梯度媒體的製作、結構、磁性以及磁記錄特性探討

Study of graded-medium properties of process, structure, magnetism, and magnetic recording

林奕宏(Yi-Hung Lin)

指導教授 ： 郭博成

共同指導教授 ： 許仁華(Jen-Hwa Hsu)

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.02907

摘要

磁記錄媒體必須具有很高的殘留磁化 (Mr，remanent magnetization)，矯頑磁場 (Hc)也要高，才能減少自我消磁效應 (self-demagnetization)以及雜散磁場 (stray fields)的去磁作用。因此，在過去使用CoCrPt基當作記錄媒體的材料，但其記錄密度在接近1 Tb/in2時，將難以再提升以克服熱擾動及雜訊變大的問題。L10-FePt材料由於具有高磁晶異向常數 (~7×107 erg/cm3) 、大的飽和磁化量 (1140 emu/cm3)、大的異向場 (116 kOe)，以及最小穩定的晶粒尺寸大約只有3 nm，所以是個很好的候選材料，因此本研究採用FePt為記錄媒體研究的材料。
　　在記錄密度不斷提升的同時，由於必須使用大的Ku材料，也產生了資料越來越難寫入的問題，熱輔助磁讀技術因此發展出來，解決寫入場過高的問題。另一個辦法，則是想辦法降低記錄媒體的翻轉場問題，在過去的雙層exchange spring媒體，可以大約降低硬磁層Hc的2倍大小，為了再進一步降低Hc的值，且不降低熱穩定因子的情況下，我們採用多層漸變的exchange spring結構。
　　本研究之多層漸變的交互彈簧 (exchange spring)結構主要有三種：利用溫度高低、成分變化，以及壓力變化，製作出三種不同的多層漸變的交互彈簧結構。結果顯示，成分漸變的結構將Hc降低了2.6倍；溫度漸變將Hc降低大約2.8倍；而壓力漸變再進一步將Hc降低大約3.3倍。溫度與壓力漸變結構具有相似的磁翻轉行為。而成分漸變結構，在去磁過程，由於不一致的磁旋轉 (non-coherent rotation)產生，會有一個大的可逆翻轉。因此，考慮降低翻轉場及磁翻轉行為，壓力漸變為最佳的漸變膜層結構。

關鍵字

梯度媒體 ； 磁性膜 ； 多層膜 ； 金屬與合金 ； 異向性 ； X光繞射 ； 磁性量測

並列摘要

Magnetic recording needs to have high remanent magnetization and high coercivity due to the effects of self-demagnetization and stray fields. Accordingly, in the past CoCrPt was used for the material of recording medium. However, because of larger thermal fluctuation and noise, it is hard to further enhance the areal density if the areal density approaches to 1 Tbit/in2. L10-FePt becomes one of the candidate materials due to the high magnetocrystalline anisotropy constant (Ku ~ 7×107 erg/cm3), high saturation magnetization (~1140 emu/cm3), high anisotropy field (~116 kOe), and minimal stable grain size as small as 3 nm. Therefore, FePt is employed as the material of recording media to be studied. Such high Ku materials also makes writing difficulty resulting to develop the technology of heat-assisted magnetic recording (HAMR) in order to solve the problems of large writing field. The other way is to reduce the switching field. In the past, double-layer of exchange spring media have been used as the recording media to reduce the coercivity by a factor of 2. Our studies concentrate on graded media in order to further decrease the coercivity and maintain the thermal stability factor.
　　Herein, we explore the magnetic behaviors of L10-FePt graded films three approaches: gradient-temperature (Tg), composition (Cg) and working pressure (Pg). As a result, Hc⊥ can be reduced by a factor of 2.6, 2.8, and 3.3 for the Cg-, Tg- and Pg-L10 FePt layers, respectively. Both Tg- and Pg-FePt have the similar reversal behavior. On the other hand, for Cg-FePt, the magnetization increases sharply upon removal of the applied field. This increment in the magnetization is due to the presence of reversible magnetization switching arising from the non-coherent rotation. Therefore, considering to reduce the switching field and magnetic reversal, Pg-FePt film may has the best gradient performance among these three structures.

並列關鍵字

graded-medium ； magnetic films and multilayers ； metals and alloys ； anisotropy ； X-ray diffraction ； magnetic measurements

參考文獻

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