Dynamic of single clusters by microwave assisted magnetization reversal

The aim of this project is to study magnetization reversal of individual magnetic nanoparticles under a frequency-modulated RF magnetic field pulse.

We have shown in previous work how a monochromatic RF pulse can, by means of a non-linear resonance, substantially reduce the required static field needed to reverse the magnetization of an individual nanoparticle. Evidence exists in atomic physics that similar “escape” processes can be more effectively triggered by a frequency-modulated field pulse. This projet will address the questions left unanswered by our previous works on RF-assited switching, as well as extend to the case of a frequency-modulated pulse and finite temperature.

Potential energy of a magnetic nanoparticle

Fig. 1 : Potential energy of a magnetic nanoparticle versus magnetization angle.The potential energy has two wells corresponding to two stable orientations of the magnetization. For small applied fields, one of the two wells is metastable. An additional applied alternative field HRF induces oscillations of the magnetization in the energy wells. When the frequency of HRF matches the precession frequency of the magnetization, energy can be pumped into the system. This can lead to magnetization reversal from the metastable to the stable well when the precessional damping is not too high.

The main questions we will attempt to answer are as follows : How can the frequency modulation of a pulse be tuned to optimize its efficiency for magnetization reversal ? What information can we then extract relative to magnetization dynamics ? Can the magnetization damping be better determined by means of such experiments on RF-assisted switching ? How cooperative are thermal and RF effects ? How does damping depend on the specific system and its environment ? How relevant are the various theoretical descriptions of magetization dynamics to individual nanoparticles having less than 1000 atoms ? How accurately can these models predict magnetization reversal under an applied oscillating field pulse ?

All these questions are relevant to the general understanding of the magnetization dynamics, but can be also useful for future devices using RF fields to have good control on the magnetization dynamics.

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