All magnetic materials are initially comprised of randomly oriented domains. The magnetization process rotates domains into common alignment and causes those aligned with the magnetizing field to grow in size. Full saturation would result in a single aligned domain if all anisotropy mechanisms can be overcome.
Practical magnetic materials have an internal self-demagnetizing field, which creates unfavorable orientations in magnetic domains near geometric extremities. Shorter magnets have a greater self-demagnetizing field (hence more unfavorably oriented domains) than longer ones. A high self-demagnetizing field effectively resets a material to its small domain, initial magnetization condition where Barkhausen response exhibits itself as noise. Step function domain alignment and growth cause Barkhausen noise in the initial magnetization phase.
MR sensors tend to be short in order to reduce track width and maximize areal density. This physical limitation typically results in units that have a high self-demagnetizing field and are subject to Barkhausen noise. To overcome this, an exchange bias film is deposited at the ends of the permalloy strip. Exchange coupling between the two films overcomes the self-demagnetizing field of the sensing element, and supports a near single domain structure. Operating in this biased condition eliminates Barkhausen noise and maximizes magnetic response to signal inputs.
To develop the biasing field, the MR element must be initialized. The initializing field can come from an electromagnet or permanent magnet with inherent advantages and disadvantages for each. An electromagnetic field can be switched on and off, but power supplies and cooling can add to clean room costs. Permanent magnet assemblies can provide the required field intensities and uniformity, but they cannot be switched off and initial cost is usually higher.
For the electromagnet approach, we can design head setters that achieve magnetic field strength of up to 30 kOe (2.4MA/m). These electromagnetic head setters have low remnant field, are ESD proof, and clean room compatible. Thermal interlocks can be built in to shut off electric circuit in case the temperature rises above a preset threshold. Stray field can be shielded from magnetically sensitive devices in the proximity. A high magnetic field for initialization can be developed with a patented permanent magnet assembly produced by Dexter. Using rare earth magnets, our design is capable of generating gap flux densities as high as 30+ kOe (2.4 MA/m). The working gap field is greater than the residual induction of the magnet material. This is accomplished by superposition of magnetic fields of individual magnet segments. Features of this design are inherent flux straightness and uniform flux density in the gap, which are by-products of flux focusing. These features ensure tight control in magnetizing parameters.
Applications for head setters include read/write head manufacturing, MRAM and other magnetic memory devices and sensors.