Fabrication of low ordering temperature L10 FePt thin films and study of its application in perpendicular magnetic recording media
|Keywords:||FePt;磁性;磁記錄;垂直磁性質;FePt;magnetic;magnetic recording;perpendicular magnetic properties||Issue Date:||2005||Abstract:||
本實驗以表面自然氧化的(100)指向矽晶圓為基板，鍍製FePt薄膜前，先對基板作300 oC烘烤後再冷卻至常溫，以去除附著在表面的水氣、氧氣等氣體以得到較為潔淨的基板表面後，再以傳統的濺鍍製程鍍製單層FePt薄膜。初鍍FePt薄膜為無序的fcc γ-FePt phase結構，晶粒尺寸大約在4 nm左右，350 oC退火後，開始有L10 FePt phase形成，400 oC退火後，L10 FePt phase已全然形成，並有最高的頑磁力。序化率S方面，L10 FePt phase的序化率會隨著退火溫度的提升而增加， 300 oC時，序化率幾乎等於0，300 oC至400 oC時，薄膜的序化程度有明顯的提升，並在400 oC 時大致序化完成。由TEM 明視野照片所計算出的平均晶粒尺寸結果得知，降低序化溫度有助於L10 FePt phase晶粒尺寸的降低，初鍍的晶粒大小約4 nm，350 oC時晶粒大小約6 nm，400 oC時晶粒大小約9 nm，600 oC時則增加至24 nm，而薄膜厚度太薄也會抑制的晶粒的成長、頑磁力的提升、序化率的增加及fcc γ-FePt phase轉變成有序L10 FePt phase的生成。晶粒尺寸4.5 nm為FePt薄膜是否會序化的臨界尺寸，當晶粒小於4.5 nm時，當晶粒尺寸大於4.5 nm後才會有硬磁性的L10 FePt phase形成，其序化的程度隨晶粒尺寸的增大而增加。經由多項的分析證明，影響L10 FePt phase的序化溫度最主要的參數為FePt薄膜的濺鍍速率及退火條件，其次才是基板表面的清潔處理。本研究利用傳統的濺鍍製程，在經過許多製程參數的調變後，成功的將單層FePt的序化溫度由文獻所記載的500 oC以上降低至350 oC左右。
本實驗以表面自然氧化的(100)指向矽晶圓及7059系列的康寧玻璃為基板，在鍍製FePt薄膜前，先對基板作RF power清潔後再以超高真空的濺鍍製程鍍製FePt/Pt/Cr薄膜，實驗結果得知，FePt / Cr雙層薄膜並不會直接引出FePt (001)的垂直磁異向性，因為FePt薄膜會與Cr底層反應而在介面處形成Cr rich的CrFePt的磊晶阻礙層，造成Cr (200)的磊晶效應在介面處被CrFePt合金阻斷，而當Pt緩衝層加入後，Pt緩衝層便扮演一座橋樑的角色，除了阻擋Cr底層原子擴散進FePt磁性層外，還會順利的承接Cr底層的(200)結構，並長成Pt(200)的優選方位，最後FePt磁性層便會沿著Pt(200)的方位而長成L10 FePt (001)的優選方位，使FePt薄膜表現出垂直磁異向性。在我們的實驗中，L10 FePt phase的序化溫度約250 oC，然而Pt緩衝層與FePt磁性層之介面間的原子排列方式也會隨Pt緩衝層厚度的增加而逐漸由半整合性介面調整成完全整合性介面，隨著FePt磁性層厚度的增加，FePt (001)的頑磁力會越來越高，在FePt膜厚約30 nm時有最高的頑磁力，同時保有垂直磁異向性，可是當FePt磁性層厚度超過30 nm時，便開始有FePt (111)的繞射峰形成而逐漸破壞FePt (001)的垂直磁異向性。
FePt / Pt / Cr三層薄膜中加入FeTaC軟磁層後，會破壞FePt (001)的優選方位，使FePt薄膜形成(111)的優選方位，但是再加入一層非晶質的Si3N4薄膜於Cr底層與FeTaC軟磁層的介面上時，Cr(200)及FePt (001)的優選方位會有增強的趨勢，但仍有FePt(111)的繞射封存在。若以Fe當軟磁層時，FePt(001)的優選方位會被保留，但由於軟硬磁之間強烈的耦合效應，或是薄膜的形狀異向性大於FePt(001)的垂直磁晶異向性等原因所致，其垂直方向的磁性質會因軟硬磁之間強烈的耦合效應或較強的形狀異向性的牽引作用而被破壞，使原本垂直膜面的易磁化方向變成了難磁化方向。若以CoCr軟磁合金薄膜當軟磁層，會抑制L10 FePt(001)的強度，並使薄膜產生L10 FePt (111)的繞射峰而破壞垂直磁異向性，但是若再加入一層Cr中間層，則會使Cr中間層沿著CoCr (11-20)的方位再度長成Cr(200)的指向，此新的Cr(200)指向便成為後續L10 FePt(001)優選方位的磊晶來源，成功的引出L10 FePt(001)的優選方位。
There are three subjects in this dissertation, we illustrate them below:
1. Fabrication of low ordering temperature L10 FePt thin films
Polycrystalline Fe52Pt48 alloy thin films were prepared by dc magnetron sputtering on preheated natural-oxidized silicon wafer substrates. The film thickness was varied from 10 to 200 nm. The as-deposited film was encapsulated in a quartz tube and post-annealed in vacuum at various temperatures for 1 hour followed by furnace cooling. Before annealing treatment, the structure of the as-deposited FePt thin films is in fcc FePt phase with ~4 nm grain size. L10 FePt phase with hard magnetic properties began to occur after annealing at 350 oC. FCC FePt phase transform completely into L10 FePt phase after annealing at 400 oC. Therefore, the ordering temperature from as-deposited soft magnetic fcc FePt phase to hard magnetic fct L10 FePt phase could be reduced down to about 350 oC by preheating substrate and furnace cooling treatment. The ordering rate of L10 FePt phase increases with increasing the annealing temperature. The ordering rate is non-existent at 300 oC. However, it is enhanced dramatically when temperature is raised above 300 oC and becomes nearly fully completed at 400 oC. From TEM bright images, it is clear that by lowering the ordering temperature it helps reduce the grain size of L10 FePt thin films. Grain grows from ~6 nm to ~9 nm when the annealing temperature increases from 350 oC to 400 oC. The magnetic properties measurements indicated that the in-plane coercivity of the films increase rapidly as annealing temperature is increased from 300 oC to 400 oC, but decrease when the annealing temperature is higher than 400 oC. The grain growth, in-plane coercivity, ordering rate, and formation of L10 FePt phase are impeded in thinner films. The critical order-disorder transformation in grain size of FePt is about 4.5 nm. L10 FePt phase and hard magnetic properties only occurred when the grain size is larger than 4.5 nm. After annealing at 400 oC, the in-plane coercivity of Fe52Pt48 thin film with film thickness of ~100 nm is 10 kOe, Ms is 580 emu/cm3, and grain size is about 9 nm.
