  Spin Hall magnetoresistance (SMR) has been investigated in Pt/NiO/YIG
structures in a wide range of temperature and NiO thickness. The SMR shows a
negative sign below a temperature which increases with the NiO thickness. This
is contrary to a conventional SMR theory picture applied to Pt/YIG bilayer
which always predicts a positive SMR. The negative SMR is found to persist even
when NiO blocks the spin transmission between Pt and YIG, indicating it is
governed by the spin current response of NiO layer. We explain the negative SMR
by the NiO 'spin-flop' coupled with YIG, which can be overridden at higher
temperatures by positive SMR contribution from YIG. This highlights the role of
magnetic structure in antiferromagnets for transport of pure spin current in
multilayers.
