  A magnet with precessing magnetization pumps a spin current into adjacent
leads. As a special case of this spin pumping, a precessing macrospin
(magnetization) can assist electrons in tunneling. In small systems, however,
the Coulomb blockade effect can block the transport of electrons. Here, we
investigate the competition between macrospin-assisted tunneling and Coulomb
blockade for the simplest system where both effects meet; namely, for a single
tunnel junction between a normal metal and a metallic ferromagnet with
precessing magnetization. By combining Fermi's golden rule with magnetization
dynamics and charging effects, we show that the macrospin-assisted tunneling
can soften or even break the Coulomb blockade. The details of these effects --
softening and breaking of Coulomb blockade -- depend on the macrospin dynamics.
This allows, for example, to measure the macrospin dynamics via a system's
current-voltage characteristics. It also allows to control a spin current
electrically. From a general perspective, our results provide a platform for
the interplay between spintronics and electronics on the mesoscopic scale. We
expect our work to provide a basis for the study of Coulomb blockade in more
complicated spintronic systems.
