Entangled magnon-pair generation in a driven synthetic antiferromagnet

Understanding, manipulating, and using magnons---the quanta of spin waves---for energy-efficient applications is one of the primary goals of magnonics. In this paper, we consider a synthetic antiferromagnet in which one of the ferromagnetic layers is driven by spin-orbit torque. We find that under specific conditions for the magnitude of the spin-orbit torque and field, magnon pairs are spontaneously produced by quantum fluctuations in a way that is similar to Hawking pair production near black-hole horizons. One of the magnons is generated near the interface with the spacer layer in one of the magnetic layers of the synthetic antiferromagnet, while the other magnon is produced in the other magnetic layer. We compute the magnon current due to these spontaneously generated magnon pairs and estimate the temperature below which they should become observable. Additionally, we find that the magnons are entangled, which makes them interesting for future applications in quantum magnonics.