FePt基交互耦合複合式垂直記錄媒體之微結構與磁性質研究

Abstract

L10序化相FePt具有絕佳的熱穩定性，但其偏高的頑磁力卻使得現今讀寫頭的磁場難以翻轉FePt之磁矩。交換耦合複合媒體 (Exchange Coupled Composite media, ECC media) 以目前現有硬碟的製造技術為基礎，只要添加軟磁層就能有效降低寫入場。因此本研究將以ECC媒體作為研究主題，探討不同膜層結構與軟磁層厚度對FePt系列ECC媒體的磁性質之影響。 本研究調控濺鍍FePt時的氬氣壓力，以得到具垂直膜面磁異向性的單層FePt薄膜。當氬氣壓力降低至4 mtorr，直接將厚度21 nm 之Fe58Pt42薄膜鍍製在玻璃基板上，高溫退火後可獲得絕佳的垂直膜面磁異向性：垂直膜面頑磁力 (Hc⊥) 為11.5 kOe，而垂直膜面角型比 (S⊥) 為0.88。後續實驗則分為兩部分：雙層ECC媒體與漸進式ECC媒體。Glass/L10 Fe58Pt42 14nm/A1 Fe58Pt42 5 nm雙層薄膜可將Hc⊥由未添加軟磁層的20.3 kOe降低至9.5 kOe，但其磁滯曲線呈階梯狀，S⊥只有0.5；另一方面，軟磁層厚度5 nm之H/S型漸進式ECC媒體，其Hc⊥約10.8 kOe，但具有極佳的垂直膜面磁異向性，S⊥高達0.96。綜合比較兩者之磁性質後，我們發現寫入場的降低與垂直膜面磁異向性是兩項需要取捨的性質：為了得到較低的寫入場，往往會犧牲垂直膜面磁異向性。 進一步的改變漸進式ECC媒體之膜層結構，我們發現在相同的軟磁層厚度下，較多擴散介面的膜層結構可進一步的降低垂直膜面方向的寫入場。將Glass/ Fe58Pt42 7 nm/ Fe40Pt60 5 nm/ Fe58Pt42 7 nm以700℃、30分鐘退火，其Hc⊥約7.9 kOe，S⊥為0.955，仍維持良好的垂直膜面磁異向性。另一方面，硬磁層與軟磁層的沉積順序也會影響漸進式ECC媒體之磁異向性：當硬磁層直接與基板接觸，可以得到較佳的垂直膜面磁異向性。 本研究提出結合漸進式ECC媒體與多層擴散介面的技術，可作為一種更有效降低寫入場的方式，並維持良好的垂直膜面磁異向性。對於具有極高磁異向能常數 (Ku) 材料的實際應用有很大的幫助。

Ordered L10-FePt has good thermal stability, however its coercivity is too high for recording head to write. Soft layer helps hard layer to rotate its magnetic moment in Exchange Coupled Composite (ECC) media, lowering the switching field of the media. Thus we choose ECC media as main subject in this work to investigate the magnetic properties of various type ECC media. Single FePt layer with perpendicular anisotrpoy can be obtained by controlling argon pressure (PAr) during FePt deposition. When PAr =4 mtorr, 21 nm Fe58Pt42 was directly deposited on glass substrate. Excellent perpendicular magnetic anisotropy was observed after post-annealing: out-of-plane coercivity (Hc⊥) is 11.5 kOe and out-of-plane squareness (S⊥) is 0.88. Discussion will be split into two parts: bilayer and graded ECC media. The Hc⊥ of Glass/L10 Fe58Pt42 14nm/A1 Fe58Pt42 5 nm bilayer film is 9.5 kOe (20.3 kOe for single hard layer) and S⊥ is only 0.5. However, step-like behavior was observed in the M-H loop of this bilayer film. On the other hand, the Hc⊥ of H/S type graded ECC media is 10.8 kOe and S⊥ is 0.96. A trade-off between perpendicular anisotropy and switching field of ECC media could be observed. Various film structure of ECC media was also investigated. We found that the switching field can be further decreased by introducing extra diffusion interfaces with fixed soft layer thickness. After annealing at 700℃ for 30 mins, the Hc⊥ of Glass/ Fe58Pt42 7 nm/ Fe40Pt60 5 nm/ Fe58Pt42 7 nm is further decreased to 7.9 kOe, and S⊥=0.955. On the other hand, the magnetic anisotropy is affected by the sequence of deposition of hard and soft layers: the perpendicular magnetic anisotropy is better when hard layer is deposited first. Graded ECC meda with multiple diffusion interfaces is proposed as an effective method to decrease the switching field without degradation of perpendicular magnetic anisotrpy in this work. It is available for the application of high Ku material with low switching field in magnetic recording technology.