Variable Field Module

The Variable Field Module (VFM) for the Asylum Research MFP-3D-Origin+ AFM system is cleverly designed to provide up to 1 [T] or 10,000 [G] of in-plane magnetic field. Unlike a conventional electromagnetic which requires high power and water cooling to achieve such fields, the VFM uses a servo motor to control the amount of magnetic flux from a permanent magnet that gets channeled to the magnetic poles. Our initial objective is to precisely position a magnetic nanoparticle (MNP) on a giant magnetoresistance (GMR) sensor and perform a field sweep to characterize the sensor's response to a nearby MNP. This experiment requires an extremely stable system. We've already characterized XY drift and Z drift and determined that it is acceptable. The next step is to determine whether or not operating the VFM module produces drift.

The VFM module and its magnetic field profile is shown in Figure 1. A magnetic field sensor positioned between the poles is used to record the magnetic field. Samples are placed on top of the poles and experiences a weaker magnetic field than the sensor. The sensor position can be calibrated such that the recorded field coincides with the field experienced by the sample, but this is difficult to do precisely and it is unnecessary for our experiment. From a preliminary assessment of our GMR sensor's response, the magnetic field at a point ~500 [μm] above the gap is approximately 1/2 of the VFM sensor reading.

Figure 1:  The (left) VFM module has (middle) field sensor between two magnetic poles. The (right) applied field depends on pole gap and sample position.

Figure 1: The (left) VFM module has (middle) field sensor between two magnetic poles. The (right) applied field depends on pole gap and sample position.

Sample alignment

It is critical to be able to position the sample consistently at the center of the gap between the magnetic poles. An alignment jig, Figure 2, was installed to assist with coarse alignment of the sample. The alignment jig consists of a laser pointer attached to the ceiling of the AFM hood via a NOGA articulating arm with a magnetic base. The articulating arm is adjusted such that the laser is pointing directly at the center of the gap. Before loading a sample, the stage must be adjusted to to position the AFM probe at the center of gap. Then place the sample on the poles and use the laser for coarse alignment. Fine alignment is achieved by using the AFM microscope and slightly nudging the sample into position. Do not move the stage to position the probe over the sample.

Figure 2:  The (left) alignment jig on the ceiling (middle) points a laser at the pole gap to (right) facilitate sample alignment.

Figure 2: The (left) alignment jig on the ceiling (middle) points a laser at the pole gap to (right) facilitate sample alignment.

Drift Experiment

VFM drift is measured by using the Asylum Research Macro Builder which helps automate a repetitive experiment. The macro does the following:

  1. Scan image and retract probe
  2. Sweep the field from 0 to 300 [G] at 300 [G/min] with ±10 [G] tolerance
  3. Scan image and retract probe
  4. Sweep the field from 300 to -300 [G] at 300 [G/min] with ±10 [G] tolerance
  5. Scan image and retract probe
  6. Sweep the field from -300 to 300 [G] at 300 [G/min] with ±10 [G] tolerance
  7. Scan image and retract probe
  8. Sweep the field from 300 to 0 [G] at 300 [G/min] with ±10 [G] tolerance
  9. Repeat from step 1
Before running the macro, make sure the image has unique objects that can be used to track the drift. Before running the experiment, it is important to let the temperature in the AFM stabilize for an hour and turn off the microscope light and hood light. The microscope light is a source of thermal energy and it does appear to increase the drift rate.

VFM Drift

The Asylum Research AFM application is built using the Igor Pro platform. The AFM data is stored as an Igor binary wave file (*.ibw). To compare drift among the several hundred images requires programming. Matlab was used to analyze the dataset since a free library is available for importing Igor binary wave files. Each image is loaded and processed to determine the position of the sensor in the image. The sensor's position with respect to time is shown in Figure 3. The graph is quite busy, but there are a few trends:

  1. The drift rate slows significantly after 1 hour of operation
  2. After a 7 hour break, the sample returns to its original position
  3. The sample position is correlated with the field magnitude
    • Black dots: field is at 0 [G]
    • Green dots: field is at 300 [G]
    • Magenta dots: field is at -300 [G]
Upon carefully analyzing the drift data, we suspect that the VFM takes approximately 1 hour of operation to reach thermal equilibrium. The drift correlated with field magnitude needs further investigation. We used a probe customized with a 500 nm MNP tip for this experiment. It will be interesting to confirm whether the probe is being moved by the magnetic field or if the drift is caused by something else. Keep in mind that what we observe and record is image drift. Image drift can be caused by movement of the stage, movement of the probe or both.

Figure 3:  Tracking the sensor position over time reveals drift in the AFM when using the VFM module.

Figure 3: Tracking the sensor position over time reveals drift in the AFM when using the VFM module.

The images for this experiment was stitched together to form a movie. Click here to view the movie. In this movie, the sensor moves towards the AFM left and AFM bottom, or -x and -y respectively. If you are facing the AFM stage, AFM left is your left and AFM bottom is towards you. The time each image is captured is annotated at the bottom right. At approximately 14 seconds into the movie, the image jumps significantly. This jump is the 7 hour rest period shown in Figure 3 where the AFM is idle.

Upon closer inspection of the data shown in Figure 3, we observe a correlation in the amount of drift and the magnitude of the applied field. Figure 4 shows that the position of the drift corresponds to the field sweep data. In the field sweep curve, the zero, 300 [G] and -300 [G] points are marked with colored circles. The XY position curve is marked the same way, corresponding to the applied field during the scan. The correlation is unmistakable.

Figure 4:  The image position is correlated with the magnitude of applied field.

Figure 4: The image position is correlated with the magnitude of applied field.

In Figure 5, we show that the amount of drift is proportional to the magnitude of the applied field. By doubling the field from 300 [G] to 600 [G], the X and Y drift magnitude doubles as well. For a field sweep experiment requiring ±600 [G], the XY fluctuation is approximately ±200 [nm].

Figure 5:  The amount of drift is proportional to the magnitude of applied field.

Figure 5: The amount of drift is proportional to the magnitude of applied field.

VFM Vibration

The VFM vibrates whenever it is asked to change fields. The noise from the VFM motor is audible and the vibrations can be measured during an AFM scan. The vibration of a sample sitting on the VFM module can be characterized by scanning a point as the VFM is performing a field sweep. Figure 6 shows the amount of noise in units of 3 standard deviation from the mean versus the angular speed of the motor. The data has been processed to show the relationship between the amount of vibration and motor acceleration.

Figure 6:  The amount of vibration in the VFM is proportional to the motor acceleration.

Figure 6: The amount of vibration in the VFM is proportional to the motor acceleration.

Analyzing the frequency spectrum of the vibration data, reveals that the sample vibrates at less than 300 [Hz], see Figure 7. Since the cantilever is oscillating at ~350 [kHz], it should be able to respond fast enough to the sample's vibration to avoid severe damage to the tip. After performing the vibration test for over an hour, the imaging quality of the tip still looks good as new.

Figure 7:  VFM induced vibration is below 300 Hz.

Figure 7: VFM induced vibration is below 300 Hz.