On Magnon Superfluidity in Ferromagnetic Films
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More than ten years ago, Bose-Einstein condensation (BEC) of magnons (or quantized spin waves) was experimentally observed in yttrium iron garnet (YIG) films at room temperature. Since BEC and superfluidity are closely related phenomena, it is natural to ask whether such magnon condensates can transport as superfluid and, if so, how such superfluid looks like. In this work, we study theoretically superfluidity of magnons in ferromagnetic films. We first give an review of the basic theory of magnons in ferromagnetic films. We then discuss BEC of magnons from both experimental and theoretical points of view. Then we study superfluidity of magnons in ferromagnetic films by starting from a model of spins in ferromagnetic films. Model in terms of magnon operators is then introduced, and a Hamiltonian describing the condensed magnons is derived. Focusing on the one-dimensional (1D) stationary case, we study behaviors of superfluid formed by the condensed magnons. We found an unconventional soliton-like profile of the magnon superfluid, as compared to a uniform superflow, which we argued to be due to the dipolar interaction. We also show by estimates that in YIG films it is possible to have a superfluid current that strongly exceeds the current of normal magnons, so the magnon superfluidity could possibly be observed.
Sun, Chen (2018). On Magnon Superfluidity in Ferromagnetic Films. Doctoral dissertation, Texas A & M University. Available electronically from