Longitudinal relaxation and thermoactivation of quantum superparamagnets

Abstract

The relaxation mechanisms of a quantum nanomagnet are discussed in the frame of linear response theory. We use a spin Hamiltonian with a uniaxial potential barrier plus a Zeeman term. The spin, having arbitrary $S$, is coupled to a bosonic environment. From the eigenstructure of the relaxation matrix, we identify two main mechanisms, namely, thermal activation over the barrier, with a time scale $\eival_1^{-1}$, and a faster dynamics inside the potential wells, with characteristic time $\eivalW^{-1}$. This allows to introduce a simple analytical formula for the response, which agrees well with the exact numerical results, and cover experiments even under moderate to strong fields in the superparamagnetic range. In passing, we generalize known classical results for a number of quantities (e.g., integral relaxation times, initial decay time, Kramers rate), results that are recovered in the limit $S\to \infty$.

submitted to Phys. Rev. B

References in corpus (5)

• Thermally Activated Resonant Magnetization Tunneling in Molecular Magnets: Mn_12Ac and others
• Spin Tunneling and Phonon-assisted Relaxation in Mn12-acetate
• Integral Relaxation Time of Single-Domain Ferromagnetic Particles
• Quantum Thermoactivation of Nanoscale Magnets
• The new integral relaxation time for thermal activation of spins

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• Equilibrium susceptibilities of superparamagnets: longitudinal & transverse, quantum & classical
• Non-linear response of single-molecule magnets: field-tuned quantum-to-classical crossovers
• Spin relaxation in phase space
• Nonlinear ac stationary response and dynamic magnetic hysteresis of quantum uniaxial superparamagnets