Reading in Achieving High Density in Magneto-Optical Disks
Han-Ping D. Shieh
|關鍵字:||熱磁超解析法;磁通量讀光寫;magnetically induced super-resolution;magnetic flux reading optical writing|
|摘要:||由於電腦、網路和多媒體的迅速發展，大量的資訊隨之產生，為了滿足高達數十億位元(Gbytes)資料儲存需求，可擦拭型光碟(erasable optical disks)需要具備更高的記錄密度與更大的容量。在過去十幾年來，許多研究致力於提昇記錄容量及資料存取速度(data transfer rate)：例如中央孔徑檢測熱磁超解析法(magnetically induced super-resolution by center aperture detection, MSR-CAD)碟片可以產生小於繞射極限的等效檢出孔徑 (effective aperture)以讀取微小的記號，進而提昇記錄密度；另外，如果能有效降低記錄時所需的外加磁場(recording bias field, Hb)，則可藉由磁調變直接覆寫(magnetic field modulation direct overwrite)技術，同時提昇磁光系統的記錄容量及資料存取速度(data transfer rate)。本研究深入探討其中兩種新型方法：利用脈衝雷射讀取法(pulsed-laser readout scheme)具有較為陡峭的溫度分佈(temperature profile)的特性，以提昇MSR-CAD的讀出效果；以磁通量讀光寫(magnetic flux reading optical writing)的交互耦合雙層(exchange-coupled double-layer, ECDL)碟片，使得系統在更高記錄密度運作時，仍能以磁通量方法(magnetic flux detection)檢測微小的記號。
在脈衝雷射光讀取法的研究中，模擬的結果顯示：此種讀取法可產生較窄小的孔徑磁壁寬度(aperture wall)，因而有效降低讀取雜訊(readout noise)，同時此種讀取法可產生較為陡峭的溫度分佈，進而降低軌間串訊(intertrack crosstalk)，提升光碟的記錄容量。實驗結果也證實：使用脈衝雷射檢讀法所得的載波雜訊比(carrier-to-noise ratio, CNR)可比直流雷射(dc laser)讀出法提升2.5 dB。因此，使用脈衝雷射光讀取法可以進一步改善MSR-CAD的讀出特性，故為一先進之高密度讀出方法。
「磁通量讀光寫」碟片有兩個基本要求，一是高磁化量(magnetization)可產生高磁通量；二是高矯頑場(coercivity)能提高記號的穩定性，故我們研究以具有高磁化量TbFeCo讀出層與另一記錄層所組成之ECDL碟片的磁性質與記錄特性，使其符合磁通量讀光寫的需求。實驗結果顯示：藉由磁性薄膜間的磁交互耦合作用力，交互耦合雙層薄膜的等效矯頑場(effective coercivity)可從1.7 kOe 提高至7.6 kOe，且該作用力使得ECDL碟片所需的記錄磁場從250 Oe 降至100 Oe。因此，交互耦合雙層薄膜具有超過300 emu/cm3的磁化量、高穩定性及低記錄磁場，故ECDL碟片能成為「磁通量讀光寫」記錄方式具備高記錄密度與速度的記錄材料。|
With the rapid progress of computer, network, and multimedia applications, the huge information is produced and continuously increased. To meet the great demand of multi-Gbyte recording media, erasable optical disk technologies should gear up for even higher density and capacity. Many techniques have been proposed to increase areal density and data rate during the past decade of years. For example, in the magnetically induced super-resolution by center aperture detection (MSR-CAD) disks, a below-diffraction-limited effective aperture is produced to read the smaller mark to increase the areal density of MO disks. Besides, if the recording bias field (Hb) of MO disks can be reduced, the areal density and data transfer rate of MO systems can be improved simultaneously by magnetic field modulation direct overwrite technique. Two of those advanced techniques have been studied in this study. The first technique is the pulsed-laser readout scheme to improve the readout characteristics of MSR-CAD disks. The other is the exchange-coupled double-layer (ECDL) disks for magnetic flux reading optical writing to resolve the low signal level resulted from small mark size in high density recording. In the study of pulsed-laser readout scheme, the simulation results indicated that the narrower aperture wall induced by pulsed laser readout results in lower readout noise than that by DC laser readout. The sharp temperature profile resulted in pulsed readout induces less crosstalk to adjacent data tracks, thus track-pitch can be reduced to reach higher density. The experimental results demonstrated that pulsed laser readout scheme produced CNR of about 2.5 dB higher than that of conventional DC readout. It can be concluded that pulsed-laser readout can further improve the readout characteristics, potentially leading to higher recording density than DC readout in MSR-CAD MO disks. As high magnetization for magnetic flux sensing and large coercivity for stable domains are desirable characteristics for magnetic flux detection, the ECDL disks consisting of a high-magnetization transition-metal (TM)-rich TbFeCo readout layer and a recording layer were studied for magnetic flux reading optical writing. The effective coercivity of ECDL with rare-earth (recording)/TM-rich (readout) films could be greatly enhanced from 1.7 to 7.6 kOe through exchange coupling between the two active layers. Moreover, the recording bias field of the ECDL disks could be reduced from 250 to 100 Oe by the exchange coupling effect. Therefore, the ECDL disks with magnetization over 300 emu/cm3, high stability and low recording bias field are potentially applicable for magnetic flux readout toward high density recording at a high writing rate.