FePd及FePd/IrMn薄膜晶體結構與磁性質之研究

Crystallographic and magnetic properties of FePd and FePd/IrMn films

張永昌(Chang, Yung Chang)

指導教授 ： 杜正恭

國立清華大學/工學院/材料科學工程學系/博士(2016年)

摘要

由於材料本身高的磁晶異向能與應用於MRAM時低的臨界電流，序化L10相的鐵鈀薄膜一直被視為重要的高密度磁紀錄媒體與自旋電子元件應用的選擇替代性材料。本論文採用鐵鈀/銥錳雙層膜材料系統研究此系統基礎的交換耦合效應，以期將來能將此材料系統應用於硬碟讀取頭與MRAM等領域。論文分為三個部分進行探討，包括於玻璃基板上成長鐵鈀薄膜、於氧化鎂單晶基板上成長鐵鈀薄膜、鐵鈀/銥錳雙層膜系統。首先，鐵鈀薄膜成長於玻璃基板上，當鈀含量(51 at.%)的多晶鐵鈀薄膜成長溫度由400oC上升到800oC時，其磁性質由軟磁轉變為硬磁，而序化度由0增加到接近1。在800oC成長的鐵鈀薄膜具有兩相共存(L10+A1)的結構與島狀的表面形貌，其矯頑磁力高達4.1kOe，飽和磁化量為814emu/cm3。就薄膜成分比例來說，鐵鈀薄膜鈀含量(<47 at.%)表現出非序化相結構與軟磁特性；薄膜鈀含量(56 at.%)的試片具單一L10相與硬磁特性；薄膜鈀含量(62 at.%)結晶結構由L10相轉變為L12相，伴隨著軟磁特性。為了得到良好垂直磁異向性，將鐵鈀薄膜成長於氧化鎂基板上，隨著成長溫度從300oC到550oC的變化，薄膜鐵含量(54 at.%)的磁性質從水平異向性轉變到垂直異向性，其序化度也由0.33增加到0.84。在薄膜成分比例來說，薄膜鐵含量介於46~54 at.%均表現出垂直磁異向性。而本質矯頑磁場與鐵鈀薄膜的L10序化程度呈現強烈關聯性，在薄膜鐵含量(54 at.%)薄膜中可得到最高序化度0.84對應到最高的本質矯頑磁場20kOe。最後，奠基於前面兩部分的研究基礎，將鐵鈀/銥錳雙層膜成長於氧化鎂基板上，在成長單層銥錳薄膜於基板時，當沉積溫度從400oC依序上升到700oC，L12相銥錳薄膜的序化度由0.17增加到0.81，但表面粗糙度也逐步上升。鐵鈀/銥錳系統的交換耦合場在底層銥錳薄膜為非序化相時為22Oe，在銥錳層漸序化成L12相的過程中遞減為0。具補償界面的銥錳層表面粗糙度上升並沒有增加鐵鈀薄膜的交換耦合場，反鐵磁性結構銥錳層的相變化(由A1到L12)才是鐵鈀交換耦合場變化的原因。

關鍵字

鐵鈀 ； 銥錳 ； 序化相轉變 ； 交換耦合

並列摘要

Due to its large magnetocrystalline anisotropy energy and low critical current density in MRAM, L10 ordered FePd thin films with perpendicular magnetic anisotropy have been probed as candidates in applications of dense magnetic recording media, spintronics devices, etc. FePd/IrMn bilayer system was used to investigate its fundamental exchange coupling effect and applications of hard disk and MRAM. First of all, FePd films were deposited on glass substrates to find out the influences of in-situ annealing temperatures and film stoichiometry on L10 phase transformation and magnetic properties of FePd thin films. When annealing temperature was raised from 400 to 800oC, the magnetic behavior changed from magnetically soft to hard for the polycrystalline FePd films with Pd contents of 51 at.%, while the ordering parameter S increased from 0 to near unity. The 800oC-deposited FePd film has two-phase (L10 + A1) structure and island-like morphology, which exhibited a coercivity of 4.1 kOe and saturated magnetization of 814 emu/cm3. In terms of the stoichiometric effect, the results showed disordered structure and soft magnetic properties for the FePd films with Pd concentration ≦ 47 at.%. For the samples with Pd content of 56 at.%, the single L10 phase with magnetically hard properties was obtained. With increasing Pd contents up to 62 at.% , the crystal structure changed from L10 to L12 phase, as accompanied with magnetic softening. Second, FePd films were grown on MgO(001) substrates to expect better perpendicular magnetic anisotropy. With substrate temperature increasing from 300 to 550oC, the magnetic behavior altered from longitudinal to perpendicular anisotropy for epitaxial (001) FePd films with 54 at. % Fe, whereas ordering parameter S increased from 0.33 to 0.84. With regard to the stoichiometric effect, XRD tests with a synchrotron radiation source showed L10 epitaxial structures of the FePd films with Fe concentrations over a wide range 40–54 at. %, unlike the equilibrium phase diagram of FePd bulk. Magnetic results revealed the presence of a perpendicular anisotropy for epitaxial FePd films with Fe contents from 46 to 54 at.%. The measured Hk was correlated with the L10 ordering of the FePd films. The greatest S, 0.84, and Hk, 20 kOe, were obtained for a film with 54 at. % Fe. The results of this work indicated that a Fe-rich FePd film might be preferable for applications that require a large magnetic anisotropy. Finally, two series of samples of single-layer IrMn and IrMn/FePd films were grown on a singlecrystal MgO substrate at different IrMn deposition temperatures (Ts=300–700 oC). XRD characterization using a synchrotron radiation source revealed that the L12 ordered IrMn phases were obtained in the epitaxial (001) IrMn films with Ts≧400oC. As Ts was raised from 400 to 700oC, the ordering parameter of L12 phase increased from 0.17 to 0.81 as surface roughness increased. The exchange bias behavior of the IrMn/FePd bilayer films with an Heb value of 22 Oe was obtained when the FePd layer was deposited on the disordered IrMn layer. Increasing the L12 ordering of IrMn layers gradually reduced the Heb values to 0 Oe. The increased surface roughness of the IrMn layers with compensated surface spins did not enhance the Heb of the FePd. The change of the antiferromagnetic structure of IrMn layers from the A1 phase to the L12 phase was responsible for the evolution of Heb of FePd.

並列關鍵字

FePd ； IrMn ； ordering transformation ； exchange coupling

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