National Sun Yat-sen University Institutional Repository:Item 987654321/38655

行政院國家科學委員會專題研究計畫 期中進度報告

整合式磁區顯微術發展與低維磁性結構之研究(1/3)

計畫類別： 個別型計畫 計畫編號： NSC94-2112-M-110-016- 執行期間： 94 年 08 月 01 日至 95 年 10 月 31 日 執行單位： 國立中山大學物理學系(所) 計畫主持人： 郭建成 共同主持人： 鄭德俊，杜立偉 計畫參與人員： 林谷亮，陳兼維 報告類型： 精簡報告 報告附件： 出席國際會議研究心得報告及發表論文 處理方式： 本計畫可公開查詢

中 華 民 國 95 年 6 月 9 日

行政院國家科學委員會補助專題研究計畫

□ 成 果 報 告

■期中進度報告

整合式磁區顯微術發展與低維磁性結構之研究(1/3)

計畫類別：■ 個別型計畫 □ 整合型計畫

計畫編號：NSC 94－2112－M－110－016－

執行期間： 94 年 8 月 1 日至 95 年 10 月 31 日

計畫主持人：郭建成

共同主持人：

計畫參與人員： 林谷亮、陳兼維

成果報告類型(依經費核定清單規定繳交)：■精簡報告 □完整報告

本成果報告包括以下應繳交之附件：

□赴國外出差或研習心得報告一份

□赴大陸地區出差或研習心得報告一份

□出席國際學術會議心得報告及發表之論文各一份

□國際合作研究計畫國外研究報告書一份

處理方式：除產學合作研究計畫、提升產業技術及人才培育研究計畫、

列管計畫及下列情形者外，得立即公開查詢

□涉及專利或其他智慧財產權，□一年□二年後可公開查詢

執行單位：國立中山大學物理學系

中 華 民 國 95 年 5 月 31 日

中英文摘要：

本專題計畫利用磁區顯微術來研究低維磁性結構的演化。整個計畫的研究重點主要集中在 研究低維磁性結構如 Fe, Mn 的磁性超薄膜在接近臨界相變狀態下的磁區演化。利用分子 束磊晶的技術，製備具有原子級平坦度的磁性超薄膜，除了可以產生接近於理論架構的系 統（如具有垂直異向性的二維海森堡模型）用以研究在臨界狀態之下的長程磁性序在相變 過程中的細微結構，而此一製備的技術也將可提供開發新材料時的依據。本計畫利用具有 不同特性的磁區顯微術同時觀測不同尺度的低維磁性結構在磁場下的演化，除了可以提供 微觀磁區結構和巨觀磁性性質彼此間關聯的直接量測外，此一知識對於自旋電子材料的研 究與發展，亦將提供關鍵的幫助。

This project employs the integrated technology for magnetic domain imaging to explore the evolution of the low-dimensional magnetic structures. It mainly focuses on the integration of the magnetic domains as well as the study for the evolution of the low-dimensional magnetic structures, such as the stripe domains for Fe, Mn magnetic ultrathin films with magnetic field around critically phase transition. This project aims at the systematic study of the evolution of magnetic domain structures for low-dimensional magnetic structures. Employing the technique of molecular beam epitaxy, the atomically flat magnetic ultrathin film can be prepared to approach the system near to the theoretical construction (for example, the 2D Heisenberg model with perpendicular magnetic anisotropy). It could help us have an insight into the details of magnetic long-range ordering under phase transition. This preparation technology can also be provided as the basis of new materials development. This project employs the different techniques for magnetic domain imaging simultaneously to observe the magnetic evolution of the low-dimensional magnetic structure for different scales. It could not only provide the direct observation of the correlation between microscopic magnetic domain structures and macroscopic magnetic properties but also contribute the knowledge for the research and development of the spintronic materials.

關鍵字：Domain imaging, low-dimensional magnetic structure, ultrathin film

計畫執行期間已發表或被接受之論文：

1. Large spin effects in Coulomb blockade of Fe/MgO/Fe tunnel junctions, W. Wulfhekel, A. Ernst, J. Henk, P. Bruno, J. Kirschner, F. Zavaliche, C. C. Kuo, and Minn-Tsong Lin, Physical Review B 72, 212407 (2005).

