  We study the 1-d isotropic Heisenberg model of two spin-1/2 systems as a
quantum heat engine. The engine undergoes a four-step Otto cycle where the two
adiabatic branches involve changing the external magnetic field at a fixed
value of the coupling constant. We find conditions for the engine efficiency to
be higher than the uncoupled model; in particular, we find an upper bound which
is tighter than the Carnot bound. A new domain of parameter values is pointed
out which was not feasible in the interaction-free model. Locally, each spin
seems to effect the flow of heat in a direction opposite to the global
temperature gradient. This seeming contradiction to the second law can be
resolved in terms of local effective temperature of the spins.
