Magnetic and structural properties of Co–Pt perpendicular recording media with large magnetic anisotropy
Introduction
Perpendicular magnetic recording has been studied for more than twenty years [1] and presented to make a demonstration for a hard disk drive system [2]. Conventional Co–Cr alloy perpendicular recording media showed a relatively small magnetic anisotropy constant Ku, in the range of 1–3×106 erg/cm3 and a relatively small saturation magnetization Ms, of 300–500 emu/cm3. A further large magnetic anisotropy is required for obtaining a higher recording density toward 1 Tbit/in2 with sufficient thermal stability. A large Ms is also needed for reducing required perpendicular coercivity Hc⊥, [3], so as to keep thermal stability and a large output signal.
Co–Pt films were first investigated for use as magneto-optical media [4] with a large magnetic anisotropy. It is known that a high perpendicular magnetic anisotropy of the film can be achieved even with a room temperature deposition process. Vacuum evaporated Co3Pt films deposited at an elevated substrate temperature exhibit a large Ku of about 2×107 erg/cm3 and a large Ms of about 1100 emu/cm3[5]. Therefore, this material could be one of the possible candidates for high density magnetic recording media. In this paper, Co–Pt films were investigated for perpendicular magnetic recording media instead of conventional Co–Cr alloy films. The effects of Ar gas pressure for Co–Pt thin film deposition on magnetic and structural properties are discussed.
Section snippets
Experimental
A Co80–Pt20 (target composition in at%, hereafter denoted Co–Pt) target was used for magnetron sputtering. Film thickness was fixed at 15 nm in this experiment. A Pt (10 nm)/Co65–Cr35 (2 nm) (at%, hereafter denoted Co–Cr) stacked film was used as an underlayer for all Co–Pt films for obtaining a high crystal orientation [6]. All films were sputter-deposited on φ2.5-inch glass disk substrates without heating during the deposition. Neither reactive gas addition such as oxygen nor application of the
Effect of Ar gas pressure
When Co–Pt films were deposited at various Ar gas pressures and room temperature, a dramatic change in Hc⊥ (Kerr) was observed as shown in Fig. 1. The similar change in magnetic property was also observed for Co–Cr system alloy media [7] at a much higher Ar gas pressure of around 70 Pa.
Crystal orientation, Δθ50, of the underlayer (Pt-FCC(1 1 1)) was kept at around 4° for all media in this figure. Δθ50 of Co–Pt films (Co–Pt-HCP (00.2)) gradually deteriorates with increasing Ar gas pressure.
Magnetization reversal mechanism
To clarify the magnetic behavior of the Co–Pt films of high Ar gas pressures deposition, angular dependence of Hc was measured. In Fig. 5, for the Co–Pt film deposited at Ar gas pressure of 1 Pa, Hc gradually increased with the applied field angle (between the sample surface normal and the direction of applied magnetic field). This indicates that the magnetization reversal mechanism of this film is inferred as a magnetic domain wall motion type. On the other hand, for the Co–Pt film deposited at
Conclusion
A Co–Pt film with large magnetic anisotropy at room temperature deposition was studied in terms of magnetic and structural properties. In conclusion, a high Ar gas pressure deposition of the Co–Pt film makes physically isolated and magnetically weak coupled grains, resulting in high coercivities with large anisotropy. Since the Co–Pt films deposited at a high Ar gas pressure and room temperature have a large magnetic anisotropy of over 1.2×107 erg/cm3, the films would be the promising candidates
Acknowledgements
The authors would like to thank Dr. S. Iwasaki, the president of Tohoku Institute of Technology, for his instruction and encouragement throughout this work. We also thank Research Institute for Technical Physics and Materials Science of Hungary Academy of Sciences for a part of TEM observation. This work was partially supported by Akita Prefecture Collaboration of Regional Entities for the Advancement of Technological Excellence, JST.
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