Chirality as generalized spin–orbit interaction in spintronics

Chirality or handedness distinguishes an object from its mirror images, such as the spread thumb, index finger, and middle finger of the right and left hand. In mathematics, it is described by the outer product of three vectors that obey a right-hand vs. left-hand rule. The chirality of ground state magnetic textures defined by the vectors of magnetization, its gradient, and an electric field from broken inversion symmetry can be fixed by a strong relativistic spin–orbit interaction. This review focuses on the chirality observed in the excited states of the magnetic order, dielectrics, and conductors that hold transverse spins when they are evanescent. Even without any relativistic effect, the transverse spin of the evanescent waves is locked to the momentum and the surface normal of their propagation plane. This chirality thereby acts as a generalized spin–orbit interaction, which leads to the discovery of various chiral interactions between magnetic, phononic, electronic, photonic, and plasmonic excitations in spintronics that mediate the excitation of quasiparticles into a single direction, leading to phenomena such as chiral spin and phonon pumping, chiral spin Seebeck, spin skin, magnonic trap, magnon Doppler, chiral magnon damping, and spin diode effects. Intriguing analogies with electric counterparts in the nano-optics and plasmonics exist. After a brief review of the concepts of chirality that characterize the ground state chiral magnetic textures and chirally coupled magnets in spintronics, we turn to the chiral phenomena of excited states. We present a unified electrodynamic picture for dynamical chirality in spintronics in terms of generalized spin–orbit interaction and compare it with that in nano-optics and plasmonics. Based on the general theory, we subsequently review the theoretical progress and experimental evidence of chiral interaction, as well as the near-field transfer of the transverse spins, between various excitations in magnetic, photonic, electronic and phononic nanostructures at GHz time scales. We provide a perspective for future research before concluding this article.