Cryogenic dissipation-noise coupling in a Fe-based nanocrystalline soft magnetic alloy: A loss-separation framework with fluctuation analysis and domain-wall insights

Understanding dissipation-noise coupling in soft magnets at cryogenic temperatures is critical for quantum sensing and space technologies. Yet a unified description of the coupled temperature dependence of loss and intrinsic noise across 77–300 K remains lacking. Here, we investigate a Fe-based nanocrystalline soft magnetic alloy using a reduced effective loss-separation model incorporating thermal activation. The model reproduces experimental losses with high fidelity (R 2 > 0.98) and shows that dissipation evolves through the interplay of a domain-wall-mediated effective loss channel and an explicitly resolved classical-eddy-current-like effective contribution governed by temperature-dependent electrical transport. By applying the fluctuation–dissipation theorem to the temperature-dependent effective loss model, we predict a strongly non-monotonic intrinsic magnetic-noise peak near 194 K. This anomaly arises from competition between the cryogenic enhancement of domain-wall-mediated effective dissipation and the linear reduction of the k B T thermal prefactor. Lorentz TEM observations of irreversible domain-wall depinning at cryogenic temperatures support enhanced pinning and provide microscopic evidence for the increased domain-wall-mediated effective dissipation. These results establish a unified reduced effective dissipation-fluctuation framework for cryogenic soft magnets and identify a temperature regime where intrinsic thermal noise may temporarily limit device sensitivity during cooldown