SEM/EDX analysis of MFM tip

A MFM tip is placed in the DBS and viewed using SEM. The MFM tip will be permanently damaged during analysis, therefore it is chosen to analyze a used MFM tip. An image of the cantilever on the mounting block is shown in figure 4.l(a). From a top zoom of the tip, shown in figure 4.l(b), it be-comes apparent that the MFM tip has a rhombohedron shape. By viewing the tip from the side as in figure 4.2(a), the tips specific dimensions can be measured. The shown MFM tip has some damage at the tip end (see figure 4.2(b)). The tip radius is much larger than the tip radius specified by the supplier ( <50 nm). This damage is probably caused by MFM scans with the tip.

(a) An overview of the cantilever (b) Top view of the MFM tip where and tip attached to the mounting the tips rhombohedron shape can

block be seen

Figure 4.1: SEM images of cantilever on mounting block and MFM tip

(a) Side view of MFM tip (b) Zoom at the top of the tip Figure 4.2: SEM images to measure the dimensions of the MFM tip

To analyze the thickness of the magnetic coating, the top part of the tip is milled. The result is shown in tigure 4.3(a). When zoomingin on the end of the tip, the magnetic coating can be distinguished from the inside of the tip due to a contrast difference, which can beseen in tigure 4.3(b). The dark triangle represents the inner material of the tip and the thick grey boarder around the triangle represents the cross-section of the magnetic coating.

The thickness of the magnetic coating can now be measured. The magnetic coating is approximately 40 nm thick, as can be seen from the measurement bars in the image.

The composition of the magnetic coating is analyzed using the Energy Dis-persive X-ray (EDX) detector. The principle of this analysis method is explained in section 3.2. The results of the EDX scans, of the tip with the top milled, are shown in tigure 4.4. Figure 4.4(a) gives a SEM image

(a) A view after milling the top (b) SEM image of cross-section of part of the tip the top of the MFM tip

Figure 4.3: SEM images to analyze the thickness and material of the magnetic coating of the MFM tip.

of the scanned area. The other figures are intensity maps of the detected elements¹. The brighter the image, the higher the density of the element at that location. In figure 4.4(b) it can be seen that the interior of the tip consists of silicon. Figures 4.4( d) and 4.4( e) show that the tips exte-rior contains cobalt and chromium. There is not much chromium present in the coating, which results in a noisy image. Noother magnetic elements are detected. Thus it can be assumed that the MFM tip has a 40 nm thick co balt-chromium ( CoCr) magnetic coating. Co balt is a known ferromagnetic metal and adding chromium gives the magnetic layer a higher coercivity[ll]. In figure 4.4( c) it can be seen that there is aluminium on the front si de of the tip (right side of the images in figure 4.4). It is not clear why the aluminium is there. Perhaps the aluminium prevents the CoCr-layer from oxidizing. The oxygen detection image shown in figure 4.4(f) however does not support this explanation. In the figure it can be seen that the oxygen concentration on the backside of the MFM tip is lower than on the front side. If the purpose of the aluminium would indeed be to prevent the CoCr-layer from oxidizing, a lower oxygen concentration is expected on the front side. Another possible explanation is that as a side-effect of coating the backside of the cantilever with aluminium for a better laser reflection, some aluminium is also deposited on the front of the tip.

4.1.2 Simulation magnetic stray field of a MFM tip

To simulate the stray field, a 40 nm thick cobalt coating, as found in the previous section, is assumed for the MFM tip. The found rhombohedron

1The images are a visualization for the detection of one spectralline of an element.

(a) SEM image (b) Silicon (c) Aluminium (d) Cobalt

(e) Chromium (f) Oxygen

Figure 4.4: Images of EDX scan for the detected elements.

shape is modeled as a prism with a triangle at each end. This shape is shown in figure 4.5.

Figure 4.5: 3D representation of simulated MFM tip, the arrows represent the tips magnetic stray field. Dimensions of the bottorn and top (tip end) triangles (base, height) are respectively (500 nm, 200 nm) and (100 nm, 90 nm).

In the manual of the LLG simulation program² there is a graph that shows the stray field strength of a MFM tip as a function of distance from the tip. The geometry of the modeled tip is shown in figure 4.6(a). The correspond-ing field strength graph is shown in figure 4.6(b). The field strengths are calculated fora tip with a cabalt coating of 10 nm thick. This is camparabie to the dimensions of the tip that will be simulated. In the graph it can be seen that at a distance of 140 nm, the strength of the stray field emanating from the tip has dropped to approximately one tenth of its initial value.

2The LLG Micromagnetics Simulator program, developed by M.R. Scheinfein is used to simulate the stray field of the MFM tip.

(a) Midplane slice of the of the LLG simulation program, adapted from [12].

To limit the number of calculations necessary for the simulation, the stray field will only be simulated up toa distance of 140 nm from the tip. Also the height of the MFM tip has been limited in the simulation. In figure 4.6(b) it can be seen that at 500 nm distance from the tip the magnetic stray field is negligible. Therefore only the last 500 nm of the end of the MFM tip is used to simulate the stray field. Any contributions from the magnetic coating located further than the simulated part are neglected. The simulated MFM tip as shown in figure 4.5 is thus 500 nm high. In the x- and y-direction, the field is simulated till a distance of approximately 250 nm.

A three dimensional simulation of the magnetic stray field emanating from a MFM tip has been performed³. Only the field in the z-direction has been simulated, since this is the component that will cause flux changes through the ring and thus is of interest for the current induced method, as explained in section 2.4.1. The result of the simulation is a three dimensional matrix Hz. Each element of the matrix contains the value for the magnetic stray field in the z-direction, at a certain spatial coordinate. The matrix is graph-ically represented in figure 4. 7.

The obtained matrix Hz is used in equation 2.26 to determine the induced voltage in the ring. Thus, the matrix will be used as input for the magnetic flux in simulations for the induced current.

3Simulation was carried out by R. Paesen, using the simulation program LLG.

o• ^{x 104}

-ll.S .Q.S

s

·I .;!!. ·1

J

.J.S ^'1.~

·2 ·2

(a) Sliced view of the stray field emanating (b) Bottom view of the image in figure from the end of the MFM tip 4.7(a)

Figure 4. 7: Graphical representation of simulated magnetic stray field in the z-direction, emanating from the MFM tip. Values for the field strength are in Oersted.