2. Patterning Co nanoclusters on thin-film Al2O3/NiAl(100), M. F. Luo, C. I. Chiang, H. W.

Shiu, S. D. Sartale, and C. C. Kuo, Nanotechnology 17, 360 (2006).

S. D. Sartale, T. Y. Wang, P. L. Chen, and C. C. Kuo, Journal of Chemical Physics 124, 164709 (2006).

4. Engineering patterns of Co nanoclusters on thin film Al2O3/NiAl(100) using scanning

tunneling microscopy manipulation techniques, Shrikrishina D. Sartale, Ku-Liang Lin, Chou-I Chiang, Meng-Fan Luo, and Chien-Cheng Kuo, Applied Physics Letters (2006), in press.

計畫成果自評

本計畫執行至今，主要成果較為集中於奈米結構的成長與操控。實驗室已建立起豐富的經 驗製作低維奈米結構，並已有完整訓練的研究生。對於磁區顯微技術的研究則為目前及今 後研究的重點，基於過去研究的經驗，相信在磁區顯微技術上也將會取得不錯的成果。

1

Engineering patterns of Co nanoclusters on thin film Al

2

O

3

/NiAl(100)

using scanning tunneling microscopy

manipulation techniques

Shrikrishina D. Sartale

Department of Physics, National Central University, Jungli, 32054 Taiwan Ku-Liang Lin

Department of Physics, National Sun Yat-Sen University, Kaohsiung, 804 Taiwan Chou-I Chiang

Department of Physics, National Central University, Jungli, 32054 Taiwan Meng-Fan Luo(a)

Department of Physics, National Central University, Jungli, 32054 Taiwan Chien-Cheng Kuo(b)

Department of Physics, National Sun Yat-Sen University, Kaohsiung, 804 Taiwan

Abstract

We present precise engineering of patterns of Co nanoclusters grown on ordered Al2O3/NiAl(100)

surface using the scanning tunneling microscopy (STM) manipulation technique. The clusters are attracted to the STM tip by lowering the bias below a threshold value and translated and relocated to another position by reversing the polarity. This facile manipulation technique in combination with the self-organized patterning on this system reported earlier might play a decisive role in nanotechnology for various applications where patterned nanoclusters are desired.

(a) Electronic-mail: [email protected] (M. F. Luo)

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One of the ultimate goals of nanotechnology is directed at controlled positioning of individual atoms and molecules for building or modifying desired nanostructures.1 Since the pioneer work of controlled positioning of xenon atoms on Ni(110) surface,2 much progress has been made in developing techniques for manipulating a variety of atom/molecules on crystal surfaces.3-8 Among all the approaches, scanning tunneling microscopy (STM) is so far the simplest and the most general technique. This advantage arises from its capability to perform atomically resolved imaging and manipulation using the same equipment. Thus it is possible to image the surface, zoom in on a particular feature of interest, and induce atomic or molecular scale modifications, and then image and study the properties of the structures. By using the STM manipulation technique most research efforts until now have been made on engineering of single atom or molecule3-8,

9-16 _{and recently few large molecules}7,8,17,18 _{supported by single crystal metals or}

semiconductor surfaces. However, for various technological applications, there is an immediate need to extend this enticing technique to engineering of the matter at macro-molecular or cluster scale on synthetic or processed surfaces, which may eventually allow fabrication of a more diverse range of nanostructures to be used in electronic, magnetic nanodevices and nanomechanical applications.

Recently we have reported patterning of Co nanoclusters formed from vapor deposition over ordered Al2O3/NiAl(100) surface.19-21 The uniform Co nanoclusters are

formed only on crystalline Al2O3 films and highly aligned by protrusion structures of the

crystalline Al2O3. Through simple thermal treatments we control the geometry of the

3

patterns of the Co nanoclusters. However, precise control of the positions of the Co nanoclusters is not possible.

