The use of molecular spin state as a quantum of information for storage, sensing and computing has generated considerable interest in the context of next-generation data storage and communication devices(1,2), opening avenues for developing multifunctional molecular spintronics(3). Such ideas have been researched extensively, using single-molecule magnets(4,5) and molecules with a metal ion(6) or nitrogen vacancy(7) as localized spin-carrying centres for storage and for realizing logic operations(8). However, the electronic coupling between the spin centres of these molecules is rather weak, which makes construction of quantum memory registers a challenging task(9). In this regard, delocalized carbon-based radical species with unpaired spin, such as phenalenyl(10), have shown promise. These phenalenyl moieties, which can be regarded as graphene fragments, are formed by the fusion of three benzene rings and belong to the class of open-shell systems. The spin structure of these molecules responds to external stimuli(11,12) (such as light, and electric and magnetic fields), which provides novel schemes for performing spin memory and logic operations. Here we construct a molecular device using such molecules as templates to engineer interfacial spin transfer resulting from hybridization and magnetic exchange interaction with the surface of a ferromagnet; the device shows an unexpected interfacial magnetoresistance of more than 20 per cent near room temperature. Moreover, we successfully demonstrate the formation of a nanoscale magnetic molecule with a well-defined magnetic hysteresis on ferromagnetic surfaces. Owing to strong magnetic coupling with the ferromagnet, such independent switching of an adsorbed magnetic molecule has been unsuccessful with single-molecule magnets(13). Our findings suggest the use of chemically amenable phenalenyl-based molecules as a viable and scalable platform for building molecular-scale quantum spin memory and processors for technological development.