The microstructure and magnetism in FePt-SiOx thin films
雙離子束濺鍍系統 ； 矯頑磁力 ； 序化 ； ion-beam deposition technique ； coercivity ； order
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本研究利用超高真空濺鍍系統製備鐵-鉑(20 nm，共鍍方式)及鐵/鉑 (多層膜濺鍍方式)薄膜，再利用雙離子束濺鍍系統在兩種不同的鐵鉑薄膜頂端鍍上不同氧含量的氧化矽(15 nm，0~41% O2/Ar)，並探討試片經熱處理後之微結構及磁性質。 X光繞射及電子顯微鏡研究顯示：共鍍及多層膜製備之鐵鉑/氧化矽薄膜，其初鍍膜由非序化之fcc 鐵鉑相(a~3.81 Å)組成，主要由於鐵鉑互相混合所致。其晶粒大小約為3~16 nm。這些初鍍膜具有軟磁之性質，其矯頑磁力約為35 Oe。 共鍍方式之鐵-鉑/氧化矽薄膜經550℃、10分鐘熱處理後產生序化之fct 鐵鉑合金相(a~3.83 Å，c~3.70 Å)，其序化度約為0.7。鐵鉑晶粒大小約為20~80 nm。且晶界之氧化矽均勻將鐵鉑晶粒隔離，主要由於氧化矽之表面能較鐵、鉑及鐵鉑低所致。此結構變化導致退火後鐵-鉑/氧化矽薄膜具有硬磁之性質，其矯頑磁力約為13 kOe。多層膜濺鍍方式之鐵/鉑/氧化矽薄膜，其結構及磁性質與共鍍方式之鐵-鉑/氧化矽薄膜接近。但氧化矽經退火後仍無法穿透至底層之鐵/鉑薄膜以分散鐵鉑晶粒，可能由於較厚之鐵/鉑多層膜，其有序排列及熱處理過程鐵鉑合金相之形成阻隔氧化矽之穿透所致。 共鍍方式及多層膜濺鍍方式之鐵鉑/氧化矽薄膜經不同退火溫度研究顯示：當退火溫度大於400℃時開始產生序化之鐵鉑合金相，且其矯頑磁力隨退火溫度上升而上升，主要由於鐵鉑產生非序化軟磁至序化硬磁鐵鉑合金相變化所致。
We have shown that the structural and magnetic properties of [Fe-Pt] thin films. The co-sputtered FePt films and [Fe/Pt]10 multilayers on SiOx substrates were prepared by a ultrahigh vacuum (UHV) magnetron sputtering system while the capping SiOx layer was prepared by using a dual ion-beam deposition technique (with mixture of O2/Ar gas that was varied from 8% to 41% O2/Ar). Samples were annealed at 550 °C for 10 mins in a UHV chamber. Then, we selected that the highest ordering parameter of specimen (30% O2/Ar) and we changed different anneal temperature (300°C ~700°C), fixed the same anneal time (10 mins),and researched the structural and magnetic properties of FePt thin films. The structures of as-deposited [Fe-Pt]-SiOx and [Fe/Pt]10/SiOx (0 to 41% O2/Ar) consisted of fcc FePt (a~ 3.81 Å) phases that resulted from intermixing of Fe and Pt during deposition. The grain sizes ranged from 2.5 to 16 nm. The coercivity of the as-deposited [Fe-Pt]-SiOx about Hc⊥~ 42 to 783 Oe, and Hc//~ 0 to 35 Oe. The coercivity of the as-deposited [Fe/Pt]10 /SiOx about Hc⊥~ 210 to 744 Oe, and Hc//~ 0 to 36 Oe. That are mainly from the magnetically soft Fe phase. The [Fe-Pt]-SiOx and [Fe/Pt]10/SiOx (annealed at 550 °C for 10 mins) exhibit ordered L10 FePt phases (a~ 3.83 Å, c~ 3.70 Å). The grain sizes ranged from 20 to 80 nm. We discovered the role of the top SiOx layer during post-annealing is to form grain boundaries and to separate the FePt grains in the [Fe-Pt]-SiOx system, but [Fe/Pt]10/SiOx still is part of continuous film. Because the structure of multilayer is more inseparable, and the capping SiOx layer is not easy to diffuse into FePt layer. The maximum coercivity [Fe-Pt]-SiOx was Hc⊥~ 13.6 kOe and H//~ 12.9 kOe (8% O2/Ar). The maximum coercivity [Fe/Pt]10/SiOx was Hc⊥~ 11.7 kOe and H//~ 13 kOe (0% O2/Ar). That is attributed to the formation of a hard L10 FePt phase from the annealing. We have shown that the magnetometry evidence of [Fe-Pt]-SiOx and indicated the negative δM value at all applied fields (dipole interaction). Comparative, the magnetometry evidence of [Fe/Pt]10/SiOx was positive δM value at all applied fields (strong ferromagnetic interaction). The magnetic domain structures of annealed [Fe-Pt]-SiOx that typical interconnected domain patterns were observed. In addition, the clear magnetic contrast is likely attributed to the strong stray field due to isolated magnetic grains. In the part of different anneal temperature, when [Fe-Pt]-SiOx and [Fe/Pt]10/SiOx (30% O2/Ar) at 300°C and have the same structure with as-deposited. The film did not order. Phase transformation from fcc to fct at temperature great than 400°C. Significant grain growth(~40 to 100 nm) with increasing annealing temperature as well as structurechanges to fct FePt. At 300°C, the film have small Hc⊥ due to soft magnetic properties (disordered FePt). Hc increases linearly with Tann> 400 °C due to ordered FePt formation (the maximum coercivity was Hc~ 16 kOe).
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