Atomic force microscopy (AFM) is a powerful tool for determining the structure of thin materials, or for determining the surface topography of a material. However, while AFM is a stand-alone technique, it is also the precursor technique for a wide range of other methods that are now used. In this article, we look at one variation of AFM – magnetic force microscopy (MFM).
There are many different variations of AFM available today. For many of these variations, the basic operating principles are the same, but the way in which the AFM tip interacts with the sample is generally the differentiating factor. Here, we look at how MFM works and the applications it is often used in.
What is Magnetic Force Microscopy (MFM)
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Conventional AFM scans the surface and touches the surface of a material. This is done through molecular attraction between the tip and the surface. When the tip moves, the laser beam deflects off the cantilever beam onto a position-sensitive photodiode (PSPD) to determine the relative position of atoms at the surface.
MFM operates using slightly different interaction principles. In MFM, the tip detects the magnetic interactions at the surface of the material and builds up a spatial distribution map of the material’s magnetic structure. It does this by a magnetic action being produced between the tip and the sample, which is then outputted as a function of potential changes in the cantilever oscillation. There are many different types of magnetic interactions that can influence the magnetic map, including magnetic dipole-dipole interactions.
MFM is commonly used with materials (and products) that exhibit a high degree of magnetism. As such, they have found a lot of use in imaging magnetic tapes, hard disks, and magneto-optical disks. It can also be used to detect the nanoscale magnetic domains in nanoparticles and proteins for anti-cancer and biomarker applications.
How MFM Works
In MFM, it is a sharp magnetized tip (often magnetized by a ferromagnetic tip coating) that scans the surface to detect the magnetic interactions at the surface of a material. Where conventional AFM can use a myriad of modes, including contact, tapping and non-contact (tapping being the most common), MFM employs a non-contact mode because the mechanical contact is a stronger force that the magnetic interactions and this would give an inaccurate representation of the sample.
In general, the tip is scanned at a defined distance above the sample, and the magnetic force can be approximated by the force gradient of a dipole interaction with the tip. The system still employs a laser and a PSPD to determine the relative position of each magnetic interaction. There a couple of ways in which the tip scans the surface – these are static and dynamic mode.
Static mode scans the surface, and the force is detected by measuring the displacement of the cantilever from its original position. In dynamic mode, the tip oscillates in the z-direction (vertically to the sample). When the tip oscillates near the sample, the change in amplitude of the spring constant backs out the magnitude of the magnetic field exhibited by the sample.
However, there are also a couple of different techniques that can be used when performing an MFM experiment. These are the force range technique and the two-pass technique and are a method of removing the influence of van der Waals forces between the sample and the tip. They do this by separating the long-range magnetic forces from the short-range van der Waals forces.
Force-Range Technique
The force-range technique scans the sample twice. In the first pass over the sample, the tip is in the region above the sample which is dominated by van der Waals forces, and a topographic image is obtained. The tip is then set to scan the area where the magnetic forces are dominant. It is a simple technique that only requires a positional change of the tip in the z-direction.
Two-Pass Technique
The two pass differs slightly to the force range technique, although it still involves two passes of the tip. The first scan over the sample in an intermittent contact mode, also known as true-non-contact mode, and this yields the topography of the surface. In the second scan, the tip follows the same line as the previous scan but at a higher distance from the sample. This time, it only scans the magnetic interactions of the sample (as the van der Waals forces are kept at a constant value), which enables the magnetic map to be determined.
Advantages and Disadvantages
As with any technique, there are advantages and disadvantages. Overall, MFM is beneficial because there is no need for the sample to be electrically conductive and the measurements can be performed in a wide range of environments, pressures, and temperatures. It is also a non-destructive technique, and long-range magnetic interactions are not influenced by surface contamination or the presence of non-magnetic thin film substrates.
However, on the negative side, the production of the image is reliant on the type of tip used and the type of coating on the surface of the tip. Additionally, there can be some issues with interpreting the image, especially when the magnetic fields of the sample and tip change each other’s magnetization.
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