Factors responsible for reducing the order-disordering temperature of L10 FePt phase include higher sputtering rates, lower annealing temperature accompanied with a longer annealing treatment, and substrate preheating. Among these three factors, the higher sputtering rate of FePt plays the dominant role to help reduce the ordering temperature.
2. Study of perpendicular anisotropy of L10 FePt thin films
FePt/Pt/Cr trilayer thin films with perpendicular magnetic properties were deposited on amorphous Corning 7059 glass substrate and natural-oxidized silicon (100) wafer substrates. Before sputtering, the base pressure of the sputtering chamber is better than 5×10-9 Torr. The perpendicular anisotropy of FePt(001) texture was not discernible right after depositing the FePt magnetic layer on top of the Cr(200) underlayer. The Cr-rich epitaxial barrier will be formed at FePt/Cr interface distorting the epitaxial growth of FePt(001) magnetic layer above the Cr(200) underlayer. After inserting a Pt buffer layer at the FePt/Cr interface, perpendicular magnetic properties with FePt(001) preferred orientation was observed. Squareness of the L10 FePt film was close to 1 when a magnetic field was applied perpendicular to the film plane. Pt buffer layer serves as a good barrier to impede diffusion of Cr into the FePt layer and modulate the lattice misfit between Cr underlayer and FePt magnetic layer. Semi-coherent epitaxial growth was initiated from the Cr (002) underlayer, continued through the Pt buffer layer and extended into the L10 FePt (001) magnetic layer. In this investigation, the ordered FePt phase in FePt/Pt/Cr film was found to show up at 250 oC substrate temperature. As the substrate temperature is increased to 300 oC, perpendicular texture and magnetic properties of L10FePt(001) become firmly established. Thus, the formation temperature of the ordered FePt(001) preferred orientation can be identified as low as ~300 oC. This is an important fact may be proven to be very useful for practical industrial perpendicular recording media application.
3. The effects on inserting a soft stability layer in perpendicular FePt/Pt/Cr trilayer thin films
When a FeTaC soft layer was inserted in FePt/Pt/Cr trilayer thin film, the FePt(001) preferred orientation of the FePt/Pt/Cr trilayer thin films become distorted. Both Cr(200) and FePt(001) orientations were enhanced by adding a Si3N4 layer between FeTaC layer and Cr underlayer. FePt(001) texture will be preserved if Fe is used as the soft layer. But, the dominant magnetic properties of the films so prepared were longitudinal. This is due fact that there exists strong exchange coupling between soft and hard magnetic phase and very large shape anisotropy in the film. If a CoCr layer was used instead as the soft layer, the intensity of FePt(001) orientation will be reduced while the that of FePt(111) orientation enhanced. The perpendicular magnetic properties of FePt/Pt/Cr were distorted when CoCr is used as the soft layer. When a Cr intermediate layer was inserted at the interface of Pt/CoCr, the Cr intermediate layer may grow along the CoCr(11-20) texture and obtain the Cr(200) preferred orientation. Thus, depositing FePt/Pt bilayer films on this new Cr intermediate layer will exhibit FePt(001) texture and obtain perpendicular magnetic anisotropy properties again.
|Appears in Collections:||材料科學與工程學系|
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