WO2015122847A1 - An improved magnetic recording medium - Google Patents

Abstract

A magnetic recording medium comprising a dual layer overcoat, which has a thickness of ≤2 nm, comprising a corrosion barrier layer on a magnetic layer of the magnetic recording medium and a protective carbon-containing layer on the corrosion barrier layer is provided to improve corrosion resistance and tribological performance of the magnetic recording medium.

Description

An improved magnetic recording medium

Technical Field

The present invention relates to a magnetic recording medium, as well as to a method of treating a surface of a magnetic layer of the magnetic recording medium.

Background

Hard disk drives comprise a magnetic medium and a read-write head, flying a few nanometers above the surface of the magnetic medium, which is responsible for writing and recovering the recorded data on the disk drive. The surfaces of the magnetic medium and the head require to be protected against corrosion and mechanical damage such as wear and tear especially when intermittent contact happens between the head and the magnetic medium. The present form of this protection is coating these surfaces with overcoats of a thin, continuous and hard material and a lubricant layer.

The demand for higher areal densities (number of bits/unit area on a disk surface) in the magnetic .hard disk drives has been consistently increasing. One way of achieving higher areal density is to reduce the thickness of the overcoats. However, decreasing the thickness of the overcoats may give rise to other problems such as a reduction in the tribological and anti-corrosion performance.

Heat assisted magnetic recording (HAMR) technology is also being used in manufacturing hard disk drives. Since HAMR uses a laser-optical system integrated into the magnetic head to locally heat a fine-grained material of high magnetic anisotropy energy density above its Curie temperature to store single bits in very small areas without being limited by the super-paramagnetic effect, the thermal stability of the carbon overcoats need to be monitored since the localised laser heating may lead to oxidation or graphitization of the overcoat material, thereby deteriorating the tribological properties.

There is therefore a need for an overcoat which has improved, thermal resistance and corrosion resistance with good tribological properties. Summary of the invention

The present invention seeks to address these problems, and/or to provide an improved magnetic recording medium.

In general terms, the invention relates to forming a dual layer overcoat formed on a magnetic layer of a magnetic recording medium, wherein the dual layer has a thickness of <2 nm. The dual layer overcoat provides protection to the magnetic layer against wear and mechanical damage, as well as acts as a diffusion barrier to prevent corrosion of the magnetic layer without compromising on the tribological properties. The dual layer overcoat is also thermally stable at elevated temperatures such as temperatures near the Curie temperature of the magnetic layer.

According to a first aspect, the present invention provides a magnetic recording medium comprising:

- a substrate;

- a magnetic layer disposed on the substrate; and

- a dual layer overcoat comprising a corrosion barrier layer on the magnetic layer and a protective carbon-containing layer on the corrosion barrier layer, wherein the dual layer overcoat has a thickness of ≤ 2 nm.

The corrosion barrier layer may comprise any suitable material. For example, the corrosion barrier layer may comprise SiN_Xl CrN_x, CrO_x, TiN_x, W_x, SiC, TiC, WC, or a combination thereof. In particular, the corrosion barrier layer may comprise SiN_x or CrN_x. Even more in particular, the corrosion barrier layer is SiN_x.

The corrosion barrier layer may have any suitable thickness. For example, the thickness of the corrosion barrier layer may be 0.4-0.8 nm. In particular, the thickness may be abput.0.4 nm.

The protective carbon-containing layer may comprise any suitable material. For example, the protective carbon-containing layer may comprise diamond-like carbon (DLC), or nitrogenated carbon (CN_X).

The protective carbon-containing layer may have any suitable thickness. For example, the thickness of the protective carbon-containing layer may be 1.2-1.6 nm. In particular, the thickness may be about .2 nm. According to a particular aspect, the dual layer overcoat may comprise a corrosion barrier layer which comprises SiN_x and a protective carbon-containing layer comprising diamond-like carbon (DLC). In particular, the dual layer overcoat may comprise a SiN_x corrosion barrier layer having a thickness of about 0.4 nm and a diamond-like carbon protective layer having a thickness of about 1.2 nm.

The dual layer overcoat may have a low coefficient of friction. For example, the coefficient of friction of the dual layer overcoat may be≤0.4.

According to a second aspect, the present invention provides a method of treating a surface of a magnetic layer of a magnetic recording medium comprising:

- depositing a corrosion barrier layer on the magnetic layer; and

- depositing a protective carbon-containing layer on the corrosion barrier layer, wherein the corrosion barrier layer and the protective carbon-containing layer have a combined thickness of ≤ 2 nm.

The corrosion barrier layer and the protective carbon-containing layer may be as described above.

The depositing of the corrosion barrier layer may be by any suitable process. For example, the depositing of the corrosion barrier layer may be by sputtering, ion beam deposition, or chemical vapour deposition. In particular, the depositing of the corrosion barrier layer may be by sputtering.

The depositing of the protective carbon-containing layer may be by any suitable process. For example, the depositing of the carbon-containing layer may be by sputtering, ion beam deposition, pulsed laser ablation (PLD), pulsed direct current sputtering, or filtered cathodic vacuum arc (FCVA) process. In particular, the depositing of the corrosion barrier layer may be by FCVA process.

For example, the FCVA process may comprise bombarding the corrosion barrier layer with energetic carbon (C⁺) ions. The bombarding may be with energetic C⁺ ions having suitable ion energy. In particular, the C⁺ ions are bombarded at an ion energy of about 20-350 eV. According to a particular aspect, the depositing of the corrosion barrier layer may be by sputtering and the depositing of the corrosion barrier layer may be by FCVA process.

The method of the second aspect may be applied to existing hard disk drives or to future hard disk drives to achieve higher areal densities, as well as better protection against wear, corrosion and an increased thermal stability of the magnetic medium.

Brief Description of the Drawings

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:

Figure 1 shows a cross-sectional view of a magnetic recording medium according to a particular embodiment of the present invention;

Figure 2 shows a process flow of a method of treating a surface of a magnetic layer of a magnetic recording medium according to a particular embodiment of the present invention;

Figure 3 shows a cross-sectional TEM image of a dual layer overcoat comprising a SiN_x corrosion barrier layer and a DLC protective carbon-containing layer according to a particular embodiment of the present invention;

Figure 4 shows the surface topographies of two samples obtained by Atomic Force Microscopy as a result of heat treatment on a surface of (A) sputtered carbon overcoat and (B) FCVA deposited carbon overcoat;

Figure 5 shows the coefficient of friction graphs for four samples after being subjected to ball-on-disk tribological testing;

Figure 6 shows the macromagnetic hysteresis loops obtained by Magneto-optical Kerr Effect Microscopy of different samples;

Figure 7 shows a bar chart showing the corrosion current densities of magnetic media having different overcoats of varying thicknesses; Figure 8 shows a flowchart of surface modification process and cross section schematics of (a) the as-received commercial disk; (b) commercial disk after Ar⁺ ion etching for COC removal; and (c) disk sample after deposition of the SiN_x/C dual layer overcoat onto the etched commercial disk;

Figure 9 shows a schematic of a typical FCVA setup for the deposition of the protective carbon-containing layer;

Figure 10 shows cross-sectional TEM images showing the thickness of the overcoats for samples (a) 16C, (b) 16SiN, (c) 4SiN12C, and (d) CM;

Figure 11 shows the average roughness (R_a) and root-mean-square roughness (Rq) values of samples 16C, 16SiN, 4SiN12C, and CM;

Figure 12 shows (a) summary of coefficient of friction data with respect to number of cycles as obtained from ball-on-disk tribological tests; (b) bar chart showing the average coefficient of friction values for each sample;

Figure 13 shows optical images of (a) to (e): counterface ball and (f) to (j) sample surface after ball-on-disk tribological tests;

Figure 14 shows MOKE hysteresis loops of samples 16C, 16SiN, 4SiN12C, and CM under an applied switching magnetic field;

Figure 15 shows potentiodynamic polarization curves of samples 16C, 16SiN, 4SiN12C, CM, and BM measured by a custom-made three-electrode electrochemical setup;

Figure 16 shows the corrosion current density ^' _Corr) values of tested disk samples extracted from Figure 15;

Figure 7 shows deconvoluted C 1s core level XPS spectra of samples (a) 16C, (b) 4SiN12C, and (c) CM. Deconvoluted (d) Si 2p and (e) N 1s core level spectra of sample 4SiN12C;

Figure 18 shows deconvoluted Co 2p₃/2 core level XPS spectra of samples (a) 16C, (b) 4SiN12C, and (c) CM. Deconvoluted Cr 2p_3/2 core level XPS spectra of samples (d) 16C, (e) 4SiN12C, and (f) CM; Figure 19 shows visible Raman spectra for samples (a) 16C, (b) 4SiN12C, and (c) CM. UV Raman spectra for samples (d) 16C, (e) 4SiN12C, and (f) CM;

Figure 20 shows schematic representation of various overcoats of varying thicknesses on magnetic layers;

Figure 21 shows the coefficient of friction versus number of cycles for bare commercial media and commercial media with 2.7 nm carbon overcoat and 1 nm lubricant layer; and

Figure 22 shows the coefficient of friction versus number of cycles for various magnetic layers with different overcoats.

