Resonant coherent quantum tunneling of the magnetization of spin- systems: Spin-parity effects

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Abstract
We perform quantum dynamical calculations to study the reversal of the magnetization for systems of a few spin- particles with a general biaxial anisotropy in the presence of an external magnetic field at T=0 and with no dissipation. Collective quantum tunneling of the magnetization is demonstrated to occur only for some specific resonant values of the magnetic field proving the invalidity of semiclassical approximations for clusters of a few spin- particles. Quantum tunneling of the magnetization direction at zero field is shown to be spin-parity dependent: whereas clusters with an odd number of particles do not exhibit quantum tunneling of the magnetization due to the degeneration between the first two levels, clusters with an even number of spins present resonant coherent quantum tunneling of the magnetization. On the contrary, resonant coherent quantum tunneling of the magnetization at finite magnetic field has no detectable parity dependence. We study the dependence of such collective tunneling of the magnetization on the magnetic field, the number of spins, and the anisotropy.

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