_{2}$ at low temperatures, raising questions about the interpretation of decades of prior data. Here, we present a complete study of the surface magnetotransport phenomena that emerge at low temperatures in high-quality single-crystal $\mathrm{Ni}{\mathrm{S}}_{2}$, which turn out to be strikingly rich. On cooling, isotropic magnetoresistance due to a field-induced shift of the first-order weak ferromagnetic ordering transition is first uncovered, i.e., metamagnetic magnetoresistance. At lower temperatures, larger, anisotropic magnetoresistance effects arise due to distinct switching events associated with the weak ferromagnetism. Strong evidence is presented that this is due to a field-driven in-plane to out-of-plane reorientation of surface spins, likely correlated with surface steps and terraces. In-plane exchange bias accompanies these effects, further supporting this interpretation. At the lowest temperatures, the spin reorientation field eventually exceeds the 9-T measurement window, generating strongly field-asymmetric magnetoresistance. Some of these unusual phenomena also manifest in the Hall channel, culminating in a sizable anomalous Hall effect at low temperatures. These results significantly demystify recent magnetoresistance and magnetic microscopy observations in $\mathrm{Ni}{\mathrm{S}}_{2}$ crystals and nanoflakes, and constitute an important step in elucidating the complex electronic and magnetic properties of this pivotal antiferromagnetic Mott insulator.