Detailed Description

The present invention provides an improved magnetic recording medium. The magnetic recording medium may be used in hard disk drives. In particular, the magnetic recording medium may have a higher resistance to corrosion, improved tribe-logical performance and higher thermal stability. Further, hard disk drives comprising the improved magnetic recording medium may have higher areal density compared to conventional hard disk drives.

Figure 1 shows a magnetic recording medium 200 comprising a substrate 202, a magnetic layer 204 disposed on the substrate 202 and a dual layer overcoat 206. The dual layer overcoat 206 comprises a corrosion barrier layer 208 and a protective carbon-containing layer 210. In particular, the corrosion barrier layer 208 is disposed on the magnetic layer 204 and the protective carbon-containing layer 210 is disposed on the corrosion barrier layer 208. ^~

The substrate 202 may be any suitable substrate. In particular, the substrate may be a non-magnetic substrate. For example, the substrate may be made of glass or glass- ceramic, metal alloys such as aluminium alloys and NiP/AI, plastic or polymer material, ceramic, glass-polymer, composite materials or other non-magnetic materials.

The magnetic layer 204 may be composed of any suitable material. For example, the magnetic layer 204 may be composed of any one of, but not limited to, cobalt, chromium, iron, platinum, or combinations thereof, such as a cobalt-based alloy, a chromium-based alloy or an iron-based alloy, like Co-Cr-Pt, CoCrPtB, or FePt. The dual layer overcoat 206 is formed on the magnetic layer 204. In particular, the dual layer overcoat 206 is formed by the deposition of a corrosion barrier layer 208 on the magnetic layer 204 followed by the deposition of a protective carbon-containing layer 210 on the corrosion barrier layer 208. The formation of the dual layer overcoat 206 will be described in detail in relation to steps 406 and 408 of Figure 2.

The dual layer overcoat 206 may have any suitable thickness. In particular, the thickness of the dual layer overcoat may be≤2 nm. For example, the thickness of the dual layer overcoat may be 1.0-2.0 nm, 1.1-1.8 nm, 1.2-1.7 nm, 1.3-1.6 nm, 1.4-1.5 nm. Even more in particular, the thickness of the dual layer overcoat may be about 1.2- 1.7 nm. The thickness of the dual layer overcoat 206 is the combined thickness of the corrosion barrier layer 208 and a protective carbon-containing layer 210.

The corrosion barrier layer 208 may comprise any suitable corrosion resistant material. For example, the corrosion barrier layer 208 may comprise nitrides or carbides of Si, Cr, Ti or W. In particular, the corrosion barrier layer 208 may be selected from, but not limited to, SiN_Xl CrN_x, CrO_x, TiN_Xl SiC, TiC, WC, or a combination thereof. In particular, the corrosion barrier layer 208 may comprise SiN_x or CrN_x. Even more in particular, the corrosion barrier layer 208 is SiN_x.

The corrosion barrier layer 208 may have any suitable thickness. For example, the thickness of the corrosion barrier layer 208 may be 0.4-0.8 nm. In particular, the thickness of the corrosion barrier layer 208 may be 0.4-0.8 nm, 0.5-0.7 nm. Even more in particular, the thickness may be about 0.4 nm.

The protective carbon-containing layer 210 may comprise any suitable material. For example, the protective carbon-containing layer 210 may comprise, but is not limited to, diamond-like carbon (DLC), nitrogenated carbon (CN_X) or a combination thereof.

The protective carbon-containing layer 210 may have any suitable thickness. For example, the thickness of the protective carbon-containing layer 210 may be 1.2-1.6 nm. In particular, the thickness may be 1.2-1.6 nm, 1.3-1.5 nm, 1.4-1.5 nm. Even more in particular, the thickness may be about 1.2 nm.

According to a particular aspect, the dual layer overcoat 206 may comprise a corrosion barrier layer 208 which comprises SiN_x and a protective carbon-containing layer 210 comprising diamond-like carbon. In particular, the dual layer overcoat 206 may comprise a SiN_x corrosion barrier layer 208 having a thickness of about 0.4 nm and a diamond-like carbon protective layer 210 having a thickness of about 1.2 nm.

According to another particular aspect, the dual layer overcoat 206 may comprise a corrosion barrier layer 208 which comprises CrN_x and a protective carbon-containing layer 210 comprising diamond-like carbon. In particular, the dual layer overcoat 206 may comprise a CrN_x corrosion barrier layer 208 having a thickness of about 0.5 nm and a diamond-like carbon protective layer 210 having a thickness of about 1.2 nm.

The coefficient of friction is defined as the ratio of the frictional force that resists the motion of the read-write head and the force that maintains contact between the read- write head and the surface of the dual layer overcoat 206. Accordingly, a lower coefficient of friction would mean there is a higher wear life by reducing the wear and tear on the surfaces of the dual layer overcoat 206 and the read-write head. The dual layer overcoat may have a low coefficient of friction. For example, the dual layer overcoat may have a coefficient of friction of ≤0.4. In particular, the coefficient of friction may be in the range 0.1-0.4, 0.2-0.3. Even more in particular the coefficient of friction may be≤ 0.2.

Overcoats are provided on magnetic layers of conventional magnetic recording medium for protecting the magnetic recording medium against corrosion and for lowering the friction during an intermittent contact between the magnetic recording medium and the read-write head. Conventionally, overcoats comprise a thick protective layer of diamond-like carbon (DLC) and a lubricant layer. However, such overcoats may not be thermally stable and therefore be unsuitable for hard disk drives manufactured using the heat assisted magnetic recording (HAMR) technology. Further, conventional carbon overcoats which are modified to be ultrathin, in order to achieve an increase in areal density of the hard disk drive, are not as effective as a diffusion barrier to prevent corrosion in the same way as thicker carbon overcoats.

In the present invention, as shown in Figure 1 , the dual layer overcoat 206 replaces the traditional overcoat. The advantage of the dual layer overcoat 206 is that it comprises the corrosion barrier layer 208 comprising a corrosion resistant material, as well as the protective carbon-containing layer 210 which is wear resistant, while itself remaining ultrathin since the overall thickness of the dual layer overcoat 206 is maintained to be ≤2 nm. In this way, the dual layer overcoat 206 is able to achieve the high areal densities which ultrathin carbon overcoats were able to achieve while also being corrosion and wear resistant.

While the corrosion barrier layer 208 provides suitable corrosion resistance to the magnetic layer 204, it is not sufficient to only provide a corrosion barrier layer 208 on the magnetic layer 204 since materials used for providing the corrosion barrier layer 208 such as SiN_x may not be wear resistant and are also prone to becoming oxidised or hydrolysed in the vicinity of oxygen and humidity of the ambient air. It is therefore necessary to include the protective carbon-containing layer 210.

The dual layer overcoat 206 may provide the adequate protection against corrosion of the magnetic layer 204 and reduces the friction between the read-write head of a hard disk drive and the magnetic recording medium 200 when the read-write head and the magnetic recording medium 200 contact each other. As a result of a reduction of the thickness of the overcoats, the head-media spacing (HMS) is reduced. This reduction in the HMS may result in an increase in the areal density of a magnetic recording device comprising the magnetic recording medium 200, such as the hard disk drive.

A method 400 of treating a surface of a magnetic layer 204 of a magnetic recording medium 200 will now be described with reference to Figure 2.

Step 402 comprises depositing a magnetic layer on a substrate. The magnetic layer and the substrate may be as described above in relation to the magnetic layer 204 and the substrate 202. Any suitable process for depositing the magnetic layer 204 on the substrate 202 may be used for the depositing of step 402. For example, the step 402 may be carried out by sputtering. The step 402 may be carried out in an inert gas atmosphere, such as an atmosphere of pure argon.

Once the magnetic layer 204 is deposited on the substrate 202, the surface of the magnetic layer 204 is optionally cleaned according to step 404. The step 404 comprises cleaning the surface of the magnetic layer 204 using any suitable solvent. For example, the solvent may be, but not limited to, isopropyl alcohol (IPA). The step 404 may also comprise etching a surface of the magnetic layer 204 by Ar⁺ ion beam or plasma etching to remove any surface oxide. The etching may not be necessary if step 402 and subsequent step 406 are performed in-situ in the same vacuum chamber.

