Assembly and Magnetic Bistability of Mn3O4 Nanoparticles Encapsulated in Hollow Carbon Nanofibers

Next-generation spintronic and data storage devices will be based on nanoscale functional materials such as magnetic nanoparticles. One particular challenge for harnessing the magnetic bistability, quantum tunnelling of magnetization, and quantum coherence of nanometer-sized magnetic objects is their coupling to the macroscopic world. Hollow carbon nanostructures with one macroscopic and two nanoscopic dimensions provide excellent materials to achieve this coupling, through the encapsulation and confinement of magnetic species. The insertion of magnetic nanoparticles into carbon nanostructures has been achieved mainly through the sublimation of a metal precursor, or the capillarity filling of a molten metal salt followed by pyrolysis of the encapsulated material. The main drawback of these approaches is a lack of control over the composition, size, and morphology of the nanoparticles formed inside the nanotubes. Since the functional properties of nanometer-sized magnetic objects are strongly dependent on these parameters, precise methods for encapsulation are required. The insertion of preformed nanoparticles with well-defined magnetic properties into carbon nanostructures, under conditions where their structures and properties are fully retained, could offer a powerful route for the development of novel architectures for spintronic devices. To date, the encapsulation of preformed nanoparticles has been reported only for nonmagnetic metals. Even though the size, shape, and composition of preformed nanoparticles can be effectively controlled by various preparation methods, the control of nanoparticle assemblies and their associated properties, combined with their confinement within carbon nanostructures, still remains a challenge. Here, we report the first example of the encapsulation of preformed, non-equiaxed, magnetic nanoparticles (NPs) within hollow carbon nanofibers (NFs), demonstrating the importance of the host-container internal structure on the NP assemblies and hence their collective magnetic properties. Two different types of hollow carbon nanofibers with different internal surface morphologies were employed to investigate the effects of confinement on the NP assembly and magnetic properties of the resultant hybrid nanostructures. The first type, a herringbone carbon nanofiber (CNF), comprised individual graphene layers tilted with respect to the main axis of the nanofiber, forming a uniform infinite stack (Figure 1A–C). The second type, a graphitized carbon