Optimizing Magnetic Force Microscopy Resolution and Sensitivity to Visualize Nanoscale Magnetic Domains

Magnetic Force Microscopy, or MFM, employs a vertically-magnetized atomic force microscopy probe to measure sample topography and local magnetic field strength with nanoscale resolution. By balancing decreasing lift height against increasing drive or oscillation amplitude, MFM spatial resolution and sensitivity can be optimized. Spin-wave computing applications of artificial spin ices rely on knowledge of the nanoelement magnetization textures as they determine the magnonic response.

High-resolution MFM enables identification of icy global magnetization states. Demonstrating the procedure will be Olivia Maryon, a current doctoral student in material science and engineering at Boise State University, a former undergraduate AFM researcher for my laboratory. To begin, open the AFM control software and select the MFM workspace under the Electrical Magnetic Lift Modes experiment category and group.

Mount an AFM probe with a magnetic coating on an appropriate probe holder by carefully placing the probe holder on a mounting block, then loading the probe onto the probe holder, aligning the probe and securing it in place with a spring-loaded clip. Ensure the probe is parallel to all edges and not touching the back of the holder's channel by inspecting it under an optical microscope. Gently manipulate the probe as necessary with a pair of tweezers.

Magnetize the probe vertically using a strong, permanent magnet for 2 to 5 seconds so that the magnetic dipole orientation of the probe tip will be perpendicular to the sample. Carefully remove the AFM head while taking care to discharge any electrostatic buildup by touching the AFM enclosure. Install the probe and probe holder by aligning the holes on the probe holder with the contact pins on the head.

Reinstall the head on the AFM and secure it in place. Align the laser onto the center of the MFM probe cantilever and into the position-sensitive detector. For optimal sensitivity, align the laser on the back of the cantilever to the location corresponding to the tip setback from the distal end of the cantilever.

Maximize the sum signal on the PSD while minimizing the left-right and the up-down deflections to center the reflected laser beam on the detector. Place the sample over the AFM chuck vacuum port. Avoid using a magnetic sample holder as this could affect the sample and/or interfere with the MFM measurement.

Turn on the chuck vacuum to secure the sample to the AFM stage. Return to the AFM control software, go to Setup and select the chosen probe type. Bring the cantilever into focus and align the crosshairs within the optical microscope view to be positioned over the back of the MFM probe cantilever where the tip is located using the known tip setback based on the selected probe.

Open the Navigate window and position the AFM stage and sample so that the region of interest is directly beneath the AFM tip. Lower the AFM head until the sample surface comes into focus in the optical view. Go back to Setup, select Manual Tune, and perform a cantilever tune by choosing start and end frequencies that will sweep the dither piezo drive frequency across a region chosen to span the expected resonance frequency of the selected probe.

Choose a drive frequency offset and target amplitude. Then tune the cantilever and set the desired amplitude set point. Engage on the sample surface and set the desired scan size depending upon the sample and features of interest.

Increase the Amplitude Setpoint in increments of one to two nanometers until the tip just loses contact with the sample surface as seen by the trace and retrace lines failing to track each other in the height sensor channel. Then decrease the Amplitude Setpoint by two to four nanometers so the tip is just in contact with the sample surface. Optimize the proportional and integral gains by adjusting them so they are high enough to force the feedback system to track the sample surface topography while minimizing noise.

Once the AFM topography imaging parameters have been optimized, withdraw a short distance from the surface and return to the probe tuning menu. Perform a second cantilever tune to be used to acquire the interleaved lift mode MFM line, making sure to unlink the results of this tune from the previous mainline parameters. In the interleaved lift mode tune, set the peak offset to 0%Choose start and end frequencies that will sweep the dry frequency across a region spanning the resonance frequency of the probe.

Adjust the interleaved lift mode target amplitude to be slightly less than the mainline target amplitude. This will enable high sensitivity MFM imaging without striking the surface when utilizing low lift heights for optimal lateral resolution. Leave the cantilever tune window to reengage on the surface.

To optimize the imaging parameters, set the initial Lift Scan Height to 25 nanometers, then gradually decrease in the increments of two to five nanometers. Once the probe begins to just strike the surface, immediately increase the scan height to preserve the probe tip and prevent the introduction of topographical artifacts. Increase the drive amplitude in small increments corresponding to two to five nanometers in oscillation amplitude until the interleave drive amplitude exceeds the mainline drive amplitude or the probe begins to contact the surface.

Then decrease the drive amplitude slightly so that no spikes are seen in the MFM phase channel. Continue iteratively optimizing the Lift Scan Height and drive amplitude by adjusting in progressively smaller increments until a high resolution MFM image free of topographical artifacts is obtained. Magnetic force microscopy is used to image twin boundaries and track their movement in response to an applied magnetic field or force.

The magnetic phase images of the polished, single crystal nickel-manganese-gallium sample show the characteristic stairstep magnetic orientation across the twin boundaries. The magnetic phase image overlaid as a colored skin on top of the sample's 3D topography shows the long direction of the magnetic domains switching at the topographical features. Optimizing MFM spatial resolution and sensitivity benefits from operating in an inert atmospheric glove box and requires balancing decreasing lift height against increasing drive or oscillation amplitude.

High resolution, high sensitivity MFM is crucial for studying the underlying magnetization configurations in artificial spin ice statuses and could also advance the rapidly developing field of spin-wave computing.