(Gd,Tb,Dy)FeCo磁光薄膜的磁性及記錄特性

Abstract

提昇磁光碟機的記錄容量及資料存取速度可從碟片的結構及記錄媒體著 手，以降低記錄時所須的外加磁場。本論文分別從(Gd,Tb,Dy)FeCo各組成 元素及膜層間的磁交互作用力，探討單層與雙層磁光薄膜的磁性與記錄特 性，以瞭解降低外加磁場的原因，及提出可降低外加磁場至100 Oe以下的 方法。當記號大小(約0.5 mm)小於雷射光點(約1 mm)的時候，讀出的載頻 雜訊比(carrier to noise ratio, CNR)會顯得不足，所以我們將採用熱 磁超解析法(magnetically induced super-resolution, MSR)提升高密度 碟片的CNR。在單層磁光薄膜的研究中，利用平均場論(mean field theory)的分析，我們發現︰稀土元素(rare earth, RE)與過渡金屬( transition metal, TM)之間的交互作用力，與薄膜的磁異向性( anisotropy energy constant, Ku)有關，此關係可由磁異向性分散( anisotropy dispersion)模型加以描述。依平均場論所計算出的稀土元素 分散角(dispersion angle, fKu)及稀土元素與過渡金屬間的交互積分( exchange integral, JRE-TM)，可決定出(Dy,Tb)FeCo磁光薄膜所須要的 外加磁場。因為磁壁的遷移率(domain wall mobility, meff)與交互耦合 係數(exchange coupling coefficient,由實驗及理論計算的結果中發現 ：磁光記錄媒體Dy23.7(Fe87Co13)76.3的meff與l分別約為Tb23.1(Fe90 Co10)76.9的2倍及1/2左右。至於所須的外加磁場大小，前者與後者則分 別為70 Oe及160 Oe。因為，meff與l分別反比於及正比於稀土元素與過渡 金屬間的磁交互作用力，因此，(Dy,Tb)FeCo磁光薄膜所須的外加磁場大 小，可由各組成元素間的交互作用力、或者meff與l估算出來。因為l不隨 溫度變化，用來評估所須的外加磁場將較其他者更為容易。而且，經由實 驗得到的l而推導出的JRE-TM值，與平均場論所得到的值相吻合，如此可 簡化平均場論的計算流程及增加計算結果的準膠b雙層磁光薄膜的研究中 ，我們發現：其記錄特性主要由界面能量(interface wall energy)的大 小所決定，而界面能量正比於膜層間的交互作用力。因為界面能量在溫度 變化的過程中，會有不連續的現象，利用此特性，就可同時達到MSR及覆 蓋效應(capping effect)的效果，繼而進一步提昇磁光碟片的效能。經由 實驗證實：當記錄訊號縮小到0.5 mm時，所須要的外加磁場僅為75 Oe， 而讀出的CNR依然可達40 dB以上。依此估算，3.5吋磁光碟片的記錄容量 可達1 Gbytes左右。 One of the most promising methods to simultaneously improve the areal density/capacity, data rate, and access time of magneto- optical disks is to reduce the required bias field (Hb). In this thesis, the required Hb of (Gd,Tb,Dy)FeCo films were correlated to the subnetwork exchange coupling in single-layered films and the inter-layered exchange coupling in exchange-coupled double- layered (ECDL) films. When the recorded magnetic domain size (about 0.5 mm) is less than the focused laser spot size (about 1 mm),The magnetic properties and the recording characteristics of single-layered (Dy,Tb)FeCo films were analyzed by mean field modeling. The experimental results showed that the required Hb and the domain wall mobility of Dy23.7(Fe87Co13)76.3 films are about 1/2 ~ 1/3 and 2 ~ 3 times those of Tb23.1(Fe90Co10)76.9 films during thermo-magnetic recording. Accordingly, the required Hb of a (Dy,Tb)FeCo film is inversely proportional to its domain wall mobility which was found to be determined by the anisotropy disperThe magnetic properties of single-layered sperimagnetic films are also governed by a finite exchange coupling coefficient (l), because of its dependence on the subnetwork coupling strength. From the study of a series of (Dy, Tb)FeCo films, it is found that l is mainly determined by the concentration of the rare earth components. The exchange integral between rare earth and transition metal subnetworks can be derived from the measured l so that the number of uncertain parameters for mean field modeling is redECDL films were adopted to simultaneously increase the areal density of MSR media by using center aperture detection (CAD) scheme and the switching field sensitivity by capping effect. The temperature dependence of interface wall energy was found to affect the recording/ playback characteristics of ECDL MSR CAD disks considerably. The interface wall energy, required by the stabilization of recorded marks, the aperture function for MSR CAD readout, and the capping effect to lower switching field, can be satis