Magnetostrictive excitation of Atomic Force Microscopy cantilevers when operating in liquid environments

Resumo

Magnetostrictive excitation of Atomic Force Microscopy cantilevers when operating in liquid environments Atomic Force Microscope (AFM) has become an important tool in nano-bio-medical studies mainly due to the fact that it can be operated in water based solutions, which is the media where biological processes take place. In addition, recent developments in AFM dynamic methods, based in the simultaneous excitation of different eigenmodes or harmonics of the fundamental resonance frequency (Multifrequency AFM), have improved the acquisition of compositional information and the image contrast, and led to force reduction when imaging. Therefore, to take full advantage of these new developments it is important to find a cantilever mechanical oscillation method that is efficient and has enough strength to excite higher vibrational modes. We have developed a novel magnetostrictive actuation technique for the cantilevers used in AFM that overcomes the different drawbacks of the current methods: the generation of spurious resonances, the inefficiency in exciting high resonance modes, or the heating of the liquid media. We have performed theoretical studies, modelings by finite elements methods and experiments in order to compare different excitation techniques (acoustic, magnetic gradient, magnetic torque, Brownian motion and magnetostrictive). We have concluded that magnetostriction not only provides the highest efficiency and strength at exciting higher flexural modes but also clean spectra. Moreover, by means of etching cantilever specific areas of the magnetostrictive coating, it is possible to further increase the oscillation amplitude and efficiency of the cantilever high resonance modes. Besides, the same effect can be achieved by means of Ga+ implantation, which locally change the magnetic properties of the material and reduces, as well, the damage on the cantilever. We have also designed and built a specific liquid cell for the magnetostrictive actuation. Finally, we have developed a different and original approach to the magnetostrictive actuation by applying short magnetic pulses. This new approach reduces the thermal effect due to the small current pulses and increases the harmonics oscillation amplitudes, which is advantageous for Multifrequency AFM. Key words: SPM, Scanning Probe Microscopy, AFM, Atomic Force Microscopy, liquids, Multifrequency AFM, magnetostriction, high eigenmodes