Multi-frequency fluxgate magnetic force microscopy
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In the recent years, progress in atomic force microscopy (AFM) led to the multifrequency imaging paradigm in which the cantilever-tip ensemble is simultaneously excited by several driving forces of different frequencies. By using multifrequency excitation, various interaction forces of different physical origin such as electronic interactions or chemical interactions can be simultaneously mapped along with topography. However, a multifrequency magnetic imaging technique has not been demonstrated yet. The difficulty in imaging magnetic forces using a multifrequency technique partly arises from difficulties in modulation of the magnetic tip-sample interaction. In the traditional unmodulated scheme, measurement of magnetic forces and elimination of coupling with other forces is obtained in a double pass measurement technique where topography and magnetic interactions are rapidly measured in successive scans with different tip-sample separations. This measurement scheme may suffer from thermal drifts or topographical artifacts. In this work, we consider a multifrequency magnetic imaging method which uses first resonant flexural mode for topography signal acquisition and second resonant flexural mode for measuring the magnetic interaction simultaneously. As in a fluxgate magnetometer, modulation of magnetic moment of nickel particles attached on the apex of AFM tip can be used to modulate the magnetic forces which are dependent on external DC fields through the nonlinear magnetic response of the nickel particles. Coupling strength can be varied by changing coil current or setpoint parameters of Magnetic Force Microscopy (MFM) system. Special MFM tips were fabricated by using Focused Ion Beam (FIB) and magnetically characterized for the purpose of multifrequency imaging. In this work, the use of such a nano-flux-gate system for simultaneous topographic and magnetic imaging is experimentally demonstrated. The excitation and detection scheme can be also used for high sensitivity cantilever magnetometry.