Thin film disk with highly faulted crystalline underlayer

Details Thin film disk with highly faulted crystalline underlayer Thin film disk with highly faulted crystalline underlayer

1998-06-26

2001-07-31

Kiliman, Leszek (Department: 1773)

Stock material or miscellaneous articles

Circular sheet or circular blank

Recording medium or carrier

C428S065100, C428S065100, C428S690000, C428S690000, C428S690000, C428S900000, C204S192200

Reexamination Certificate

active

06268036

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of data storage devices such as disk drives having thin film magnetic disks. More particularly the invention relates to microstructure of the thin films and their effect on magnetic recording characteristics.
BACKGROUND OF THE INVENTION
The thin film magnetic recording disk in a conventional drive assembly typically consists of a substrate, an underlayer consisting of a thin film of chromium (Cr) or a Cr alloy, a cobalt-based ferromagnetic alloy deposited on the underlayer, and a protective overcoat over the magnetic layer. The word “magnetic” will be used to mean ferromagnetic, antiferromagnetic, ferrimagnetic or any other magnetic material suitable for magnetic recording. A variety of disk substrates such as NiP-coated AlMg, glass, glass ceramic, glassy carbon, etc., have been used. The microstructural parameters of the magnetic layer, i.e., crystallographic preferred orientation (PO), grain size, anisotropy and magnetic exchange decoupling between the grains, play key roles in the recording characteristics of the disk. The Cr underlayer is mainly used to control such microstructural parameters such as the PO, the unit cell size and grain size of the cobalt-based magnetic alloy.
One variation of the layer structure described above uses a very thin initial seed layer on the substrate to establish an appropriate nucleation base for the underlayer. Various materials have been used or proposed for seed layers, for example, Al, Cr, Ni
3
P, Ta, C, W, FeAl and NiAl. Laughlin, et al., have described use of an NiAl seed layer followed by a Cr underlayer and a CoCrPt magnetic layer. The NiAl seed layer with the Cr underlayer was said to induce the [10{overscore (1)}0] texture in the magnetic layer. (See “The Control and Characterization of the Crystallographic Texture of Longitudinal Thin Film Recording Media”, IEEE Trans. Magnetic. 32(5) September 1996, p. 3632).
The PO of the various materials forming the layers on the disk, as discussed herein, is not necessarily an exclusive orientation which may be found in the material, but is merely the most prominent orientation. When the Cr underlayer is sputter deposited at a sufficiently elevated temperature on a NiP-coated AlMg substrate a [200] PO is usually formed. This PO promotes the epitaxial growth of [11{overscore (2)}0] PO of the hexagonal close-packed (hcp) cobalt (Co) alloy, and thereby improves the magnetic performance of the disk. The [11{overscore (2)}0] PO refers to a film of —hexagonal structure whose (11{overscore (2)}0) planes are predominantly parallel to the surface of the film. (Likewise the [10{overscore (1)}0] PO refers to a film of hexagonal structure whose (10{overscore (1)}0) planes are predominantly parallel to the surface of the film).
In the prior art the optimal underlayer structure was believed to be one with as little deviation from the target PO as possible. For example, if [200] PO was the design goal for the underlayer, then it was thought that the more narrow the distribution of the orientation of the grains, the better and ideally every grain would be [200].
Alloys of chromium have been used for the underlayer. For example, CrTi and CrV have been used. The addition of limited amounts of titanium or vanadium modifies the lattice parameters by atomic substitution, but the crystalline nature of the underlayer is not modified.
SUMMARY OF INVENTION
An improved thin film magnetic recording medium with an underlayer with deliberately induced strain, crystalline defects and dislocations (collectively “faults”) in the lattice structure of the grains is described. The relatively high density of induced faults in the underlayer results in surprising improvements in recording characteristics such as resolution, soft error rate and signal-to-noise ratio (SNR) of the thin film disk. Although ion implantation is one way to create the faults in the underlayer, it is preferred to sputter deposit the underlayer under conditions which cause the faults to form as the film grows. The film is preferably sputter deposited using materials and parameters selected to grow grains of the underlayer material which tend to be highly faulted by incorporation of a second material which has sufficiently different characteristics to disrupt the lattice structure of the underlayer material without altering the basic crystallographic orientation. Thus, for example, a bcc structure would still be present in a Cr based underlayer even though it is highly faulted. Preferably for any particular sputtering system, the sputtering equipment and/or conditions can be adjusted to control the energy characteristics of the sputtering gas ions/atoms to ensure that the atoms of the sputtering gas will be incorporated in the film to cause the desired density of lattice faults. In the following, the underlayer of the invention will be referred to as a “Hi-fault” underlayer for convenience.


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D.E Laughlin et al., “The Control and Characterization of the Crytallographic Texture of Longitudinal Thin Film Recording Media”, IEEE Trans. on Magnetics, vol. 32, No. 5, Sep. 1996, pp. 3632-3637.
Lal et al, “Effects of Cr and Magnetic Bias on Read/Write and Noise Characteristics of CoCrTa/Cr Longitudinal Thin-Film Media” J. Appl. Phys. (USA), vol. 81, No. 8, Apr. 15, 1997, pp. 3934-6.

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