稀土鐵石榴石與鈣鈦礦奈米材料之結構、磁性和應用

Abstract

鈣鈦礦和稀土石榴(REIG)薄膜具有優異的光學和磁光特性。因此，將這兩種材料結合在一起可以創造出具有可調控光學和磁性性能的異質結構，並應用於光學通信、光學記憶和磁光元件等領域。在本研究中，我們將深入探討鈣鈦礦和REIG薄膜各自的潛在價值。近年來，一些研究表明使用稀土元素(RE)元素代替釔(Y)來調節石榴石薄膜的應變誘導磁異向性。REIG薄膜(~100 nm)藉由脈衝雷射沉積法製備於(111)取向的釔鋁石榴石(YAG)基板上。釤、钬和釔鐵石榴石(SmIG, HoIG, and YIG)具有垂直於膜面的壓縮應變，而鉺和铥鐵石榴石(ErIG and TmIG)具有弱的拉伸應變。由於負磁致伸縮常數，因此SmIG和HoIG薄膜表現出相對強的垂直磁異向性(PMA)。隨著技術的發展，對高存儲容量和快訪問速度的需求不斷增加。因此我們選擇對擁有相對強PMA的SmIG薄膜進一步研究。藉由降低SmIG薄膜厚度，可使其具有更強的壓縮應變，進而獲得更強的PMA。相比之下，YIG在30-120奈米區間仍展現水平磁異向性(IMA)。這一發現表明磁性能受Y:Sm比的顯著影響。隨後，我們製備了一系列不同厚度、Sm摻雜濃度的SmYIG薄膜。振動樣品磁力計揭露隨著厚度的遞減和Sm摻雜濃度的增加，可使SmYIG薄膜具有較強的PMA。此外，我們展示了在不同Sm摻雜濃度下，SmYIG薄膜的臨界厚度。為基於REIG薄膜的高密度磁信息存儲鋪平道路。YIG與反鐵磁材料的結合因其在自旋泵等應用中的潛力而備受關注。因此，我們於YIG薄膜上沉積氧化鈷(CoOx)薄膜以研究介面效應。由於CoOx薄膜於高溫缺氧環境下製備，所以其表面區域由純CoO組成，界面區域則為CoO和Co的混合物。CoOx/YIG薄膜不僅表現出低溫下由CoO提供的磁耦合，還表現出由鐵磁Co提供室溫負交換偏置(RT-NEB)。與CoOx/YIG薄膜相比，我們於YIG薄膜上製造了進一步氧化的CoO薄膜，並觀察到室溫正交換偏置(RT-PEB)。RT-PEB隨著外加磁化場增加而增加，並在外加磁化場為500 Oe時飽和。隨著溫度降低，PEB 逐漸轉變為 NEB。這些結果清楚地表明 CoO/YIG 雙層系統中PEB和NEB共存，而PEB歸因於CoO界面自旋的反平行耦合，而NEB歸因於AFM-FM耦合。有機-無機鈣鈦礦(MAPbBr3)/鐵磁異質結構在光控自旋電子元件中已被廣泛探討。然而使用金屬鐵磁層作為底部電極仍然是一個挑戰。因此，我們提出插入氧化鋁(AlOx)或石墨烯(Gr)層的超薄異質界面來改善均勻性。通過原子力顯微鏡和掃描電子顯微鏡，我們觀察到MAPbBr3層成功地形成了緻密的連續薄膜。此外，AlO¬x或Gr層的存在可以有效地防止鈣鈦礦和鐵磁金屬薄膜之間的氧化和界面擴散。然而，MAPbBr3層在環境下很容易受溫度、濕度、氧氣濃度影響而分解。因此，我們製備了全無機銫鉛溴化物鈣鈦礦量子點(CsPbBr3 QDs)來替代鐵磁層上方的 MAPbBr3，並研究了藍光雷射對磁性的影響。隨著雷射照射時間的增加，CsPbBr3 QDs的表面形貌和特徵尺寸發生了顯著變化並逐漸演變，引發了一系列氧化還原和界面擴散過程，特別是在 CsPbBr3 QDs/Co異質結構的界面處。這些結果開啟了鈣鈦礦/鐵磁異質結構在自旋電子學應用研究。

