The magnetization process as a response to a sweeping magnetic field in the thermal environment is investigated by using the quantum master equation. In a slow sweeping velocity region where the system shows quasi-adiabatic motion, a magnetic plateau in the magnetization process has been observed in the recent experiment of V15, [Phys. Rev. Lett. 84 (2000) 3454]. Although the experiment was explained from a view of small capacity of the phonon heat bath, i.e., the phonon bottleneck effect, we found that a magnetic plateau appears almost regardless of the properties of the heat bath in the quasi-adiabatic transition and the appearance of the plateau is quire universal. We call it 'Magnetic Foehn effect'. On the other hand, the magnetic plateau also appears in a fast sweeping velocity region by the Landau-Zener-Stückelberg mechanism which is different from the above mechanism in a slow sweeping region, and we study the crossover between them. We also propose an experiment which clarifies the structure of coupling to the thermal reservoir.
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