In this letter, we show that engineering of nanocluster patterns on ordered oxide surfaces is possible by using the STM manipulation technique. In particular, we demonstrate precise engineering of Co nanocluster patterns on Al2O3 thin films grown on

NiAl(100) surface. We placed the tip over a specific cluster and reduced the applied bias below a threshold value to attract the cluster and subsequently remove it from the patterns. Systematically through this approach, we tailored the patterns of the supported Co clusters. The removed clusters can be relocated to other positions by reversing the polarity. This technique in combination with the self-organized patterning reported previously20,21 enable one to fabricate desired cluster patterns and consequently investigate their physical properties, for instance, the magnetic properties of patterned Co clusters, which is a critical issue in the development of nano-storage systems.22

The experiments were performed in an ultra high vacuum (UHV) environment with base pressure lower than 10-9 mbar. The detailed experimental procedure for the growth of Al2O3 films on NiAl(100) and formation of Co clusters over it has been

reported elsewhere.19-21 In short the NiAl(100) surface, cleaned by repeated cycles of Ar ions bombardment (2 keV, 4 µA sample current, 300 K, 45 minutes) and annealing at 1000 K for 45 min, was exposed to 1000 L (Langmuir) O2 at 1000 K and annealed at

1000-1100 K for 1h. The sample was quenched to 300 K to deposit Co atoms by evaporating a Co rod using an e-beam evaporator (Oxford scientific). The Co atoms nucleate on crystalline Al2O3 surface with preferable uniform cluster size19-21 (a diameter

4

stripes. In contrast, in the amorphous region, sizeable Co clusters appear only at high Co coverages.20,21 The STM images reported here were taken at 90 K with an RHK UHV STM-300 unit in constant current mode using electrochemically etched tungsten tips. The cluster manipulation was conducted at the same temperature. The applied bias refers to the sample voltage with respect to the tip.

Fig. 1(a)-(c) present a sequence of STM images illustrating removal of the Co clusters by reducing the bias during scanning. Fig. 1(a) shows the STM image obtained at 2.4 V bias and 0.8 nA tunneling current, where aligned Co nanoclusters are formed on crystalline Al2O3.20 We zoomed in the area shown by the square and reduced the bias to

different values (Fig. 1(b)) during scanning (from left to right). As the tunneling current was kept the same, this process brought the tip close to the clusters, as illustrated schematically in the cartoon inset. We found a threshold bias value of 0.8 V to induce motion of the Co clusters. We can see in parts of the image where the bias was above the threshold value 0.8 V the Co clusters were in fixed and well-defined locations. On the contrary, the regions where the bias was below the threshold value are seen clear. Fig. 1(c) is the same surface region as shown in Fig. 1(a) scanned just after zoom out with 2.4 V bias. The part of the surface where the bias was below the threshold value is seen clear without the Co clusters. When a higher tunneling current was set, we found a higher threshold bias. For instance, when 1.6 nA is used, the threshold bias is 1.1 V.

Based on this result we can engineer precisely desired Co clusters patterns. Fig. 2(a)-(g) demonstrate potential of this approach. Fig. 2(a)&(b) show that specific clusters were pulled out from the edge of crystalline region to create voids or cavities in the pattern. The arrows in Fig. 2(a) stand for the process that the tip was first moved over the

5

specific clusters (the end sides of the arrows) and brought close to them, by reducing the bias, to induce the motion of the Co clusters, and the tip was translated to the other location to remove them from the pattern. Fig. 2(c)-(e) show that we can trim a long cluster chain and fabricate cluster chains with various lengths. Fig. 2(d) is the zoom in image of Fig. 2(c) and Fig. 2(e) is the image after the trimming processes denoted by the arrows in Fig. 2(d). This approach is effective not only for small-scale engineering but also for large-scale pattern modification. Fig. 2(f) shows two sections of the rectangular clusters chains were removed. Fig. 2(g) displays long clusters chains were trimmed into short ones by removing rows of Co clusters. The uncovered protrusion stripes are clearly seen and aligned with the remained Co clusters chains, confirming directly that the protrusion structures are preferable nucleation centers.20 These results not only promise precise engineering of the cluster patterns but also imply an opportunity to study pattern-dependent physical properties of the nanostructures. For example, one may study the length-dependent electronic structures of the clusters-composed nanowires or the magnetism variations induced by the interactions between the magnetic nanoclusters.22-25 We believe that the tip attracts the clusters when it is brought closer by lowering the bias below the threshold value, and carry them with it. It might resemble the removal of Pd atoms on Al2O3/NiAl(110) surface during scanning with lower bias.26 Although we

