The application of vacuum SPM for non-contact measurements considerably enhances the sensitivity of electrostatic and magnetic interactions. The improved sensitivity is accomplished as a result of increase in cantilever quality factor, or Q-factor, in vacuum environment. Q-factor increases by over 10 times at a pressure of less than 10-1 Torr, which can be achieved even by applying vacuum pump. However, following increase in the vacuum level, there is a gradual change in the cantilever Q-factor. Solver HV and NTEGRA Aura from NT-MDT enable measurements to be performed in vacuum below the pressure of 10-1 Torr.
Preparation and Characterization of Sample
As illustrated in Figure 1, the sample used in the experiments is ferromagnetic particles ordered array, with the following parameters: height of 7 nm, 120 nm period, and diameter of ~35–40 nm. Electron beam lithography was adopted to make such an array on the CoPt film of a height of 7 nm with perpendicular magnetic anisotropy.
Figure 1. SEM image of sample.
Figure 2 illustrates the sample MFM image acquired by one-pass technique that enables the MSM image to be obtained immediately after the first pass. In this case, the magnetic measurements are carried out at a specific Z-scanner position without feedback control. (A standard two-pass technique involves carrying out topography measurements during first pass and long-range interaction during second pass). The benefit of adopting one-pass technique is the absence of tip-sample contact, thereby minimizing the possibility of undesired reversal magnetization at the time of scanning. Hence, the preliminary adjustment of sample slope is mandatory for this technique to minimize the difference in tip-sample separation at different X and Y positions. This can be easily performed by measuring the adjustment of head legs.
Figure 2. MFM image of sample.
Figure 3 depicts the MFM image obtained at different distances between the tip and the sample. The bright spots indicate repulsion force when the direction of tip magnetization is opposite to that of the magnetic particle. The dark spots indicate attraction force near particles, where magnetization is aligned in the direction same as that of the tip magnetic moment.
Figure 3. MFM pictures obtained at different tip-sample distance.
Factors Influencing MFM Writing and Reading Processes
The thickness of the magnetic layer at the tip and the tip-sample separation are the most significant parameters affecting the MFM reading and writing processes. Uncontrolled magnetic reversal can be caused if the tip-sample distance is very small or if the magnetic layer at the tip if very thick. In contrast, very large tip-sample distance or very thin tip layer makes the system unsuitable for writing.
This condition is clearly demonstrated in Figure 3. If the cantilever is covered by a CoCr-alloy film measuring 50 nm, then the magnetic state of particles is easily switched: on some particles, repulsion changes to attraction during scanning (Figure 3a). (Slow scanning was performed bottom-up.) Although the increased tip-sample distance results in scanning without switching, here, the resolution of the final image is poor (Figure 3b).
To carry out bit-by-bit writing, the sample was initially magnetized in the direction opposite to the tip magnetization direction. Subsequently, the MFM image exhibits only repulsion.
Figure 4 illustrates the outline of controllable switching of particle magnetization by the tip. The magnetic tip is moved close to the sample to carry out the local changes of the particle magnetic moment. Magnetic reversal takes place when the local magnetic field of the tip surpasses the coercivity of the particle. The consequence is clearly visible in the MFM image as dark spots (attraction) in the light background (repulsion). Thus, data writing is performed by magnetic reversal of specific particles, and data reading is carried out through one-pass scanning.
Figure 4. Scheme of magnetic writing.
Following cautious fitting of the thickness of the tip magnetic layer, the 30-nm CoCr-alloy layer was discovered to be the most appropriate for controllable local magnetic switching. For testing the results, the four individual particles located in determined positions were switched by using such a tip (Figure 5).
Figure 5. Controllable switching in ordered magnetic particles array
This experiment exhibits high sensitivity and reliability with respect to the nanoscale particles read/write processes carried out in vacuum.
This information has been sourced, reviewed and adapted from materials provided by NT-MDT Spectrum Instruments.
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