鐵超薄膜在鉑(111)上之表面結構與磁性研究

Surface structures and magnetic properties of ultrathin Fe films on Pt(111)

陳耀榮(Yao-Jung Chen)

指導教授 ： 沈青嵩

國立臺灣師範大學/理學院/物理學系/博士(2007年)

摘要

本論文的實驗是以歐傑電子能譜術、低能量電子繞射儀、紫外光電子能譜術及表面磁光科爾效應來研究鐵超薄膜在鉑(111)面上的成長模式、表面結構、合金形成及磁性性質。分別討論在低溫(LT:180 K)和室溫(RT:300 K)下所成長的鐵超薄膜的結構與磁性的相關性。首先在室溫中經由歐傑電子能譜及低能量電子繞射儀的測量中發現鐵超薄膜在鉑單晶上是以3個原子層(ML)的層狀成長轉成3維島狀的成長模式。當鐵薄膜厚度為1 ML時並無任何科爾訊號，而在鐵膜厚1.5 ML時;平行及垂直薄膜面皆有科爾磁滯曲線，當鐵膜厚達到2 ML後只有平行薄膜面方向的磁滯曲線表示其磁化易軸在平行鐵薄膜面上。其磁性的變化是與鐵薄膜的表面結構在膜厚增加到2 ML時將由fcc轉成bcc的結果是互相符合的。
在低溫時;鐵超薄膜在鉑單晶上是以3維島狀的模式成長。當鐵薄膜厚度為0.75 ML時可偵測到垂直方向的科爾磁滯曲線;接著當鐵膜厚達1.3 ML時其磁化易軸由垂直轉至平行薄膜面方向，發生自旋指向轉變。此結果顯示鐵超薄膜在鉑(111)上發生結構變化;其臨界厚度是與成長溫度息息相關，其臨界厚度在低溫與室溫分別為1.3 ML及2 ML。
對1.2 ML鐵/鉑(111)樣品進行高溫退火處理時;其在平行面上的科爾強度與矯頑力分別增加到160%及30%。而對5 ML鐵/鉑(111)樣品退火處理時;則在平行面上的科爾強度與矯頑力分別增加到150%及1100%。當1.2 ML鐵/鉑(111)其退火溫度介於600 K與700 K時;甚至出現垂直方向的科爾磁滯曲線。退火處理時也由歐傑電子能譜術、低能量電子繞射儀與紫外光電子能譜術量測發現鐵-鉑合金的產生。有關退火處理時磁性的變化是與鐵原子擴散及鐵與鉑形成合金有著強烈的關係。
將銀蒸鍍覆蓋在鐵/鉑(111)上;可由表面磁光科爾效應偵測到自旋指向轉變(SRT)發生。同時樣品的垂直方向的磁化強度與矯頑力皆與銀覆蓋厚度有顯著的關係。當銀覆蓋厚度達到1 ML時;其磁化易軸將由平行面方向完全轉至垂直面方向，同時垂直方向的磁化強度與矯頑力兩者皆達到最大值。當銀覆蓋層慢慢被氬離子濺射移除時;其磁化易軸也會慢慢轉迴至平行面方向。其間其介面的組合成份的變化是經由歐傑電子能譜術量測。經由銀與鐵介面間的電子態雜錯交互作用而導致介面磁異向能的改變;其可能為由銀所引發自旋指向轉變的機制。

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 ； 鐵 ； 銀 ； 鉑 ； 表面磁性 ； 磁異向性

並列摘要

Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and surface magneto-optical Kerr effect (SMOKE) are used to investigate the growth mode, alloy formation, and magnetic properties of ultrathin Fe films deposited on Pt(111) surface. The correlation between structural and magnetic properties is explored for Fe films grown at two different temperatures, 180 K (low temperature: LT) and 300 K (room temperature: RT). The growth mode of Fe films on Pt(111) at RT is at least 3 ML layer-by-layer growth before three-dimensional (3-D) island growth begins. When the thickness of Fe films (dFe) is 1 ML, no Kerr signal is observed at RT. Both the polar and longitudinal Kerr signals can be detected when dFe is 1.5 ML. When dFe ? 2 ML, only the longitudinal Kerr signal can be observed, the easy axis of the magnetization is in the in-plane direction. It is corresponded with a fcc to bcc structural transformation at dFe = 2 ML. For LT-growth, the growth mode of Fe deposited on Pt(111) is 3-D island growth. A polar Kerr signal can be detected when dFe is 0.75 ML then the easy axis of the LT-grown films changes from out-of-plane to in-plane at 1.3 ML Fe. The results show that the Fe films on Pt(111) has a structural change, starting from a critical thickness, which depends on the deposition temperature. The critical thickness is estimated as 1.3 ML and 2 ML for Fe films deposited at LT and RT, respectively.
The formation of Fe–Pt surface alloy for 1.2 ML and 5 ML ultrathin Fe films on a flat Pt(111) surface after high temperature annealing, the longitudinal Kerr signals enhance to 160% and 150%, in the meantime the coercivities also enhance to 30% and 1100%, respectively. A polar Kerr hysteresis loop can be observed for 1.2 ML Fe/Pt(111) when the annealing temperature is between 600 K and 700 K. The formation of Fe–Pt surface alloy is confirmed by AES and LEED. The evolutions of the magnetic properties are strongly related to the diffusion process of Fe thin films into Pt substrate.
The spin-reorientation transition (SRT) is observed when Ag overlayers deposited on Fe/Pt(111) by magneto-optical Kerr effect measurements. The polar Kerr intensity and its coercivity as a function of Ag coverage are investigated through SRT. The easy axis of the magnetization changes completely from the in-plane to out-of-plane direction after 1 ML of Ag coverage. Both the polar Kerr intensity and perpendicular coercivity have their maximal value at dAg = 1 ML. The easy axis of magnetization can be reversed back to the in-plane direction after the Ag overlayers are removed by sputtering. The chemical compositions of the interfaces are measured by Auger electron spectroscopy. The hybridization of electron states at the Ag/Fe interface causing the change in interface magnetic anisotropy may be the mechanisms of the spin-reorientation transition induced by Ag.

並列關鍵字

Auger electron spectroscopy ； low-energy electron diffraction ； ultraviolet photoelectron spectroscopy ； surface magneto-optic Kerr effect ； iron ； silver ； platinum ； surface magnetism ； spin-reorientation transition

參考文獻

[1] H. A. Shute, 2nd North America Perpendicular Magenetic Recording Conference. (NAPMRC) Monterey (2003).

[3] C. P. Luo, D. J. Sellmyer, IEEE Trans. Magn. 31, 2764 (1995).

[9] Terry W McDaniel, J. Phys.: Condens. Matter 17, R315 (2005).

[11] David J. Sellmyer, Minglang Yan, Yingfan Xu, and Ralph Skomski, IEEE Trans. Magn. 41, 560 (2005).

[13] W. C. Lin, P. C. Huang, K. J. Song, and M. T. Lin, Appl. Phys. Lett. 88, 153117 (2006).