The cleaned surface of the magnetic layer 204 is then subjected to a step 406 which comprises depositing a corrosion barrier layer on the magnetic layer 204. The corrosion barrier layer may be as described above in relation to the corrosion barrier layer 208. The depositing of the corrosion barrier layer 208 on the magnetic layer 204 may be by any suitable process. For example, the depositing of step 406 may be by sputtering, ion beam deposition, chemical vapour deposition, or the like. In particular, the depositing of step 406 is by sputtering. The advantage of sputtering is that damage of the magnetic layer 204 is prevented since sputtering avoids high-energy embedment of the corrosion barrier layer onto the magnetic layer 204. The step 406 comprises depositing the corrosion barrier layer until the corrosion barrier layer has a thickness of about 0.4-0.8 nm. In particular, the step 406 may be carried out until the thickness of the corrosion barrier layer 208 is 0.4-0.8 nm, 0.5-0.7 nm. Even more in particular, the thickness may be about 0.4 nm. In particular, the depositing of step 406 comprises depositing a corrosion barrier layer comprising SiN_x or CrN_x. Even more in particular, the depositing of step 406 comprises depositing a corrosion barrier layer comprising SiN_x having a thickness of about 0.4 nm.

Following the step- 406, a further depositing step 408 is carried out to deposit a protective carbon-containing layer on the corrosion barrier layer 208. The protective carbon-containing layer may be as described above in relation to the protective carbon- containing layer 210. The depositing of the protective carbon-containing layer 210 on the magnetic layer 204 may be by any suitable process. For example, the depositing of step 408 may be by sputtering, ion beam deposition, pulsed laser ablation (PLD), filtered cathodic vacuum arc (FCVA) process, or the like. In particular, the depositing of step 408 is either by PLD or by the FCVA process. Even more in particular, the depositing of step 408 is by FCVA process.

The advantage of the FCVA process is that fully ionised C⁺ plasma may be achieved. This enables optimization oMhe ion energy of the C⁺ ions so that better properties of the protective carbon-containing layer 210 may be_. achieved, such as tribological and corrosion performance. The depth of penetration of the ions is determined by the energy of the impinging ions. The ion energy can be controlled by applying proper negative bias to the substrate. The FCVA process may comprise bombarding the corrosion barrier layer 208 with energetic carbon (C⁺) ions. For example, the step 408 may be carried out in a vacuum atmosphere. By using the FCVA process, a uniform and continuous film may be achieved in the protective carbon-containing layer. In particular, the energetic ions may have ion energy in the range of 20-350 eV during the step 408. In particular, the energetic ions may have an ion energy of about 20-350 eV, 40-320 eV, 50-300 eV, 90-270 eV, 100-250 eV, 120-220 eV, 150-200 eV, 70-180 eV. Even more in particular, the energetic ions may have an ion energy of about 20 eV.

The step 408 comprises depositing the protective carbon-containing layer until the protective carbon-containing layer has a thickness of about 1.2-1.6 nm. In particular, the step 408 may be carried out until the thickness of the protective carbon-containing layer 210 is 1.2-1.6 nm, 1.3-1.5 nm, 1.4-1.5 nm. Even more in particular, the thickness may be about 1.2 nm. In particular, the depositing of step 408 comprises depositing a protective carbon-containing layer comprising diamond-like carbon (DLC). Even more in particular, the depositing of step 408 comprises depositing a protective carbon- containing layer comprising DLC having a thickness of about 1.2 nm.

Following the step 408, the combined thickness of the deposited corrosion barrier layer 208 and the deposited protective carbon-containing layer 210 is≤2 nm.

In the method 400, the sputtering and FCVA source for the steps 406 and 408, respectively, may be located within a same deposition chamber. In particular, the surface of the magnetic layer 206 is subjected to both of the plasma sources without breaking the vacuum of the chamber between steps 406 and 408.

According to a particular embodiment, the, method 400 comprised: depositing a 0.4 nm thick corrosion barrier layer comprising SiN_x by sputtering on a magnetic layer; and depositing a 1.2 nm thick protective carbon-containing layer comprising DLC by FCVA process on the corrosion barrier, layer. The treated magnetic layer comprising a dual layer overcoat 206 of corrosion barrier layer 208 and protective carbon-containing layer 210 is shown in Figure 3.

Although it has been shown in the art that embedment of a magnetic layer with energetic C⁺ ions at higher energies may improve the wear resistance of a magnetic recording medium, it has been proven that such embedment may deteriorate the magnetic properties of the magnetic recording medium. However, in the present invention, depositing a corrosion barrier layer 208 with proper adhesion to the magnetic layer 204 effectively helps prevent any deterioration of the magnetic properties while maintaining desirable tribplogical properties of the protective carbon-containing layer.

While a carbon rich layer is more effective in protecting a magnetic layer against wear, the corrosion barrier layer is more effective in protecting the magnetic layer against corrosion. As will be shown in the examples, when the surface of a magnetic layer of a magnetic recording medium is treated to provide a corrosion barrier layer and a protective carbon-containing layer on the corrosion barrier layer, desirable corrosion and wear protection was achieved. In contrast, when only a corrosion barrier layer was provided having the same thickness as the combined thickness of the corrosion barrier layer and the protective carbon-containing layer, while corrosion of the magnetic layer was low, the corrosion barrier layer was not tribologically as protective as a protective carbon-containing layer of the same thickness. Thus, having a dual layer overcoat comprising a corrosion barrier layer and a protective carbon-containing layer on the magnetic layer is advantageous in obtaining a magnetic recording medium with desirable corrosion and wear protection.

It is well known that elevated temperatures result in oxidation of pure sputtered carbon overcoats (COCs) or result in hydrogen depletion in the conventional chemical vapour deposited (CVD) hydrogenated COCs which consequently leads to graphitization of the overcoat. As seen from Figures 4(A) and 4(B), the carbon film deposited by the FCVA method (Figure 4(B)) showed more resistance against oxidation and graphitization as compared with a sputtered carbon film (Figure 4(A)) or conventional CVD hydrogenated carbon overcoat found in commercial magnetic recording medium. In particular, a surface of sputtered and FCVA deposited COCs of the same thickness were heated by shining a laser on the surface to increase the temperature up to 760K and the surface of both samples were probed by Micro Raman spectroscopy and Atomic Force Microscopy. As seen in Figures 4(A) and 4(B), while a considerable amount of the sputtered COC vanished due to oxidation at high temperature, no change in the thickness or structure of the FCVA deposited COC was detectable. Accordingly, the dual layer overcoat comprising a corrosion barrier layer and a protective carbon-containing layer on the magnetic layer is also thermally stable under HAMR conditions. This is because of increased sp³ carbon content (i.e. the number of 1 sp³ C-C bonds) of the dual layer overcoat which in turn increases the thermal stability of the overcoat. In particular, the protective carbon-containing layer results in higher sp³ bonds being formed within the protective carbon-containing layer, thus showing more resistance against oxidation and graphitization. The depositing of the protective carbon-containing layer on the corrosion barrier layer by embedment of energetic C⁺ ions results in the formation of a dense atomically mixed layer, such as a mixture of Si or Cr, and C atoms, containing strong carbide bonds (Si-C or Cr-C) which are thermally stable. Formation of these thermally stable bonds in addition to the formation of an atomically dense interlayer due to the embedment of the carbon ions into the corrosion barrier layer further enhances the formation of C-C sp³ bonds and consequently improves the thermal stability of the dual layer overcoat.

Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention. For example, the method described above can equally apply to treating the surface of a magnetic recording medium head in any magnetic device such as a magnetic tape drive or hard disk drive. The method and dual layer overcoat may also be applied to micro-electro-mechanical systems (MEMS) with accurate tolerances to protect the moving parts against wear and corrosion.

Thus, the dual layer overcoat can also be formed as part of other magnetic recording devices or applied to any surface requiring corrosion and mechanical protection, improvement in adhesion, reduction in friction and/or increase in thermal stability. In particular, the dual layer overcoat of the present invention which has a thickness of ≤2 nm provides protection against wear and corrosion with comparable tribological performance to thicker conventional DLC coatings (about 2.7 nm) of existing commercial media.

Having now generally described the invention, the same will be more readily understood through reference to the following examples which are provided by way of illustration, and are not intended to be limiting. EXAMPLES

Example 1

An experiment was carried out to measure the thickness of the dual layer SiN_x/C overcoat by cross-sectional Transmission Electron Microscopy (TEM) imaging. To do this, a magnetic layer coated with the dual layer overcoat was diced into two small pieces of about 2 mm by 4 mm in size. These two pieces were then glued face to face using a cured mixture of epoxy resin and hardener and later polished down to a thickness of approximately 50 μητι. Subsequently, the polished samples were mounted onto a copper slot and milled down by Ar⁺ ions in an ion miller system to create a dimple at the interface^ such that the interface becomes thin enough to be imaged by TEM. The imaging of the cross-sectional thickness of the dual layer overcoat was performed in a JEOL JEM-200EX TEM with an accelerating voltage of up to 200 kV. The image is shown in Figure 3. As can be seen, the measured thickness of the dual layer SiN_x/C overcoat (1.6 ± 0.2 nm) is close to the expected thickness of 1.6 nm.

