How systems of single-molecule magnets magnetize at low temperatures

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We model magnetization processes that take place through tunneling in crystals of single-molecule magnets, such as Mn₁2 and Fe₈. 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 ∝ sqrt(t) at time t after H is applied, as observed in Fe₈, but only for 1+2₁0(hd/hw) time decades, where hd 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 (-hw,hw). However, the sqrt(t) behavior is not universal. For BCC and FCC lattices, m ∝ tᵖ, but p ≃ 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 epsilonₐ is larger than the tunneling window's energy width (i.e., if epsilonₐ gμB hw S). Thermal processes come in only later on to drive further magnetization growth.