Magnetic Force Microscopy (MFM)
Updated: Apr 7, 2022
1. History of MFM
Scanning probe microscopy (SPM) techniques such as scanning tunneling microscopy (STM) and atomic force microscopy are examples of scanning probe microscopy (SPM) (AFM). The STM was the first of the SPM methods to be developed, by Binnig, Rohrer, and collaborators, in the early 1980s. A modest tunneling current travels from an atomically flat metallic or semiconducting surface to a fine tip positioned close by in this way. This approach has a high spatial resolution and is frequently used to scan surface crystallographies in real-time with atomic detail.
Scanning tunneling spectroscopy has evolved from the STM method, which is sensitive to the density of electronic states in both the surface and tip. Binnig and Rohrer were awarded the Nobel Prize for Physics in 1986 for developing the STM; Binnig was also involved in the invention of the AFM. This modification of SPM analyzes the atomic forces between a tip on a soft cantilever and a sample's surface, with no restrictions on the type of material investigated. It is possible to detect forces as little as pN. The electrostatic forces between the tip and the sample surface cause the deflection of the tip. Martin and Wickramasinghe developed the MFM for the first time the following year, by replacing the AFM tip to a magnetic one. The deflection of the cantilever may be translated onto the domain structure of the sample using magnetic dipolar forces (or force gradients) between a magnetic tip and a magnetic sample, such as a thin film. This enables for the detection of both normal cantilever deflection and torsional motion.
Magnetic force microscopy, or MFM, is a scanning probe microscopy technology that is used to probe objects with magnetic characteristics or magnetic materials and reveal features such as magnetic domains and domain walls. This approach is commonly used for quality control in the field of magnetic storage media.
The magnetic forces exerted by the sample on a sharp, magnetized tip are measured in MFM. The tip is elevated off the surface during this measurement to distinguish long-range magnetic forces from short-range atomic forces between the tip and the sample. Amplitude modulation mode, a sort of dynamic force mode in which a cantilever with a thin magnetic coating is pushed at its resonance frequency, often in the tens or hundreds of kilohertz, is used in magnetic force microscopy (this mode is also referred to as tapping mode). These cantilevers are low-cost and widely accessible. The phase and frequency of the oscillating cantilever are mapped by MFM as it passes over the sample at a predetermined height.
The resonance curve will move to a higher frequency as a result of a repulsive magnetic force gradient, which will also induce a phase shift rise (bright contrast). An attractive magnetic force gradient, on the other hand, causes the resonance curve to move to a lower frequency, with a reduction in phase shift (dark contrast). Lower noise and improved resolution are two advantages of using MFM in dynamic mode. For optimal MFM functioning, the tip-sample distance is an important metric to tune. The resolution will be affected if the tip is too far away from the sample. The topography will be tangled into the MFM signal if the tip is too close to the sample, making it much more difficult to analyze.
MFM can function in a single pass or dual pass mode. The MFM tip travels over the sample at a consistent height in the single-pass arrangement. The tip is closer to the sample in the single-pass mode, resulting in increased sensitivity and resolution in the magnetic force measurement, although spatial resolution may suffer. This kind of MFM implementation is the quickest, has the least amount of tip wear, and has a more obvious theoretical explanation.
The cantilever in the dual-pass configuration passes twice over each line in the picture. The MFM tip is in touch with the sample during the first pass as it maps out the topography in amplitude modulation mode. The tip is then elevated by an amount indicated by the user over the sample surface for the second pass. The tip follows the contours of the terrain in the second pass, but this time at a distance above the sample. Every picture optimizes the tip-sample distance, which is generally a few or tens of nanometers. Optimization requires striking a balance between getting the tip as close to the sample as feasible while avoiding colliding with it.
2. Examples of magnetic force microscopy
Vortex Imaging LT-MFM on BSCCO single crystal at 4.5K
MFM image of HDD @77K, k=8