  Two-dimensional magnetic materials have recently attracted great interest due
to their unique functions as the electric field control of a magnetic phase and
the anomalous spin Hall effect. For such remarkable functions, a spin-orbit
coupling (SOC) serves as an essential ingredient. Here we report a giant
positive magnetoresistance in a layered magnetic semiconductor AgCrSe2, which
is a manifestation of the subtle combination of the SOC and Zeeman-type spin
splitting. When the carrier concentration approaches the critical value of
2.5\times10^18 cm^-3, a sizable positive magnetoresistance of ~400 % emerges
upon the application of magnetic fields normal to the conducting layers. Based
on the magneto-Seebeck effect and the first-principles calculations, the
unconventional magnetoresistance is ascribable to the enhancement of effective
carrier mass in the SOC induced J = 3/2 state, which is tuned to the Fermi
level through the Zeeman splitting enhanced by the p-d coupling. This study
demonstrates a new aspect of the SOC-derived magnetotransport in
two-dimensional magnetic semiconductors, paving the way to novel spintronic
functions.
