Interfacial spin-order engineering mitigates the permeability-loss trade-off in amorphous soft magnetic composites.

High magnetic permeability and low power loss are critical requirements for amorphous soft magnetic composites in high-frequency electronics. They are also typically mutually exclusive; low power loss requires insulating interfaces between magnetic particles, but conventional insulating interfaces cause magnetic dilution and decoupling, reducing permeability. Here, we introduce an interfacial spin-order engineering strategy that departs from traditional ferromagnetic interface solutions. By exploiting a thermally-electrically activated oxygen migration process, we construct a gradient oxygen-deficient functional layer between heterogeneous interfaces, enabling atomic-scale control of interfacial spin configurations. This engineered interface leads to a 150% increase in magnetic permeability and a 76% reduction in power loss, outperforming state-of-the-art counterparts. Neutron dark-field imaging provides direct, bulk-sensitive evidence that oxygen-defect-mediated spin ordering induces enlarged magnetic domains with intergranular magnetic connectivity. Mechanistically, defect-mediated super-exchange interactions reverse the spin orientation of Ce and Fe atoms at the heterointerface, transforming disordered or antiferromagnetic alignment into ferromagnetic coupling and thereby eliminating magnetic coupling barriers across particles. Our work demonstrates interfacial spin-ordering as a generalizable design principle, mitigating the long-standing permeability-loss trade-off.