Database Reference
In-Depth Information
understand the effectiveness of various implementations with respect to abso-
lute performance and scalability across a broad range of key scientific domains.
1.2 Fundamentals of Magnetic Storage
Magnetic recording technology was first demonstrated in 1898. Magnetic stor-
age has been closely associated with digital computing since the dawn of the
industry. Its performance and cost-effectiveness ensure that it will continue
to play an important role in nonvolatile storage for some time to come. The
primary factor that consistently drives down the cost of storage media is the
ability to double storage density consistently on an annual basis as shown in
the storage trends in Figure 1.3. As we reach physical limits of the media,
the storage density trends can no longer continue and the market may change
dramatically. Until then, magnetic storage will continue to play a leading role
in nonvolatile storage technology. This section elucidates the physics underly-
ing the magnetic storage technology and the current challenges to improving
storage density.
The underlying media for magnetic recording are ferromagnetic materi-
als. During the write operation, a magnetic field is applied to the recording
medium, which aligns the magnetic domains to the orientation of the field.
The write head uses an electrical coil to induce the magnetic field in the head
to magnetize regions of the medium, and the read head senses the polarity
of magnetization in the underlying media using either magnetic induction or
the magnetoresistive effect. Figure 1.1(a) shows how the current is applied to
the write head to record bits onto the medium and how the read head sub-
sequently recovers the data when the magnetically polarized media induces
electrical impulses into the read head as it passes over the magnetically po-
larized media. The green lines shown under the read head in Figure 1.1(a)
show the magnetic flux lines. The current is induced in the read head when
the flux lines change direction, thereby making the boundaries between bits
detectable.
Figure 1.1(b) shows the hysteresis curve of typical magnetic media, which
enables the magnetic flux of the write head (H) to induce a reversible change
in the magnetization (M) of the underlying medium. The coercivity of the
media refers to the amount of magnetic flux that must be applied to the
media to change its magnetization. Higher coercivity media improves the sta-
bility and density of the storage (e.g., reducing the instabilities caused by
superparamagnetism). The magnetic recording industry employs a number
of technologies to improve the recording density of the media. These include
improvements to the coercivity of ferromagnetic materials that comprise the
media, improved read-head technology (magnetoresistive recording), and im-
provements to the recording technology (perpendicular recording). In the next
