We systematically investigate the impact of spin current $\left(J_{\mathrm{s}}\right)$ direction on terahertz (THz) emission intensity in Co/Al/Pt trilayer spintronic heterostructures. Contrary to the conventional expectations that reversing the $J_{\mathrm{s}}$ direction merely inverts the THz signal polarity without altering its magnitude, we observe a striking asymmetry: Pt/Al/Co trilayers generate THz signals that are over two orders of magnitude stronger than those from Co/Al/Pt stacks with reversed layer sequence. In contrast, Co/Pt bilayers and Cu-based trilayers show no such dependence, indicating that the effect is specific to Al-based systems. Moreover, the spin attenuation length in Al differs by a factor of $\sim 17$ depending on the $J_{\mathrm{s}}$ direction, and ferromagnetic resonance (FMR) measurements further reveal a corresponding anisotropy in the Gilbert damping coefficient $\left(\alpha \right)$. Cross-sectional high-resolution transmission electron microscopy measurements confirm that directional Pt-Al alloying occurs in Pt/Al/Co but is suppressed in Co/Al/Pt, which is responsible for the observed asymmetric THz signal. Our findings demonstrate that stacking-order-dependent interfacial alloying critically governs ultrafast spin transport and THz emission, offering new strategies for engineering efficient and tunable spintronic THz emitters.

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• Spintronics
• Magnetic multilayers
• Metals
• Ferromagnetic resonance
• Terahertz techniques