展透性FePt-MgO垂直記錄媒體的製作及磁化反轉行為

Fabrication and magnetic reversal behaviors of FePt-MgO percolated perpendicular medium

蔡昀志(Yun-Chih Tsai)

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/碩士(2007年)

https://doi.org/10.6342/NTU.2007.02490

摘要

在過去的granular perpendicular medium (GPM)，晶粒尺寸決定了磁翻轉雜訊的大小，為了減少雜訊必需縮小晶粒尺寸，但卻會造成媒體熱穩定性的下降，因此GPM媒體的記錄密度極限只有500 Gbits/in2。為了解決此問題，並繼續提高記錄密度，percolated perpendicular media (PPM)這種既能夠提升記錄密度又不喪失熱穩定性的記錄媒體便產生了，PPM藉由非磁性材料密集地散佈在記錄媒體中形成pinning sites，使磁翻轉介面變得較為平滑，進而降低雜訊，並且晶粒間強大的鐵磁耦合作用力也可有效地保持熱穩定性，因此可將記錄密度提升至1 Tbits/in2。
本研究以磁控濺鍍的方式，在基板溫度530℃及580℃時，共同濺鍍Fe、Pt、MgO於MgO(001)薄膜底層上，製備percolated perpendicular FePt-MgO薄膜，MgO的添加量由0到10.07 vol.%。X光繞射儀及高解析穿透式電子顯微鏡分別用來分析薄膜的相結構及顯微結構，振動式磁儀及超導量子干涉儀用來分析薄膜的各項磁性質。
在磁性層濺鍍溫度為530℃時，MgO添加量從0增加到0.76 vol.%，序化因子維持在0.96左右，垂直方向的角形比也皆為0.88，但水平方向的角形比則從0.35增加到0.45，矯頑磁場從9.5僅增加到9.7 kOe，異相性場從7.75增加到9.25 kOe，磁晶異相性常數從2.76×107增加到3.21×107 erg/cm3，α值從2.3下降到2.17；另外當MgO添加量分別增加到2.61及4.43 vol.%時，FePt(002)繞射峰開始向低角度偏移，序化因子分別下降到0.89及0.75，垂直方向的角形比下降到0.87及0.84，水平方向的角形比皆為0.45左右，矯頑磁場開始下降到9.2及8.7 kOe，異相性場下降到7.45 及6.9 kOe，磁晶異相性常數下降到2.55×107及2.32×107 erg/cm3，α值下降到1.99及1.87。
從以上結果來看，在530℃下製備的FePt-MgO薄膜其展透效應並不顯著，因此將磁性層濺鍍溫度升高到580℃。當MgO添加量從0增加到1.23 vol.%時，矯頑磁場明顯地從10.6增加到13 kOe，並且磁翻轉機制由磁壁移動轉往磁區翻轉，證明MgO形成pinning sites阻礙了磁壁的移動。在580℃下製備之FePt-MgO薄膜的序化因子、垂直方向的角形比、異相性場、磁晶異相性常數及α值都較530℃時高，同時水平方向的角形比也降低了；從樣品微結構分析知道，未添加MgO時，FePt為連續膜，MgO增加到0.76 vol.%時，MgO在FePt的裡會以柱狀和條狀的形式析出，FePt晶粒逐漸縮小，當MgO增加到6.13 vol.%時，FePt-MgO薄膜開始轉變成包覆膜。由於磁性層濺鍍溫度的提高，FePt(001)方向不易被添加的MgO破壞，使FePt-MgO薄膜擁有較佳的垂直磁性質，因此PPM的效應也開始顯現出來。
為了添加更多的MgO形成柱狀形的pinning sites，使其完全達到PPM功效，而不是在FePt晶界上偏析成為FePt-MgO顆粒膜，在低溫下共鍍Fe、Pt、MgO，之後快速高溫退火，應可解決MgO在FePt晶界上偏析的問題，密集分佈的MgO pinning sites將會使媒體的雜訊大幅下降、記錄密度大幅提升。

