  We investigate the observation of negative spin Hall magnetoresistance (SMR)
in antiferromagnetic Cr2O3/Ta bilayers at low temperature. The sign of the SMR
signals is changed from positive to negative monotonously from 300 K to 50 K.
The change of the signs for SMR is related with the competitions between the
surface ferromagnetism and bulky antiferromagnetic of Cr2O3. The surface
magnetizations of Cr2O3 (0001) is considered to be dominated at higher
temperature, while the bulky antiferromagnetics gets to be robust with
decreasing of temperature. The slopes of the abnormal Hall curves coincide with
the signs of SMR, confirming variational interface magnetism of Cr2O3 at
different temperature. From the observed SMR ratio under 3 T, the spin mixing
conductance at Cr2O3/Ta interface is estimated to be 1.12*10^14 (ohm^-1*m^-2),
which is comparable to that of YIG/Pt structures and our early results of
Cr2O3/W. (Appl. Phys. Lett. 110, 262401 (2017))
