Elsevier

Tribology International

Volume 33, Issue 9, September 2000, Pages 611-621
Tribology International

Surface and subsurface damages and magnetic recording pattern degradation induced by indentation and scratching

https://doi.org/10.1016/S0301-679X(00)00077-3Get rights and content

Abstract

Magnetic recording pattern degradation due to head–disk impact and scratching are simulated by static indentation and scratch testing, respectively, on a pre-recorded thin film magnetic recording disk. Different magnitude of controlled stresses were used to induce stress and physical damage to the magnetic recording disk resulting in erasure and distortion of the magnetic recording pattern. Both nanoindentation and scratching resulted in the elastic–plastic deformation of the multilayer coating of the magnetic recording disk but in different relative magnitude and types of in-plane stresses (which are effective in causing magnetization changes). For residual indentation and scratch depths of the order of the magnetic disk coating thickness, magnetization changes in the recording pattern were observed even though the protective carbon overcoat was not damaged. Large magnetic pattern distortion and erasure results where cracks and pileups, and delamination and buckling damages were observed for deeper indentation marks and scratch grooves, respectively.

Introduction

In today's high density hard-disk drives, the recording head is made to fly extremely close to the magnetic recording disk for better signal resolution and at an increasingly high velocity for better data rate. The surface of the magnetic disks is made very smooth to avoid head-asperity collision when the recording head is flying. To avoid the stiction problem of a very smooth disk, dynamic load–unload technology is introduced [1]. The protective carbon overcoat is also made extremely thin to reduce spacing loss [2]. The increasing use of hard-disk drive in mobile computers also exposes the magnetic recording disk to shock. All these factors make the magnetic recording disk surface particularly vulnerable to damage due to head–disk contact. At high velocity, potentially large stresses due to head–disk impact during the load/unload process or shock from the environment can result in head-slap, dents and scratches in the magnetic recording disk surface with potential for data loss [3], [4], [5]. Very small and hard particles generated from components within a disk drive that can get into the head–disk interface also result in scratches that can cause the disk drive to fai1 [6], [7].

Depending on the magnitude of the stress introduced by head–disk contact, and due to the extremely hard and elastic protective overcoat, no physical damage to the magnetic recording disk surface may occur. However, magnetic pattern degradation can still occur as a result of inverse magnetostriction or the Villari effect [8]. In inverse magnetostriction, magnetization changes can arise from the dimensional change in the magnetic crystals due to externally applied stress. The magnetization change due to stress is dependent on the direction of the applied stress, the magnetostriction coefficients, and also on the distribution of the magnetic anisotropy axes. In longitudinal magnetic recording media, where the anisotropy axes are in-plane, it is expected that stress demagnetization effect will be important for stresses applied in-plane to the magnetic thin film disk media. Mauri et al. [9] reported significant reduction in magnetization when they introduced a large stress, by applying a large bending strain of the order of 10−3, to various magnetic Co–alloy thin films. In particular, due to the negative magnetostriction constants of these films, tensile stress was observed to be more effective in reducing the magnetization. If the introduced stress exceeds the yield strength of the multilayer thin film of the magnetic recording disk physical damage to the surface, such as the displacement and removal (wear or delamination) of the magnetic storage layer, can occur. This results in permanent data loss.

The use of micro-mechanical techniques, such as scratching and nanoindentation, to introduce deformation and to study the surface damage in multilayer magnetic disk was reported by Wu and Frommer [10]. In both scratching and indentation, stress occurs at a much longer time scale compared to a head–disk impact event. Hence, influence such as frictional heating during head–disk contact is not simulated in such quasi-static experiments. Nevertheless, nanoindentation and scratching are very useful ways to introduce tailored deformation on the magnetic recording disk for studying stress erasure of the magnetic recording pattern.

In this paper we investigated the erasure or degradation of the magnetic recording pattern due to stresses and surface damage introduced on a pre-recorded magnetic disk using a low load nanoindenter with indentation and scratching function. The magnetic pattern degradation was examined using a magnetic force microscope (MFM) in the constant field gradient mode. The surface damage was examined using an atomic force microscope (AFM), scanning electron microscope (SEM) and scanning Auger microprobe (SAM). Subsurface damage was revealed by the focused-ion beam (FIB) cross-sections of the physical damage on the surface of the magnetic recording disk. The mechanism resulting in the magnetic pattern degradation due to indentation and scratching was inferred from the surface and subsurface damages.

Section snippets

Experimental approach

Parallel tracks of magnetic recording patterns with a linear density of about 4720 bits/mm (120 kbpi) were written on a 95 mm multilayer thin film magnetic recording disk using a spinstand. The recorded disk was then cut into four pieces for nanoindentation and scratching. The layer structure of the magnetic recording disk is shown in Fig. 1. The total thickness of the sputtered coating (carbon overcoat, magnetic Co–alloy, and Cr underlayer on the NiP plated Al–Mg substrate) is about 39.5 nm.

Magnetic bit pattern degradation due to indentation

Indentation was performed on a magnetic recording disk with the layer structure shown in Fig. 1. The disk has a coercivity of 3000 Oe and Mrt value of 1.4–1.5 memu/cm2. The linear recording density is 4720 bits/mm (120 kbpi) and the track width is about 1 μm.

The measured elastic–plastic response of the multilayer magnetic recording disk to nanoindentation is shown in the load–displacement curves, Fig. 3(a), for indentation depths of 160 nm, 250 nm and 320 nm. Significant plastic deformation is

Conclusion

The surface and subsurface damage of the multilayer sputtered coating magnetic recording disk induced by indentation and scratching and their influence on the magnetic recording pattern degradation was investigated.

  • 1.

    We found that the relatively large surface and subsurface damage from heavy indentation and scratching resulted in large magnetization changes and loss. The mechanism of magnetization changes is displacement of materials due to the formation of dislocations and cracks in the thin

Acknowledgements

The authors wish to thank Ms G.L. Chen and Ms L.P. Tan of the Data Storage Institute for assistance with the AES mapping and also MFM imaging, and Mr Djohni Chandra and Mr Victor Tan of Seagate International (S) Pte. Ltd. for providing the recorded magnetic disk.

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