  Graphene grain boundaries have attracted interest for their ability to host
nearly dispersionless electronic bands and magnetic instabilities. Here, we
employ quantum transport and universal conductance fluctuations (UCF)
measurements to experimentally demonstrate a spontaneous breaking of time
reversal symmetry (TRS) across individual GBs of chemical vapour deposited
graphene. While quantum transport across the GBs indicate
spin-scattering-induced dephasing, and hence formation of local magnetic
moments, below $T\lesssim 4$ K, we observe complete lifting of TRS at high
carrier densities ($n \gtrsim 5\times 10^{12}$cm$^{-2}$) and low temperature
($T\lesssim 2$ K). An unprecedented thirty times reduction in the UCF magnitude
with increasing doping density further supports the possibility of an emergent
frozen magnetic state at the GBs. Our experimental results suggest that
realistic GBs of graphene can be a promising resource for new electronic phases
and spin-based applications.