關鍵字

展透性

並列摘要

In the present granular perpendicular medium (GPM), the transition jitter noise is essentially determined by the grain size. In order to reduce signal-to-noise ratio (SNR), the magnetic grain size must be scaled down. But it would cause a degradation of the medium thermal stability. As a consequence, the largest area density of GPM is expected to be below 500 Gbits/in2. To overcome the problem and further increase the area density, a new type of recording medium referred to as percolated perpendicular medium (PPM) is proposed. The PPM, consisting of densely distributed nonmagnetic pinning sites, ensure relatively smooth transition boundaries and reduced SNR. Besides, the ferromagnetic exchange coupling between the grains provides sufficient strength to resist thermal fluctuation.
In this study, percolated perpendicular FePt-MgO films were prepared by alternating magnetron sputtering Fe, Pt, MgO on MgO(001) underlayer at 530 and 580℃ in an ultra-high vacuum chamber. The MgO content was varied between 0 to 10.07 vol.%. X-ray diffraction (XRD) using Cu-Kα radiation and high-resolution transmission electron microscopy (HRTEM) were used to study the crystalline phases and microstructure of the films. A vibrating sample magnetometer (VSM) and superconducting quantum interference device (SQUID) were used to measure the magnetic properties.
When the deposition temperature of FePt-MgO was 530℃ and MgO content was increasing from 0 to 0.76 vol.%, the ordering parameter (Sorder) was maintained at about 0.96. The out-of-plane squareness (S⊥) remained at 0.88, but in-plane squareness (S∥) was increased from 0.35 to 0.45. However, the out-of-plane coercivity (Hc⊥)was only increased from 9.5 to 9.7 kOe. The anisotropy field (Hk) and the crystalline anisotropy constant (Ku) were increased from increased from 77.5 to 92.5 kOe and 2.76×107 to 3.21×107 erg/cm3 respectively. The α value, which gives the degree of coupling between magnetic grains, was decreased from 2.3 to 2.17. Further increasing MgO content, the FePt(002) peak shifted to low angle accompanied with the rapid reduction of Sorder and S⊥ although S∥ was always kept at 0.45. For example, Sorder was 0.89 and 0.75 for 2.61 and 4.43 vol.% and S⊥ changed from 0.87 to 0.84 from 2.61 to 4.43 vol.%. Hc⊥ was decreased to 9.2 and 8.7 kOe. Hk was decreased to 74.5 and 69 kOe. Ku was decreased to 2.55×107 and 2.32×107 erg/cm3, and α value was decreased to 1.99 to 1.87.
Consequently, the PPM effect of FePt-MgO film deposited at 530℃ was not conspicuous. However, when the deposition temperature was increased to 580℃, the PPM effect becomes much more apparent. Hc⊥ raised largely from 10.6 to 13 kOe when the MgO content was increased from 0 to 1.23 vol.% and the magnetization reversal process transfers from domain wall motion to magnetization rotation . It demonstrated that the MgO formed as pinning sites hindered the motion of domains. The values of Sorder, S⊥, Hk, Ku and α were all increased with a reduction of S∥. From the microstructure analysis, FePt formed as a continuous film without MgO additive. With increasing MgO content, MgO columns and flakes were present in the FePt matrix with the decreased size of FePt grains. However, the FePt-MgO film will transfer into granular film as the MgO content was increased above 6.13 vol.%, losing the unique properties of PPM.
Thus, in order to distribute more MgO pinning sites in FePt matrix and avoid forming the FePt-MgO granular film, alternating sputtered Fe, Pt, MgO at lower temperatures with a rapid post thermal annealing should be adopted. The densely distributed MgO pinning sites in the FePt matrix is expected to markedly decrease SNR and increase the area density of medium.

並列關鍵字

percolated

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