釤釔鐵石榴石薄膜的磁異向性研究

Magnetic anisotropy of samarium yttrium iron garnet thin films

張鄴壬(Chang Ye-Ren)

指導教授 ： 駱芳鈺

國立臺灣師範大學/理學院/物理學系/碩士(2022年)

https://doi.org/10.6345/NTNU202201647

摘要

本論文是探討使用脈衝雷射蒸鍍法來製備生長於 YAG(111)基板上摻雜釤的釔鐵石榴石薄膜(x = 0 ~ 3.0)之磁異向性研究。從中可得知薄膜的晶體結構、表面形貌以及磁光、磁性性質。釤釔鐵石榴石的薄膜厚度是 30 至 120 nm，鍍膜溫度為 750 ℃、鍍膜氧氣壓力為3×10-1 mbar，雷射轟擊靶材的能量為 3.5 J/cm2，之後在大氣環境中使用爐管升至1050 ℃來進行 4 小時的鍍膜後熱退火。
使用 X 光繞射來得知薄膜的晶體結構，從各個樣品皆有觀察到 YAG 基板(111)以及釤釔鐵石榴石薄膜的(444)繞射峰。隨著摻雜釤的比例以及薄膜厚度增加，晶格常數都會變大，晶粒大小則變化不明顯，而釔鐵石榴石與釤鐵石榴石的晶格所受的垂直應變在 x = 3.0 時從拉伸應變轉為壓縮應變。從原子力顯微術測得之表面形貌能發現到，在未摻雜釤時，表面粗糙度隨薄膜厚度的變化不明顯，但在相同厚度時，表面粗糙度則隨著摻雜釤的比例增加而遞增。而在表面顆粒方面，隨著厚度增加，顆粒也會增大。
磁光法拉第效應在波長 300 ~ 500 nm 間法拉第旋轉角會隨著磁場而變化，在波長360 和 445 nm 量得明確的磁滯曲線，代表此研究的樣品有垂直面的磁異向性。隨著摻雜釤的比例與薄膜的厚度增加，磁滯曲線訊雜比會提高，法拉第旋轉角會增加，磁光磁滯曲線方正度則會隨著厚度增長而減少。振動樣品磁量儀量測平行磁場(IP)和垂直磁場(OP)的薄膜磁性，其磁滯曲線方正度用於判斷磁易軸方向。釤釔鐵石榴石薄膜的磁異軸方向則隨著比例和厚度轉變，隨著摻雜釤的比例增加，其磁異軸由 OP 轉為 IP 的臨界厚度會隨著增加。釤釔鐵石榴石薄膜的磁異向性變化為應變產生的磁異向能所造成，且可經由薄膜厚度以及摻雜釤元素的比例的變化來誘發應變。

關鍵字

脈衝雷射蒸鍍法 ； 釤鐵石榴石 ； 釔鐵石榴石 ； 垂直磁異向性

並列摘要

In this study, we investigated magnetic anisotropy of Sm-substituted YIG (Y3-xSmxFe5O12,
x = 0 ~ 3.0) thin films on YAG(111) substrates grown by pulsed laser deposition. We studied
the structural, morphological, magneto-optical and magnetic properties of these thin films,
whose thickness ranged from 30 nm to 120 nm. The thin films were grown at a heater
temperature of 750 ℃under the oxygen pressure of 3×10-1 mbar with the laser energy fluence of 3.5 J/cm2. The thin films were subjected to post-growth thermal annealing at 1050 ℃ for 4 hours in ambient atmosphere in a tube furnace.
The structural properties was investigated by x-ray diffraction, where (444) characteristic
peaks were observed for all films and YAG substrates. The lattice constants increases as Sm
content and film thickness increase, and the crystallite size shows no change. Due to doping of Sm, the strain of film on the substrate shifts from tensile strain to the compressive strain at x =3.0. For the morphology recorded by atomic force microscopy, surface roughness will be larger as Sm content increases, while the surface grain size increases with increasing thin film
thickness.
The magneto-optical Faraday effect (MOFE) are observed in wavelengths between 300
nm and 500 nm. MOFE loops at wavelengths of 360 nm and 445 nm were measured, and clear hysteresis loops were observed, indicating perpendicular magnetic anisotropy (PMA). The signal to noise ratio and the saturation Faraday rotation angle increase as the Sm content and thin film thickness increase, while magneto-optical hysteresis loop squareness decreases as film thickness increases. Magnetic properties of thin films in parallel (IP) and perpendicular (OP) magnetic fields was measured by vibrating sample magnetometry, and hysteresis loops were recorded for all samples. The squareness of hysteresis loop was used to determine the direction of magnetic easy axis of each thin film, which serves to determine the magnetic anisotropy. As Sm content increases, PMA becomes stronger, and the critical thickness of magnetic anisotropy changing from OP to IP increases. The change of magnetic anisotropy in films were resulted from magnetic anisotropic energy induced by strain, and the strain can be triggered by variation of film thickness and Sm content.

並列關鍵字

pulsed laser deposition ； samarium iron garnet ； yttrium iron garnet ； perpendicular magnetic anisotropy

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