Macroscopic quantum effect in single domain antiferromagnetic particles inan external magnetic field

The effect of the external magnetic field on macroscopic quantum tunneling in biaxial single domain antiferromagnetic particles is studied with the he lp of the instanton method. It is shown that the decay rate increases with increasing external magnetic field applied along the easy axis of the magne tization, and that the tunneling splitting oscilates with respect to the ex ternal magnetic field in the direction of the medium axis. Our results may be helpful for experimental observation of the macroscopic quantum coherenc e and the tunneling decay in single domain antiferromagnetic particles.