An experiment was carried out to evaluate the thermal stabilities of the sputtered and FCVA deposited carbon overcoats. An optical pump-probe setup was used to irradiate the sputtered (A) and FCVA (B) deposited carbon films on iron platinum (FePt) media. A schematic and working principle of such a setup can be found in Kundu S. et al., (ACS Appl. Mater. Interfaces, 2015, 7:158-165). A pump laser was used to locally heat a spot of about 8 im in diameter of the media with the overcoats up to the Curie temperature (T_c). A peak temperature of > 700 K was used. In terms of the heating cycle, the heating rate was about 10⁶ K s with peak temperature dwell time of 500 s. The total heating cycle lasted for 2 s. Figures 4(A) and 4(B) show the surface topography of the irradiated region of the sputtered and FCVA deposited carbon overcoats on FePt media respectively, as measured by Atomic Force Microscopy (AFM).

An experiment was carried out to evaluate the tribological properties and wear resistance of a commercial disk without any protective overcoats (CM-bare), two different commercial disks (CM1 - a magnetic medium with a single layer DLC overcoat of about 2.7 nm thickness; and CM2 - a magnetic medium with a single layer DLC overcoat of about 2.7 nm thickness and a perfluoropolyether (PFPE) lubricant layer of about 1.0 nm thickness), as well as a surface-modified commercial disk containing a dual layer overcoat of thickness of about 1.6 nm comprising a corrosion barrier layer comprising SiN_x and a protective carbon-containing layer deposited on top of the magnetic medium (CM+4ASiN+12AC). The tribological and wear behaviour of these samples were investigated by ball-on-disk tribological tests. In each test, a sapphire (Al₂0₃) ball with a diameter of 2 mm was used as the counterface. The ball was kept in constant, fixed contact with the sample surface, while the sample rotated under the ball at a constant linear speed of 1 cm s^"1. The rotation created a wear track on the sample surface with diameter of 1.2 mm. The normal load applied by the ball on the sample surface was 20 mN, and the test was carried out for 10,000 sliding cycles (rotations) and then stopped. From the test, the coefficient of friction was measured and recorded. For each set of samples, the test was repeated at least three times on different locations to ensure consistency in the results. The results are as shown in Figure 5.

An experiment was carried out to measure the macromagnetic properties of various commercial media containing different overcoats using a custom-made magneto- optical Kerr effect (MOKE) system. For each sample, a magnetic field was applied from 0 Tesla to +1 Tesla to -1 Tesla and back to 0 Tesla, while the Kerr signal was recorded simultaneously. The Kerr signal values obtained were then normalized to the maximum Kerr signal value (maximum normalized Kerr signal value is 1 ), to obtain the resulting magnetic hysteresis loop of normalized Kerr rotation versus applied field.

The following samples were measured: (a) CM, which is commercial disk containing a conventional carbon overcoat with thickness of about 2.7 nm but without a PFPE lubricant layer, (b) CM/16ASiN, which is an etched magnetic medium containing a single layer SiN_x overcoat with thickness of about 1.6 nm, and (c) CM/4ASiN/12AC, which is an etched magnetic medium containing a dual layer overcoat with thickness of about 1.6 nm, comprising SiN_x with thickness of about 0.4 nm followed by carbon with thickness of about 1.2 nm. The hysteresis loops for CM, CM/16ASiN and CM/4ASiN/12AC are as shown in Figure 6. From the results, it can be concluded that the macromagnetic properties of the CM, CM/16ASiN and CM/4ASiN/12AC samples did not suffer any deterioration despite CM/16ASiN and CM/4ASiN/12AC both having been subjected to magnetic layer etching process and overcoat deposition processes of sputtering (for SiN_x) and/or FCVA (for carbon). An experiment was carried out to evaluate the corrosion protection of the various overcoats deposited on magnetic media. To do this, the coated media samples were placed in a custom-made corrosion setup to measure the corrosion current density Ocorr) of each sample by electrochemical corrosion testing. A schematic and working principle of such a setup can be found in Yeo R.J. et al., (Diamond Relat. Mater., 2014, 44:100-108). This setup consists of a holder containing a silver/silver chloride (Ag/AgCI) reference electrode and a platinum counter-electrode covered with cotton wool that is soaked with an electrolyte of 0.1 M NaCI solution. The holder was held tight against the tested disk sample, which acted as the working electrode, such that a geometric surface area of about 0.24 cm² was exposed to the electrolyte. The open circuit potential was monitored for 1 hour before commencement of each potentiodynamic polarization test, which was always initiated from the sample's open circuit potential and swept at a rate of 0.1 mV s^"1. Positive and negative potential sweeps were conducted at different locations on the sample. For each positive sweep, the potential was swept from the open circuit potential to +0.8 V versus Ag/AgCI, whereas for each negative sweep, the potential was swept from the open circuit potential to -0.4 V versus Ag/AgCI. A minimum of two measurements were performed for each positive and negative sweep on each disk until consistent data was obtained.

The following samples were measured: (a) CM1 , which is a commercial disk containing a conventional carbon overcoat with thickness of about 2.7 nm with a PFPE lubricant layer with thickness of about 1.0 nm, (b) CM3a+etched+C350/90, which is etched magnetic medium containing a FCVA deposited carbon overcoat using a bi-level ion energy deposition/embedment process of carbon ions at 350 eV followed by carbon ions at 90 eV, (c) CM+4SiN-12C, which is etched magnetic medium containing a dual layer overcoat with thickness of about 1.6 nm, comprising SiN_x with thickness of about 0.4 nm followed by carbon with thickness of about 1.2 nm, (d) CM+4SiN-9C, which is etched magnetic medium containing a dual layer overcoat with thickness of about 1.3 nm, comprising SiN_x with thickness of about 0.4 nm followed by carbon with thickness of about 0.9 nm, (e) CM+4SiN-6C, which is etched magnetic medium containing a dual layer overcoat with thickness of about 1.0 nm, comprising SiN_x with thickness of about 0.4 nm followed by carbon with thickness of about 0.6 nm, (f) CM+4SiN-3C, which is etched magnetic medium containing a dual layer overcoat with thickness of about 0.7 nm, comprising SiN_x with thickness of about 0.4 nm followed by carbon with thickness of about 0.3 nm, (g) CM+4SiN-12C (2^nd trial), which is another sample of etched magnetic medium containing a dual layer overcoat with thickness of about 1.6 nm, comprising SiN_x with thickness of about 0.4 nm followed by carbon with thickness of about 1.2 nm, (h) CM+0SiN-12C, which is etched magnetic medium containing a single layer overcoat of carbon with thickness of about 1.2 nm, (i) CM+0SiN-16C, which is etched magnetic medium containing a single layer overcoat of carbon with thickness of about 1.6 nm, and (j) CM+16SiN-0C, which is etched magnetic medium containing a single layer overcoat of SiN_x with thickness of about 1.6 nm. Figure 7 summarizes the jcorr results for all the above samples in a bar chart.

From the results, the following conclusions can be made: 1 ) With the same thickness of SiN_x and decreasing carbon thickness, the jcorr increases, indicating a lower corrosion resistance with decreasing carbon thickness while SiN_x thickness is maintained (decreasing overall thickness of overcoat); 2) Between the same thickness of single layer SiN_x overcoat and single, layer carbon overcoat (CM+OS1N-I6C and CM+16SiN+0C), the SiN_x overcoat gives a lower j_corr, indicating that SiN_x is a better corrosion protection layer than carbon even at the same thickness of 1.6 nm; 3) Between the same overall overcoat thickness of the dual layer overcoat and single layer carbon overcoat (GM+4SiN-12C (2^nd trial) and CM+0SiN-16C), the dual layer overcoat shows a slightly lower j_corr value, indicating a slight improvement in the corrosion protection.

Example 2

An experiment was carried out on five types of magnetic recording medium with different overcoats to investigate the difference in their properties;

Samples

The five samples were as follows: Sample Sample type Overcoat nomenclature structure

Ar+ etched commercial C (16 A)

16C

disk (COC removed)

Ar+ etched commercial SiNx (16 A)

16SiN

disk (COC removed)

Ar+ etched commercial SiNx(4 A)/C(12 A)

4SiN12C

disk (COC removed)

as-received commercial commercial COC

CM

disk (without lubricant) (about 27 A)

as-received commercial no overcoat (bare

BM disk (without COC and media)

without lubricant)

Table 1 : Description and nomenclature of samples used in Example 2

Methods of preparing samples

The sample preparation process for fabricating the SiN_x/C dual layer overcoat on commercial media disk is shown in Figure 8. Commercial 2.5" disks were used as the starting substrates in this example, consisting of (from bottom to top) a glass disk substrate, a multilayer structure of various materials for optimized recording performance, a CoCrPt:Oxide magnetic recording layer, a carbon overcoat (COC) layer, and finally a lubricant layer. A cross-section schematic of the commercial disk structure is provided in Figure 8(a). Prior to deposition of the SiN_x/C dual layer overcoat, the pre-existing commercial COC and lubricant layer were removed by Ar⁺ ion beam etching at an ion energy of 300 eV, as seen in Figure 8(b). A secondary ion mass spectrometer (SIMS) detector was used to calibrate the etching rate to remove the commercial COC. It should be noted that this etching process was applied as a necessary step when fabricating these overcoats for the present experiment. However, it is not required in a conventional hard disk manufacturing process.

