Title: Magnon spin capacitor

Abstract: In this work we show that a magnon spin capacitor can be realized at a junction between two exchange coupled ferromagnets. In this junction, the buildup of magnon spin over the junction is coupled to the difference in magnon chemical potential, realizing the magnon spin analogue of an electrical capacitor. The relation between magnon spin and magnon chemical potential difference directly follows from considering the magnon density-density interaction between the two ferromagnets. We analyse the junction in detail by considering spin injection and detection from normal metal leads, the tunneling current across the junction and magnon decay within the ferromagnet, showing that such a structure realizes a magnon spin capacitor in series with a spin resistor. Choosing yttrium iron garnet as the ferromagnet, we numerically calculate the capacitance, which ranges from picofarad to microfarad, depending on the area of the junction. We therefore conclude that the magnon spin capacitor could directly be of use in applications.