  Trigonal zigzag graphene nanodisk exhibits magnetism whose spin is
proportional to the edge length of the nanodisk. Its spin can be designed from
1/2 to a huge value. The spins form a quasiferromagnet, which has intermediate
properties between a single spin and a ferromagnet. In other words, the
ferromagnet order has a relatively long life time, and yet the nanodisk spin
can be rotated by external field or current. We consider a nanodisk connected
with two leads. This system acts as a spin filter just as in a
metal-ferromagnet-metal junction. In this way we can generate a spin current.
Furthermore we can manipulate spin current by spin valve, spin switch and other
spintronic devices made of graphene nanodisks. We also show that nanodisk spins
are robust against the effect of randomness in site energy, transfer energy and
lattice defects.
