Regensburg 2000 – scientific programme

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AM: Magnetismus

AM 9: Molekularmagnete

AM 9.3: Talk

Monday, March 27, 2000, 17:45–18:00, H22

Ac-susceptibility of Mn₁₂-acetate in the thermally-activated tunneling regime — •Teemu Pohjola^1,2 and Herbert Schoeller³ — ¹Materials Physics Laboratory, Helsinki University of Technology, Finland — ²Institut für Theoretische Festkörperphysik, Universität Karlsruhe, Germany — ³Forschungszentrum Karlsruhe, Institut für Nanotechnologie, Germany

In this work, we study the spin dynamics of Mn₁₂-acetate (Mn₁₂) molecules in the regime of thermally assisted tunneling. The interest in such molecular magnets lies in that, at low temperatures, they can display quantum tunneling of magnetization on a macroscopic scale. [1] This phenomenon is attributed to the dynamics of noninteracting spins – the ground state configuration of single molecules has a total spin S=10 and a high anisotropy barrier between opposite spin orientations. The spins may tunnel through the anisotropy barrier due to crystalline anisotropies and an external magnetic field. At not too low temperatures, the spins interact with the phonons present in the surrounding crystal and are excited to higher energies where the tunneling is enhanced. Here we take the tunneling into account exactly by diagonalizing the spin Hamiltonian and, in the leading order in the spin-phonon coupling, consider all components of the density matrix. We calculate the relaxation rates as well as the dynamic susceptibility of the molecules as a function of the external magnetic field. The relaxation rate displays a series of resonances whose width is determined by the tunneling. The resonant structure is also reflected in the susceptibility.

[1] L. Thomas et al., Nature (London), 383, 145 (1996).