Exploring Origin and Formation Mechanisms of Strong Perpendicular Magnetic Anisotropy in FeNi Films

本研究嘗試以臨場退火方式在鎳鐵薄膜中誘發該硬磁性序化相形成、並且比較不同基板/底層對鐵鎳合金序化行為的影響。長程目標是製作能使用於MRAM等自旋電子學元件作為垂直硬磁層的鐵鎳薄膜。L10相之鎳鐵合金，Ku與稀土系合金相當、阻尼係數又低，適合作為MRAM紀錄單元的硬磁層。 選用的底層有分別有康寧7059玻璃、鉑(111) 、氧化鎂(200)、銅(200)。鐵50鎳50合金以共濺鍍之方式、高真空(HV)環境中沉積於加熱之底層上，基板溫度由200到600攝氏度的範圍不等。樣品的成分是以掃描電子顯微鏡(SEM)外掛之EDAX功能做鑑定。以Kerr磁光儀(MOKE)、震動樣品磁性計(VSM)檢測磁性質。以X光繞射儀(XRD)鑑定晶體結構以及薄膜織構。需要分析交互擴散的場合，則使用歐傑電子能譜儀(AES)作縱深分布圖形之量測。 在非晶態基板上鍍製的樣品，展現研究目標的可達成性。少量的高Ku相生成，增進磁硬度。為了最佳化系統的垂直磁異向性，選用具有晶格結構的底層引導晶粒指向、促進晶格畸變。雖然白金具有適當的晶格參數，對於鐵鎳合金的序化卻有不良影響。因為鐵-鉑系的合金是更容易生成。氧化鎂因為與鐵、鎳的高親和力，引入的應力強、引導晶粒指向效果佳。然而其引入的應力是拉伸鐵鎳晶格、不利於系統發展具垂直磁異向性的晶體結構。參考製作L10-鐵鉑的經驗，經證實不甚有效。銅與鎳鐵的晶格匹配度佳，卻會與之互溶，不適用於較高溫的製程。所幸，低溫製程中三元合金生成不多、還達到減低消磁場的效果，這一系列的樣品在低溫下即表現較佳的磁硬度、垂直磁異向性。擴散障礙層的實驗證實，銅底層可以誘導鐵鎳層在低溫下發生硬磁性、甚至少量的強磁晶異向性得以生成。L10相鐵鎳研究，將在於尋求適當的底層以引導晶粒指向與提升序化度。The aim of the research is to induce formation of the hard magnetic, ordered phase in iron-nickel films, by means of in-situ annealing method. Various substrates/ underly-ers were compared on ability to affect the order-disorder transition. It’s expected that, ultimately, such iron-nickel films serve as perependicular hard magnet in spintronics de-vices. L10-phase iron-nickel alloys have competently high Ku (2 × 107 erg/cm3 ) and gratefully low damping constant, hence a promising candidate for hard magnetic layer in MRAM. Corning 7059 cover glass, Pt(111) underlayer, MgO(200) single crystal, and Cu(200) underlayer were chosen. Then Fe50Ni50 alloy films were co-sputtered on the heated underlayers. The substrate temperature varies from 200 ℃ to 600 ℃. Composi-tion was checked by X-ray energy dispersive spectrum (EDS) on SEM. Magnetic prop-erties were probed by magneto-optical Kerr effect magnetometer (MOKE) and vibrating sample magnetometer (VSM). X-ray diffractometer (XRD) was used to investigate crys-talline structure. When inter diffusion was analyzed, Auger electron spectroscopy (AES) was used. FeNi films which deposited on amorphous substrates showed that the goal is realis-tic. Few high-Ku crystallites formed and enhanced hard magnetism. To optimize PMA, underlayers with crystalline structure were chosen. Thry were expected to engrow (001) texture and introduce lattice distorsion. Platinum has suitable crystal parameter, but its incorporation inhibited L10-type ordering of nickel-iron. Their combination into ternary alloy was favored. There is affinity between magnesium oxide and nickel/iron, but its over-sized crystalline structure introduced tensile stress. Therefore, incorporation of MgO reduces PMA. It was concluded that, to manufacture L10-FeNi, to use methods fabricating L10-FePt does not help. Copper, on the other hand, matches FeNi well on crystalline parameter. Its solubility with NiFe destroyed texture during annealing. In spite of this, rare ternary alloys form during low-temperature deposition, and demagnet-ization field from boundary was greatly reduced. As a result, the series showed better PMA below 300 ℃deposition temperature. Insertion of diffusion barrier proved that, few hard-magnetic FeNi formed on Cu or Cu-Ta underlayer, below 300 ℃deposition temperature. In conclusion, conditions fabricating hard magnetic FeNi films were found. It is predicted that highly ordered and highly textured L10-FeNi could be made by such recipes, on suitable underlayer