This study investigates the giant magnetothermal resistance (GMTR) effect in a fully-bcc epitaxial $\mathrm{C}{\mathrm{o}}_{50}\mathrm{F}{\mathrm{e}}_{50}$/Cu multilayer through both experimental and theoretical approaches. The applied magnetic field results in a giant change of the cross-plane thermal conductivity (Δκ) of 37 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, which reaches 1.5 times larger than the previously reported value for a magnetic multilayer and records the highest value at room temperature among the other solid-state thermal switching materials working on different principles. We investigated the electron thermal conductivity for exploring the remarkable Δκ by the two-current-series-resistor model combined with the Wiedemann-Franz law. However, the result shows the electron contribution accounts for only 35% of the Δκ, indicating the presence of additional spin-dependent heat carriers. Further investigation of the lattice thermal conductivity, which is expected to be spin independent, using nonequilibrium molecular dynamics simulations suggests a striking contrast: the additional spin-dependent heat carrier contribution is significantly enhanced in the parallel magnetization configuration but nearly negligible in the antiparallel configuration. These findings provide a fundamental insight into the origin of large GMTR effect and highlight its potential of active thermal management technologies for future electronic devices.

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• Giant magnetoresistance
• Heat transfer
• Magnetotransport
• Spintronics
• Thermal conductivity