Antiferromagnetically coupled thin films for magnetic recording

Details Antiferromagnetically coupled thin films for magnetic recording Antiferromagnetically coupled thin films for magnetic recording

2001-11-09

2003-05-20

Rickman, Holly (Department: 1773)

Dynamic magnetic information storage or retrieval

Record transport with head stationary during transducing

Disk record

C428S690000, C428S690000, C428S212000, C428S216000, C428S611000, C428S666000, C428S668000, C428S900000, C427S128000, C427S131000

Reexamination Certificate

active

06567236

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to magnetic thin film media with antiferromagnetically coupled ferromagnetic layers and more particularly to materials used for the plurality of thin films in such media.
BACKGROUND OF THE INVENTION
A typical prior art head and disk system
10
is illustrated in block form in FIG.
1
. In operation the magnetic transducer
20
is supported by the suspension
13
as it flies above the disk
16
. The magnetic transducer
20
, usually called a “head” or “slider,” is composed of elements that perform the task of writing magnetic transitions (the write head
23
) and reading the magnetic transitions (the read head
12
). The electrical signals to and from the read and write heads
12
,
23
travel along conductive paths (leads)
14
which are attached to or embedded in the suspension
13
. The magnetic transducer
20
is positioned over points at varying radial distances from the center of the disk
16
to read and write circular tracks (not shown). The disk
16
is attached to a spindle
18
that is driven by a spindle motor
24
to rotate the disk
16
. The disk
16
comprises a substrate
26
on which a plurality of thin films
21
are deposited. The thin films
21
include ferromagnetic material in which the write head
23
records the magnetic transitions in which information is encoded.
The conventional disk
16
includes substrate
26
of glass or AlMg with an electroless coating of Ni
3
P that has been highly polished. The thin films
21
on the disk
16
typically include a chromium or chromium alloy underlayer and at least one ferromagnetic layer based on various alloys of cobalt. For example, a commonly used alloy is CoPtCr. Additional elements such as tantalum and boron are often used in the magnetic alloy. A protective overcoat layer is used to improve wearability and corrosion resistance. Various seed layers, multiple underlayers and laminated magnetic films have all been described in the prior art. The laminated magnetic films have included multiple ferromagnetic layers that are ferromagnetically coupled and more recently antiferromagnetic coupling has been proposed. Seed layers have been suggested for use with nonmetallic substrate materials such as glass. Typically the seed layer is a relatively thin layer which is the first crystalline film deposited in the structure and is followed by the underlayer. Materials proposed for use as seed layers include chromium, titanium, tantalum, MgO, tungsten, CrTi, FeAl, NiAl and RuAl. The use of pre-seed layers
31
is relatively recent practice. The pre-seed layer is a noncrystalline thin film which provides a base for growing the subsequent crystalline films that is superior to the substrate for this purpose. It is known that substantially improved SNR can be achieved by the use of a laminated magnetic layer of two (or more) separate magnetic layers that are separated by a nonmagnetic spacer layer. The reduced media noise is believed due to reduced exchange coupling between the magnetic layers. The use of lamination for noise reduction has been extensively studied to find the favorable spacer layer materials, including Cr, CrV, Mo and Ru, and spacer thicknesses, from a few Angstroms upward, that result in the best decoupling of the magnetic layers and the lowest media noise.
In U.S. Pat. No. 6,280,813 to Carey, et al. (which is commonly assigned with the present application) a layer structure is described that includes at least two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic coupling/spacer film. In general, it is said that the exchange coupling oscillates from ferromagnetic to antiferromagnetic with increasing coupling/spacer film thickness and that the preferred
6
Angstrom thickness of the ruthenium coupling/spacer layer was selected because it corresponds to the first antiferromagnetic peak in the oscillation for the particular thin film structure. Materials that are appropriate for use as the nonferromagnetic coupling/spacer films include ruthenium (Ru), chromium (Cr), rhodium (Rh), iridium (lr), copper (Cu), and their alloys. Because the magnetic moments of the two antiferromagnetically coupled films are oriented antiparallel, the net remanent magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in the thermal stability of the recording medium because the volumes of the grains in the antiferromagnetically coupled films add constructively. An embodiment of the structure includes two ferromagnetic CoPtCrB films, separated by a Ru spacer film having a thickness selected to maximize the antiferromagnetic exchange coupling between the two CoPtCrB films. The top ferromagnetic layer is designed to have a greater Mrt than the bottom ferromagnetic layer, so that the net moment in zero applied magnetic field is low, but nonzero. The Carey '813 patent also states that the antiferromagnetic coupling is enhanced by a thin (5 Angstroms) ferromagnetic cobalt interface layer added between the coupling/spacer layer and the top and/or bottom ferromagnetic layers. The patent mentions, but does not elaborate on the use CoCr interface layers.
SUMMARY OF THE INVENTION
The applicants disclose an antiferromagnetically coupled layer structure for magnetic recording wherein the top ferromagnetic structure is a bilayer structure including a relatively thin first sublayer of ferromagnetic material in contact with the coupling/spacer layer. The first sublayer has a higher magnetic moment than the second sublayer. The second sublayer has a lower magnetic moment and is much thicker than the first sublayer with a composition and thickness selected to provide the Mrt when combined with first sublayer that is needed for the overall magnetic structure. The layer structure of the invention results improved manufacturability and improved performance. A preferred embodiment of a layer structure according to the invention is:
a pre-seed layer preferably of CrTi
a seed layer preferably of RuAl;
an underlayer preferably of CrTi;
a bottom ferromagnetic layer preferably of CoCr;
an antiferromagnetic coupling/spacer layer preferably of Ru; and
a top ferromagnetic structure including:
a thin first sublayer of material preferably of CoCr, CoCrB or CoPtCrB, and
a thicker second sublayer of material preferably of CoPtCrB with a lower moment than the first sublayer.


REFERENCES:
patent: 5341261 (1994-08-01), Dieny et al.
patent: 5462796 (1995-10-01), Teng et al.
patent: 6280813 (2001-08-01), Carey et al.
Abarra et al., “Longitudinal MAgnetic Recording Media with Thermal Stabilization”, IEEE International Magnetics Conference, Apr. 2000.

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