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Magnetic storage was first invented by
Valdemar Poulsen in 1898, first applied for audio purposes on a plastic strip in
the German Magnetophon in 1936, and has since its first use for data storage in
1951 evolved to become the most important means for digital storage in modern
computers, particularly in the form of hard disks.
Hard Disk
Storage
The storage medium of the hard disk has two
preferential magnetisation directions. The magnetic field of the hard disk write
head determines in which of these directions the medium is magnetised. The head
writes ones and zeros by reversing or not reversing the magnetisation direction
of the medium at regular intervals. Where the magnetisation reverses, a magnetic
stray field emanates from the medium, which can be detected by various
means.
Magnetic
Force Microscopy Imaging
The Nanosurf Mobile S and EasyScan 2 (with mode extension) can
image the magnetic stray field in the MFM (Magnetic Force Microscopy) imaging
mode. In this mode, the stray field is detected by sensing the magnetic force it
exerts on a magnetically coated cantilever tip. This force causes a change in
the cantilever resonance frequency and thereby shifts the phase of the
cantilever vibration. The MFM image is measured by recording the phase contrast
image when scanning a plane parallel to the surface in the same location, but a
few nanometers away from the sample.
Figure 1. Surface topography measurement (dynamic force mode). The recording
tracks, as evidenced in the MFM image, run parallel to the grooves. Scan range 2
ƒÊm ~ 2 ƒÊm / Z range 5 nm
Figure 2. MFM measurement (phase contrast mode). White and black areas connote
magnetisation reversal, where white stands for repulsive forces and black for
attractive forces. Scan range 2 ƒÊm ~ 2 ƒÊm / Phase range 5‹
The measurements shown here were taken with
a Nanosurf Mobile S Large Scan head using NanoWorld MFMR
cantilevers. The sample was a 10GB single head 3.5" hard disk magnetised in the
plane of the medium with a track distance of 600 nm and a bit length of 70 nm,
which corresponds to 42k TPI and 363k BPI. |