MgO/NiO/Ni3Fe系統的界面缺陷對交換偏壓的影響

The Effect of Interfacial Defects on the Exchange Bias in the MgO/NiO/Ni3Fe System

李自原(Li, Tzuyuan)

指導教授 ： 歐陽浩

國立清華大學/工學院/材料科學工程學系/碩士(2013年)

https://doi.org/10.6843/NTHU.2013.00508

摘要

交換偏壓(exchange bias)是一種發生在界面的磁性現象。當鐵磁和反鐵磁材料形成的雙層膜在高於反鐵磁層的尼爾溫度(Néel temperature)進行場冷後，這兩個材料會在界面進行磁耦合。因此，鎳鐵/氧化鎳雙層膜可以發生磁耦合並產生交換偏壓。這樣的現象和界面的原子磁矩有關。然而，實驗上很難觀測試片中各個原子的磁矩方向，而要在鎳鐵/氧化鎳這樣的多晶薄膜去解析原子的排列更為艱困，因此我們透過理論模擬計算的方式，企圖將NiFe/NiO系統在界面發生交換偏壓之各原子的磁化方向模擬出來，並與該雙層膜長在MgO單晶基板上的實驗結果比較。根據隨機場模型(Random field model)，可由外加磁場改變方向前後的界面能量，再配合鐵磁層的厚度和磁化量，來計算交換偏壓，並將模擬計算的結果與實驗值(感謝加拿大prof. Johan van Lierop提供數據)以及高解析度電子顯微鏡影像(HRTEM)的結果進行比較，可以得到比較合理的原子磁矩排列。由VASP的模擬結果得到界面無粗糙度和兩種粗糙度(2.20Å和4.43Å)的交換偏壓分別為3.08±4.59(Oe)、-24.27±3.13(Oe)和-195.53±10.88(Oe)，而另一組外加場方向得到的結果分別為0.00±3.88(Oe)、-4.17±1.96(Oe)和-11.02±3.12(Oe)。模擬計算結果顯示，界面粗糙度可以強化交換偏壓，這與實驗的結果符合。因此，我們可以推測，當界面粗糙度存在的情況下，原子磁矩的排列方式應與真實情形較為接近，如同高解析度電子顯微鏡所示。隨後，我們也針對界面附近的差排對交換偏壓的影響進行模擬計算。當差排加入粗糙度為2.20Å結構界面的NiO時，交換偏壓從-24.27±3.13(Oe)提升為-143.46±4.37(Oe)，在另一組外加磁場方向下則從-4.17±1.96(Oe)增為-82.30±7.29(Oe)；當差排加入粗糙度為2.20Å結構界面的Ni3Fe時，交換偏壓從-24.27±3.13(Oe)變為-23.42±2.71(Oe)，另一組外加磁場方向則從-4.17±1.96(Oe)變為-6.29±2.09(Oe)。顯示出在界面的NiO差排可以提升交換偏壓。最後，我們研究各種粗糙度和差排對未補償自旋數目的影響，發現粗糙度提升會使界面的未補償自旋密度增加，且界面的NiO差排亦能增加未補償自旋密度，而Ni3Fe的差排則對此沒有影響。基本上，交換偏壓是一個鐵磁自旋與反鐵磁自旋在界面的交互作用，所以任何對於界面自旋排列的改變(如反鐵磁在界面的為抵消自旋數量，以及鐵磁和反鐵磁自旋的相對方向)都會影響最終的交換耦合。因此，我們可以合理地推論，界面粗糙度是藉由提升缺陷產生的未抵消自旋，來強化介面交換耦合。這與最近蒙地卡羅(Monte Carlo)的模擬計算結果相符(J. Spray and U. Nowak, J. Phys. D: Appl. Phys. 39 (2006) 4536)。

關鍵字

交換偏壓 ； 氧化鎂基板 ； 第一原理計算 ； VASP模擬 ； 鎳鐵/氧化鎳

並列摘要

Exchange bias(EB) is an interfacial magnetic phenomenon. After a bilayer film made of a ferromagnet and antiferromagnet goes through a field-cooling process from above the antiferromagnet's Néel temperature, these two materials will couple magnetically at their interface. Thus, a nickel-iron/nickel-oxide bilayer can magnetically couple and enable EB. This phenomenon is related to the interfacial arrangement of atomic magnetic moments. However, it is quite difficult to experimentally observe the direction of magnetic moments for individual atoms, and even harder to resolve the necessary atomic arrangement in polycrystalline films such as NiFe/NiO. Thus, we try to simulate the magnetic moments of each atom coupling to permit EB at the interface of nickel-iron/nickel-oxide system by theoretical simulations and mapping these results onto experiments on the films grown on a crystalline MgO substrate. According to random-field model, we can obtain the exchange bias from the difference of interfacial energies before and after the change of magnetic field direction, then incorporating the thickness effects of the ferromagnetic layer and its magnetization. Afterwards, we compared the simulated results with the experimental data and determined reasonable configurations of atomic magnetic moments by comparison with results from HRTEM (high resolution transmission electron microscopy). The result of VASP simulations for exchange bias without roughness and with two kinds of roughness (2.20Å and 4.43Å) at the interface is 3.08±4.59(Oe), -24.27±3.13(Oe) and -195.53±10.88(Oe), respectively, and 0.00±3.88(Oe), -4.17±1.96(Oe) and -11.02±3.12(Oe) for different directions of applied field. All simulations revealed that interfacial roughness can strengthen the EB, which is consistent with experimental results. Thus, we can conclude that the configuration of atomic magnetic moments is similar to real case with the interfacial roughness as also shown from HRTEM. Afterwards, we calculated for the effect of interfacial dislocations on EB. When dislocations appear around the interface with 2.20Å roughness, EB increased from -24.27±3.13(Oe) to -143.46±4.37(Oe), and from -4.17±1.96(Oe) to -82.30±7.29(Oe) for different directions of applied field. As EB is essentially a spin-spin interaction between ferromagnet and antiferromagnet at the interface, any kind of modification of the interfacial spin configuration (e.g. quantity of the uncompensated interfacial antiferromagnetic spins and the relative orientation between antiferromagnetic and ferromagnetic spins) will affect the final exchange coupling. Thus, it is reasonable to conclude that an interface roughness can strengthen the interfacial exchange coupling by enhancing the defect-generated uncompensated condition. This is in agreement with recent Monte Carlo simulations [J. Spray and U. Nowak, J. Phys. D: Appl. Phys. 39, 4536 (2006)].

並列關鍵字

exchange bias ； MgO substrate ； first-principle calculation ； VASP simulation ； NiFe/NiO

參考文獻

[1] W. H. Meiklejohn, C. P. Bean, Phys. Rev., 102, 1413, 1956.

[7] J. Nogués and Ivan K. Schuller, J. Magn. Magn. Mater., 192 (1999) 203.

[8] W.H. Meiklejohn, J. Appl. Phys., 33 (1962) 1328.

[9] D. Mauri, H. C. Siegmann, P. S. Bagus, E. Kay, J. Appl. Phys., 62,1987 3047.

[10] A. P. Malozemoff, Phys. Rev. B, 35, 3679, 1987.