Deposition of the SiN_x and carbon layers on the etched commercial disk substrates were carried out in situ after etching, using a VEECO Deposition System equipped with a pulsed filtered cathodic arc source, sputtering source and Ar⁺ ion beam etching capability. A 99.99% pure silicon target was used for the deposition of the SiN_x corrosion barrier layer, while a 99.999% pure graphite rod was used for the deposition of the protective carbon-containing layer by FCVA. Deposition was carried out at a background pressure of about 10^"7 Torr. First, a 4 A ultrathin layer of SiN_x was deposited by pulsed DC reactive sputtering in a gaseous mixture of Ar + N₂, with a ratio of 67% Ar to 33% N₂ and at a duty cycle of 0.7. Next, the sample was transferred to the FCVA chamber in situ under vacuum. A schematic of a FCVA setup for deposition of COC is shown in Figure 9.

A 12 A layer of carbon was subsequently deposited above the SiN_x corrosion barrier layer by pulsed FCVA deposition, giving a total overcoat thickness of 16 A. During the FCVA deposition process, an arc was struck between the anode and cathode using a high current pulsed power supply with a duty cycle of 0.001. The resulting arc discharge was transported using a single 90° bend filter coil which magnetically confined and guided the plasma towards the substrate. This filter helped to remove any neutrals or macroparticles in the plasma which did not react with the coil's magnetic field. The plasma exiting the filter coil then passed through a plasma shaping coil before reaching the substrate. No substrate bias was applied during the FCVA deposition process, hence the average energy of the C⁺ ions arriving at the substrate was around 25 eV.

The deposition rates of both the sputtered SiN_x and FCVA-deposited carbon layers were calibrated individually by X-ray reflectivity (XRR). The deposition rate of carbon by FCVA was found to be 0.063 A/pulse. By adjusting the number of pulses, the COCs of desired thicknesses were deposited in samples 16C and 4SiN12C. In addition, the etching rate uniformity as well as the thickness uniformity of the SiN_x and C layers were qualified prior to deposition.

The schematic of the resultant disk sample is shown in Figure 8(c). For the purpose of evaluating the performance of the SiN_x/C dual layer overcoat, single layer overcoats of reactive sputtered SiN_x and FCVA-deposited carbon, each with thickness of 16 A, were also deposited on similar etched magnetic media substrates. The performance of each of these samples was compared to a reference commercial media disk sample with its commercial COC but without the lubricant layer. For completeness, a specially prepared commercial disk without any protective overcoat over the magnetic media was also used in this experiment to provide a benchmark to show the effectiveness of using a protective overcoat on commercial magnetic media. Characterization methods

High resolution cross-section TEM (Philips FEG CM300) was used to image the microstructure and thickness of the overcoats after deposition. Before imaging, all samples (16C, 16SiN, ^"4SiN12C and CM) were prepared for cross-section TEM measurements.

To investigate any surface modification induced changes in the smoothness of the media surface, the surface roughness of samples 16C, 16SiN, 4SiN12C and CM were measured with the help of tapping mode atomic force microscopy (AFM, Bruker Innova). The measurements were conducted at a scan area of 2 pm x 2 pm at three different points on each sample surface, from which a mean value was taken.

One of the methods to explore the wear resistance of the overcoats on magnetic media is to investigate their tribological properties through ball-on-disk tribological tests. Ballon-disk tribological tests were carried out on all the samples using a nano-tribometer (CSM Instruments). Sapphire (Al₂0₃) was used as the counterface ball material and a contact load of 20 mN (the minimum load which can be applied by the tribometer) was kept constant in all the tests. The sample was rotated such that the ball slid across the sample surface in a circular motion with a radius of 1.2 mm and at a linear speed of 1.0 cm s^~ . Each test was carried out on at least two locations on each sample for up to 10,000 cycles while the coefficient of friction, μ, was measured. After the test, the wear track and ball images were captured by an optical microscope.

During surface modification, the C⁺ ions may also interact with the magnetic recording media. Hence, it is crucial to investigate any change in the macromagnetic properties of the magnetic media. The macromagnetic properties of the disk samples after surface modification were measured using a custom-made magneto-optic Kerr effect (MOKE) setup to investigate whether the surface modification process had significantly affected the magnetic performance of the magnetic media. The hysteresis loops of Kerr rotation with respect to the applied magnetic field for samples 16C, 4SiN12C and CM were recorded and compared.

At ultralow thicknesses, protection of the underlying hard disk media from corrosion or oxidation becomes critical to prevent the degradation of the magnetic material over time which would lead to the loss of stored data. To understand the effectiveness of the overcoats in protecting the underlying media from corrosion, a custom-made three- electrode corrosion setup was used to perform electrochemical potentiodynamic polarization measurements on all the samples (Yeo RJ et al, Diamond Relat. Mater., 2014, 44: 100-108). An electrolyte solution of 0.1 M NaCI was used during the test, and a geometric surface area of 0.24 cm² of the sample surface was exposed to the electrolyte. Each electrochemical test consisted of an anodic sweep and a cathodic sweep where the potential was varied and the corresponding current was measured. In the anodic sweep, the potential was swept 0.4 V above the open circuit potential, whereas in the cathodic sweep the potential was swept 0.4 V below the open circuit potential. Every sweep was conducted at a different location on the sample, with at least three sets of tests (6 sweeps) conducted on each sample to obtain consistent readings. The test best representing the consistent result is shown below.

X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical bonding and oxidation resistance performance of the protective carbon-containing layer in the dual layer overcoats of samples 16C, 4SiN12C and CM. XPS measurements were performed using a VG ESCALAB 220I-XL spectrometer with an ΑΙ-Κα source. The microstructures of the protective carbon-containing layer in these samples were probed with visible and UV Raman spectroscopy (Jobin Yvon LABRAM-HR) at laser excitation wavelengths of 488 nm and 325 nm respectively. To avoid damage to ultrathin COCs due to the laser heating of the sample surface, the laser power was kept low and other parameters such as the charge coupled device (CCD)'s exposure and data acquisition time were synchronized to obtain a reasonable signal-to-noise ratio for all samples. The relative intensity ratios of the COCs were compared from various locations of the sample surfaces to distinguish the nature of the carbon bonding with an accuracy of <2%. The XPS and Raman data is discussed briefly to correlate the microstructural properties with the functional performance of the overcoats based on the tribological and corrosion results.

Results and discussion

The cross-sectional TEM images of samples 16C, 16SiN, 4SiN12C and CM are shown in Figure 10. From the images, the thicknesses of the overcoats in samples 16C, 16SiN, and 4SiN12C were each measured to be about 1.6 ± 0.1 nm, while the thickness of the COC in sample CM was measured to be 2.7+ 0.1 nm. It is evident that the deposited overcoat thicknesses in samples 16C, 16SiN and 4SiN12C matched well with the initial calibration, revealing that the process can be precisely controlled even at ultrathin levels. In sample 4SiN12C, however, owing to the extremely low thickness of the SiN_x layer and similar contrast to carbon, it is difficult to distinguish it from the FCVA-deposited carbon layer. Consequently, only the total overcoat thickness of about 1.6 ± 0.1 nm was measured. The presence of SiN_x in sample 4SiN12C was subsequently confirmed by XPS analysis, which will be discussed later.

The average roughness (R_a) and root-mean-square roughness (R_q) of the four samples were measured, as shown in Figure 11. It can be seen that the R_q of the COC in commercial disks (sample CM) is about 0.25 nm. Surprisingly, the disks which had undergone the surface modification process of overcoat, deposition by FCVA carbon exhibited at least 20% lower R_q compared to the commercial disk sample, with values between about 0.19-0.20 nm. The decrease in the surface roughness is contributed by the surface etching process before overcoat deposition, which may have smoothened the surface of the underlying rough magnetic media. The FCVA process may have also contributed to the improvement in surface smoothness. A low surface roughness is a desirable feature for better tribological and corrosion performances, and at the nanoscale regime, it is beneficial for reducing the magnetic spacing for higher storage densities.

