氧與氧化鈷在鈷／矽(111)超薄膜上之磁性研究

Investigations of magnetic properties for oxygen adsorption and ultra-thin CoO films on Co/Si(111)

張惟祐(Michael)

指導教授 ： 蔡志申

國立臺灣師範大學/理學院/物理學系/碩士(2007年)

摘要

摘要

本實驗在超高真空的環境中，使用蒸鍍的方式將Co膜成長在Si (111)-7×7表面上，通入高純度之O2來研究曝氧效應對於Co/Si(111)超薄膜之影響，以歐傑電子能譜儀分析其表面成份，以低能量電子繞射儀以及反射式高能量電子繞射儀觀察其表面週期性結構，以表面磁光柯爾效應儀量測其磁性質。

在純Si (111)基板以及CoSi2介面上，O2會有弱物理吸附而不形成化合態；在鍍上Co膜後，O2吸附之效應較強，且吸附效果隨著Co膜厚成正相關。而O2的吸附作用將改變Co/Si(111)超薄膜之表面磁性層的電子組態改變，故MS、MR與其磁滯曲線角型比皆呈現下降之趨勢。此外由於O2的吸附之效應，一方面降低了有效磁性Co的層數，因而降低了HC；另一方面形成了釘紮區域(pinning sites)阻礙磁化的反轉，因而提高HC，而本系統所觀測到HC之變化為此兩種效應互相競爭的結果。

另一方面，以氧壓下鍍Co的方式製作超薄反鐵磁CoO膜於11 ML Co/Si (111)上，其易磁化軸由原本的水平膜面轉變至傾斜出膜面。且經過場冷卻至150 K可發現水平膜面與垂直膜面兩方向皆有交換偏壓的現象產生。於 CoO膜厚為20 ML時，此系統有最大水平膜面交換偏向場為258 Oe，且其阻隔溫度為200 K；而於CoO膜厚為15 ML時，此系統有最大垂直膜面交換偏向場為924 Oe，且其阻隔溫度為164 K。

關鍵字

曝氧 ； 矽 ； 鈷

並列摘要

Abstract

The purpose of this research is to study effect of oxygen exposure and ultrathin antiferromagnetic CoO films on the magnetic properties of ultrathin Co/Si(111)-7×7 surface. All experiments were in-situ performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 2.5×10-10 torr, which was equipped with Auger electron spectroscopy (AES) for surface composition analysis, low-energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED) for surface structure analysis, and surface magneto-optic Kerr effect (SMOKE) for magnetic properties measurement.
For Si (111)-7×7 surface and CoSi2 interface, it is found that oxygen is weakly adsorbed. As the thickness of Co films increases to above 5 monolayers (ML), pure cobalt islands start to accumulate on the surface and the amount of oxygen on the surface increases with increasing the oxygen exposure time. The adsorbed oxygen influences the electronic density of states of Co and leads to the decrease of saturated and remanent magnetization and poor squareness of demagnetization. The trend of coercivity with oxygen exposure is mainly governed by two competed factors: one is the reduction in the effective ferromagnetic Co films, resulting in the decrease of the coercivity; another is the action of oxygen as pinning sites to impede the magnetization reversal, leading to the enhancement of the coercivity.
On other hand, the method to fabricate the antiferromagnetic CoO film is the evaporation of Co in oxygen atmosphere. As ultrathin CoO film is grown on 11 ML Co/Si(111) films, the direction of easy axis is transformed from in-plane to canted-out-of-plane. Interestingly, both longitudinal and perpendicular exchange bias phenomena are observed for 15 ML CoO/ 11 ML Co/Si(111) films measured at T = 150 K after cooling in a field H = 700 Oe from T = 300 K, and the maximum perpendicular exchange bias field of 924 Oe and the blocking temperature of 164 K are attained for 15 ML CoO, while the maximum longitudinal exchange bias field of 258 Oe and the blocking temperature of 200 K are obtained for 20 ML CoO.

並列關鍵字

exchange bias

參考文獻

[3] B.Dieny , V.S.Speriosu et al.,”Grant magnetoresistive in soft ferromagnetic multilayers”, Phys.Rev.B 43,1297 (1991)

[4] C.A.Ross,”Fabrication of patterned media for high density magnetic storage”, Microelectronic Engineering.53,67 (2000)

[5] R.C.O’Handley , Modern Magnetic Materials. John Wiley & Sons inc, (NewYork, 2000)

[6] W.H.Meiklejohn & C.P.Bean , “New Magnetic Anisotropy”, Phys. Rev.102,1413 (1956)

[11] J.S.Tsay and Y.D.Yao, “Magnetic phase diagram of ultrathin Co/Si(111) film studied by surface magneto-optic Kerr effect”, Appl. Phys. Lett. 74,1311 (1999)