Macroscopic Quantum Phenomena And Topological Phase Interference Effects In Single-domain Magnets

In this work, a detailed investigation has been made on the phenomena of macroscopic quantum coherence (MQC) and tunneling (MQT) in single-domain magnetic nanoparticles and the associated topological phase interference effects, which may be helpful to further experimental studies on the magnetic quantum tunneling.The phenomena of MQT and MQC have been investigated theoretically in single-domain FM and AFM particles. Several models of MQC and MQT have been studied for single-domain FM particles in the spin-coherent-state path-integral representation. By applying the instanton technique, the tunneling rate and the ground-state tunneling level splitting have been evaluated for these models. It is noted that in these models, the external magnetic field can be varied in the range of 0 < H < Hc, which is beyond the low-barrier ( H-Hc) limit in the previous works. This provides a controllable parameter for experimental observation of the phenomena of MQT and MQC in single-domain FM particles. Based on the two-sublattice model and the instanton technique in the spin-coherent-state path-integral representation, the general formulas have been presented to evaluate the tunneling rate and the tunneling level splitting for single-domain AFM particles, without assuming a specific form of the magnetocrystalline anisotropy and the external applied magnetic field. By applying these formulas, the previous results for simple biaxial crystal symmetry have been extended to those for all the major crystal symmetries. Both the WKB exponents and the preexponential factors are evaluated in the tunneling rates and the tunneling level splittings for these systems, which will be more applicable for experimental checks.One of the most striking effects in magnetic MQC is that the tunneling behaviors seem sensitive to the parity of the total spin in single-domain magnets for some spin systems with high symmetries. A systematic investigation has been made in the topological phase interference effects for single-domainmagnets with the complex trigonal, tetragonal and hexagonal crystal symmetry by applying the effective Hamiltonian approach. The low-lying tunneling-level spectrum and the thermodynamic properties of the magnetic tunneling states have been evaluated for each kind of crystal symmetry. The effects induced by the external magnetic field applied along the medium axis or the hard axis have been studied. Possible relevance to experiments has also been discussed.Finally, the quantum tunneling behaviors of the magnetization vector (or the Neel vector) have been investigated in single-domain FM (or AFM) particles placed in an external magnetic field at an arbitrarily directed angle in the ZX plane. For single-domain FM particles, the magnetocrystalline anisotropy has the trigonal and hexagonal crystal symmetry. For AFM case, the magnetocrystalline anisotropy has the biaxial, trigonal, tetragonal and hexagonal crystal symmetry, respectively. By applying the instanton technique in the spin-coherent-state path-integral representation, the tunneling rates and the crossover temperatures have been evaluated in the low-barrier limit for three angle ranges of the external applied magnetic field It has been found that the tunneling rates and the crossover temperatures depend on the orientation of the external applied magnetic field distinctly, which provides an independent experimental test for MQT and MQC of the magnetization vector (or the Neel vector) in single-domain magnets.