observed the tip could also pull or push the Co clusters on the oxide surface, in most cases the clusters are attracted and adsorbed by the tip. These adsorbed clusters can be relocated on the surface by reversing the bias polarity when the tip is brought closer to the surface. Fig. 3(a)&(b) show that we picked up a Co cluster from the crystalline Al2O3

6

sometimes causes multi-tip effect or deteriorates the imaging. We can refresh the tip simply by removing the adsorbed clusters. Fig. 3(c) shows a number of the Co clusters were unloaded from the tip. It is noted that the Co clusters unloaded from the tip remain sizes similar to those on the surface. In the context of the STM atom/molecule manipulation, the process reported here is more like the vertical manipulation by making atom/molecule mechanical contact.6 As the tip-cluster distance is reduced to below 2.5 Å during patterning (estimated by a simple tunneling model27,28), the tip-cluster interaction involves a combination of the electric field and chemical boding, which must be stronger than the adhesion force between the Co cluster and the oxide surface, about 35 eV/Å for a single cluster with the mean size,20,21 derived from density-functional-theory calculations. The electric field might provide a directional driving force and unload the cluster after reversing the bias polarity.6 In line with the argument, the STM tip may not move the clusters while competing with much stronger cluster-oxide interactions, such as Pt clusters on ordered Al2O3/NiAl(100) observed previously.20

In conclusion, we have shown that it is possible to engineer precisely the patterns of Co nanoclusters grown on ordered Al2O3/NiAl(100) surface by using the STM

manipulation technique. The STM tip can attract and carry the clusters in order to tailor the patterns. The attracted clusters on the tip can also be relocated on the surface. This manipulation technique in combination with the self-organized patterning on this system may open a door to fundamental researches and applications in nanodevices where patterned nanoclusters are desired.

We thank K. R. Hu for technical support. The work was supported by National Science Council of Taiwan under Grant No. NSC 94-2112-M-008-032.

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Figure captions

FIG. 1 STM images illustrating removal of the Co clusters by reducing the bias during scanning. (a) STM image of Co clusters grown on ordered Al2O3/NiAl(100) surface

(2.4 V, 0.8 nA). (b) Zoom in the area shown by the square in (a). The bias was reduced to different values as indicated in the figure (I = 0.8 nA) during scanning. (c) STM image of the same surface region as in (a) after zoom out (b) (2.4 V, 0.8 nA). The inset cartoon illustrates the procedure.

FIG. 2 The cluster patterns tailored by the STM manipulation technique. (a) and (b), creation of void or cavity in the pattern by removing the specific Co clusters. (a) before, and (b) after removal of the clusters. (c)-(e), fabrication of clusters chains by trimming a long Co clusters chain on a crystalline Al2O3 strip. (c) and (d) before, (e) after trimming

the chain. (f) STM image for the rectangular cluster chains cut by removing sections of the chains. (g) STM image after removing rows of the Co clusters from the long clusters chains. (0.8 V, 0.8 nA) and (1.1 V, 1.6 nA) were used for the removal of the Co clusters and (2.4 V, 0.8 nA) was used for the imaging.

FIG. 3 The clusters unloaded from the tip to the surface. (a)&(b) relocation of the specific cluster from the crystalline Al2O3 to the amorphous region. (a) before picking up the

cluster, and (b) after relocating it on the amorphous region. The cluster is indicated by the circle. The inset manifests the creation of void after removing the specific cluster. (c) STM image for a number of Co clusters released from the tip. All the images were obtained at 2.4 V and 0.8 nA.