Macroscopic quantum coherence and tunneling of an antiferromagnetic nanoparticle in a magnetic field at an arbitrarily directed angle

Abstract

Based on the two-sublattice model, we investigate the quantum tunneling behaviors of the N\'eel vector in single-domain antiferromagnetic nanoparticles placed in an external magnetic field at an arbitrarily directed angle in the ZX plane. We consider the magnetocrystalline anisotropy with biaxial, tetragonal, and hexagonal crystal symmetry, respectively. By applying the standard instanton technique in the spin-coherent-state path-integral representation, we calculate the tunneling level splittings, the tunneling rates, and the crossover temperatures in the low barrier limit for three angle ranges of the external applied magnetic field $({\ensuremath{\theta}}_{H}=\ensuremath{\pi}/2,$ $\ensuremath{\pi}\ensuremath{\ll}{\ensuremath{\theta}}_{H}\ensuremath{\ll}\ensuremath{\pi},$ and ${\ensuremath{\theta}}_{H}=\ensuremath{\pi}).$ Our results show that the tunneling level splittings, the tunneling rates, and the crossover temperatures depend on the orientation of the external applied magnetic field distinctly, which provides a possible experimental test for macroscopic quantum tunneling and coherence of the N\'eel vector in nanometer-scale single-domain antiferromagnets.