4. How do they work?

Digital data stored and used on computers is in binary form, either a 0 or a 1.

Writing data

Figure 1

Figure 1

Figure 2

Figure 2

Figure 3

Figure 3

Figure 4

Figure 4

The writing diagram above shows how the 0 and 1 is represented on the cylinder (see figure 1).

The computer sends the binary values to the hard drive controller (see figures 2 and 4) via the interface channel; this then converts the binary value to a current on the coil of the write head. "The current in the coil reverses at each 1 and remains the same at each 0"[4].

This current causes the magnetisation in the cylinder (see figure 3).

Reading data


When the read head senses a magnetic change in the cylinder, the read head becomes in an excited state. These voltage changes are then converted into a binary sequence of 0 and 1 again.

"The so-called Wallace's spacing loss factor postulates that the loss of magnetic signal power will be proportional to the media - head separation. This requires magnetic heads to fly as close to the disk surface as possible, which forces modern heads to fly at a few nanometers only (compare this to 20,000 nanometers back in 1957!)."[4] (See figure below).

Side View

Read and Write head

Modern magnetic heads tend to contain a magneto-resistive (MR) or giant MR(GMR) reading head and a thin-film inductive write head.

MR heads are based on the property of metals changing their resistivity in the presence of a magnetic field.

An alloy of Ni and Fe (81%/19%) is largely found in MR heads, it is called Permalloy. MR heads are ideally suited for extremely high bit density. They also have superior signal-to-noise ratio when compared to inductive read heads.[4]


The first magnetic media was called particulate media because it included particles of iron oxide as the magnetic medium and aluminium oxide for increased durability. Today's magnetic media is known as thin-film media and is made from extremely thin layers with a total thickness approximately 50 nm. The figure below shows a not to scale sketch of a possible thin-film media.


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Gregg Williams, 294805, CS_134 Web Report