Stability-enhancing underlayer for exchange-coupled magnetic structures, magnetoresistive sensors, and magnetic disk drive systems

Abstract

An exchange-coupled magnetic structure includes a ferromagnetic layer, a coercive ferrite layer, such as cobalt-ferrite, for biasing the magnetization of the ferromagnetic layer, and an oxide underlayer, such as cobalt-oxide, in proximity to the coercive ferrite layer. The oxide underlayer has a lattice structure of either rock salt or a spinel and exhibits no magnetic moment at room temperature. The underlayer affects the structure of the coercive ferrite layer and therefore its magnetic properties, providing increased coercivity and enhanced thermal stability. As a result, the coercive ferrite layer is thermally stable at much smaller thicknesses than without the underlayer. The exchange-coupled structure is used in spin valve and magnetic tunnel junction magnetoresistive sensors in read heads of magnetic disk drive systems. Because the coercive ferrite layer can be made as thin as 1 nm while remaining thermally stable, the sensor satisfies the narrow gap requirements of high recording density systems.

Description

RELATED APPLICATIONS [0001] This is a divisional application of the application bearing Ser. No. 09 / 841,942 filed Apr. 24, 2001 which has been allowed. Another divisional of the same parent is 10 / 931,315 filed on Aug. 31, 2004.FIELD OF THE INVENTION [0002] This invention relates generally to magnetic devices such as spin valve magnetoresistive (MR) sensors and magnetic tunnel junctions. More particularly, it relates to exchange-coupled magnetic structures containing underlayers that enhance the stability of coercive ferrite layers used to bias the magnetic moment of adjacent ferromagnetic layers. BACKGROUND ART [0003] Computer systems generally use auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data are recorded on concentric, radially spaced tracks on the ...

AI Technical Summary

Problems solved by technology

Smaller sensors require thinner layers, which tend to produce lower signals.

NiO pinning layers are unsatisfactory in these thickness regimes because of their low magnetic anisotropy energy, which leads to a weak pinning field and a high critical layer thickness.

The low ordering temperature of NiO also causes thermally unstable pinning.

While cobalt-ferrite provides a number of advantages over NiO and other standard AFM pinning layer materials, it also introduces two problems.

First, coercive ferrites are thermally unstable in the thickness regime of approximately 30 nm or less, which is required for 50-nm gap sensors.

Second, unlike AFM pinning layers, ferrites exhibit a substantial magnetic moment that contributes to the overall device moment, making it difficult to balance the device moment as required for stable and consistent operation.

Nickel-ferrite has a relatively low coercivity; for example, it is generally not possible to grow nickel-ferrite with coercivities of 1 kOe.

While nickel-ferrite / NiO spin valves display increased blocking temperature (temperature at which the exchange field drops to zero) and improved thermal stability, they cannot fit within a 50 nm sensor gap.

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