Quantum Nucleation of Antiferromagnetic Bubbles with Tetragonal and Hexagonal Symmetries

We study the quantum nucleation in a nanometer-scale antiferromagnetplaced in a magnetic field at an arbitrary angle.We consider the magnetocrystalline anisotropy with tetragonal symmetryand that with hexagonal symmetry, respectively.Different structures of the tunneling barriers can be generated by themagnitude and the orientation of the magnetic field.We use the instanton method in the spin-coherent-state path-integralrepresentation to calculate the dependence of the rate of quantumnucleation and the crossover temperature on the orientation and strengthof the field for bulk solids and two-dimensional films of antiferromagnets,respectively.We find that the rate of quantum nucleation and the crossovertemperature from thermal-to-quantum transitions depend on the orientationand strength of the external magnetic field distinctly, which can be testedby use of existing experimental techniques.