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Magnetic Force Microscopy

A Non-Destructive Method that Easily Measures the Critical Performance Parameters of Magnetic Storage Media

Manufacturers of magnetic equipment and storage media are striving for greater reliability in recording and playback, higher bit and track density, and long term stability of recorded data. The most relevant parameter to the quality of your product is the stray magnetic field read by the head. MFM directly measures that stray magnetic field.

How MFM Can Help You

A major advantage of MFM is that the probe can be positioned to measure the magnetic field at the same height as the head that reads it. This allows you to "see" what the reading head "sees." Additionally, measurements can be taken at different elevations above the surface allowing the magnetic field to be mapped three dimensionally.

Figure 1. The Stray Magnetic Field at 50 nm (2 µin) above the surface of the Hard Disk. The tracks are 11.5 µm in length and spacing between tracks is 2.5 µm. The smallest bits are 1.4 µm wide.

The density, track spacing and quality of the magnetic bits (Figure 1) are quickly and easily determined from the MFM output. The density and track spacing are measured directly. The quality of the recording can be determined by analyzing the dimensions and the definition of the bits. If the current for the recording head is too weak, the bits will lack definition, if the current is to strong, the edges of the bits will be distorted.

On rewritten recordings, the MFM output can show residual magnetic bits that were not fully erased. These residual magnetic bits reduce the quality of playback because of media noise and reduce the stability of the recording because of self-erasure.

Figure 2. Surface of Hard Disk showing polishing lines. This image was recorded simultaneously with Figure 1. The surface roughness is 17.0 nm RMS.

The surface features of the magnetic film are also important (Figure 2). The smoother the film, the closer the heads can ride above the surface. For as deposited films, surface roughness (i.e., grain size) corresponds to magnetic roughness. Maximum storage density, both bit and track density will be affected by the grain size of the film.

MFM provides a cost-effective way to get the information that is most critical to your product. It does not require thinning the sample, removing protective layers, or ultra-high vacuum environ-ments.

How MFM works

A probe, (Figure 3), magnetized perpendicular to the sample is scanned over the surface of the sample. Very near the surface the topography of the sample dominates (Figure 2), above the surface the stray magnetic field dominates (Figure 1). Using "One Pass Imaging"™, the topography (i.e., the surface roughness) and the stray magnetic field are measured at one or more positions above the surface during a single scan. As the probe moves over the sample, it is either attracted toward or repelled from the surface, depending on the direction of the field lines.

Figure 3. SEM Image of MFM tip.

For further information call Dr. John M. Mikrut at (714) 895-4465 to discuss how MFM can increase your productivity and/or improve the quality of your product.