The frictional curves (μ versus number of cycles) measured during the tribological tests are shown in Figure 12(a), and the variation of μ for different samples Is shown in Figure 12(b). The optical images of the counterface ball and the wear track region are shown in Figures 13(a)-13(e) and 13(f)— 130) respectively. It is evident that without any protective overcoat, sample BM exhibits the highest μ of about 0.70+ 0.10 over the whole duration of the test with large fluctuations. The bare media has soft magnetic layers, which are easily wears upon sliding with the counterface balls. The excessive wear in sample BM can be seen in optical images as shown in Figures 13(e) and 13Q). Sample 16SiN gives a μ value of between 0.55 ± 0.05, which is characteristic of monolithic SiN_x. It was also observed that there were severe wear tracks and debris on the sample surfaces and counterface ball respectively. Thus, it can be seen that the tribological performance of the monolithic SiN_x layer is poor. On the other hand, the μ of the commercial disk COC in sample CM was found to be about 0.35 and showed large fluctuation throughout the test. The optical images of sample CM (Figures 13(d) and 13(i)) also showed considerable amount of material transfer to the ball and severe wear track, indicating the poor wear resistance of commercial COC. However, samples 16C and 4SiN12C, which have FCVA-deposited COCs, exhibited μ values lower than that of CM which contains commercially grown COC. While sample 16C showed lower μ of about 0.25 ± 0.03, negligible material transfer to the counterface ball and a very faint wear track, the best tribological performance was observed for sample 4SiN12C with the SiN_x/C dual layer overcoat. Sample 4SiN12C exhibited a μ value of about 0.18 ± 0.02 which was found to be very stable until the end of test, along with a negligible amount of material transfer to the ball and no visible wear track. These results highlight the efficacy of FCVA-deposited carbon in acting as an excellent wear- resistant layer with low friction, and the improvement in the frictional properties conveys the benefit of the addition of an atomically thin SiN_x corrosion barrier layer. The improved tribological characteristics provided by the SiN_x/C dual layer overcoat can be attributed to the increased interfacial bonding and enhanced sp³ carbon hybridization.

The macromagnetic properties of the hard disk can be characterized based on its magnetization behaviour under an applied switching magnetic field. The magnetization behaviour was measured by MOKE, where the Kerr rotation - which is directly related to the sample magnetization - was detected under a changing magnetic field. The magnetization hysteresis loops of samples 16C, 16SiN, 4SiN12C and CM obtained from the MOKE measurements are shown in Figure 14. It is apparent that the hysteresis loops for samples 16C, 16SiN and 4SiN12C are almost identical and overlap with each other. As compared to the as-received commercial disk sample CM, the macromagnetic properties such as the coercivity and switching field distribution show little difference. This shows that the surface modification and dual layer overcoat deposition process did not significantly affect the macromagnetic properties of the magnetic media.

The use of the SiN_x corrosion barrier layer provides a chemically inert diffusion barrier layer which hinders the diffusion of small molecules and ions into the magnetic media. In doing so, the rate of oxidation and corrosion of the magnetic media is reduced. After potentiodynamic polarization tests were conducted on the samples, the measured potential and current data were extracted from the anodic and cathodic sweeps and plotted on a semi-logarithmic plot of potential versus current density, as shown in Figure 15. The corrosion performance of the samples can be compared by a method known as Tafel extrapolation, where the linear portions of the anodic and cathodic curves are extrapolated towards the open circuit potential. The intersection of the extrapolations at a certain current density would indicate the corrosion current density G_corr), which is a measure of the propensity of the sample to undergo corrosion. Hence, the j_corr is inversely proportional to the corrosion resistance of the sample, i.e. the higher the j_corr value, the lower the corrosion resistance of the sample. Figure 16 shows the j_corr values extracted from the curves in Figure 15. Not surprisingly, for bare magnetic media without COC, the average j_corr is the highest of 15 x 10^"9 A cm^"2. On the other hand, the average j_corr of sample 16SiN is the lowest among the four samples which shows that monolithic SiN_x acts as a good corrosion protection barrier. When comparing sample 4SiN12C with sample 16C, sample 4SiN12C has a lower average j_COr_r. This can be attributed to the additional corrosion protection provided by the SiN_x layer, as well as the generation of a higher sp³ carbon bonding fraction which improves the density of the carbon overcoat layer in sample 4SiN12C (which will be discussed later). Furthermore, although samples 16C, 16SiN and 4SiN12C have about 40% lower overcoat thickness compared to sample CM, it can be seen that the corrosion resistance has not been adversely affected. This shows that the single layer FCVA- deposited COC, single layer SiN_x overcoat and the SiN_x/C dual layer overcoat still provide good corrosion protection of the media at thicknesses of < 2 nm.

The corrosion performance can be succinctly summarized and compared using the concept of protective efficiency (PE), which gives a measure of the overcoat's corrosion protection capability with respect to bare magnetic media. The formula for the PE of the overcoat is given by:

where J°_corr is the value of the extrapolated corrosion current density of bare magnetic media (sample BM). The calculated PEs of the tested samples are summarized in Table 2. Based on the results shown in Table 2 and taking the tribological results into account, the SiN_x/C dual layer overcoat used in sample 4SiN12C is found to be the best protective overcoat material.

In order to investigate the chemical structure and oxidation within the carbon-containing overcoats, XPS measurements were performed at a photoelectron take-off angle of 65° with respect to the analyzed surface. Background correction was performed before fitting the XPS core level spectra of each element to remove any background noise. Figures 17(a)-(c) show the C 1s core level spectra of samples 16C, 4SiN12C and CM. The fitting of the C 1s core level spectra was performed with a Gaussian curve, where all the samples exhibited four constituent peaks corresponding to the sp² bonding in carbon (sp²C, at 284.1 ± 0.1 eV), sp³ bonding in carbon (sp³C, at 284.9 eV), C-O bonding (at 286.2+ 0.1 eV) and C=0 bonding (at 287.9± 0.1 eV). An additional peak at about 283.0 eV was observed only in sample 4SiN12C, which is assigned to C-Si bonding. The formation of C-Si bonding can be attributed to the interaction of C⁺ ions with the SiN_x underlayer at the SiN_x/C interface. In order to further confirm the interfacial bonding of C with Si and C with N, the Si 2p and N 1s core level spectra of sample 4SiN12C were also fitted with various Gaussian components, as shown in Figures 17(d) and 17(e). The four constituent peaks observed in the Si 2p core level spectrum are assigned to Si-Si bonding (at 99.4 eV), Si-C bonding (at 100.6 eV), Si-N bonding (at 101.5 eV) and Si-O/Si-N-0 bonding (at 102.8 eV). Similarly, five constituent peaks found in the N 1s core level spectrum are assigned to N-Si bonding (at 397.7 eV), N-sp³C bonding (at 398.9 eV), N-sp²C bonding (at 400.4 eV) and N-O_x bonding (at 402.4 eV and 403.7 eV).

After analysing the Si 2p, N 1s and G1s core level spectra, the Co 2p_3/2 core level spectra of the magnetic media for samples 16C, 4SiN12C and CM were also examined. As shown in Figures 18(a)-(c), the Co 2p₃₂ spectra for all three samples were fitted with various Gaussian-Lorentzian components. The peaks observed in each Co 2p_3/2 core level spectrum can be assigned to Co-Co bonding (at 778.0 ± 0.1 eV), Co oxide bonding in Co₂0₃ (at 779.2± 0.1 eV), cobalt oxide/hydroxide bonding in CoO, Co₃0₄, CoOOH or Co(OH)₂ (at 780.3 ± 0.3 eV), and cobalt oxide/hydroxide bonding in CoO, Co₃0₄ or Co(OH)₂ (at 781.5 ± 0.2 eV). In addition, a fifth peak at 778.5 eV corresponding to Co-Si bonding was observed only in sample 4SiN12C, attributed to the interfacial bonding between Si in the SiN_x layer and Co in the magnetic recording media. It should be noted that this peak may also be influenced by metallic Co bonding. Similarly, the Cr 2p₃₂ core level spectra were also analyzed and fitted with various Gaussian-Lorentzian components, and are shown in Figures 18(d)-(f). The deconvoluted peaks as shown in the Cr 2p_3/2 core level spectra can be assigned to Cr- Cr (at 573.7 ± 0.1 eV and 574.3 ± 0.1 eV), Cr-oxide (at 576.2 ± 0.4 eV) and Cr- oxide/hydroxide (at 577.8 ± 0.3 eV) bonding, respectively.

Extra care was taken to analyze the peaks located at 575.2 ± 0.1 eV and 574.9 eV. For samples 16C and CM, the peak observed at 575.2 ± 0.1 eV is ascribed to Cr-Cr bonding, whereas in sample 4SiN12C, the peak observed at 574.9 eV may also have some contribution from Cr₂N bonding at the interface in addition to the Cr-Cr bonding thus explaining the slight shift in the binding energy. It is important to note that the reason for fitting more than one peak to Cr-Cr bonding was only to get the best fit to the data.

The constituent peaks in all the core level spectra obtained for all the samples were quantitatively analyzed using an area ratio method, which compares the areas under the constituent peaks. The bonding fractions are summarized in Table 3.

