利用者:加藤勝憲/磁気ディスクヘッド

プラッタ上のハードディスクのアームと、その先端についているヘッド。

ハードディスクドライブの磁気ディスクヘッドを顕微鏡で観察した写真。前縁の大きさは約0.3×1.2 mmである。中央の丸いオレンジ色の部分がヘッドとして機能する。金メッキパッドに接続されたワイヤーにも注目。

2013年に製造された3TBハードディスク・ドライブの読み書きヘッド。暗い長方形の部品はスライダーで、長さは1.25mmである。読み書きヘッドのコイルはスライダーの左側にある。プラッター表面はヘッドを右から左に通過する。

磁気ディスクヘッドは、ディスク（円盤）状のプラッタの上を移動し、プラッタの磁界を電流に変換する（ディスクを読み取る）か、逆に電流を磁界に変換する（ディスクに書き込む）機能を持った小さな部品である。この数年、ヘッドは幾度も変遷を遂げてきた。

ハードディスク・ドライブでは、ヘッドがディスク表面上をわずか3ナノメートルの隙間を保って浮上する。より高いデータ記録密度を実現するため、技術の世代が新しくなるごとにヘッドの高さは低くなっている。ヘッドの浮上する高さは、スライダーのディスク面にエッチングされたエア・ベアリングの設計によって制御される。エアベアリングの役割は、ヘッドがディスク表面上を移動する際、浮上する高さを一定に保つことである。エアベアリングは、プラッターの中心からのヘッドの距離によって速度が異なるにもかかわらず、プラッター面全体で同じ高さを維持するように注意深く設計されている。もし、ヘッドがディスクの表面にぶつかるようなことがあると、ヘッドクラッシュを起こす可能性がある。

磁場感応部分

Inductive heads use the same element for both reading and writing.

Traditional head

ヘッド自体は、テープレコーダーのヘッドと同じように、パーマロイやフェライトのような磁化しやすい材料に細いワイヤーコイルを巻いた小さなCの字形をした単純な装置から始まった。 書き込み時には、コイルに通電され、Cの字形の切れている隙間部分に強い磁場が形成され、隙間に隣接する記録面が磁化される。 読み取り時には、磁化された材料が発生する磁場をヘッドが通過すると、コイルに電流が発生する。隙間では磁場は非常に強く、かなり狭い。 その隙間は、記録面の磁気メディアの厚さにほぼ等しい。 この隙間によって、ディスク上の記録領域の最小サイズが決まる。 フェライトヘッドは大きく、かなり大きなフィーチャーを書き込む。 また、表面からかなり遠くに飛ばす必要があるため、より強い磁界とより大きなヘッドが必要となる[3]。

The heads themselves started out similar to the heads in tape recorders—simple devices made out of a tiny C-shaped piece of highly magnetizable material such as permalloy or ferrite wrapped in a fine wire coil. When writing, the coil is energized, a strong magnetic field forms in the gap of the C, 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. In the gap the field is very strong and 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

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 by thin-film heads.

Thin-film heads

First introduced in 1979 on the IBM 3370 disk drive, thin-film technology use photolithographic techniques similar to those used on semiconductor devices to fabricate HDD heads with smaller size and greater precision than the ferrite-based designs then in use. Thin-film heads are electronically similar to ferrite heads and used the same physics. Thin layers of magnetic (Ni–Fe), insulating, and copper coil wiring materials are built on ceramic substrates that are then physically separated into individual read/write heads integrated with their air bearing significantly reducing the manufacturing cost per unit. Thin-film heads were much smaller than MIG heads and therefore allowed smaller recorded features to be used. Thin-film heads allowed 3.5 inch drives to reach 4 GB 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.

Magnetoresistive heads (MR heads)

The next head improvement in head design was to separate the writing element from the reading element allowing the optimization of a thin-film element for writing and a separate head element for reading. The separate read element uses the magnetoresistive (MR) effect which changes the resistance of a material in the presence of magnetic field. 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 (Anisotropic MR) is used to distinguish it from the later introduced improvement in MR technology called GMR (giant magnetoresistance) and "TMR" (tunneling magnetoresistance).

The transition to perpendicular magnetic recording (PMR) media has major implications for the write process and the write element of the head structure but less so for the MR read sensor of the head structure.

AMR heads

The introduction of the AMR head in 1990 by IBM led to a period of rapid areal density increases of about 100% per year.

GMR heads

In 1997 GMR, giant magnetoresistive heads started to replace AMR heads.

Since the 1990s, a number of studies have been done on the effects of colossal magnetoresistance (CMR), which may allow for even greater increases in density. But so far it has not led to practical applications because it requires low temperatures and large equipment size.

TMR heads

In 2004, the first drives to use tunneling MR (TMR) heads were introduced by Seagate allowing 400 GB drives with 3 disk platters. Seagate introduced TMR heads featuring integrated microscopic heater coils to control the shape of the transducer region of the head during operation. The heater can be activated prior to the start of a write operation to ensure proximity of the write pole to the disk/medium. This improves the written magnetic transitions by ensuring that the head's write field fully saturates the magnetic disk medium. The same thermal actuation approach can be used to temporarily decrease the separation between the disk medium and the read sensor during the readback process, thus improving signal strength and resolution. By mid-2006 other manufacturers have begun to use similar approaches in their products.

See also

• Applied Magnetics Corporation, once the largest supplier of disk heads
• Tape head

外部リンク

• The PC Guide: Function of the Read/Write Heads
• IBM Research: GMR introduction, animations Archived 2012-01-11 at the Wayback Machine.
• Hitachi Global Storage Technologies: Recording Head Materials

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