The majority of hard disk drives found on the market today use a recording technology called longitudinal, where bits are stored side by side on the magnetic surface. This recording technique has been used since the first hard disk drives. A new recording technology, called perpendicular, is being used on newer hard disk drives, allowing a higher recording density. In this tutorial we will explain you everything you need to know about perpendicular recording and how data is stored on the hard disk drive magnetic surface.

Data is read and write on magnetic disks thanks to the electromagnetism physics phenomena. In 1820 a physicist called Hans Christian Oersted observed while he was preparing a lab class for his Physics students that an electrical current flowing in a wire moved the needle of a compass located near this wire. When the electrical current was shut down, the compass needle went back showing the location of Earth’s magnetic north pole. With that, he came to the conclusion that all conductors (wires) create a magnetic field around them when an electrical current is flowing. When the direction (polarity) of this electrical current is reversed, so is the polarity of the magnetic field.

In 1831 another physicist called Michael Faraday found out that the inverse was also true, i.e., if a magnetic field strong enough was created near a wire, electrical current was to be produced (inducted) in this wire. If the direction of this magnetic field was reversed, the direction of the electrical current was reversed too.

To understand how data are read and written on hard disk drives and other magnetic devices, keep in mind these two electromagnetic properties:

  • All conductors created a magnetic fiels around them when there is na electrical current flowing.
  • A strong magnetic field can generate (induct) electrical current on a wire.

This is all you need to know in order to undestand how data is read and written on a hard disk drive. An upside down U-shaped conductive material with a coil on it makes the hard disk drive read/write head. On the process of writing data to the hard disk drive, an electrical current is applied to the coil, creating a magnetic field around the read/write read. This field magnetizes the platter surface right below the head, aligning the magnetic particles to the left or to the right, depending on the polarity of the electrical current that was applied. Keep in mind that reversing the electrical current polarity will also reverse the polarity of the magnetic field. A stored bit is a sequence of magnetized particles.

Longitudinal RecordingFigure 1: Read/write read.

On the process of reading data from the hard disk drive, when the head passes on a magnetized area either a positive or a negative current will be inducted, allowing the drive control circuit to read the stored bits.

On next page we will explain how perpendicular recording works and we will compare it to longitudinal recording.