Dual-functional soft magnetic interface materials for quantum packaging

We report the development of soft magnetic interface materials (MIMs) designed for localized chip-scale magnetic shielding in superconducting quantum systems. These composites, composed of Mu-metal nanoparticles dispersed in a cryogenically stable epoxy matrix, are engineered to fill interfacial voids in magnetic shielding assemblies, where discontinuities can disrupt magnetic flux closure and lead to leakage. Analogous to thermal interface materials for electronic packaging, MIMs are intended to restore magnetic flux continuity by reducing interfacial magnetic reluctance. Our magnetic measurements show that these composites exhibit soft ferromagnetic behavior, with relative permeabilities reaching up to 104 at low fields and remaining above 102 at 160 A/m, a field range critical for maintaining qubit coherence. A percolation-like magnetic transition emerges near 5 vol. % filler loading under low applied fields, enabling high permeability at minimal particle concentrations. Interestingly, the apparent magnetic percolation threshold depends strongly on the applied field, appearing at higher filler loadings as the external field increases. This behavior arises from the strongly nonlinear field-dependent intrinsic permeability of the Mu-metal fillers. Additionally, the incorporation of Mu-metal fillers reduces the base polymer's coefficient of thermal expansion nearly linearly with increasing filler content, enhancing compatibility with conventional cryogenic substrates. These MIMs offer a scalable approach for integrating dual magnetic and thermal compliance into advanced quantum device packaging.