Quantum Magnon Casimir effect

Magnons are the quanta of spin-wave excitations in magnetically ordered media. Spin-up and spin-down magnons coexist in antiferromagnets (AFs) where they act similarly as photons in many different phenomena. We find that the zero-point quantum fluctuation of magnons in AFs can play a significant role in determining the properties of magnetic thin films. When two ferromagnets are separated by an insulating AF, they can couple and exert spin torques on each other even at absolute zero temperature, thanks to the zero-point quantum fluctuation of magnons confined in the AF. This is in perfect analogy to the Casimir effect of photons confined between two plates. Moreover, we find that in a ferromagnet/AF bilayer, the magnonic Casimir effect leads to a counterintuitive temperature dependence of the overall magnetization.