Anomalous Chiral Exchange Drag in Synthetic Antiferromagnets

Abstract

Long-range interactions between quasiparticles give rise to a ‘drag’ that affects the fundamental properties in condensed matter physics like Coulomb drag [1]. Drag typically involves the exchange of linear momentum between quasiparticles affecting transport properties. We recently discovered a new form of drag, “Chiral Exchange Drag”, that involves the exchange of angular momentum between two current-driven magnetic domain walls (DWs) from synthetic antiferromagnets [2]. Previously we identified a powerful new torque, exchange coupling torque, in synthetic antiferromagnets that can move composite DWs very efficiently [3]. I will present our new findings that the composite DWs motions are correlated and determined by the strength of the exchange drag between component DWs. For example, when the drag is below a threshold value, the DWs move together at a constant intermediate velocity with a steady leakage of angular momentum from the faster to the slower wall. On the other hand, I will show that when the drag exceeds a threshold value, an entirely different dynamic can take place in which the faster DW’s magnetization oscillates synchronously with a precessional motion of the slower DW’s magnetization, and angular momentum is continuously transferred between them: “Chiral Exchange Drag Anomaly”. In this talk, I will discuss a new method for delivering spin angular momentum remotely to magnetic entities showing analogues with massive Dirac fermions. [1] B. N. Narozhny, and A. Levchenko, “Coulomb drag”, Rev. Mod. Phys. 88, 025003 (2016). [2] S.-H. Yang, C. Garg, and S. S. P. Parkin, “Chiral Exchange Drag and Chirality Oscillations in Synthetic Antiferromagnets”, Nat. Phys. 15, 543 (2019). [3] S.-H. Yang, K.-S. Ryu, and S. S. P. Parkin, “Domain-wall velocities of up to 750 m s− 1 driven by exchange-coupling torque in synthetic antiferromagnets”, Nat. Nano. 10, 221 (2015).