Unidirectional spin Hall magnetoresistance (USMR), arising from the interaction between nonequilibrium spin accumulation and magnetization, has been proposed as a simple two-terminal method for electrically detecting magnetic states in heavy-metal/ferromagnet bilayers. However, conventional spin polarization along the $y$ direction restricts USMR to responding only to the transverse component of in-plane magnetization when a charge current is applied along the $x$ direction. As a result, the electrical readout of longitudinal magnetic states via USMR has remained elusive. Here, we overcome this limitation by demonstrating an unconventional USMR induced by an $x$-polarized spin current in (001)-oriented epitaxial IrMn/FeNi bilayers, which enables electrical detection of the longitudinal magnetic state via USMR. By applying charge currents along different crystal orientations, we show that the $x$-polarized spin current exhibits a fourfold symmetry, consistent with the crystal mirror symmetry of (001)-oriented IrMn. This unconventional spin current further generates a large spin-orbit torque efficiency, an order of magnitude higher than that reported in antiferromagnetic ${\mathrm{Mn}}_3\mathrm{Pt}$ and ${\mathrm{MnPd}}_3$ systems. Our Letter not only highlights the crucial role of intrinsic crystal symmetry in controlling spin polarization, but also extends the applicability of USMR to the electrical detection of longitudinal magnetization in two-terminal magnetic memory and spin-logic devices.

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• Crystal structure
• Spin-orbit coupling
• Spintronics
• Transport phenomena
• Magnetic thin films