The generation of spin currents and their use in controlling magnetic states are central to spintronics. In particular, out-of-plane spin polarization is highly desirable for ultrahigh density, low-power magnetic memory because it enables field-free switching of perpendicular magnetization. This can be achieved through low-symmetry crystals and symmetry-broken interfaces, which generate out-of-plane spin currents, enabling deterministic field-free switching. In this work, we investigate the spin-orbit torque (SOT) in high-quality epitaxial ${\mathrm{MnPd}}_3$ films grown on MgO substrates, with a particular emphasis on the role of crystal symmetry in generating unconventional spin polarizations. Our measurements reveal a conventional SOT arising from the $y$-polarized spin current, along with unconventional SOT originating from the $x$- and $z$-polarized spin currents. By injecting charge currents along different crystal orientations, we further reveal a strong correlation between the unconventional torques and crystal orientation. Notably, the damping-like torque generated by the out-of-plane polarized spin current is found to comprise two distinct contributions: a bulk contribution and an interfacial one. Even after the insertion of a Cu spacer layer, this significant damping-like torque efficiency persists. This work provides valuable insights for designing and optimizing spintronic devices with enhanced performance.

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• Magnetotransport
• Spin torque
• Spin-orbit torque
• Spintronics
• Antiferromagnets
• Ferromagnetic resonance
• Landau-Lifschitz-Gilbert equation
• Sputtering
• X-ray diffraction