無電鍍核(鎳)/殼(氧化鎳)奈米柱之界面鐵磁/反鐵磁交換耦合研究

Abstract

本論文利用陽極氧化鋁模板與無電鍍沉積法製作核(鎳)/殼(氧化鎳)奈米陣列結構，並藉由退火來調變氧化鎳殼層的厚度。我們發現鐵磁鎳奈米柱的磁滯曲線，因為引入氧化鎳層而產生抑制的效果，這是由於反鐵磁(氧化鎳殼層)的序化排列遮蔽了鐵磁(鎳核層)的有序性。另外，我們將核(鎳)/殼(氧化鎳)結構經過場冷處理，發現界面處有交換耦合的現象，包含:磁滯曲線的橫向與縱向偏移。

此外，實驗結果顯示核(鎳)/殼(氧化鎳)結構的矯頑場和溫度有相依性，在100K時的鐵磁/反鐵磁界面會有最劇烈的交換耦合作用，而100K以下/以上的磁性質表現分別由反鐵磁相/鐵磁相所主導。由於鐵磁/反鐵磁界面的耦合行為會隨氧化鎳層厚度與奈米柱的深寬比有關，這兩個變數使得我們可以間接控制此核殼奈米結構的磁性質。本研究擴展我們對於二維薄膜結構至一維奈米陣列結構之鐵磁/反鐵磁界面耦合現象的了解，期許相關見解能促進往後技術的應用。

Ni-NiO core-shell nano-arrays were fabricated by means of electroless-deposition and an anodic aluminum oxide template.The NiO shell of the Ni-NiO arrays displayed tunable thickness upon annealing.The magnetic hysteresis (M-H) curve of the Ni-NiO arrays was found to be suppressed compared with that of the pure Ni arrays. This can be attributed to the screening effect of the NiO shell, whose antiferromagnetic (AFM) ordering masked the core Ni’s ferromagnetic (FM) ordering. Severely competing AFM (NiO shell)-FM (Ni core) interactions were found to occur at the arrays’surfaces, which were characterized by vertical shift in M-H curve and asymmetric exchange bias upon field cooling.

Temperature-dependent coercive field (Hc) revealed that the strongest AFM-FM interfacial coupling took place at 100K, where the magnetic properties of the arrays were dominated by the AFM/FM below/above this temperature. The AFM-FM coupling was sensitive to the variation of the arrays’ shell thickness and aspect-ratio, leading to a fair control over the magnetic properties of the core-shell structure. The work extended the understanding of the AFM-FM coupling from two-dimensional film-structure to one-dimensional array-structure, in a hope to facilitate the related technologies utilizing such perception.