具垂直磁異向性之鈀/鈷鐵硼/氧化鎂三層結構之研究

Investigation of Pd/CoFeB/MgO trilayer structure with perpendicular magnetic anisotropy

王鼎碩(Ding-Shuo Wang)

指導教授 ： 賴志煌

國立清華大學/工學院/材料科學工程學系/博士(2015年)

https://doi.org/10.6843/NTHU.2015.00211

摘要

本研究專注於製作出應用於高效能磁性記憶體之自由層磁性材料，並探討產生純自旋流的方法藉以提升元件的操作表現。論文共分為三個研究子題，分別就材料特性及元件操作來進行研究。
第一部分為關於鐵磁性鈷鐵硼的材料特性研究及改良，使其更能適用在垂直式磁性記憶體的自由層中。我們研究出在鈀/鈷鐵硼/氧化鎂三層結構中加入一層高活性的金屬鋁做為覆蓋層並佐以快速升溫退火，可以大幅度調整退火後鈷鐵硼/氧化鎂介面的鐵-氧混成軌域數量及膜層中的硼含量，進而調控此結構的磁性表現。最終我們開發出使鈷鐵硼材料能同時具低阻尼常數及高熱穩定性的製程，同時也探討了掌控垂直式鈷鐵硼材料在製程中之熱債的方法。
論文的第二部分為在同樣的鈀/鈷鐵硼/氧化鎂三層結構中，嘗試以微波誘發之自旋幫浦效應產生由鐵磁層注入鄰近非鐵磁層的純自旋流。在此種磁矩垂直於膜面的系統內，由於磁矩進動時角動量改變的方向與膜層間界面平行，通常無法產生自旋幫浦效應及自旋流注入的效果。但我們藉由改變所施加磁場的角度破壞磁矩在垂直膜面方向進動的對稱性，成功地以自旋幫浦效應在鈀金屬層中注入了純自旋流，並以反自旋霍爾效應分析此純自旋流對於磁場角度的關係。我們更深入探討了在此種垂直系統內，磁異向性及磁矩大小對於自旋幫浦效應的影響，指出了以自旋幫浦效應產生純自旋流應用在垂直式元件的可能性。
在第三部分的實驗中，我們嘗試將電流通過具有高自旋-軌道耦合力的鈀金屬層，藉由自旋-軌道的交互作用可以把電流轉換成純自旋流並注入鄰近的鈷鐵硼層裡，同時對鈷鐵硼之磁矩作用一自旋傳輸力矩。我們使用鎖相向量量測的方式，測出了受此純自旋流作用下分別在平行及垂直於電流方向上磁矩所受到的力矩及等效磁場，並分析出在本系統內的自旋-軌道耦合作用力來源。最終我們成功使用水平於膜面的電流影響了垂直於膜面的磁矩達到去磁狀態。本實驗結果顯示了使用高自旋-軌道耦合力材料所產生的純自旋電流能夠應用在實際的元件上，並且具有大幅提升磁記憶體表現的潛力。

關鍵字

自旋電子學 ； 鈷鐵硼 ； 自旋幫浦 ； 自旋霍爾效應

並列摘要

This research work includes fabrication of high-quality free-layer magnetic materials with perpendicular magnetic anisotropy and generation of pure spin current in the magnetic multilayer structures for high-performance magnetic memory devices. In this dissertation, we study three different topics focusing on improving the material properties and developing new ways for device operation.
The first topic focuses on improving the magnetic properties of ferromagnetic CoFeB. We studied its magnetic properties and modified the fabrication process to make it more suitable for device application. The way to manage the thermal budget of perpendicular-anisotropy CoFeB were also investigated. We found out that by adding an additional high-reactivity Al capping layer in Pd/CoFeB/MgO structure accompanying with rapid thermal annealing, we could fine-tune the interfacial Fe-O orbital hybridization as well as the amount of boron in bulk CoFeB, and optimize the magnetic properties of this structure. Eventually we developed one modified fabrication process that make CoFeB to possess low Gilbert damping constant and high thermal stability simultaneously.
In the second topic, we tried to inject a pure spin current via microwave-induced spin-pumping in the Pd/CoFeB/MgO trilayer structure. Typically it is not allowed to have spin current injection in such perpendicular-anisotropy system via spin-pumping since the direction of angular momentum change during magnetization precession is parallel to the non-magnet/ferromagnet interface. However, by varying the orientation of applied field to break the symmetry of magnetization precession, we successfully injected a pure spin current into the non-magnetic Pd layer by spin-pumping, and we studied the relationship between the injected spin current and field orientation by using inverse spin Hall effect. We further investigated the roles of magnetic anisotropy and saturation moment on affecting spin-pumping. Finally we revealed the possibility to apply spin-pumping for spin current injection in such perpendicular-anisotropy devices operation.
In the third topic, we passed a charge current through the high-spin-orbit-coupling Pd layer in the above mentioned trilayer structure and convert the charge current into pure spin current via spin-orbit interaction, and we found out that the pure spin current can also be injected into the nearby ferromagnet CoFeB layer and apply spin-transfer-torques to the magnetic moments of CoFeB. We analyzed the current induced effect field at two directions, in-plane parallel or perpendicular to the current, by lock-in vector measurement, and figured out the origins of spin-orbit coupling effects in this system. Finally, we demagnetized the out-of-plane magnetic moments by applying an in-plane current. This result proves that the pure spin current generated by high-spin-orbit-coupling materials has great potential on real device application, and the new operation mode can have huge opportunity to further improve the device performance in the near future.

並列關鍵字

spintronics ； CoFeB ； spin-pumping ； spin Hall effect

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