Controllable patterning of magnetic bits with atomically sharp boundaries.

Magnetic information bits in magnetic media are approaching their fundamental thermodynamic limits, creating an urgent need for high-precision nanoscale fabrication methods. Here we establish a proof-of-concept demonstration for fabricating, controlling, and testing closely spaced individual magnetic bits at the nanometre scale. An aberration-corrected atomic-sized electron probe directly sculpts ferrimagnetic (FM) spinel NiFe2O4 bits within an antiferromagnetic (AFM) rock-salt spacer by beam-induced phase transition. Through in situ nanobeam electron magnetic circular dichroism (EMCD), which probes the out-of-plane magnetization component, we perform single-domain magnetometry. This demonstrates detectable magnetic signals from a domain with a lateral width of ~4 nm, thermodynamically supported by the surrounding AFM matrix via exchange bias coupling. Furthermore, we achieve the deterministic writing of binary information ("1" and "0") by reversing the localized magnetization direction of individual FM bits under an applied external magnetic field. By establishing a complete, closed-loop workflow from atomic-scale fabrication to single-bit magnetic readout, this study demonstrates a pathway toward engineering future high-density magnetic architectures.