Study of magnetic properties and microstructures of FePt island films

連續式磁記錄薄膜的晶粒尺寸受到超順磁現象的限制，阻礙了超高密度記錄媒體的發展，而圖案化記錄媒體是能有效克服此障礙的方法之一。本研究的目的是開發一種製程相對較簡易、較經濟而且可大面積製作圖案化記錄媒體的方法。 為了改善磁記錄媒體的訊雜比，磁性薄膜的晶粒尺寸必須大幅縮小，且磁性晶粒間的交換耦合效應也需減小才可有效降低媒體雜訊。不連續磁性薄膜中的磁性島狀物之間的交換耦合效應較低，可降低媒體雜訊，進一步調整實驗參數得到尺寸較小且分布均勻的島狀磁記錄點即可大幅提高磁記錄密度。 以(Fe/Pt)n為主要膜層結構鍍製FePt薄膜(15 nm)於玻璃基板上經700 ℃退火30分鐘後，當FePt薄膜內的Fe原子百分比為59 at.%時，FePt薄膜擁有較佳的垂直膜面磁異向性與序化度，此時L10 FePt phase之易磁化軸[001]沿著垂直膜面的方向成長，適合應用於垂直式磁記錄媒體。 將不同厚度FePt薄膜於玻璃基板上以700 ℃退火30分鐘，經XRD測試發現，當FePt薄膜厚度為5 nm已經有明顯的fct-FePt (001)及fct-FePt (002)序化相繞射峰出現。當FePt薄膜厚度增加到15 nm時，有最強的fct-FePt (001)及fct-FePt (002)序化相繞射峰，顯示在此厚度下的FePt薄膜有很好的垂直膜面磁異向性。當厚度為30 nm時，fct-FePt (001)及fct-FePt (002)序化相繞射峰的強度開始減弱，而fct-FePt (111)序化相繞射峰會增強，顯示當FePt薄膜厚度大於15 nm後，薄膜的磁異向性將由垂直膜面磁異向性逐漸傾向為平行膜面磁異向性，但整體薄膜序化相的含量隨著薄膜厚度的增加而增加。 厚度1 nm的FePt薄膜於玻璃基板上以700 ℃退火10分鐘後可形成島狀FePt顆粒，顆粒的尺寸介於2.5至5 nm之間，島狀FePt顆粒的密度高達1.37×1013 island/inch2，而且奈米尺寸的島狀FePt顆粒分佈均勻，如此不連續島狀磁性薄膜中的磁性島狀物之間交換耦合效應較低，可降低媒體雜訊並大幅提高磁記錄密度。 由於FePt形成島狀物的驅動力為基板與FePt合金的表面能差異，FePt島狀物的成核數目會隨表面能差異增大而增加。因此，鍍製厚度1 nm的FePt薄膜於鍍碳銅網上以700 ℃退火10分鐘後產生的FePt島狀物密度(1.44×1013 island/inch2)會略大於厚度1 nm的FePt薄膜鍍製於玻璃基板上經700 ℃退火10分鐘後所產生島狀FePt顆粒的密度，但在此條件下仍有fcc-FePt相存在，因此相同厚度的FePt薄膜鍍製在不同基材上以相同退火條件退火所到的島狀FePt顆粒其序化程度明顯不同。 FePt(1 nm~7.5 nm)/Ag(100 nm)雙層薄膜經700 ℃退火30分鐘後所形成的金屬島狀顆粒尺寸並不均勻，粒徑尺寸分佈由數十奈米到數微米，這可能是由於FePt(1 nm~7.5 nm)/Ag(100 nm) 雙層薄膜整體膜厚太厚導致金屬島狀物成核成長非常容易，但添加Ag底層有幫助FePt薄膜序化的效果。Ag底層的添加，可能導致FePt薄膜在膜厚較薄的情況下即發生由介穩態的垂直膜面磁異向性轉向穩定之平行膜面磁異向性，其原因可能是Ag底層與FePt膜層之間的晶格不匹配提高應變能促進穩定的fct-FePt(111)面產生。在FePt(1 nm)/Ag(1 nm~3 nm)雙層薄膜也發現，具有較大擴散係數的Ag原子在熱處理的過程中可使金屬島狀物的真圓度提升。 研究中發現厚度1 nm的FePt薄膜於玻璃基板上以700 ℃退火10分鐘後可得到具垂直膜面磁異向性且顆粒尺寸分佈均勻的島狀FePt顆粒，適合應用於超高密度磁記錄媒體，但其垂直膜面矯頑磁力約為20 kOe，如此大的矯頑磁力，很難使用目前市面上的磁頭來翻轉其磁矩並儲存資料，因此我們在島狀FePt薄膜上添加不同厚度的Fe軟磁層(0.5~5 nm)，藉由添加的軟磁層與原本的磁記錄層產生交互耦合作用，達到降低島狀FePt薄膜寫入磁場的目的。The development of recording media of continuous films is restricted by super-paramagnetic limit. Patterned media has been suggested as a potential solution for this physical limit. In this study, we describe a simple and cheap method to fabricate large-area patterned media. The grain size of the magnetic films must be reduced to improve the signal-to-noise ratio. The exchange coupling effect between magnetic grains should be minimized in order to decrease transition noise. This discontinuous nano-size island magnetic film was suggested to reduce the exchange coupling effect between magnetic islands and increased the recording density. In order to increase the recording density, the size of magnetic islands must be reduced by modifying the experiment condition. FePt films (15 nm) were fabricated with (Fe/Pt)n multilayer on amorphous glass substrates then post-annealed at 700 ℃ for 30 mins to obtain better perpendicular magnetic anisotropy and ordering degree. The chemical composition of FePt films is Fe59Pt41. The magnetic easy axis [001] of L10 FePt phase in the FePt films was perpendicular to the film plane. This nano-size island-shape FePt film with perpendicular magnetic anisotropy may be a good candidate for ultra-high density recording