於陽極氧化鋁表面自組化奈米金屬點陣之成形與物性研究

Abstract

本研究於具有奈米孔洞的陽極氧化鋁模板表面，以磁控濺鍍及蒸鍍的方式，製作自組化且垂直排列的Pt及Fe奈米點陣，並對陽極氧化鋁模板尺寸對於堆積原子表面形貌的影響進行探討。孔洞的外環對於堆積原子扮演著障礙物的角色，阻礙連續膜的形成。一般而言，相較於堆積原子的晶粒尺寸，若孔洞之間距離較長或成核點位置較接近，有助於連續膜之形成。若孔洞為緊密堆積，較短的孔間間距或較短的角偶間距，亦即較小的表面積，會使異質成核所需的表面能不足，限制堆積原子的成長。然而，若僅有角偶間距較大，個別分離的點陣將取代連續膜而成形於這些限制區域。更進一步的，點陣的直徑隨著膜厚增加，導致間距減小，使得可成長個別分離點陣的陽極氧化鋁模板尺寸範圍因此縮小。最後歸納出一奈米點陣成形相圖，進一步的了解奈米點陣於陽極氧化鋁模板上成形的形式。 以蒸鍍的方式於陽極氧化鋁模板表面製作的奈米點陣，其外型近似於倒錐形體。藉由控制Fe相對膜厚(tn)由400至5 nm，表面形貌由連續膜轉變為個別分離點陣，使得磁翻轉機制由磁區壁移動轉變為磁矩旋轉所主導。當tn < 59 nm時，表面形貌為個別分離而非相互連接。於此範圍內，矯頑磁力於tn = 47 nm及點陣直徑大約52 nm時有一特別的變化，可得大約470 Oe的最大值，因此定義此為單磁區點陣臨界尺寸。當tn < 27 nm時，熱擾動使得淨磁力與交互作用力降低，並且磁矩偏離水平方向而隨機分布於三維空間。此外，藉由添加Pt種子層，使Fe成長的區域面積擴大並且縮短間距，製作Fe/Pt雙層膜結構，進一步探討Fe奈米點陣之磁翻轉機制。Fe奈米點陣之淨磁力及交換作用力與其尺寸相關，當作用力增加時會使得單磁區所主導的磁矩旋轉的翻轉機制轉變為合力方式翻轉。 於新穎性發展上，於陽極氧化鋁模板表面自組化的個別分離奈米點陣，藉由其可傾斜式磁異向性，可進一步應用於傾斜式磁記錄媒體。自組化的Fe點陣，藉由傾斜蒸鍍的方式得到可傾斜式磁異向性。孔洞的外環對於堆積原子引起堆積差異，阻礙於表面並遮蔽於孔洞內壁，有助於形成個別分離的點陣且同時具有延伸的末端結構。此末端結構垂直成形於未遮蔽的孔洞內壁，誘發垂直形狀異向性，藉由與頂端單磁區點陣的耦合作用，主導整體的磁異向性。此可傾斜機制實現於Fe單磁區點陣時，當鍍膜傾斜角增加及相對膜厚降低分別至50o及16 nm時，而使末端結構之形狀異向性增加，進一步得到高磁異向性(1.38 × 10^7 ergs/cm3)並傾斜至垂直方向。由翻轉場隨角度變化的結果發現此點陣為獨立翻轉，證明了點陣間去耦合的作用力，並且於相對於易軸45o的角度可得最小值。 於相對膜厚16 nm及傾斜27.6o鍍膜時，可得傾斜45o磁異向性且獨立磁翻轉之個別分離Fe點陣。因此，此可傾斜的機制符合傾斜式磁記錄媒體的需求。大面積的奈米點陣有助於未來的研究與應用，例如垂直式及傾斜式磁記錄媒體方面的發展。

Vertically aligned and self-assembled Pt and Fe nano-size arrays are fabricated on the top of nanoporous anodic aluminum oxide (AAO) templates by magnetron sputtering and evaporation. In this study we focus on the size dependence of AAO template on the morphology of stacked atoms. The rims of the pores, which act as obstacles to the stacking of atoms, prevent them from forming continuous films. The continuous films are commonly formed because of the larger interpore distance and/or the closer nucleation sites with smaller periodic distance, compared to the grain size of stacked atoms. Between closely-distributed pores, the shorter interpore distance, or even the shorter corner distance, namely smaller surface area, indicates insufficient surface energy for heterogeneous nucleation, restricts the growth of stacked atoms. Nevertheless, if the corner distance is larger and interpore distance is smaller than the grain size, instead of continuous films, isolated arrays are formed on these constricted regions. Furthermore, the diameter of the arrays increases with the thickness, leading to the decrease in the spacing. The dimension of AAO templates for the formation of isolated array is reduced. A nano-size array formation diagram is deduced, and understanding of the formation of nano-size arrays on the AAO template is furthered. The Fe arrays with quasi-inverted-cone shape on the top of AAO templates by evaporation are presented. By controlling the Fe nominal thicknesses (tn) from 400 to 5 nm, the morphology is changed from continuous film to isolated arrays, leading to the change of the predominant magnetization reversal from domain wall motion to spin rotation. For tn < 59 nm, isolated, rather than interconnected, morphology is formed. In this range, the coercivity shows a spectacular change for tn = 47 nm, with an array diameter of about 52 nm, achieving a maximum of about 470 Oe. The critical dimension of single-domain array is therefore determined. The magnetostatic and exchange interactions are reduced due to the thermal fluctuation, and the magnetization leaves from the in-plane direction to be randomly distributed in 3-D, for tn < 27 nm. In addition, the magnetization reversal mechanisms of Fe arrays are investigated by inserting Pt seed arrays to form Fe/Pt bilayer arrays in order to extend the area and shorten the interval of the formation regions of Fe arrays. The magnetostatic and exchange interaction of Fe arrays are size-dependent, and the enhanced interactions cause the departure of the predominant spin rotation of single domain to be cooperative rotation. A novel advance in nano-size arrays, isolatedly self-assembled on nanoporous AAO templates with tiltable magnetic anisotropy, is proposed as a potential tilted magnetic recording media. Tiltable magnetic anisotropy of self-assembled Fe arrays has been obtained via oblique evaporation. The rims of the pores, which induce a stacking variation to the stacked atoms, obstructed on the top and shadowed on the inner-wall, aid the formation of isolated arrays with extended 'sterns.' The sterns, formed perpendicularly on the unshadowed inner-wall inducing out-of-plane shape anisotropy, dominate the magnetic anisotropy via the coupling to the magnetization of the topmost single-domain array. The tiltable mechanism materializes on Fe single-domain arrays, allowing the high magnetic anisotropy (1.38 × 10^7 ergs/cm3) to be tilted to perpendicular by the stern at a nominal thickness of 16 nm via 50o-oblique deposition with an increase in shape anisotropy, as a result of the increased oblique angle and decreased nominal thickness. The results of angular-dependent switching field show the independent rotation, verifying the decoupling of inter-array interaction and the achievement of minimum at 45o with respect to the easy direction. The 45o-tilted magnetic anisotropy of isolated Fe arrays with independent magnetization reversal is obtained at 16-nm nominal thickness of via about 27.6o-oblique deposition. The demands for tilted magnetic recording can be met with this tiltable mechanism. These large area nano-size arrays are useful for future research and applications, such as perpendicular and tilted magnetic recording media.