  A quantum thermal device based on three nearest-neighbor coupled spin-1/2
systems controlled by the magnetic field is proposed. We systematically study
the steady-state thermal behaviors of the system. When the two terminals of our
system are in contact with two thermal reservoirs, respectively, the system
behaves as a perfect thermal modulator that can manipulate heat current from
zero to specific values by adjusting magnetic field direction over different
parameter ranges, since the longitudinal magnetic field can completely block
the heat transport. Significantly, the modulator can also be achieved when a
third thermal reservoir perturbs the middle spin. We also find that the
transverse field can induce the system to separate into two subspaces in which
neither steady-state heat current vanishes, thus providing an extra level of
control over the heat current through the manipulation of the initial state. In
addition, the performance of this device as a transistor can be enhanced by
controlling the magnetic field, achieving versatile amplification behaviors, in
particular substantial amplification factors.
