  Spintronic devices using antiferromagnets (AFMs) are promising candidates for
future applications. Recently, many interesting physical properties have been
reported with AFM-based devices. Here we report a butterfly-shaped
magnetoresistance (MR) in a micrometer-sized triangular-lattice antiferromagnet
Ag$_2$CrO$_2$. The material consists of two-dimensional triangular-lattice
CrO$_2$ layers with antiferromagnetically coupled $S$ = 3/2 spins and Ag$_2$
layers with high electrical conductivity. The butterfly-shaped MR appears only
when the magnetic field is applied perpendicularly to the CrO$_2$ plane with
the maximum MR ratio ($\approx$ 15%) at the magnetic ordering temperature.
These features are distinct from those observed in conventional magnetic
materials. We propose a theoretical model where fluctuations of partially
disordered spins with the Ising anisotropy play an essential role in the
butterfly-shaped MR in Ag$_2$CrO$_2$.
