  Spin Hall magnetoresistance (SMR) refers to a resistance change in a metallic
film reflecting the magnetization direction of a magnet attached to the film.
The mechanism of this phenomenon is spin exchange between conduction-electron
spins and magnetization at the interface. SMR has been used to read out
information written in a small magnet and to detect magnetization dynamics, but
it has been limited to magnets; magnetic ordered phases or instability of
magnetic phase transition has been believed to be indispensable. Here, we
report the observation of SMR in a paramagnetic insulator
Gd$_{3}$Ga$_{5}$O$_{12}$ (GGG) without spontaneous magnetization combined with
a Pt film. The paramagnetic SMR can be attributed to spin-transfer torque
acting on localized spins in GGG. We determine the efficiencies of spin torque
and spin-flip scattering at the Pt/GGG interface, and demonstrate these
quantities can be tuned with external magnetic fields. The results clarify the
mechanism of spin-transport at a metal/paramagnetic insulator interface, which
gives new insight into the spintronic manipulation of spin states in
paramagnetic systems.
