Composite Head

The ferrite or composite head is composed primarily of a ferrite core with a wire coil and a glass-filled gap, and the material used to bond the core to the slider. Ferrite is a hard material which makes it difficult to meet today's physical and electrical characteristics required for higher areal densities.

Conventional ferrite heads, such as this, have reached the limits of their performance with regard to the machining processes and magnetic characteristics in the high-frequency domain.

MIG-head

An advancement of the ferrite head is the MIG-head (metal-in-gap) in which metal alloys (e.g. AlFeSil) are sputtered in the magnetic gap of the head. This increases substantially the magnetic field strength, allowing higher bit densities to be achieved on highly coercive data carriers.

Thin-film head

Thin film heads are created using vacuum deposition, photolithographic and other similar techniques used in semiconductor wafer processing, to achieve highly controllable dimensions and characteristics. Through this process, the coil winding is deposited on a ceramic base, creating extremely small heads with equally small and precise gaps between the pole pieces of the magnetic recording material.

Thin film heads perform best at lower flying heights, achieving higher areal densities and faster data transfer rates.

Magneto Resistive head

One of the most significant new generations in head technology is the magneto resistive head. This uses similar technology to thin film heads, but features a dual element design comprising a magneto resistive read element which is formed first, to which an inductive write element is added. By incorporating dedicated elements in this way, each part of the head is optimised for performance. Conventional heads use a dual function head, in which the same element controls both read and write: performance, therefore, has to be compromised between the two functions.

A key advantage of the magneto resistive head is that it automatically senses flux, whereas an inductive element -- such as used in a thin film head -- has to be energised by the flux. This allows the magneto-resistive head to sense smaller magnetic domains which in turn facilitates smaller bit cells and narrower track widths with both contributing to increased capacities.

Designs such as the magneto resistive head certainly seem to point the way forward, particularly for high performance drives.

Flying height

As we have discussed in this topic, the head "flies" at sub-micron spacing above the disk surface on an air bearing created by the geometry of the head and the rotation of the disk. The flying height has a major effect on storage densities. With lower flying heights (i.e. the head closer to the disk), less fringing occurs, enabling more bits to be stored per inch and the tracks to be closer together, both resulting in increased disk capacity. Lower flying heights also permit higher frequencies which enable higher data rates to be achieved.

Much of the research into head design at Fujitsu is therefore connected with reducing the flying height. Since the early '80s it has been brought down from 2.5 to just 0.15 micrometres today - that's 30 times smaller than a smoke particle. An obvious goal then would appear to be total contact heads.

Total contact heads though, are not new. They have been used in floppy disk drives for many years and the two technologies each have their own distinct set of requirements. With a floppy disk drive, for example, the read/write head cannot "fly" because it is not situated in a clean environment therefore total contact is the only answer which results in floppy disks have to spin at a much slower rate: 360rpm compared to the industry standard for hard disks of 3,600rpm.

It is unlikely that total contact heads will appear in high-performance hard disk drives, but virtual or near contact heads will. It must be appreciated that when heads come into constant contact with media, wear will inevitably occur. Virtual contact heads will, therefore, be limited to low-cost, infrequently used drives (such as those used in low end PCs and laptops etc.). With other, harder working drives such as for minis and mainframes, operating 24 hours a day flying heights will continue to reduce, but consistent with safe flying and with due regard to data integrity.