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Interactions of magnetic elements with graphene may lead to various electronic states that have potential applications.We report an in-situ experiment in which the quantum transport properties of graphene are measured with increasing cobalt coverage in continuous ultra-high vacuum environment.The results show that e-beam deposited cobalt forms clusters on the surface of graphene,even at low sample temperatures.Scattering of charge carriers by the absorbed cobalt clusters results in the disappearance of the Shubnikov-de Haas (SdH) oscillations and the appearance of negative magnetoresistance (MR)which shows no sign of saturation up to an applied magnetic field of 9 T.We propose that these observations could originate from quantum interference driven by cobalt disorder and can be explained by the weak localization theory.