Spin Hall magnetoresistance (SMR), the variation in resistance of a heavy metal (HM) with the magnetization orientation of an adjacent ferromagnet (FM), has been extensively studied as a powerful probe of surface magnetic moments in a variety of magnetic materials. While conventional SMR relies on the angular rotation of an assumed rigid magnetization of fixed magnitude, we report a SMR arising from magnetic-field modulation of spin fluctuations in the FM, with its magnetization direction kept fixed parallel to the spin Hall accumulation in the HM. In contrast to conventional SMR, which scales with the magnetization and vanishes near the Curie temperature ($T_c$), such “longitudinal” SMR, though suppressed at low temperatures, becomes critically enhanced at $T_c$, reaching a magnitude comparable to conventional SMR amplitudes. Our work highlights the overlooked importance of the magnetization-magnitude degree of freedom in spin transport and opens a promising avenue for electrically detecting enhanced spin fluctuations in magnetic systems.

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• Spin Hall magnetoresistance
• Spin fluctuations
• Spintronics
• Ferromagnets
• Green's function methods
• Holstein-Primakoff method