We investigated perpendicular exchange bias and spin canting phenomena in Pt/Co/Pt/Cr2O3 thin films grown on Si (111) substrates. Unusual decrease of perpendicular exchange bias has been observed below 160 K in ultrathin Co (5 angstrom) and Cr2O3 (0001) exchange coupling system. We also examined the possible deviation of antiferromagnetic spin axis from the c-axis of Cr2O3 (0001) layer by using a thicker Co (50 angstrom) film with a magnetic easy axis lies on the sample plane. The temperature-dependent exchange bias measurements of both samples exhibited that the canting of uncompensated Cr spins at the Cr2O3 surface is responsible for the unusual behavior of perpendicular exchange bias in Co (5 angstrom)/Cr2O3 (0001) ultra-thin film. Furthermore, in-plane angle (phi)-dependent exchange bias experiments with thicker Co (50 angstrom)/Cr2O3 (0001) sample indicated that neither the deviation of antiferromagnetic spin axis from the c-axis of Cr2O3 nor spin-flop transition mechanisms is present in antiferromagnetic Cr2O3 layer. We also found that uncompensated canted Cr spins at the Cr2O3 surface are magnetically disordered (paramagnetic-like) and reduce the exchange coupling between the ferromagnetic Co and antiferromagnetic Cr2O3 (0001) layers. The magnetically disordered uncompensated canted Cr spins negatively affect the perpendicular exchange bias starting from 220 K down to 160 K, and further decrease of temperature dramatically reduces perpendicular exchange bias in Pt/Co/Pt/Cr2O3 thin films. (C) 2015 Elsevier B.V. All rights reserved.