Field‐Free Exchange‐Bias Reversal in a Substrate‐Directed Self‐Assembled 2‐(1‐3)‐Type Ferrimagnet/Antiferromagnet Nanocomposite

Self‐assembled nanocomposites typically form through the phase separation of immiscible components, yet controllable synthesis with tailored functionalities remains challenging. Here, we demonstrate a substrate‐directed self‐assembled 2‐(1‐3)‐structured nanocomposite via topotactic phase separation, preserving the crystallographic orientation from the substrate. It comprises a horizontal antiferromagnetic (AFM) rocksalt layer (2) at the substrate and vertical rocksalt nanopillars (1) embedded in a ferrimagnetic (FiM) spinel matrix (3). The nanopillars arise from kinetic trapping of late‐arriving rocksalt species unable to diffuse through the growing spinel layer, forming “frozen diffusion channels” under continuous pulsed laser deposition. Importantly, the comparable magnitudes of AFM anisotropy and AFM‐FiM interfacial exchange coupling enable two phenomena: switching between coercivity enhancement and exchange bias regimes, and field‐free manipulation of AFM order. This approach offers precise control of magnetic interactions in self‐assembled nanocomposites for advanced spintronic applications.