  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.
