Nonrelativistic and Relativistic Contributions to Spin-Momentum Locking and Spin Photogalvanic Effect in an Altermagnetic Multiferroic

Giuseppe Cuono; Subhadeep Bandyopadhyay; Amar Fakhredine; Mathews Benny; Xujia Gong; Paolo Barone; Andrea Droghetti; Carmine Autieri; Silvia Picozzi

arXiv · 2609.37763

Nonrelativistic and Relativistic Contributions to Spin-Momentum Locking and Spin Photogalvanic Effect in an Altermagnetic Multiferroic

Giuseppe Cuono·Subhadeep Bandyopadhyay·Amar Fakhredine·Mathews Benny·Xujia Gong·Paolo Barone·Andrea Droghetti·Carmine Autieri·Silvia Picozzi

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Abstract

Altermagnetic multiferroics provide a platform where non-relativistic spin-momentum locking (SML) coexists with relativistic spin textures, yet disentangling their different origins remains challenging. Here, using first-principles density-functional-theory calculations and symmetry analysis of the multipolar expansion of the spin polarization, we investigate the altermagnetic multiferroic BaCuF4. Its dominant non-relativistic altermagnetic d-wave SML originates primarily from a CuF6 octahedral rotation coupled to the polar distortion and can therefore be reversed by ferroelectric switching. We show that spin-orbit coupling generates additional spin multipoles and, depending on the Neel-vector orientation, induces spin canting and weak ferromagnetism. By exploiting the group-subgroup relations between spin groups and magnetic space groups, we then distinguish non-relativistic altermagnetic, canting-associated, and purely relativistic contributions to the SML. We further show that this hierarchy is encoded in the nonlinear spin-photogalvanic response, whose spin projection selectively probes contributions of different origins. These results establish a direct connection between the microscopic spin texture and nonlinear spin transport in altermagnetic multiferroics.

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Giuseppe Cuono, Subhadeep Bandyopadhyay, Amar Fakhredine, Mathews Benny, Xujia Gong, Paolo Barone, Andrea Droghetti, Carmine Autieri, Silvia Picozzi. 2026-09-29. Nonrelativistic and Relativistic Contributions to Spin-Momentum Locking and Spin Photogalvanic Effect in an Altermagnetic Multiferroic. https://arxiv.org/abs/2609.37763

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