▎ 摘　　要

Monitoring magnetoelectric transport behavior of graphene material is critical in establishing a foundation for engineering next-generation spintronic devices. Here we report a temperature-dependent crossover between positive and negative magnetoresistance (MR) in graphene nanocrystallines embedded carbon film (GNC film), two sign changes of MRTotal (where MRTotal presents the measured MR value) value from positive MRTotal(thorn) to negative MRTotal() and then back to MRTotal(thorn) are observed as temperature varies from 2 K to 400 K. The crossover can be ascribed to the competitions among different origins including wave function shrinkage, spin-dependent Coulomb blockade effect and Lorentz force, which play dominant roles in low, medium and high temperatures, respectively. Moreover, it is revealed that GNC film with smaller GNs has larger positive contribution to MRTotal behavior, resulting in maximum MRTotal(thorn) of 8.1%. In contrast, GNC film with larger GNs dramatically contributes to negative MRTotal behavior, which obtains maximum MRTotal() of -1.1%. This work deepens understanding about the origin of MR of nanometer-sized graphene, which provides guidance for further magnetoelectric property improvement, and paves a way towards specific design of graphene derived spintronic devices with controllable MR behavior. (c) 2020 Elsevier Ltd. All rights reserved.