Field-free control of magnetism and electronic states in ZGNR/h-BN heterojunctions via topological line defects

The pursuit of fully spin-polarized currents remains a central challenge in spintronics. Zigzag graphene nanoribbon/hexagonal boron nitride (ZGNR/h-BN) heterojunctions offer a promising platform due to their stabilized edge states and tunable electronic properties. However, their inherent antiferromagnetic (AFM) order and limited means of external-field-free control hinder practical applications. Here, we demonstrate a deterministic strategy to achieve field-free manipulation of both magnetic order and electronic phases in ZGNR/h-BN heterojunctions by embedding pentagon-octagon (558) topological line defects. Density functional theory calculations reveal that this defect engineering spontaneously drives a transition from the pristine AFM state to a ferrimagnetic (FiM) ground state, originating from spin polarization reconstruction at the defect sites and asymmetric edge moments. Furthermore, the electronic properties exhibit programmable phase transitions: the system evolves from a conventional half-metal to a Dirac half-semimetal with Fermi velocities up to 3 × 10⁵ m s^-1, and eventually to a full metal, as the ZGNR width increases. Remarkably, shifting the defect position enables continuous modulation of the spin-down bandgap (0-0.18 eV) and can even induce a ferromagnetic state. The FiM order and half-metallicity are robust under biaxial strain within ±5%, while a transition to ferromagnetism can be triggered by specific uniaxial strains. This work establishes a versatile pathway, integrating defect engineering with heterojunction design, for the on-demand, external-field-free control of spin and charge degrees of freedom in carbon-based nanostructures, paving the way for low-power spintronic devices.