Magnetic transitions in graphene derivatives

The magnetic transitions in graphene oxide (GO) have been investigated experimentally. Micron-sized GO flakes exhibit dominant diamagnetism accompanied by weak ferromagnetism at room temperature. However, when the lateral dimensions of GO flakes are reduced from micron-size to nano-size, a clear transition from dominant diamagnetism to ferromagnetism is observed. After reducing the GO chemically or thermally, the dominant magnetic properties are not altered markedly except for the gradual enhancement of ferromagnetic components. In contrast, at 2 K, significant paramagnetism is present in both the micron-sized and nano-sized GO sheets. The effects of different functional groups on magnetic transitions in graphene derivatives have been further investigated using on hydroxyl-, carboxyl-, amino- and thiolfunctionalized graphene. The results reveal that significant diamagnetism with weak ferromagnetism is present at room temperature in all of these functionalized graphene derivatives and the ability of different functional groups to introduce magnetic moments follows the order -SH > -OH > -COOH, -NH2. Notably, at 5 K, diamagnetism, paramagnetism and ferromagnetism coexist in thiol-, hydroxyland carboxyl-functionalized graphene, while amino-graphene exhibits dominant paramagnetism, analogous to the low-temperature magnetism in GO. These results indicate that diamagnetism, paramagnetism and ferromagnetism can coexist in graphene derivatives and magnetic transitions among the three states can be achieved which depend on edge states, vacancies, chemical doping and the attached functional groups. The results obtained may help settle the current controversy about the magnetism of graphene-related materials.