Unconventional Phase Shift in Spin Hall Magnetoresistance of Antiferromagnetic Insulators

Antiferromagnetic spintronics offers a transformative route toward high-density and ultrafast memory technologies. However, probing and manipulating spin dynamics in antiferromagnets remain highly challenging due to their nearly vanishing net magnetization and intrinsically complex magnetic structures. A long-standing puzzle in this field is the anomalous phase shift observed in spin Hall magnetoresistance (SMR) measurements, which has recently been attributed to altermagnetic spin splitting effect. In this work, we demonstrate that such an anomalous phase shift also emerges in nonaltermagnetic materials. By investigating the microscopic interplay between the Néel order and canted spin polarization in fully epitaxial BiFeO₃/SrRuO₃ heterostructures, we provide a comprehensive explanation for this intriguing phenomenon. Through a combination of angle-dependent transport measurements and first-principles calculations, we show that the SMR signal is governed by two competing mechanisms: a robust contribution originating from the antiferromagnetic Néel vector (NSMR) and a highly temperature-sensitive component arising from ferromagnet-like canted spin polorization (PSMR) which is constrained by the symmetry-allowed Dzyaloshinskii-Moriya coupling. We further reveal that the elusive phase shift stems from the rapid enhancement of the canted spin polarization at low temperatures, which fundamentally alters the symmetry of spin-current absorption. Our findings establish a unified physical framework for disentangling complex spin interactions in antiferromagnetic materials.