Perovskite and rare earth iron garnet (REIG) thin films have excellent optical and magneto-optical properties. Therefore, combining these two materials can result in heterostructures with tunable optical and magnetic properties, which can be used in the fields of magneto-optical components, optical communication, and optical memory. In this study, we will deeply explore the respective potential values of perovskite and REIG thin films.In recent years, several studies have shown that the strain-induced magnetic anisotropy of garnet films could be modulated by substituting RE elements for yttrium (Y). The REIG films (~100 nm) are prepared by pulsed laser deposition on (111)-oriented yttrium aluminum garnet (YAG) substrates. Samarium, holmium, and yttrium iron garnets (SmIG, HoIG, and YIG) show out-of-plane compressive strain, while erbium and thulium iron garnets (ErIG and TmIG) show weak out-of-plane tensile strains. Due to the negative magnetostriction constant, SmIG and HoIG films exhibit rather significant perpendicular magnetic anisotropy (PMA). High storage capacities and quick access times are becoming increasingly important as technology advances. Therefore, we selected to investigate SmIG films with rather strong PMA. By reducing the thickness of the SmIG film, it can have a stronger compressive strain and thus a stronger PMA. In contrast, the YIG film remains to display in-plane magnetic anisotropy (IMA) in the 30-120 nm thickness range. This finding shows that the magnetic properties are significantly influenced by the Y:Sm content ratio. Subsequently, we prepared a series of SmYIG films with different thicknesses and Sm-doped contents. The vibrating sample magnetometer reveals that the SmYIG films exhibit stronger PMA with decreasing thickness and increasing Sm-doped content. Furthermore, we demonstrate the critical thickness of SmYIG films with various Sm-doped content. These findings pave the way for high-density magnetic information storage based on REIG thin films.The combination of YIG with antiferromagnetic materials has attracted much attention due to its potential in applications such as spin pumps. Therefore, we deposited cobalt oxide (CoOx) films on YIG films to investigate the interfacial effect. Since the CoOx thin film is fabricated in a high-temperature and oxygen-deficient environment, its surface region is composed of pure CoO, and the interface region is a mixture of CoO and Co. The CoOx/YIG films not only exhibit the magnetic coupling provided by CoO at low temperatures but also exhibit the room temperature negative exchange bias (RT-NEB) provided by ferromagnetic Co. Compared with CoOx/YIG films, we fabricated further oxidized CoO films on YIG films and discovered room temperature positive exchange bias (RT-PEB). The PEB increases with increasing applied magnetic field (HAF) and saturates at HAF = 500 Oe. The PEB gradually turns into NEB as the temperature decreases. These results clearly show that PEB and NEB can coexist in the CoO/YIG bilayer system; PEB is explained by the antiparallel coupling of CoO interfacial spins, whilst NEB is explained by the AFM-FM coupling.Organic-inorganic perovskite (MAPbBr3)/ferromagnetic heterostructures have been extensively explored in light-controlled spintronic devices. However, it is still a challenge to apply a metallic ferromagnetic layer as the bottom electrode. We therefore propose using ultrathin aluminum oxide (AlOx) or graphene (Gr) films as heterointerfaces to improve uniformity. Through atomic force microscopy and scanning electron microscopy, we discovered that the MAPbBr3 layer is successful in forming a dense and continuous film. In addition, the presence of AlOx or Gr layers can effectively prevent the oxidation and interfacial diffusion between MAPbBr3 and ferromagnetic metal films. However, the MAPbBr3 layer is easily decomposed by changes in temperature, humidity, and oxygen concentration in the environment. Hence, we fabricated all-inorganic cesium lead bromide quantum dots (CsPbBr3 QDs) to replace MAPbBr3 on the ferromagnetic layer and investigated the blue laser effect on magnetism. The surface morphology and feature size of CsPbBr3 QDs influenced dramatically and gradually evolved with an increase in laser irradiation time. Thus, led to numerous of redox and interfacial diffusion processes, particularly at the interface of CsPbBr3 QDs/Co heterostructure. These results pave the way for the application of perovskite/ferromagnetic heterostructures in spintronics.