鎳超薄膜在鉑(111)基板上之表面結構及表面磁光性質研究

Structural and magneto-optic properties of Ni-based ultrathin films on Pt(111) surface

蘇炯武(Chiung-Wu Su)

指導教授 ： 沈青嵩 博士

國立臺灣師範大學/理學院/物理學系/博士(2002年)

摘要

本實驗是利用歐傑電子能譜術、低能量電子繞射儀、紫外光電子能譜術以及利用表面磁光科爾效應來研究鎳金屬超薄膜在鉑金屬(111)表面上的結構及磁光性質。討論的範圍首先著重在鎳超薄膜在鉑(111)表面上的磊晶成長模式、結構相圖和合金形成。我們經由歐傑電子能譜、低能量電子繞射及紫外光電子能譜的測量中發現鎳超薄膜在鉑單晶上是以2個原子層的層狀模式成長，且在磊晶的過程中我們更利用低能量電子繞射發現鎳超薄膜在鉑(111)表面上有一些有趣且特殊的結構：偽(1×1)超結構、(√3×√3)R30º、Ni(1.1×1.1)非同調性磊晶、衛星點結構以及(2×2)超結構。鎳原子發現在高溫時會擴散與鉑形成合金，當我們在進行0.8到3.0個鎳原子層熱處理時，結果發現當鎳的厚度愈高，鎳與鉑開始形成合金的溫度也就愈高。為了提高系統鎳超薄膜的膜厚準確度，我們利用兩種理論模型來計算並決定鎳超薄膜膜厚。此外，當經過高溫回火的鎳/鉑(111)表面經由離子濺射技術後亦發現表面組成大多為鉑原子所佔據，此部分確立了鎳原子與鉑原子形成合金的事實。
第二部分我們在鎳/鉑(111)表面上覆蓋銀原子層來研究鎳鉑合金形成因其所受到之影響並與未加銀原子層來做比較。結果發現，覆蓋銀原子層的鎳薄膜層必須上升到更高溫時才與鉑原子形成合金，而且銀原子層在熱處理的過程中並不擴散進入基底且都位於表面的最上層，更有趣的是我們發現在1 ML Ag/1 ML Ni/Pt(111) (ML:原子層)的樣品中經由高溫處理後形成有趣的(2×2)表面超結構，經由晶格常數計算、以及離子濺射實驗後，我們初步推斷最上層的銀原子以1/4的覆蓋率形成(2×2)超結構之後剩餘的3/4銀原子與最上層的1/4殘餘鎳原子形成Ag(75%)Ni(25%)的合金原子層，剩餘的則為鎳鉑合金層。
第三部分我們利用表面磁光科爾效應來探測鎳超薄膜在鉑(111)表面上的磁光性質。鎳超薄膜在諸多系統中都發現具有dead layer的磁性質，故當我們在磊晶過程中探測鎳薄膜的磁光訊號中發現，將近有7層覆蓋率的鎳原子在室溫裡是沒有磁性的，累積到將近24層的鎳原子測得之最大科爾旋轉角也只有0.02º，並且在熱處理的實驗當中，我們發現膜厚與系統的居禮溫度有很大的關連性，甚至極有可能低於室溫。
此外，當磁性超薄膜鎳/鉑(111)表面間加進鈷原子層後，初步發現鎳原子會有初期升溫的過程中先與鈷原子在鉑表面上混合，高溫時再擴散進入鉑基底的特性。經由深度分析的實驗，雖然證明了鎳鈷原子都會與鉑形成合金，但是我們發現鎳原子卻擴散的比鈷原子更為深層。1 ML Ni/1 ML Co/Pt(111)樣品在垂直磁光效應的測量中，也同樣發現在鎳鈷原子混合時磁光訊號有微量的增加，然而之後主要的磁光訊號大增主因來自於鈷鉑形成合金所致，科爾旋轉角在高溫回火後增加為原先的兩倍之多，當我們對於1 ML Ni/1 ML Co-Pt 合金表面進一步的研究中發現，系統的居禮溫度隨著鎳鉑原子在表面的相對組成而有強烈的變化，而且接近甚至低於室溫，在表面化學組成計算後可以初步推論，若鉑原子含量在表面層愈多、鎳原子愈少的狀況下，系統的居禮溫度就愈低。結果發現，1 ML Ni/1 ML Co-Pt 合金表面樣品在經由830 K高溫回火後所測得之系統居禮溫度為275 K，此時所對應的表面化學組成為Pt(69%)Co(29%)Ni(2%)合金層。
最後，鏡射系統1 ML Co/1 ML Ni/Pt(111)的磁光訊號測量也發現許多有趣且不同於1 ML Ni/1 ML Co/Pt(111)系統的物理現象，在升溫的過程當中發現，特定的溫度範圍對於兩種系統有著截然不同的行為，我們發現在600 K到725 K的磁光訊號變化中對於Ni/Co/Pt薄膜有一極大值，然而對於Co/Ni/Pt薄膜卻發現有一極小值。此外，Co/Ni/Pt薄膜發現具有比Ni/Co/Pt薄膜更大的的矯頑磁場，我們初步認為這些有趣的物理現象來自於表面鎳鈷鉑原子的相對組成，以及許多特定穩定合金結構的形成，所以當我們又利用紫外光電子能譜來觀看這兩種磁性超薄膜系統經過高溫熱處理後時，我們可以確定表面原子態大多來自於鉑原子，換句話說，高含量的表面鉑原子是促成系統具有相當低的居禮溫度的主因。