media. The X-ray diffraction patterns of the annealed FePt films with heat treatment on amorphous glass substrates shown that the fct-FePt(001) peak and fct-FePt(002) peak appeared in 5-nm FePt film. However, the intensity of the fct-FePt(001) peak and fct-FePt(002) peak were weaker as the thickness of FePt film increasing to 30 nm, but the intensity of the fct-FePt (111) peak enhanced. As the thickness of FePt films was increased, the easy axis orientation of FePt films would turn from perpendicular to parallel to the film plane. However, the ordering degree of the FePt film would increase with FePt film thickness. The particle size of 1-nm thick FePt film on glass that annealed at 700 ℃ for 10 minutes distributed between 2.5 to 5 nm, and the density of islands is 1.37×1013 islands/inch2. A discontinuous and well-separated nano-size island magnetic film can reduce the exchange coupling of the media and increase the recording density. The driving force of FePt island formation was the surface energy difference between the substrate and FePt alloy. The nucleation site number of FePt islands increased with the surface energy difference between the substrate and FePt alloy. Therefore, the density of islands of 1-nm thick FePt film annealed at 700 ℃ for 10 minutes on carbon film (1.44×1013 islands/inch2) was slightly larger than that of 1-nm thick FePt film annealed at 700 ℃ for 10 minutes on glass. But, the fcc-FePt phase still existed in the 1-nm thick FePt film which annealed at 700 ℃ for 10 minutes on carbon film. Due to the thick Ag under-layer, the grain growth and island cluster occurred easily in the FePt (1 nm~7.5 nm)/Ag (100 nm) bi-layer films, and the size distribution of metallic islands would be very broad. However, the Ag under layer could improve the ordering of FePt film. The lattice mismatch between Ag under layer and FePt film would provide additional strain energy and promote the occurrence of the fct-FePt (111) preferred orientation. The orientation of magnetic easy axis of FePt (1 nm~7.5 nm) films with Ag (100 nm) under-layer would turn from perpendicular to parallel to the film plane. Due to the Ag atom has larger diffusion coefficient during annealing process, the metallic islands had better roundness in FePt (1 nm)/Ag (1 nm ~3 nm) bi-layer films. Investigation of the microstructures and magnetic properties of the ordered FePt films revealed that the 1 nm-FePt film annealed at 700 ℃ for 10 minutes had perpendicular magnetic anisotropy and formed well-separated FePt nano-size islands. It is suitable for ultra-high density magnetic recording media. But, the out-of-plane coercivity of the well-separated FePt nano-size islands was about 20 kOe, and it was too large for recording head to reverse its moments. In order to reduce the writing field of FePt nano-island film, we introduced Fe capping layer (0.5~5 nm) on the FePt nano-island films. The exchange coupling effect between Fe capping layer and FePt nano-island would decrease the writing field of the island films