- 學位論文
具有垂直異向性的CoFeB/MgO/CoFeB穿隧結構中耦合效應之探討
The exchange coupling between ferromanetic layer in CoFeB/MgO/CoFeB tunnel structure with perpendicular magnetic anisotropy
摘要
本論文探討Co40Fe40B20/MgO/Co20Fe60B20磁穿隧結(p-MTJ, perpendicular magnetic tunnel junction)之介面耦合效應(IEC, inter -layer exchange coupling)主要分為下列兩部進行探討: 1. Co40Fe40B20(1.2)/MgO(x)/Co20Fe60B20(1.2)，x=0.9~1.5單位為nm，每隔0.05nm做一組樣品，並經退火處理溫度為225℃和255℃。 首先，此系列樣品改變MgO厚度並觀察耦合效應與MgO厚度的關係。觀察到當改變MgO厚度時耦合效應會呈現類似週期性的變化，當MgO為1nm時沒有耦合現象，MgO為1.2 nm時為反鐵磁耦合，而1.25 nm時為鐵磁耦合。 再由以上實驗中取出MgO厚度x=1、1.1、1.2、1.3、1.4、1.5的樣品進行較詳細的退火實驗，退火溫度為225~315℃，並且觀察耦合效應與退火溫度的關係。MgO厚度為1、1.1、1.3、1.4nm的樣品在退火溫度為225℃時呈現鐵磁耦合，當退火溫度增加，耦合現象會從鐵磁耦合轉變為反鐵磁耦合。MgO厚度1.2nm的樣品則在退火溫度為225 ℃已經呈現反鐵磁耦合，當退火溫度增加時反鐵磁現象更加明顯。MgO為1.5nm的樣品在退火溫度為225 ℃時呈現反鐵磁耦合。當退火溫度低於285℃時，耦合現象隨溫度增加略呈鐵磁耦合改變，當退火溫度高於285℃時耦合現象隨溫度增加往反鐵磁耦合改變。此外我們也觀察到樣品的異向能常數(Ku值)和交換耦合能(J值)的變化趨勢不同，亦即當Ku值增加時耦合強度會逐漸呈現反鐵磁耦合的狀態，也同時顯示異向能從水平(未退火)轉為垂直(退火)的過程中也同時會影響耦合作用從鐵磁轉為反鐵磁，此結果與最近文獻中Orange peel Model所計算出的結果相符。 2. Co40Fe40B20(1.2)/MgO(1)/Co20Fe60B20(x)以及Co40Fe40B20(x)/MgO(1) /Co20Fe60B20(1.2) x=0.9~1.5單位為nm，每隔0.05nm做一組樣品，退火處理溫度為225和255℃。此系列樣品主要改變上下層鐵磁層厚度並觀察耦合效應與鐵磁層厚度的關係。當退火溫度為225℃時我們觀察到耦合現象隨CoFeB厚度改變而呈週期性的震盪，但是當退火溫度達255℃時卻無明顯週期的現象，我們利用Fabry-Perot-type interference model與這些實驗結果比較，發現定性上是相符合的。
並列摘要
In this thesis we study the interlayer-exchange-coupling, as a function of the thickness of MgO spacer layers and the thickness of CoFeB, in the Co40Fe40B20/MgO/Co20Fe60B20 perpendicular magnetic tunnel junction structures. The main work is divided into two parts: (1) In Co40Fe40B20(1.2)/MgO(x)/Co20Fe60B20(1.2) structure (all units are in nm), the MgO thickness varies from 0.9 to 1.5 nm at annealing temper -atures are 225℃ and 255℃, respectively. We found that the coupling strength and sign (corresponded to par -allel or antiparallel coupling) varied depending upon MgO layer thick -ness. These samples show parallel(ferromagnetic) coupling at the thickness of MgO ~ 1.25nm while show antiparallel (antiferromagnetic) coupling at the thickness of MgO ~ 1.2 nm, and show no coupling at the thickness of MgO ~ 1nm. Samples with MgO layer thickness of 1, 1.1, 1.2, 1.3, 1.4, and 1.5nm are chosen for further annealing test and the annealing temperature is in the range of 225℃- 315℃. The results show clear variation of the coup -ling field as a function of annealing temperatures. The samples of MgO 1, 1.1, 1.3, 1.4nm show ferromagnetic coupling at 225℃ but change to antiferromagnetic after the annealing temperature is increased. The sample of MgO 1.2nm show antiferromagnetic coupling at 225℃ and the antiferromagnetic coupling strength is increased after the annealing temp -erature is increased. The sample of MgO 1.5nm show antiferromagnetic coupling at 225℃ and changes to ferromagnetic at the annealing temper -ature increases from 225℃ to 285℃. However, it changes back to antiferromagnetic at T > 285℃. In addition, we also found that the anisotropy energy constant (Ku) and the exchange coupling energy (J) shows inverse relation as the annealing temperature varies. The anisotropy energy constant increases as temperature increases while the exchange coupling decreases from ferromagnetic to antiferromagnetic as temperature increases. This effect seems to be consistent with the recent report of the calculation which is based on an Orange peel Model. (2) In Co40Fe40B20(1.2)/MgO(1)/Co20Fe60B20(x) and Co40Fe40B20(x)/MgO(1)/Co20Fe60B20(1.2) structures, the ferromagnetic layer thickness is changed in the range of 0.9 - 1.5nm and annealing temperatures of 225 - 255℃, respectively. We measure the variation of the coupling field as a function of ferro -magnetic layer thickness and find an oscillatory behavior of exchange coupling as a function of the ferromagnetic layer thickness at annealing temperature 225℃. However, this oscillatory behavior disappears after the annealing reaches 255℃. This effect may be explained by a Fabry-Perot- type interferences model.