Field-free perpendicular magnetization switching by altermagnets with collinear spin current

The generation of collinear spin current (CSC) in the out-of-plane direction, where both the propagation direction and spin-polarized direction aligned perpendicularly to the applied charge current, is crucial for efficiently manipulating systems with perpendicular magnetic anisotropy used in high-density magnetic recording. However, the efficient generation of CSC remains a challenge. In this work, based on the symmetry analysis, we propose that CSC can be effectively generated using altermagnets when the charge current is aligned along specific directions, due to spin-dependent symmetry breaking. This proposal is supported by density functional theory and Boltzmann transport equation calculations on a series of altermagnetic materials, including RuO₂, Mn₅Si₃, KRu₄ O₈, and CuF₂, where unusually large CSC is produced by the charge current along certain orientations. Furthermore, we introduce a physical quantity, the spin-splitting angle, to quantify the efficiency of CSC generated by the charge current. We find that the spin-splitting angle ranges from 0.24 to 0.57 in these altermagnets, which is significantly larger than the spin-Hall angle typically observed in the anomalous spin-Hall effect, where the spin-Hall angle is generally less than 0.1. Our findings provide an effective method for manipulating spin currents, which is advantageous for the exploration of altermagnetic spintronic devices with field-free perpendicular magnetization switching.