MULTILEVEL AND THREEDIMENSIONAL NANOMAGNETIC RECORDING - Bio-Inspired and Nanoscale Integrated Computing - page 183

substantially more information could be recorded per unit area and thus the
fundamental data density limit could be extended substantially further. This
means that this technology has strong potential to revive the current,
relatively slow progress in the multibillion-dollar data storage industry.
c. Long-term Potential
For the reason mentioned above (exploiting not only a surface but the volume
of a device), the technology has potential to be extended much further into
the future compared to any other alternative data storage technology.
d. Multilevel Signal
The exploitation of multilevel signals will substantially loosen fabrication
requirement. In other words, unlike conventional magnetic and silicon
technologies, this effort will not be limited by the many problems that arise
during fabrication of sub-100-nm devices. In this context, multilevel
implies the ability to record more information per unit surface area.
As mentioned above, previous research has identified the multilevel mode as a
likely form of 3D recording to impact the future world of memory and storage
applications [37-39]. In this mode, a varying recording field was used to sequen-
tially record across the media thickness, as illustrated in Figure 6.6a. The research
effort has triggered industry-wide interest in the new topic [40]. The current study is
aimed at bringing the research to the next level. The chapter studies the feasibility
of multilevel magnetic recording and attempts to consider the physical limitations
of the technology designed to function at densities above 10Tbit/in 2 .
Previously, the authors developed 3D media similar to the popular perpendi-
cular media configuration (with the magnetization perpendicular to the plane of
the disk) [41]. A recording system with a single pole head and a recording media
with a soft underlayer (SUL) (Fig. 6.6b [42]) was used to generate adequate
perpendicular field. SUL was used to force the magnetic flux to flow in the
perpendicular direction. Typical FIB-modified write head with a 80-nm track-
width and a magnetic force microscopy (MFM) nanoprobe used to write and read
information from 3D media, respectively, are shown in Figure 6.6c and d. The
write head was in the form of a single pole [43], while the MFM nanoprobe was
designed to read a certain component of the magnetization [44, 45].
A schematic illustrating the concept of 3D recording is shown in Figure 6.7a.
For simplicity, only two magnetic layers across the thickness are discussed. The
layers can be deposited via regular sputtering systems. Continuous and patterned
versions of 3D media could be used. For illustration purposes and prototype
development, a focused ion beam (FIB) was used to further pattern 3D media
within the plane of the disk, as illustrated in Figure 6.7a. A straightforward MFM
experiment can be performed to demonstrate the presence of more than one signal
level. In this example, a CoCrPt-based 2-layer 3D media was sputter-deposited
and then patterned via FIB into a square periodic arrays with a linear period of
80 nm [46]. Then, the media was demagnetized using an alternating external
magnetic field. AFM (left) and MFM images of the surface of the fabricated