How systems of single-molecule magnets magnetize at low temperatures

Abstract

We model magnetization processes that take place through tunneling in crystals of single-molecule magnets, such as Mn_12 and Fe_8. These processes take place when a field H is applied after quenching to very low temperatures. Magnetic dipolar interactions and spin flipping rules are essential ingredients of the model. The results obtained follow from Monte Carlo simulations and from the stochastic model we propose for dipole field diffusion. Correlations established before quenching are shown to later drive the magnetization process. We also show that in simple cubic lattices, m \propto sqrt(t) at time t after H is applied, as observed in Fe_8, but only for 1+2\log_10(h_d/h_w) time decades, where h_d is some near-neighbor magnetic dipolar field and a spin reversal can occur only if the magnetic field acting on it is within some field window (-h_w,h_w). However, the sqrt(t) behavior is not universal. For BCC and FCC lattices, m \propto t^p, but p \simeq 0.7 . An expression for p in terms of lattice parameters is derived. At later times the magnetization levels off to a constant value. All these processes take place at approximately constant magnetic energy if the annealing energy epsilon_a is larger than the tunneling window's energy width (i.e., if epsilon_a \gtrsim gμ_B h_w S). Thermal processes come in only later on to drive further magnetization growth.

11 LaTeX pages, 8 figures. Submitted to PRB on 5 September 2003. Accepted on 30 October 2003

References in corpus (13)

• Theory of the spin bath
• Low-Temperature Quantum Relaxation in a System of Magnetic Nanomolecules
• Observation of the distribution of molecular spin states by resonant quantum tunneling of the magnetization
• The origin of second-order transverse magnetic anisotropy in Mn12-acetate
• Crossover between Thermally Assisted and Pure Quantum Tunneling in Molecular Magnet Mn12-Acetate
• Dislocation-induced spin tunneling in Mn-12 acetate
• Ordering of dipolar Ising crystals
• Symmetry of Magnetic Quantum Tunneling in Single Molecule Magnet Mn12-acetate
• Dipolar interaction and incoherent quantum tunneling: a Monte Carlo study of magnetic relaxation
• Magnetization Process of Single Molecule Magnets at Low Temperatures
• Tunneling of a large spin via hyperfine interactions
• Quadratic transverse anisotropy term due to dislocations in Mn12-Ac directly obtained by EPR spectroscopy
• Answer to the comment of Chudnovsky: On the square-root time relaxation in molecular nanomagnets

Cited by in corpus (9)

• Landau-Zener tunneling in the presence of weak intermolecular interactions in a crystal of Mn4 single-molecule magnets
• Self-organized patterns of macroscopic quantum tunneling in molecular magnets
• Fronts of spin tunneling in molecular magnets
• Time relaxation of interacting single--molecule magnets
• Reply to the Comment on the 'Hole-digging' in ensembles of tunneling molecular magnets
• Comment on "Hole digging in ensembles of tunneling molecular magnets"
• Dipolar-controlled spin tunneling and relaxation in molecular magnets
• Magnetic Tunnel Window's Imprint and Beyond
• Reply to Comment on "Magnetization Process of Single Molecule Magnets at Low Temperatures"