Harnessing band symmetry for giant tunneling magnetoresistance and tunable spin transport in strain-ferroelectricity coupled multiferroic tunnel junctions

Multiferroic tunnel junctions (MFTJs) exhibit rich spin transport behavior through the coupling between magnetism and ferroelectricity. While tunneling magnetoresistance (TMR) is commonly tuned by reversing the ferroelectric polarization (${P}_{z}$), frequent switching undermines device stability. Using density functional theory, we reveal that spin transport in a $\mathrm{SrRu}{\mathrm{O}}_{3}/\mathrm{BaTi}{\mathrm{O}}_{3}/\mathrm{SrRu}{\mathrm{O}}_{3}$ MFTJ model can be effectively modulated through the coupled effects of band symmetry, in-plane strain (${\ensuremath{\delta}}_{xy}$), and ${P}_{z}$, without flipping polarization. At ${\ensuremath{\delta}}_{xy}=0%$ and ${P}_{z}=11.68\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$, the TMR reaches 87.6%, surpassing the previously reported 75%. Under the conventional TMR definition, these values correspond to $\ensuremath{\sim}1413%$ and $\ensuremath{\sim}600%$, indicating a more than twofold enhancement. The tunneling spin polarization (TSP) also shifts from \ensuremath{-}80% to 57.6% as ${\ensuremath{\delta}}_{xy}$ varies from \ensuremath{-}3% to 2%, with a sign reversal near ${\ensuremath{\delta}}_{xy}\ensuremath{\sim}\ensuremath{-}2%$ and ${P}_{z}=35\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$. Detailed DFT analysis reveals that these effects originate not from dielectric effects along, but from symmetry-dependent responses of wave functions to ${\ensuremath{\delta}}_{xy}$ and ${P}_{z}$. Our work uncovers the missing symmetry factor in strain-polarization-TMR coupling and suggests a pathway toward strain-sensitive, energy-efficient spintronic and neuromorphic devices.