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Super-Poissonian shot noise of squeezed-magnon mediated spin transport

The magnetization of a ferromagnet (F) driven out of equilibrium injects pure spin current into an adjacent conductor (N). Such F$|$N bilayers have become basic building blocks in a wide variety of spin based devices. We evaluate the shot noise of the spin current traversing the F$|$N interface when F is subjected to a coherent microwave drive. We find that the noise spectrum is frequency independent up to the drive frequency, and increases linearly with frequency thereafter. The low frequency noise indicates super-Poissonian spin transfer, which results from quasi-particles with effective spin $\hbar^* = \hbar (1 + δ)$. For typical ferromagnetic thin films, $δ\sim 1$ is related to the dipolar interaction-mediated squeezing of F eigenmodes.