Magnetic force microscopy (MFM) is one of the modes of scanning probe microscopy (SPM). As the name suggests, it is used for mapping magnetic properties. MFM probes local magnetic fields on the nanoscale, resulting in images that contain information on a sample’s localized magnetic properties including the mapping of magnetic domains and walls. It is primarily a qualitative, contrast-based technique and has been widely used to characterize magnetic storage media, superconductors, magnetic nanomaterials, and even biological systems.
In addition to single point measurement at localized regions of interest, maps of force curves are also possible. This enables the correlation of localized mechanical property information with topographical or other information obtained by the various imaging methods.
How does it work?
MFM operates in dynamic force mode with phase contrast. A cantilever with a thin magnetic coating is driven near its resonance frequency f0, where you have a 180° phase shift going from below to above the resonance frequency. Typical resonance frequencies are in the tens to hundreds of kilohertz. MFM maps the phase of the oscillating cantilever as it passes over the sample at a prescribed height. An attractive magnetic force gradient is obtained when the magnetic moments of tip and sample are parallel. This results in the resonance curve shifting to a lower frequency, accompanied by a decrease in phase shift at the excitation frequency (Fig. 1). Conversely, a repulsive magnetic force gradient by anti-parallel orientation of the magnetic moments will cause the resonance curve to shift to a higher frequency, accompanied by an increase in phase shift at the excitation frequency. The cantilever predominantly responds to out-of-plane fields.
Magnetic forces acting on the cantilever by the sample are measured as the tip is lifted off the surface thus separating the long-range magnetic forces from the short-range atomic forces between tip and sample. The tip-sample distance is a crucial parameter to optimize for effective MFM operation. If the tip is too far from the sample, the resolution will be compromised. If the tip is too close to the sample, the topography will be convoluted into the MFM signal, significantly complicating its interpretation.
As with all SPM based methods, the key to MFM is the probe, which is magnetized. The silicon-based probes, coated with different magnetic thin layers, (e.g. cobalt alloys) are commercially available. A variety of MFM probes can be purchased including those that are high-moment, low-moment, and low coercivity.
MFM can operate in either a single or dual pass technique. In the single pass setup, the MFM tip passes over the sample at a constant height. A slope between the sample and the scanner should be compensated for to maintain a constant gap. This implementation of MFM is the fastest and minimizes tip wear. Since the topography is not measured, the tipsample distance cannot be kept constant. The lift height must consequently be set by the highest areas of the sample to prevent topography convolution in these higher parts. This deteriorates resolution and contrast.
In the dual pass mode, the cantilever passes twice over every line in the image. During the first pass, the MFM tip collects topography information in dynamic mode. The tip is then lifted over the sample surface for the second pass by an amount prescribed by the user. In the second pass, the tip follows the contours of the topography keeping the tip-sample distance constant during every scan line. The amplitude of the second scan can be reduced to stay closer to the surface than in the first, topography recording scan. Optimization entails a tradeoff between having the tip as close to the sample as possible, but not being too close as to crash into the sample.
A. Magnetic tape: One of the most popular applications of MFM is to study magnetic storage media, which have features on the nanoscale that cannot be imaged with other techniques. MFM is widely used for quality control in this industry because it can detect defects and feature morphology necessary to optimize device performance. An example from the data storage industry is the AFM image in Fig. 2 of a digital backup tape with magnetically stored data. The 50 x 50 μm2 image clearly shows the magnetized domains of data within this magnetic storage tape. This is a straightforward sample that can be used to get acquainted with the MFM operation mode
B. Hard disk drive: The most wide-spread application of MFM on magnetic storage devices is the measurement of hard disk drives. In Fig. 3, AFM and MFM are used to successfully analyze a hard disk drive. Images of 2 x 2 µm2 are shown below with the topography image at the top and MFM phase image at the bottom. The topography image, collected in dynamic force mode, shows diagonal grooves parallel to the recording tracks. But the topography does not reveal any of the magnetization information, which are clearly observed in the corresponding MFM phase image. The red and blue areas in the MFM image represent magnetic domains where red depicts repulsive forces and blue for the attractive forces, acting on the tip. One could deduce a track distance of ~600 nm with a bit length of ~70 nm, a length scale that can only be imaged by a technique such as scanning probe microscopy.
C. MFM on steel: An example from the materials science sector of the powerful use of MFM to visualize magnetic domains is shown in Fig. 4. Polished stainless steel has a rich morphology of various domains. An 80 x 80 µm2 topography image is shown on the top, revealing some polishing scratches and perhaps a hint of some underlying morphology in the form of round dark domains. The corresponding MFM phase image, collected in dual pass MFM, shows a much more complete understanding of the polished steel surface, revealing domains with low MFM contrast (red) and domains with a distinct wormlike contrast, interspersed among the red domains. The red domain is identified as the paramagnetic austenite phase that exhibits uniform contrast, while the wormlike domain is identified as the ferrite phase due to its ferromagnetic behavior. The pattern and contrast of the wormlike domains depend on the orientation of the magnetic field.
D. Thin permalloy films: Another striking example of how applicable the MFM in visualizing magnetism in different materials, is the imaging of magnetic domains in thin ferromagnetic films. In this case – permalloy (Fe0.2Ni0.8), notable for its extremely high magnetic permeability. In the 30 x 30 µm2 image of Fig. 5, we see the remnant magnetization of the film where the domains of opposite orientations form a pattern of stripes with a period of ~0.5 µm. The period of the pattern depends on the magnetic properties of the film and is increasing with the square root of its thickness.
Magnetic force microscopy, a variant of atomic force microscopy detects the tip-sample magnetic interactions to reconstruct the magnetic structure of the sample. Despite MFM primarily being a qualitative technique, it has proven to be an excellent characterization tool in research and industrial applications. MFM has been widely used for characterization of magnetic nanostructures and imaging the magnetic field distribution. The primary advantages of MFM are its high spatial resolution and sensitivity, ability to operate in different environments and the capability to apply in situ magnetic fields to study magnetization processes.