關鍵字

歐傑電子能譜術 ； 低能量電子繞射 ； 紫外光電子能譜術 ； 表面磁光科爾效應 ； 鉑 ； 鎳 ； 銀 ； 鈷 ； 合金 ； 表面結構 ； 表面磁性 ； 磁異向性 ； 磁滯曲線 ； 成長模式

並列摘要

Structural and magneto-optic (MO) properties of Ni-based ultrathin films on Pt(111) were studied by means of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS) and surface magneto-optic Kerr effect (SMOKE). The epitaxial growth mode, structural phase diagram and alloy formations of Ni on Pt(111) were first presented in this paper. Ni ultrathin films show some interesting surface structures: the pseudo-(1×1) structure; the (√3×√3)R30º commensurate superstructure; the incoherent epitaxy of Ni(1.1×1.1) structure; the incoherent satellite structure, and the (2×2) superstructure. The starting temperatures for alloy formation of Ni-Pt are higher with the increasing coverage of Ni from 0.8 to 3.0 ML. The coverage determination for Ni layers on Pt by two theoretical models was used for a better thickness degree of accuracy. The chemical composition after high temperature annealing by a depth profile experiment showed enrichment of Pt atoms in the surface layers. The electron band structure dependent on the coverage and annealing temperatures was also measured for confirming the initial growth as a layer-by-layer mode and alloy formation of Ni–Pt.
The Ag capping layer on Ni/Pt(111) was studied to compare with the alloy formation of Ni–Pt in pure Ni/Pt(111) system. We observed the alloying temperatures comparatively higher than Ni/Pt(111) system. Furthermore, the capping layer of Ag in the system is located at the top layers during the annealing. Hence, the Ag capping layer promotes the stability of the Ni layer on Pt(111) surface. Interestingly, a new structural phase of (2×2) LEED pattern was first observed when 1 ML Ag/1 ML Ni/Pt(111) was annealed at high temperature. From the depth profile experiment and the calculation of lattice constants, we propose that the possible interface structures of the annealed 1 ML Ag/1 ML Ni/Pt(111) are consist of Ag(2×2) structure on the topmost layer, the Ag(75%)Ni(25%) surface alloy in the second layer, and the remaining layers are the Ni–Pt alloys.
SMOKE was used for examining the magneto-optic property of Ni on Pt(111). Ni film has a known “dead layer” property in ultrathin film system. The nonmagnetic layers of Ni in our system are estimated about 7 ML at room temperature. Kerr rotation for the 24 ML Ni/Pt(111) is only about 0.02º. The Curie temperature (Tc) strongly altered by the thickness of Ni within 24 ML even smaller than room temperature. Thinner Ni films than 4 ML may possess lower Tc lower than 200 K or less.
Besides, the intercalating high magneto-optic response Co layer in the ultrathin film Ni/Pt(111) was further studied for a comparison. A surface mixing effect of Ni–Co occurs and both atoms successively diffuse into bulk when annealing temperature is higher than 600 K. The diffusion volume of Ni is larger than that of Co after a depth profile analysis. The Co layer just forms surface alloy with Pt and less Ni about top few layers but considerable quantities of Ni diffuses deeper. The magneto-optic property of 1 ML Ni/1 ML Co/Pt(111) was performed by perpendicular MOKE. We observed a minor increase of Kerr rotation (θk) in the mixing of Ni and Co and a major enhancement of θk contributed to the alloying effect of Co–Pt in the annealing. The Kerr rotation is two times larger than the as-deposited layered film after annealing. The Curie temperature in the system is strongly controlled by the interface composition of Ni and the segregation layer of Pt. The Tc even shifts lower than room temperature. When we assumed a linear chemical composition within surface region, the Pt-rich surface and the Ni concentration on the top layers is the reason of the alloy film possessing low-Tc. The chemical composition of the lowest-Tc of 275 K is estimated as a Pt(69%)Co(29%)Ni(2%) alloy film.
The MO properties of the mirror 1 ML Co/1 ML Ni/Pt(111) film was also used to compare the 1 ML Ni/1 ML Co/Pt(111). A very different behavior for the Co/Ni/Pt layer occurred in 600~725 K annealing. In this range, θk has a maximum about 0.06º for Ni/Co/Pt film but for Co/Ni/Pt layer has an almost zero minimum. In addition, the coercivity of film is large than that of Ni/Co/Pt film. This unusual and interesting phenomenon may be attributed to the diffusion volume of Ni, Co and the segregation layer of Pt. Ultimately, both electronic d-band structures of 1 ML Ni/1 ML Co/Pt(111) and 1 ML Co/1 ML Ni/Pt(111) were examined comparatively. From the evolution of UP spectra as function of temperature, high concentration of Pt on the surface layers can explain the magnetic behavior with low–Tc.

並列關鍵字

Auger electron spectroscopy ； low-energy electron diffraction ； ultraviolet photoelectron spectroscopy ； surface magneto-optic Kerr effect ； platinum ； nickel ； silver ； cobalt ； alloys ； surface structure ； surface magnetism ； magnetic anisotropy ； hysteresis ； growth mode