Table 3: Quantitative measure of bonding fractions in samples using an area ratio method.

From the XPS analyses, it is evident that the introduction of the 4 A SiN_x underlayer causes a significant change in the microstructure of the carbon film by creating higher amounts of interfacial bonding between the carbon and SiN_x layer, increasing the sp³C bonding fraction, and reducing the amount of oxidation of the underlying magnetic media. When comparing the sp³C bonding fraction of FCVA-deposited carbon films with and without the SiN_x underlayer (i.e. samples 16C and 4SiN12C, respectively), while keeping total coating thickness constant at 16 A, sample 4SiN12C shows higher sp³C bonding with an absolute difference of about 9% as compared to sample 16C (corresponding to about a 25% increase in relative terms). When compared with- the commercial COC in sample CM, the FCVA-deposited carbon in the SiN_x/C dual layer overcoat of sample 4SiN12C shows about 13% higher sp³C bonding in absolute terms (corresponding to a relative increase of about 40%), despite about a 40% reduction in the total overcoat thickness. This difference is quite significant, especially at the ultrathin film level. The enhanced sp³C bonding in the SiN_x/C dual layer overcoat helps to improve oxidation protection of the underlying media, which is reflected by the reduction in the oxidation level of the Co and Cr. Still, even without a SiN_x underlayer, the monolithic FCVA-deposited COC in sample 16C exhibited better oxidation protection than a thicker commercial COC in sample CM.

The introduction of the SiN_x underlayer between the magnetic layer and COC in the SiN_x/C dual layer overcoat gives rise to two advantageous effects. Firstly, it enhances the adhesion between the COC and the magnetic layer by promoting the interfacial bonding. XPS analysis shows the formation of strong interfacial bonds at the SiN_x/C and SiN_x/magnetic layer interface (such as Co-Si, Cr₂N, Si-C and C-N). This suggests that it is a good adhesion layer. At the same time, the SiN_x underlayer functions as a barrier between the magnetic layer and COC and helps in enhancing the sp³C bonding fraction. The enhancement of the sp³C fraction can be explained based on the interaction of the arriving energetic C⁺ ions with the magnetic media substrate, with and without the SiN_x layer. When the carbon film is grown directly onto the magnetic layer (like in sample 16C), the Co and Pt atoms present in the magnetic layer promotes sp² carbon bonding at the media-carbon interface due to a catalytic effect. However, when an ultrathin layer of SiN_x is introduced over the magnetic layer, it acts as a barrier between the magnetic layer and the arriving C⁺ ions, which reduces this catalytic reaction to some extent. The interaction of the C⁺ ions with Si and N in the SiN_x layer also reduces the probability of carbon interaction with Co and Pt in the magnetic layer. The suppression of the catalytic reaction to some extent helps in enhancing the sp³C bonding. Hence, the combined usefulness of the SiN_x underlayer as both an adhesion layer and barrier layer has led to the improved corrosion/oxidation resistance and better tribological properties in terms of low coefficient of friction and high wear resistance in the SiN_x/C dual layer overcoat. Visible and UV Raman spectroscopy measurements were performed to gain further insight into the microstructures of the carbon-containing overcoats. The visible Raman spectra for samples 16C, 4SiN12C and CM are shown in Figures 19(a)-(c), while their UV Raman spectra are shown in Figures 19(d)-(f).

The visible Raman spectra of these samples each show two characteristic peaks corresponding to the disorder (D) peak centered in the wavenumber range of 1380- 1405 cm^"1 and the G peak centered in the range of 1565-1580 cm^"1. The D peak is contributed by the breathing mode of sp²C atoms in aromatic rings, whereas the G peak arises from the in-plane bond stretching motion of all pairs of sp²C atoms in both rings and chains. Hence, the D and G peaks are both associated with sp²C bonding. UV Raman spectroscopy was conducted using higher energy photons than visible Raman so as to probe the vibrational mode of sp³-bonded carbon, which cannot be observed using visible excitation wavelengths. In addition to its characteristic D and G peaks, the UV Raman spectra of the three samples showed an additional peak centered in the range of 1050-1100 cm^"1, albeit weak. This peak is known as the T peak. The T peak corresponds to the vibration of the sp³C bonds in amorphous carbon films. It should be noted that the T peak has a slightly higher intensity in sample 4SiN12C than in samples 16C and CM. This indicates that sample 4SiN12C has relatively higher sp³C bonding than the other two samples. This finding agrees well with the XPS analysis.

While the G peak is associated with sp²C bonding, the relative shifts in the G peak position can be used to probe changes in sp³C and sp²C bonding under UV excitation. On the other hand, visible Raman spectroscopy can be employed to explain the nature of sp² carbon clustering in different samples due to its predominant sensitivity to sp²C bonding. The visible and UV Raman spectra of the samples were fitted with two Gaussian components, and their exact D and G peak positions and the ratios of the D- to-G peak intensities (/D//_g ratios) were determined. The G peak positions in the UV Raman spectra of samples 16C, 4SiN12C and CM were found to be 1586 cm^"1, 1588 cm^"1 and 1582.0 cm^"1, respectively. According to the three-stage model proposed by Ferrari and ^Robertson (Ferrari AC and Robertson J, Philos. Trans. R. Soc, A, 2004, 362:2477-2512), the shift in the G peak position towards higher wavenumbers during the third stage implies an increase in the sp³C bonding fraction. Since the XPS results reveal the trend of the sp³C bonding fraction to be 4SiN12C > 16C > CM, the obtained Raman results corroborate well with the XPS analysis. Furthermore, the i_G ratios (at excitation wavelength of 488 nm) in samples 16C, 4SiN12G and CM were found to be 0.5, 0.4 and 0.6, respectively. This indicates that sample 4SiN12C inhibits the development of sp² carbon clusters as compared to other samples. Hence, as interpreted from the Raman and XPS spectra, it can be seen that the introduction of a 4 A thick SiN_x underlayer between the media and an FCVA-deposited COC helps to increase the sp³ carbon bonding and lower the sp² carbon clustering within the COC.

Conclusion

A dual layer overcoat of reactively sputtered 4 A of SiN_x corrosion barrier layer followed by 12 A of FCVA-deposited protective carbon-containing layer (SiN_x/C dual layer overcoat) has been explored as an overcoat structure for high storage density application. The SiN_x/C dual layer overcoat is shown to enrich the sp³C content to obtain the benefits of good corrosion/oxidation protection with good tribological properties at ultra-low thicknesses of < 2 nm.

From the above, it can be seen that when an atomically thin interfacial corrosion barrier layer, such as a SiN_x layer, is applied between the magnetic layer (such as CoCrPt:oxide based magnetic layer) and a protective carbon-containing layer (such as a FCVA-deposited COC), its advantage is threefold. Firstly, it acts as an adhesion layer due to increased interfacial bonding, which improves the adhesion between the magnetic layer and the protective carbon-containing layer. Secondly, due to the dense structure and chemical inertness of the corrosion barrier layer, it acts as a barrier layer to hinder the diffusion of metallic ions and small molecules (such as water and oxygen) through the layer and reduces corrosion and oxidation in the magnetic layer. Thirdly, it suppresses the catalytic reaction of Co and Pt in the magnetic layer with the incoming energetic C⁺ ions of the protective carbon-containing layer. The suppression of the catalytic effect helps to increase the sp³ carbon fraction in the protective carbon- containing layer of the dual layer overcoat by about 40% when compared to conventionally deposited COC in today's commercial magnetic disks (a change from about 33% sp³C in conventional COC to about 46% sp³C in the 16 A SiN_x/C dual layer overcoat), and by about 25% when compared to monolithic 16 A FCVA-deposited COC (a change from about 37% sp³C in 16 A FCVA-deposited COC to about 46% sp³C in the 16 A SiN_x/C dual layer overcoat). This is even more remarkable given that the dual layer overcoat has about 40% lower thickness than commercial disk COC. With reduced diffusion through the corrosion barrier layer, higher sp³ carbon content and improved interfacial bonding, it is seen that the SiN_x/C dual layer overcoat was able to provide a low and stable coefficient of friction as well as better wear and corrosion/oxidation protection of the magnetic layer. This was confirmed by ball-on-disk tribological tests, XPS and electrochemical corrosion measurements. It was also observed that the etching and deposition process during fabrication of the monolithic and bi-layer overcoats did not cause surface roughening or degrade the macromagnetic properties of the magnetic media.

The fabrication of the dual layer overcoat has demonstrated enhanced sp³ carbon content at ultrathin overcoat thickness (< 2 nm) with desirable corrosion and tribological performance, without degrading the magnetic properties of a magnetic layer on which it is deposited. With the high sp³ carbon content and low thickness of the dual layer overcoat, it is suitable for providing low magnetic spacing and high thermal stability - which are critical factors for applications such as high density storage and HAMR applications in hard disk drives.

