Disk read-and-write head

Disk read/write heads are mechanisms that read data from or write data to disk drives. The heads have gone through a number of changes over the years.

In a hard drive, the heads fly above the disk surface with clearance of as little as 3 nanometres. The "flying height" is constantly decreasing to enable higher ariel density. The flying height of the head is controlled by the design of an air-bearing etched onto the disk-facing surface. The role of the airbearing is to maintain the flying height constant as the head moves over the surface of the disk.

The heads themselves started out similar to the heads in tape recorders -- simple devices made out of a tiny U-shaped piece of highly magnetizable material called ferrite wrapped in a fine wire coil. When writing, the coil is energized, a strong magnetic field forms in the gap of the U, and the recording surface adjacent to the gap is magnetized. When reading, the magnetized material rotates past the heads, the ferrite core concentrates the field, and a current is generated in the coil. The gap where the field is very strong is quite narrow. That gap is roughly equal to the thickness of the magnetic media on the recording surface. The gap determines the minimum size of a recorded area on the disk. Ferrite heads are large, and write fairly large features. They must also be flown fairly far from the surface thus requiring stronger fields and larger heads.

Metal in Gap (MIG) heads are ferrite heads with a small piece of metal in the head gap that concentrates the field. This allows smaller features to be read and written. MIG heads were replaced with thin film heads. Thin film heads were electronically similar to ferrite heads and used the same physics. But they were manufactured using photographic processes and thin films of material that allowed fine features to be created. Thin film heads were much smaller than MIG heads and therefore allowed smaller recorded features to be used. Thin film heads allowed 3.5 in drives to reach 4GB storage capacities in 1995. The geometry of the head gap was a compromise between what worked best for reading and what worked best for writing.

The next head improvement was to optimize the thin film head for writing and to create a separate head for reading. The separate read head uses the magnetoresistive effect which changes the resistance of a material in the presence of magnetism. These MR heads are able to read very small magnetic features reliably, but can not be used to create the strong field used for writing. The term AMR (A=anisotropic) is sometimes used instead of MR. In introduction of the MR head in 1996 lead to a period of rapid ariel density increases exceeding 100% per year. In 2000 GMR, Giant Magnetoresistive, heads started to replace MR/AMR read heads. In 2005, the first drives to use TMR, tunneling MR, heads were introduced by Seagate allowing 400 GB drives with 3 disk platters.

In 2005, Seagate introduced TMR heads using integrated heaters to control the shape the pole-tip region of the head during operation. The heater is activated prior to the start of a write operation to ensure close proximity the the write pole to the media. This improves the written magnetic transitions by saturating the media.

The PC Guide: Function of the Read/Write Heads (http://www.pcguide.com/ref/hdd/op/heads/opFunction-c.html)

IBM Research: GMR introduction, animations (http://www.research.ibm.com/research/gmr.html)

Hitachi Global Storage: Recording Head Materials (http://www.hitachigst.com/hdd/research/recording_head/headmaterials/)