  Plasmons, which are collective charge oscillations, offer the potential to
use optical signals in nano-scale electric circuits. Recently, plasmonics using
graphene have attracted interest, particularly because of the tunable plasmon
frequency through the carrier density $n$. However, the $n$ dependence of the
plasmon velocity is weak ($\propto n^{1/4}$) and it is difficult to tune the
frequency over orders of magnitude. Here, we demonstrate that the velocity of
plasmons in graphene can be changed over two orders of magnitude by applying a
magnetic field $B$ and by the presence/absence of a gate; at high $B$, edge
magnetoplasmons (EMPs), which are plasmons localized at the sample edge, are
formed and their velocity depends on $B$ and the gate screening effect. The
wide range tunability of the velocity and the observed low-loss plasmon
transport encourage designing graphene nanostructures for plasmonics
applications.