Example 3

An experiment was carried out to evaluate the performance of a set of samples with various thicknesses of a dual layer overcoat comprising a corrosion barrier layer comprising CrN_x and a protective carbon-containing layer compared to a commercial disk having a carbon overcoat but without PFPE lubricant. Figure 20 shows the schematic of various samples used. Prior to deposition of the various overcoats, the magnetic media was etched by applying a substrate bias, so as to sputter off the thin oxide layer present on the magnetic media surface. For deposition of CrN_x corrosion barrier layer, chromium was deposited by reactive sputtering in an Ar/N₂ gas atmosphere. For deposition of the protective carbon-containing layer, carbon was deposited by pulsed direct current (DC) sputtering in the same sputtering chamber. The samples were as follows: Table

3

Subsequently, the samples were tested for their tribological properties and wear resistance by ball-on-disk tribological tests. In each test, a sapphire (Al₂0₃) ball with a diameter of 2 mm was used as the counterface. The ball was kept in constant, fixed contact with the sample surface, while the sample rotated under the ball at a constant linear speed of 1 cm s^' The rotation created a wear track on the sample surface with diameter of 1.2 mm. The normal load applied by the ball on the sample surface was 20 mN, and the test was carried out for 10,000 sliding cycles (rotations) and then stopped. From the test, the coefficient of friction was measured and recorded. For each set of samples, the test was repeated at least three times on different locations to ensure consistency in the results. Optical images of the counterface ball and wear track were also taken to observe the severity of the wear and debris.

Figure 22 shows the graphs of coefficient of friction versus number of cycles and optical images of the ball and wear track for: (a) sample L1 - etched magnetic medium with 0.7 nm carbon overcoat, (b) sample L2 - etched magnetic medium with 1.2 nm carbon overcoat, (c) sample L3 - etched magnetic medium with 1.7 nm carbon overcoat, (d) sample L6 - etched magnetic medium with 1.7 nm CrN_x overcoat, (e) sample L4 - etched magnetic medium with a dual layer overcoat of 0.5 nm CrN_x followed by 0.7 nm carbon, and (f) sample L5 - etched magnetic medium with a dual layer overcoat of 0.5 nm CrN_x followed by 1.2 nm carbon.

Similarly, ball-on-disk tribological tests and optical imaging were performed on two other types of commercial disks for comparison purposes: one commercial disk having a bare etched magnetic medium surface without any carbon overcoat or lubricant layer, and another commercial disk containing a carbon overcoat of about 2.7 nm and a PFPE lubricant layer of about 1.0 nm. Figure 21 shows the graphs of coefficient of friction versus number of cycles and optical images of the ball and wear track for the above two samples.

As shown in Figure 21 , the bare commercial magnetic medium with no overcoat and lubricant layer exhibits the highest coefficient of friction (μ) of about 0.70 over the whole duration of the test and also shows severe wear track and large amount of material transfer to the counterface ball. Further, the commercial media with 2.7 nm COC and 1 nm lubricant has also shown higher μ value of between 0.40-0.50 with a severely damaged worn region and significant material transfer to the ball.

Among the monolithic carbon films studied (0.7 nm COC, 1.2 nm COC and 1.7 nm COC), COCs with thickness of 1.2 nm and 1.7 nm have shown good tribological properties with μ of about 0.2-0.3 (Figure 22 (b) and Figure 22 (c)) while the 0.7 nm thick COC has shown higher μ value of 0.4-0.5 beyond about 3000 cycles of sliding with severe wear track and a lot of material transfer to the counterface ball (Figure 22 (a)).

Figure 22 (d) shows that the monolithic 1.7 nm CrN_x exhibits poor tribological properties with μ of about 0.5, giving rise to a severe worn region and significant amount of material transfer to the counterface ball. This result is similar to that observed for monolithic SiN_x layer. Thus, it can be concluded that the corrosion barrier layer comprising CrN_x does not function well as a standalone tribological and corrosion protective coating. In particular, it functions well as an interlayer to promote adhesion between the magnetic layer and protective carbon-containing overcoat.

Two samples i.e. L4 and L5, were used with dual layer coatings of CrN_x and COC (L4: 0.5 nm CrN_x + 0.7 nm COC and L5: 0.5 nm CrN_x + 1.2 nm COC) and the tribological results are shown in Figures 22 (e) and (f), respectively. The tribological data has shown that CrN_x+COC with a total thickness of 1.7 nm demonstrated excellent tribological properties with a μ value of about 0.2, without the presence of a wear track and very minimal material transfer to the counterface ball. This result is similar to that observed for the dual layer overcoat of SiN_x+COC of 1.6 nm thickness which was explained in Example 2. Thus, it can be concluded that the presence of CrN_x also enhanced the adhesion between the COC and the magnetic media. Conclusions:

A dual layer overcoat of reactively sputtered 5 A of CrN_x corrosion barrier layer followed by 12 A of pulsed DC sputter deposited protective carbon-containing layer (CrN_x/C dual layer overcoat) was tested as an overcoat structure for high storage density application. The CrN_x/C dual layer overcoat with 1.7 nm thickness has shown better tribological properties when compared to the same thickness of a monolithic CrN_x layer and also showed similar and/or slightly better tribological properties as compared to the monolithic COC layer of the same thickness. Additionally, the dual layer overcoat of CrN_x/C even demonstrated better tribological properties than present day commercial magnetic media with 2.7 nm thick COC and with 1 nm thick PFPE lubricant despite being much thinner as compared to the overcoats and layers on commercial magnetic media.

Claims

Claims

1. A magnetic recording medium comprising:

- a substrate;

- a magnetic layer disposed on the substrate; and

- a dual layer overcoat comprising a corrosion barrier layer on the magnetic layer and a protective carbon-containing layer on the corrosion barrier layer,

wherein the dual layer overcoat has a thickness of ≤2 nm.

2. The magnetic recording medium according to claim 1 , wherein the corrosion barrier layer comprises SiN_x, CrN_X) CrO_x, TiN_x> SiC, TiC, WC, or a combination thereof.

3. The magnetic recording medium according to claim 1 or claim 2, wherein the corrosion barrier layer has a thickness of about 0.4-0.8 nm.

4. The magnetic recording medium according to any preceding claim, wherein the protective carbon-containing layer comprises diamond-like carbon (DLC), nitrogenated carbon (CN_X), or a combination thereof.

5. The magnetic recording medium according to any preceding claim, wherein the protective carbon-containing layer has a thickness of about 1.2-1.6 nm.

6. The magnetic recording medium according to any preceding claim, wherein the dual layer overcoat has a coefficient of friction of < 0.2.

7. The magnetic recording medium according to any preceding claim, wherein the dual layer overcoat comprises a SiN_x corrosion barrier layer and a diamond-like carbon protective layer.

8. The magnetic recording medium according to claim 7, wherein the dual layer overcoat comprises a SiN_x corrosion barrier layer having a thickness of about 0.4 nm and a diamond-like carbon protective layer having a thickness of about 1.2 nm.

9. A method of treating a surface of a magnetic layer of a magnetic recording medium comprising:

- depositing a corrosion barrier layer on the magnetic layer; and - depositing a protective carbon-containing layer on the corrosion barrier layer,

wherein the corrosion barrier layer and the protective carbon-containing layer have a combined thickness of < 2 nm.

10. The method according to claim 9, wherein the depositing of the corrosion barrier layer is by sputtering, ion beam deposition, or chemical vapour deposition.

11. The method according to claim 9 or 10, wherein the depositing of the protective carbon-containing layer is by sputtering, ion beam deposition, pulsed laser ablation (PLD) or filtered cathodic vacuum arc (FCVA) process.

12. The method according to any of claims 9 to 11, wherein the depositing of the corrosion barrier layer is by sputtering and the depositing of the protective carbon- containing layer is by FCVA process.

13. The method according to claim 12, wherein the FCVA process comprises bombarding the corrosion barrier layer with energetic carbon (C⁺) ions.

14. The method according to claim 13, wherein the FCVA process comprises bombarding the corrosion barrier layer with energetic C⁺ ions at ion energy of about 20- 350 eV.

15. The method according to any of claims 9 to 14, wherein the depositing of the corrosion barrier layer comprises depositing a corrosion barrier layer having a thickness of about 0.4-0.8 nm.

16. The method according to any of claims 9 to 15, wherein the depositing of the protective carbon-containing layer comprises depositing a protective carbon-containing layer having a thickness of about 1.2-1.6 nm.

17. The method according to any of claims 9 to 16, wherein the corrosion barrier layer comprises SiN_x> CrN_X) CrO_x, TiN_x, SiC, TiC, WC, or a combination thereof.

18. The method according to any of claims 9 to 17, wherein the protective carbon- containing layer comprises diamond-like carbon (DLC), nitrogenated carbon (CN_X), or a combination thereof.