Angular-dependent spin tunneling in mesoscopic biaxial antiferromagnets

An imaginary-time path integral study is presented for spin tunneling in the biaxial antiferromagnetic grain with noncompensated sublattices placed in a magnetic field with an arbitrary direction in the plane of easy and medium axes. Different structures of the tunneling barriers can be generated by the magnitude and the orientation of the magnetic field. By calculating the nonvacuum instantons or bounces, we analytically obtain the dependence of decay rates from excited levels as well as ground-state levels and the temperature of the crossover from the thermal to quantum regime on the direction and strength of the field in a wide range of angles of the applied magnetic field. It is found that the WKB exponent and the crossover temperature strongly depend on the orientation of the field, which can be tested with the use of existing experimental techniques. In the large noncompensation limit, our results reduce to spin tunneling in ferromagnetic particles.